ORIGINAL RESEARCH published: 14 August 2019 doi: 10.3389/fpls.2019.01026 Distinct Morphological, Physiological, and Biochemical Responses to Light Quality in Barley Leaves and Roots Karel Klem 1*, Albert Gargallo-Garriga 1,2, Wutthida Rattanapichai 3, Michal Oravec 1, Petr Holub 1, Barbora Veselá 1, Jordi Sardans 1,2,4, Josep Peñuelas 1,2,4 and Otmar Urban 1 1 Global Change Research Institute, Czech Academy of Sciences, Brno, Czechia, 2 Centro de Investigación Ecológica y Aplicaciones Forestales (CREAF), Barcelona, Spain, 3 Department of Soil Science, Kasetsart University, Bangkok, Thailand, 4 Global Ecology Unit CREAF-CSIC-UAB, Barcelona, Spain Light quality modulates plant growth, development, physiology, and metabolism through a series of photoreceptors perceiving light signal and related signaling pathways. Although the partial mechanisms of the responses to light quality are well understood, how plants orchestrate these impacts on the levels of above- and below-ground tissues and molecular, physiological, and morphological processes remains unclear. However, the re-allocation Edited by: of plant resources can substantially adjust plant tolerance to stress conditions such as Tibor Janda, Centre for Agricultural Research reduced water availability. In this study, we investigated in two spring barley genotypes the (MTA), Hungary effect of ultraviolet-A (UV-A), blue, red, and far-red light on morphological, physiological, and Reviewed by: metabolic responses in leaves and roots. The plants were grown in growth units where the Péter Poór, University of Szeged, Hungary root system develops on black filter paper, placed in growth chambers. While the growth Muhammad Abass Ahanger, of above-ground biomass and photosynthetic performance were enhanced mainly by Northwest A&F University, China the combined action of red, blue, far-red, and UV-A light, the root growth was stimulated *Correspondence: particularly by supplementary far-red light to red light. Exposure of plants to the full light Karel Klem klem.k@czechglobe.cz spectrum also stimulates the accumulation of numerous compounds related to stress tolerance such as proline, secondary metabolites with antioxidative functions or jasmonic Specialty section: acid. On the other hand, full light spectrum reduces the accumulation of abscisic acid, which This article was submitted to Plant Abiotic Stress, is closely associated with stress responses. Addition of blue light induced accumulation of a section of the journal γ-aminobutyric acid (GABA), sorgolactone, or several secondary metabolites. Because these Frontiers in Plant Science compounds play important roles as osmolytes, antioxidants, UV screening compounds, or Received: 06 May 2019 growth regulators, the importance of light quality in stress tolerance is unequivocal. Accepted: 23 July 2019 Published: 14 August 2019 Keywords: light quality, photosynthesis, morphology, root system architecture, metabolomics Citation: Klem K, Gargallo-Garriga A, Rattanapichai W, Oravec M, Holub P, INTRODUCTION Veselá B, Sardans J, Peñuelas J and Urban O (2019) Distinct Plants growing under natural conditions are exposed to light that varies in intensity, duration, and Morphological, Physiological, and Biochemical Responses to Light spectral quality due to changes in canopy filtering, cloud cover, and diurnal and seasonal variations. Quality in Barley Leaves and Roots. In addition, it can be challenging to determine the optimal light spectrum and intensity when Front. Plant Sci. 10:1026. cultivating plants under artificial conditions, such as in greenhouses or growth chambers (Ouzounis doi: 10.3389/fpls.2019.01026 et al., 2015). Light-mediated photosynthesis is the predominant source of energy for plants, but Frontiers in Plant Science | www.frontiersin.org 1 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley light also provides important sensory signals for plants because it Besides the effects of light on primary metabolism, its can promote acclimation to different environmental conditions, spectral quality also influences the biosynthesis of secondary modulate growth and development, alter physiological functions, metabolites (particularly carotenoids, phenolic compounds, and regulate biochemical pathways (Kami et al., 2010). and anthocyanins) that function as antioxidants or UV Research during recent decades has identified three screening compounds (Bian et al., 2015). UV and blue light major groups of sensory photoreceptors in plants (Chen (perceived by UVR8 and CRYs) affect the biosynthesis of et al., 2004): the phytochromes (PHYs), which absorb numerous secondary metabolites including phenolic acids light strongly in the red (600 to 700 nm) and far-red (700 (e.g., chlorogenic acid, p-coumaric acid, caffeic acid), flavonoids to 800 nm) regions (Casal et al., 1998); the cryptochromes (e.g., kaempferol, quercetin, rutin, anthocyanin), and carotenoids (CRYs), phototropins (PHOTs), and Zeitlupe family (lutein, violaxanthin, zeaxanthin, neoxanthin, β-carotene; reviewed proteins, which absorb light strongly in the blue (400 to by Ouzounis et al., 2015). These compounds, in turn, enhance the 500 nm) and ultraviolet-A (UV-A; 315 to 400 nm) regions photoprotective capacity of plants against high radiation stress (Galvão and Fankhauser, 2015); and UVB-RESISTANCE (Klem et al., 2015). Although the transition between regulatory locus 8 (UVR8), which absorbs light strongly in the UV-B effect and stress response to light intensity and quality is gradual (280 to 315 nm) region (Tilbrook et al., 2013). In contrast (Robson et al., 2015), particularly at high intensities of UV-B to the many photobiology studies on above-ground plant radiation, the stress responses involving DNA damage, lipid organs, very few studies have investigated the role of light peroxidation, photoinhibition, and/or growth inhibition often spectral quality on roots (Gundel et al., 2014). However, occur in plants (Hideg et al., 2013). there is evidence that the red:far-red (R:FR) ratio mediates Based on a review of the literature, Gundel et al. (2014) plant–plant interactions, in that it allows a plant to detect suggested that light spectral quality considerably affects neighboring plants, and promotes rapid root growth and root growth, morphology, and functional interactions with escape from the main zone of competition (Ballaré, 2009). soil microorganisms. Light spectral quality can thus play an Light spectral quality is also an important signal that allows important ecological role by altering root–shoot interactions. plants to acclimate to changing environmental conditions, For instance, Sadras et al. (1989) showed that the roots of crops particularly abiotic stressors (Catalá et al., 2011). Light- grown at high density penetrate deeper soil layers than crops induced metabolic reprogramming can lead to accumulation grown at low density because of differences in the above-ground of anti-stress compounds, such as osmolytes, antioxidants, R:FR ratio. Light spectral quality can therefore greatly impact the and stress-responsive proteins (Obata and Fernie, 2012). efficiency of the uptake of resources (water and nutrients) from Plant growth largely depends on the availability of the soil and affect the way the plants adjust to competition for carbohydrates synthesized during photosynthesis, and this is soil resources. However, a meta-analysis by Poorter et al. (2012) limited by numerous environmental factors in addition to light, found that the response of roots to the R:FR ratio depended on such as temperature, water availability, nutrient availability, plant ontogeny (Green-Tracewicz et al., 2012), intensity of light and pathogens (Kangasjärvi et al., 2012). The sugar-mediated (McLaren and Smith, 1978), proximity of other plants (Murphy regulation of plant growth interacts with multiple phytohormones and Dudley, 2009), and nutrient availability (Aphalo and Lehto, (Das et al., 2012) and the circadian clock (Lastdrager et al., 2014), 2001). Gundel et al. (2014) proposed that the R:FR ratio affects and both are connected to light signaling (de Lucas and Prat, root system architecture (RSA) by altering the levels of the 2014). Thus, it is likely that crosstalk between sugar production plant hormone auxin. Auxin is a well-known regulator of root and light signaling also regulates plant growth. Monochromatic development (Blilou et al., 2005) and architecture (Péret et al., red light stimulates the accumulation of glucose and fructose, 2009) and can also induce directional root growth away from while combined red and blue light significantly enhances potential competitors. the accumulation of sucrose and starch (Li et al., 2017). Such The blue and UV light photoreceptors (CRYs, PHOTs, changes are mainly associated with activities of invertases and and UVR8) also alter signaling pathways connected to the explain also the effects on photosynthetic performance and plant phytohormone-mediated regulation of growth, development, morphology. Sugar, and particularly sucrose allocation, plays a physiological functions, and biochemical pathways, so it crucial role in long-distance communications between shoot seems likely that they also affect RSA (Gelderen van et al., and root and coupling of carbon and nitrogen metabolisms 2018a). Photoreceptors have important roles in plant resource involving cytokinin, auxin, and small peptide signals (Wang and management because they control the allocation of resources Ruan, 2016). Since the phosphorylated hexoses are required in needed for growth and the transition of metabolism between different metabolic processes and plant development, hexose- growth and stress-coping states based on light availability. Thus, phosphorylating enzymes (mainly hexokinase) are essential crop scientists have a renewed interest in photoreceptors because in sugar sensing and orchestration of photosynthetic carbon of their effect on root–shoot interactions and the possible fixation and water and nutrient uptake (Granot et al., 2014). Light implications of these responses to economically important crops. intensity and daytime have a major effect on sugar transport in Besides the direct effect of light intensity and quality on the phloem, but it was significantly affected also by light quality balancing plant growth and defense mechanisms, the circadian at the medium photosynthetic rates (Lanoue et al., 2018). Sugar- clock and redox rhythm serve also as an important regulatory mediated regulation of plant growth and photosynthesis can be mechanism. Such mechanisms involve especially biosynthesis of thus attributed to the effects of both light intensity and quality. the major antioxidant (glutathione) and plant defense hormones Frontiers in Plant Science | www.frontiersin.org 2 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley [jasmonic acid (JA) and salicylic acid], thus adjusting plant of 25/20°C and a day/night relative humidity of 60%/80%. Photon immunity (Karapetyan and Dong, 2018). flux density in the range of 400–700 nm (PAR) was measured The main aim of this study was to test the hypothesis that using light meter Li-250 (LI-COR Biosciences, Lincoln, NE, US) light spectral quality changes the allocation of plant biomass with quantum sensor Li-190 (LI-COR Biosciences). The emission between above-ground parts and roots, alters the RSA, affects spectra of LEDs and fluorescent lamps were measured using interactions among neighboring plants, and impacts the ability spectroradiometer AvaSpec-2048-USB2 (Avantes BV, Apeldoorn, of plants to cope with reduced availability of water or nutrients. NL), with grating and a measurement range of 200–1100 nm. The effect of spectral quality was tested on two barley varieties. R:FR ratio was calculated by integration photon flux density Barke variety is considered as highly sensitive to oxidative stress, between 655–665 nm UV/VIS/NIR (R) and 725–735 nm (FR). while the variety Bojos is commonly grown in Central European conditions without symptoms of oxidative stress damage. We also Root and Shoot Morphology hypothesized that light spectral quality affects the accumulation Roots and shoots were scanned with a large area scanner (LA2400, and allocation of metabolites involved in stress tolerance, such Regent Instruments, Quebec, CA) at a resolution of 300 pixels as plant hormones, antioxidants, and osmolytes, and thereby per inch. Total (projected) leaf area (TLA) and leaf length were modulates tolerances to different stressors. then measured using ImageJ software. For subsequent statistical analyses, TLA and length of the youngest completely developed leaf (4th leaf) were determined because these were most sensitive MATERIAL AND METHODS to light regimens. The RSA parameters were evaluated using WinRHIZO Plant Material and Experimental Design software (Regent Instruments). The parameters measured were: Seeds of spring barley (Hordeum vulgare) varieties Barke total root length (TRL), total seminal root length (TSL), total (sensitive to oxidative stress; Klem et al., 2012) and Bojos (tolerant lateral root length (TLL), total root surface area (TSA), and to oxidative stress) were provided by the barley gene bank of the average branching angle (BA). Plant material was subsequently Agricultural Research Institute Kroměříž Ltd., Czech Republic. used for metabolomic analyses. Seeds were germinated on moistened germination paper for 48 h at 26°C in the dark. Germinating seeds were then transplanted into the growth unit, in which the roots grew on black filter paper Plant Physiology that was between two black plastic sheets, and a micro-irrigation All physiological measurements were conducted between 10:00 system recirculated a nutrient solution through the roots (Figure and 14:00 Central European Time (CET). Basic photosynthetic S1; for details, see Rattanapichai and Klem, 2016). The nutrient parameters were measured on the 4th leaf using a Li-6400 gas- solution was Hoagland’s No. 2 basal salt mixture (Sigma Aldrich exchange system (LI-COR Biosciences) at a CO2 concentration of Chemie, Steinheim, DE; concentration: 1.6 g L−1, pH range: 4.5 400 μmol CO2 mol−1. During these measurements, leaf temperature to 5.2). The solution was replaced every 7 days throughout the (25°C) and humidity (60%) were kept constant in the growth 26-day experiments. chamber. Light-saturated rates of the following photosynthetic The growth units were placed in five large growth chambers (FS- parameters were measured at a photosynthetic photon flux density SI-3400, PSI, Drásov, CZ), which allowed continuous regulation of of 1200 μmol photons m−2 s−1 after 3 to 5 min of acclimation: light- light intensity in the red, blue, far-red, and UV-A spectral regions. saturated CO2 assimilation rate, Amax; light-saturated stomatal Light-emitting diodes (LEDs) were the source for radiation in conductance, GSmax; and light-saturated transpiration rate, Tr. the red (R; maximum emission: 640 nm), blue (B; maximum Chlorophyll a fluorescence (Chl-F) parameters were emission: 450 nm), and far-red (FR; maximum emission: 740 nm) estimated on the light-adapted 4th leaves using a pulse amplitude regions, and fluorescent lamps (LT 30W T8/010UV, Narva, DE) modulated fluorometer (PAM 2500, H. Walz, Effeltrich, DE). were the source for UV-A radiation (maximum emission: 370 Before these measurements, the leaves were acclimated to a nm). The following five light regimens were used: R, R light only; photosynthetic photon flux density of 130 μmol photons m−2 s−1 R-B, R and B lights at 1:1 ratio of photon flux density; R-FR, R and for 3 min, and the same photon flux density of actinic light was FR lights; R-UVA, R and UV-A lights; and R-B-FR-UVA, R and used for the measurement. The Chl-F signal at the red band B radiation in a 1:1 ratio of photon flux density with additional (near 690 nm) was measured using short flashes (10 ms pulses, FR and UV-A light. The intensity of photosynthetically active intensity approximately 0.003 μmol photons m−2 s−1) that were radiation (PAR, 400 to 700 nm) was the same for all treatments (130 800 ms apart (FS). Maximum Chl-F (FM′) was recorded during μmol photons m−2 s−1). The intensity of UV-A was set to 4 W m−2, 1 s application of a saturating light pulse (approx. 5,000 μmol and the intensity of FR light was adjusted to provide a 0.6 ratio of photons m−2 s−1). Minimum Chl-F in the light-adapted state (F0′) R:FR. Each light regimen and barley variety had four replicates was estimated after a saturating pulse when the actinic light was (four separate growth units), which were randomized within each off, and the FR light was on for 5 s. The actual quantum yield growth chamber. The growth units and plants were transferred of photosystem (PS) II photochemistry [ΦPSII = (FM′ – FS)/FM′] between growth chambers while maintaining the same light and thermal energy dissipation [D = 1 – (FM′ – F0′)/FM′] were regimen, and newly randomized every 7 days to avoid possible calculated according to Demmig-Adams et al. (1996). artifacts from individual growth chambers. All plants were grown Chlorophyll and epidermal flavonols were estimated indirectly under a 12/12 h day/night regimen, with a day/night temperature as chlorophyll and flavonol indices, based on light transmittance Frontiers in Plant Science | www.frontiersin.org 3 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley and UV screening of Chl-F (Dualex Flav, Force A, Orsay, FR). Two Statistica 12 software (StatSoft, Tulsa, US). Tukey’s post hoc test (p = measurements in the central part of the 4th leaf were performed. 0.05) was used to identify significant differences between the means. The peak area corresponding to each metabolite was normalized based on the total peak area in the sample. Neutral masses Metabolomic Analyses obtained in positive and negative modes were evaluated to avoid The upper three leaves (2nd–4th leaf) and roots from each replicate duplicates (same RT and neutral mass in different modes) and to were sampled between 11:00 and 14:00 (CET) and immediately retain the greatest number and largest peaks. The differences of frozen in liquid nitrogen for metabolomic analyses. The samples were the metabolomes of each barley variety that received different light homogenized using a mortar and pestle with liquid nitrogen and regimens were identified by a permutational analysis of variance then extracted using methanol:chloroform:H2O solution (1:2:2). An (PERMANOVA) (Anderson et al., 2008) of the HPLC–HRMS aliquot of the upper (polar) phase was used to analyze metabolites data for each replication using Euclidean distances, in which light using an UltiMate 3000 high-performance liquid chromatograph regimen and variety were the fixed factors, and there were 2000 (HPLC) (Thermo Fisher Scientific, US/Dionex RSLC, Dionex, US) permutations. We also used principal component analysis (PCA) coupled with an LTQ Orbitrap XL high resolution mass spectrometer to determine the relationships of light regimen, barley variety, and (HRMS) (Orbitrap, Thermo Fisher Scientific, Waltham, MA, US) root and shot metabolites most responsible for these differences that was equipped with a heated electrospray ionization source. between different levels of experimental factors. The PCA was All samples were analyzed in the positive and negative polarity of performed using R v2.12 Core functions. Orbitrap, operated in full-scan mode over a range of m/z 50 to 1,000 (positive mode) and 65 to 1,000 (negative mode). A Hypersil Gold column (C18; 150 mm × 2.1 mm, 3 μm; Thermo Fisher Scientific, US) was used for chromatographic separation. The RESULTS flow rate of the mobile phase was 0.3 ml min−1, and the column temperature was 30°C. The mobile phase consisted of A) acetonitrile Leaf Area and Length and B) water with 0.1% acetic acid. Both mobile phases (A and B) The Barke variety generally had a greater TLA than the Bojos were filtered and degassed for 10 min in an ultrasonic bath before variety by 23%, but this difference was only significant for the use. Gradient elution chromatography was performed starting with R-UVA regimen (Figure 1A). The R-B, R-FR, and R-UVA regimens 10% A and 90% B for 5 min. From 5 to 20 min, the proportion of A led to greater TLA than the R regimen (by 29%, 40%, and 13%, increased steadily to 90% as B declined to 10%, and this condition respectively), but these differences were not statistically significant was maintained for 5 min. Then, the system was equilibrated to the for the R-B and R-UVA regimens. Relative to the R regimen, the initial condition (10% A and 90% B) over 5 min. The absorbances R-B-UVA-FR regimen significantly increased TLA in both varieties at 254, 272, 274, and 331 nm were monitored. Acetonitrile hyper- (by 54%), but the R-FR regimen significantly increased the TLA grade for liquid chromatography - mass spectrometry (LC-MS) only in the Bojos variety. Generally, the R-B-UVA-FR regimen led LiChrosolvR was from Merck KgaA (Darmstadt, DE), acetic acid to the greatest TLA. The length of the 4th leaf was similar for the was from Sigma Aldrich Chemie, and the Purelab Classic (ELGA different light regimens, and there were no differences between LabWater, High Wycombe, Bucks, UK) was used to generate high- varieties (Figure 1B). Relative to the R regimen, the R-FR and R-B- purity water for preparation of the aqueous mobile phase. FR-UVA regimens led to significantly greater length of the 4th leaf First, all metabolites were identified using a standard library in both varieties (by 25% and 30%, respectively). The length of the (>200 compounds) and confirmed by mass, retention time (RT), 4th leaf was also significantly greater under the R-B regimen than isotopic pattern, and ring double-bon+d parameters. Then, under the R regimen in the Bojos variety (by 26%). identifications were confirmed by comparing chromatogram peaks with the KEGG database (Kanehisa and Goto, 2000) using a built-in Root System Architecture M/Z MINE utility with a 5 ppm threshold. The third approach of Similar to the TLA results, TSA, TRL, and lengths of the seminal metabolite identification used the MASSBANK databases (Horai and lateral roots (TSL and TLL) were greater in the Barke variety et al., 2010), with searches for each mass spectra (MS) and tandem than the Bojos variety under most light regimens (by 42%, 32%, mass spectra (MS/MS). We tentatively assigned metabolites to 41%, and 26%, respectively; Figures 2A, B, C, D); however, these molecular ions and exact masses corresponding to metabolites in varieties only had statistically significant differences in TSA, TRL, these databases. Data, including detailed interpretations of the MS/ and TSL for the R-FR regimen. Relative to the R regimen, the R-FR MS spectra that supported our tentative identifications of masses, are regimen significantly increased TRL (by 217%), TSA (by 225%), referred to as not positively identified throughout the manuscript. TSL (by 206%), and TLL (by 222%) in both varieties, but this The raw data from the HPLC-HRMS system were processed and regimen significantly decreased the BA in the Barke variety (by compared using the XCMS processing platform for peak detection 21%) (Figure 2E). Also, relative to the R regimen, the R-UVA and and matching (Smith et al., 2006). R-B-UVA-FR regimens substantially increased TRL (by 65% and 59%, respectively), TSA (by 135% and 81%, respectively), and TSL (by 157% and 104%, respectively) in the Barke variety, although Statistical and Bioinformatic Analyses the effects were statistically insignificant in the Bojos variety. Morphological and physiological data were analyzed using a The TSA : TLA ratio was significantly greater under the R-FR two-way fixed-effect analysis of variance (ANOVA) model using (by 136%) and R-UVA (by 73%) regimens than the R regimen. Frontiers in Plant Science | www.frontiersin.org 4 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 1 | Effect of 26 days exposure to different light regimens on total leaf area per plant (A) and length of the last completely developed (4th) leaf (B) of spring barley varieties Barke (dark grey) and Bojos (light grey). Here and in subsequent figures: R, only red radiation; R-B, red and blue radiation (1:1 ratio of photon flux density); R-FR, red and far-red radiation; R-UVA, R and UV-A radiation; R-B-FR-UVA, red and blue radiation (1:1 ratio of photon flux density) with far-red and UV-A radiation. All treatments used a photosynthetic photon flux density of 130 µmol m−2 s−1. Column height indicates mean, vertical bars indicate standard deviations (n = 4), and different letters indicate significant differences (p ≤ 0.05) based on Tukey’s ANOVA post hoc test. The effect of the R-FR regimen on this ratio was only significantly Furthermore, relative to the R regimen, the R-UVA regimen led greater that of the R-UVA in the Barke variety (Figure 2F). to a greater chlorophyll index in the Barke variety (by 47%), and the R-B-UVA-FR regimen led to a greater chlorophyll index in the Bojos variety (by 28%). The light regimen had a stronger effect on Chlorophyll and Epidermal Flavonols the flavonol index (Figure 3B) in the Barke variety than the Bojos The chlorophyll index (Figure 3A) was greater under the variety. Relative to the R regimen, the flavonol index was only R-B regimen than the R regimen in both varieties (by 35%). greater in the Bojos variety under the R-B-UVA-FR regimen (by Frontiers in Plant Science | www.frontiersin.org 5 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 2 | Effect of 26 days exposure to different light regimens on total root length (A), total root surface area (B), total seminal root length (C), total lateral root length (D), average branching angle (E), and ratio of total root area:total leaf area (F) of spring barley varieties Barke (dark grey) and Bojos (light grey). Additional details are in the Figure 1 legend. 51%); the R-B-UVA-FR, R-UVA, and R-B regimens increased the non-photochemical energy dissipation were generally small and flavonol indexes in the Barke variety (by 110%, 58%, and 44%, statistically insignificant in both varieties (Figure 4B). respectively). Relative to the R regimen, the R-B-UVA-FR regimen The light-saturated rates of CO2 assimilation (Amax), led to the greatest increase of the flavonol index. transpiration (Trmax), and stomatal conductance (GSmax) were similar to the FPSII results (Figures 5A, B, C). In both varieties, Photosynthetic Parameters these parameters were significantly increased under the R-B (by The actual quantum yield of PSII photochemistry (FPSII) was 151%, 65%, and 118%, respectively), R-UVA (by 106%, 52%, and significantly greater in the Barke variety under the R-UVA (by 73%, respectively), and R-B-UVA-FR (by 157%, 68%, and 124%, 94%), R-B (by 145%), and R-B-UVA-FR (by 231%) regimens respectively) regimens relative to the R regimen. In addition, relative to the R regimen (Figure 4A). The results were similar the R-FR regimen significantly increased the Amax and GSmax in but less pronounced in the Bojos variety (by 39%, 44%, and 85%, the Barke variety (by 77% and 48%, respectively). Water use respectively); the only significant increase in this variety was under efficiency (ratio of Amax and Trmax) was significantly less under the the R-B-UVA-FR regimen. The effects of both light regimens on R regimen than under the other four regimens (the regimens with Frontiers in Plant Science | www.frontiersin.org 6 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 3 | Effect of 26 days exposure to different light regimens on chlorophyll index (A) and flavonol index (B) of spring barley varieties Barke (dark grey) and Bojos (light grey). Additional details are in the Figure 1 legend. the addition of B, FR, and UV-A increased water use efficiency by the plant organ (root vs. shoot) explained almost half of the 28–51%; Figure 5D). variance (pseudo-F = 49.03, p < 0.001; Table 1). The light regimen explained less of the total variance, but its effect was also significant (pseudo-F = 3.26, p < 0.001). Barley variety Metabolite Profiling had a significant but smaller effect on the metabolite profile Overall, we detected 1,785 metabolites and identified 73 of (pseudo-F = 2.19, p < 0.05). Analysis of all interactions indicated them. Among the identified metabolites, amino acids were the that the only significant interaction was between light regimen most abundant, followed by carbohydrates, organic acids, and and plant organ (pseudo-F = 2.57, p < 0.05). secondary metabolites. When the entire metabolomic data set A PCA analysis of metabolites in barley leaves also indicated was analyzed jointly for leaves and roots using PERMANOVA, a major effect of light regimen and small differences between Frontiers in Plant Science | www.frontiersin.org 7 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 4 | Effect of 26 days exposure to different light regimens on actual quantum yield of PSII photochemistry (ΦPSII, A) and non-photochemical energy dissipation (B) of spring barley varieties Barke (dark grey) and Bojos (light grey). Additional details are in the Figure 1 legend. varieties (Figure 6). PC1 mainly separated the effect of the greater accumulation of glucose, serine, proline, isoleucine, and R-B-UVA-FR regimen from other regimens, and PC2 showed phenylalanine in the Barke variety than the Bojos variety. the dominant effect of the R-B regimen. The R-B-UVA-FR Figure 7 shows selected metabolites in barley leaves that had regimen was associated with an accumulation of D-xylose large differences among the different light regimens. These results and D-fructose and some secondary metabolites (isovitexin show that there were high levels of proline and p-coumaric acid or kaempferol). The R, R-FR, and R-UVA regimens led to the under the R-B-UVA-FR (by 365% and 983% relative to the R accumulation of glycine-betaine, abscisic acid (ABA), mannitol, regimen) and R-B regimens (by 123% and 445% relative to the R and ferulic acid. On the other hand, the R-B regimen led to the regimen); however, the R-B regimen only had a significant effect accumulation of γ-aminobutyric acid (GABA), sorgolactone, on p-coumaric acid (Figures 7A, B). and several secondary metabolites (homoorientin, myricetin, The Barke variety was generally more sensitive to the R-B- aucubin, and acacetin). Analysis of barley variety indicated UVA-FR regimen than the Bojos variety, in terms of proline Frontiers in Plant Science | www.frontiersin.org 8 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 5 | Effect of 26 days exposure to different light regimens on light-saturated CO2 assimilation rate (Amax, A), light-saturated stomatal conductance (GSmax, B), transpiration rate (Tr, C), and water use efficiency (WUE, D) of spring barley varieties Barke (dark grey) and Bojos (light grey). Additional details are in the Figure 1 legend. TABLE 1 | PERMANOVA results for the entire metabolomic data set. Df SS MS Pseudo-F R2 P(perm) Barley variety 1 0.1818 0.1818 2.189 0.01551 0.05 * Light regimen (Light) 4 1.085 0.2713 3.265 0.09256 0.001 *** Plant organ 1 4.0714 4.0714 49.013 0.34734 0.001 *** Variety:Light 4 0.2971 0.0743 0.894 0.02535 0.582 Variety:Organ 1 0.1722 0.1722 2.073 0.01469 0.054 . Light:Organ 4 0.8539 0.2135 2.57 0.07285 0.002 ** Variety:Light:Organ 4 0.3254 0.0814 0.979 0.02776 0.484 Residuals 57 4.7348 0.0831 0.40394 Total 76 11.7217 1 *P(perm) 0.05>=0.01, **P(perm) 0.01>=0.001, ***P(perm) <=0.001. (by 516% in Barke and 211% in Bojos relative to the R regimen) small effect of barley variety on root metabolites (Figure 8). In and JA (by 741% in Barke and 346% in Bojos relative to the R particular, PC1 separated the effect of the R-B-UVA-FR regimen regimen) accumulation (Figures 7A, C), but the Barke variety from the other regimens, and PC2 mainly separated the effects accumulated less ABA (by 19% relative to Bojos; Figure 7D). of the R-B and R-UVA regimens. The R-B-UVA-FR regimen was Analysis of sorgolactone indicated increased accumulation associated with greater levels of thymidine, D-sorbose, GABA, under the R-B regimen relative to the R regimen (by 2,818%), aucubin, and D-lyxose, but the addition of UV-A radiation but the R-B-UVA-FR regimen only had a weak effect (by 103%; increased the accumulation of L-malic acid, fructose, and Figure 7E). On the contrary, relative to the R regimen, the R-B- acacetin in roots. UVA-FR regimen led to significant accumulation of the flavonoid We also analyzed the changes in metabolite allocation isovitexin (by 847%; Figure 7F). between leaves and roots, expressed as leaf:root ratios Analysis of roots indicated that they had substantially lower (Figure 9). PC1 mainly separated the effects of the R-FR levels of metabolites than leaves. A PCA analysis indicated a and R-B-UVA-FR regimens. In particular, these treatments Frontiers in Plant Science | www.frontiersin.org 9 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 6 | Principal component analysis (PC1 vs. PC2) of leaf metabolite concentrations of two barley varieties grown for 26 days under different light regimens. Here and in Figures 8 and 9: values for each variety (circles, Bojos; triangles, Barke) and light regimen (red, R; R-B, cyan; R-B-UVA-FR, orange; R-FR, brown; R-UVA, purple) are presented as means and standard deviations (error bars). Loadings of PC1 and PC2 for individual metabolite concentrations are represented by abbreviations. The data from individual replicates (n = 4) were used for the analysis. Biochemical families are represented by colors: dark blue, sugars; green, amino acids; yellow, compounds involved in the metabolism of amino acids and sugars; cyan, nucleotides; brown, phenolics; and violet, others. Abbreviations of metabolites: uracil (Ura), adenine (Ade), thymidine (Thy), adenosine (Aden), D-ribose (Rib), D-lyxose (Lyx), D-xylose (Xyl), D-fructose (Fru), 5-methylthio-D- ribose (RibO), D-sorbose (Sor), D-glucose (Gluc), D-galactose (Gae), D-(+)-trehalose dehydrate (Trha), lysine (Lys), glutamine (Gln), leucine (Leu), isoleucine (Iso), phenylalanine (Phe), methionine (Met), tryptophan (Trp), valine (Val), acid aspartic (Asp), tyrosine (Tyr), proline (Pro), serine (Ser), succinic acid (SucA), citrate acid (CitA), malic acid (Mal), lactic acid (Lac), quinic acid (Qui), pyruvate (Pyr), p-coumaric acid (CouA), jasmone (Jas), gallic acid (Gal), maleic acid (MaA), fumarate (Fum), trans-caffeic acid (Caf), aconitate (Suc), quinic acid (QA), trans-ferulic acid (Fer), syringic acid (Syr), oxaloglutarate (Oxa), jasmonic acid (JA), cinnamic acid (Cin), chlorogenic acid (Chl), γ-aminobutyric acid (GABA), glycine-betaine (GB), choline (Cho), sodium salicylate (Sali), (-)-abscisic acid (ABA), resveratrol (Res), acacetin (Aca), kaempferol (Kae), epicatechin (Epi), alpha-ketoglutaric acid (Ket), quercetin (Que), epigallocatechin (Epg), myricetin (Myr), aucubin (Auc), isovitexin (Isx), homoorientin (Hom), lucenin (Luc), malvidin (Malv), naringenin (Nar), flavan-3-ol (Fla), catechin (Cat), sorgolactone (Srg), (+)-orobanchol (Oro), (+)-strigol (Strl), polyols (Pol), mannitol (Mnt), sorbitol (Sbl). were mainly associated with more allocation of fructose, (Barke variety) and the R-UVA regimen (Bojos variety) myricetin, naringenin, mannitol, and glutathione to roots altered the leaf:root ratios (Figure 10A). Relative to the R than leaves. PC2 mainly separated the effects of the R-B and regimen, leaf fructose level was significantly greater under R-UVA regimens, which were associated with more allocation the R-B-UVA-FR regimen (by 1,371%) but lower under of JA, kaempferol, p-coumaric acid, and D-lyxose to roots the R-B and R-FR (by 86% and 87%, respectively), and than leaves. On the other hand, the R regimen was associated this led to significant effects on the leaf:root ratio (Figure with more allocation of glucose, sorbitol, and ABA to roots 10B). The light regimen also had a significant effect on the than leaves. leaf:root ratio for mannitol, with a particularly high ratio Figure 10 shows the selected metabolites that had under the R-FR regimen and a decreased ratio under the large differences in roots and root/leaf ratios among the R-UVA regimen (Figure 10C). Similarly, root naringenin different light regimens. Although the glucose content was only present at very low levels under the R-FR and R-B- had large variance in roots and leaves, the R-FR regimen UVA-FR regimens (Figure 10D). Frontiers in Plant Science | www.frontiersin.org 10 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 7 | Effects of 26 days exposure to different light regimens on the levels of a representative amino acid (proline, A), plant hormones or compounds with regulatory effects (jasmonic acid, C; abscisic acid, D; sorgolactone, E), and phenolic compounds (p-coumaric acid, B; isovitexin, F) in leaves of spring barley varieties Barke (dark grey) and Bojos (light grey). Additional details are in the Figure 1 legend. DISCUSSION thaliana (Gelderen van et al., 2018b), and reduced root hair density in Arabidopsis thaliana (de Simone et al., 2000). However, Responses to Far-Red Light these findings differ from our results for barley, which showed Phytochromes in a plant shoot sense changes in the R:FR ratio that a low R:FR ratio stimulated most RSA traits, although it and signal that neighboring plants are competing for light. This reduced root BA. Lee et al. (2015) showed that the response of signal allows competing plants rapid elongation of leaves and root biomass to the R:FR ratio is non-linear and that decreasing stems and upward reorientation of leaves (Franklin, 2008). In the R:FR ratio beyond a certain level stimulated the growth of monocotyledonous plants, a low R:FR ratio also stimulates apical root biomass and increased the root-to-shoot ratio. An interactive dominance and reduces tiller formation (Kebrom and Brutnell, effect of circadian clock and R:FR ratio on root and shoot growth 2007). In accordance with these results, we found that the two mediated by phytochrome-interacting factor 4 (PIF4) and PIF5 varieties of barley studied here had significant increases of leaf transcription factors also has to be taken into account. While the length and TLA under the R-FR regimen relative to the R regimen. PIF4 and PIF5 expression peaks in the morning hours of long Less is known about the effect of the R:FR ratio on root days, the maximum expression occurs in the night when the development. However, there is evidence that a low R:FR ratio photoperiods are short (Farré, 2012). leads to a reduced root-to-shoot biomass ratio in soybean Light-induced regulation of plant growth, physiology, and (Kasperbauer, 1987), reduced lateral root density in Arabidopsis metabolism involves multiple hormonal pathways such as Frontiers in Plant Science | www.frontiersin.org 11 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 8 | Principal component analysis (PC1 vs. PC2) of root metabolite concentrations of two barley varieties grown for 26 days under different light regimens. Additional details are in the Figure 6 legend. gibberellins, ABA, auxins, ethylene, and cytokinins; however, Previous research indicated that JA (Suzuki et al., 2011) and the molecular links between these pathways and light signaling photosynthetic products (Kircher and Schopfer, 2012) might link are not yet completely understood (Lau and Deng, 2010). A the detection of the R:FR ratio by the shoot with morphogenesis low R:FR ratio stimulates the accumulation of at least three in the root. In accordance with other research (Park and Runkle, plant hormones: gibberellins (de Lucas et al., 2008), auxin 2017), we found that the presence of FR stimulated photosynthesis. (Procko et al., 2014), and ethylene (Kurepin et al., 2010). A Nevertheless, our metabolomic analyses indicated an increased low R:FR ratio also desensitizes plants to defense-associated content of glucose in roots and reduced content in leaves in the plant hormones, such as JA and salicylic acid. Auxin is the presence of FR. A high glucose content can affect root length, a dominant physiological regulator when plants are growing in number of lateral roots, and root growth direction because of the an environment with a low R:FR ratio (Yang and Li, 2017), and strong interactions of glucose with auxin (Mishra et al., 2009). its polar transport affects root growth and development (Saini et On the contrary, we found that fructose and mannitol levels were al., 2013). The complex nature of auxin shoot-to-root transport reduced in roots under the R-FR regimen, which indicates that and its interactions with other plant hormones may lead to a different sugars have different effects on root growth. Although wide range of shoot and root responses (Lavenus et al., 2013), as the function of fructose as sugar regulator is less explored, its observed under the different light regimens used in this study. interactions with signaling pathways of ABA and ethylene suggest Moreover, a recent study showed that JA, whose abundance an inhibitory role in root growth (Cho and Yoo, 2011). In contrast, was affected by the R:FR ratio, links the molecular mechanisms sucrose promotes cell division in root meristems, modifies the responsible for plant growth and immunity (Campos et expression of numerous genes related to P deficiency, and thus leads al., 2016). This suggests that the R:FR ratio determines the to altered root growth and morphology (Horacio and Martinez- allocation of plant resources to growth or defense. However, Noel, 2013). On the other hand, mannitol and other sugar alcohols our results showed that FR light had only a small effect on JA play a role in reactive oxygen species (ROS) scavenging rather than biosynthesis in leaves, in contrast to B light. in plant growth regulation (Bolouri-Moghaddam et al., 2010). Frontiers in Plant Science | www.frontiersin.org 12 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 9 | Principal component analysis (PC1 vs. PC2) of root-to-leaf ratios of metabolite concentrations of two barley varieties grown for 26 days under different light regimens. Additional details are in the Figure 6 legend. Responses to Blue Light Many studies demonstrated that supplementary blue light Blue light is an essential environmental signal that also controls increased photosynthetic performance (Hogewoning et al., plant cell elongation (Huché-Thélier et al., 2016). Blue light 2010; Wang et al., 2016), chlorophyll (Chl) content, and the represses gibberellin synthesis and auxin synthesis and affects Chl a/b ratio (Hogewoning et al., 2010; Kopsell and Sams, several genes involved in growth repression (Folta et al., 2003). 2013). Similarly, we found that the addition of blue light (R-B Although the “shade avoidance syndrome” is most often regimen) significantly increased most of the photosynthetic associated with a reduced R:FR ratio, some studies have shown parameters in our two barley varieties. An increased CO2 that a reduced level of blue light had a similar effect and suggested assimilation rate under blue light is particularly associated that a PHY-B–independent hormonal cascade can also lead to the with increased stomatal conductance (Boccalandro et al., shade avoidance syndrome (Keller et al., 2011). The blue light– 2012) being mediated by phototropins PHOT1 and PHOT2 induced shade avoidance syndrome is predominantly mediated (Kinoshita et al., 2001). However, high photosynthetic carbon by CRY1 and brassinosteroid hormones. Previous studies showed uptake may lead, under the conditions of limited C sink, to the that a high blue-to-red light ratio increased biomass in wheat accumulation of soluble sugars and consequently to stomata (Goins et al., 1997) but decreased biomass in lettuce (Kang et al., closure. Such feedback regulation is mediated by hexokinase 2016) and rice (Ohashi-Kaneko et al., 2006). Folta and Childers within the guard cells of stomatal pores (Granot and Kelly, (2008) reported that responses to blue light are organ-specific 2019). If there is sufficient sink for sugars in roots, these (mainly and depend on the total photon flux density and blue-to-red light sucrose) are rapidly transported by phloem and stimulate root ratio. In our study, the R regimen and the R-B regimen had similar growth, the process mediated by auxin and cytokinin (Wang effects, in that the R-B regimen only significantly increased leaf and Ruan, 2016). Re-allocation of sugars between roots and length in the Bojos variety. The blue light also had no significant shoots is tightly coupled with plant N metabolism. N uptake effect on RSA parameters in either variety of barley. controls the availability of sugars because carbon skeletons are Frontiers in Plant Science | www.frontiersin.org 13 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley FIGURE 10 | Effect of 26 days exposure to different light regimens on allocation of selected sugars (glucose, A; fructose, B; mannitol, C) and a selected phenolic compound (naringenin, D) to leaves and roots of spring barley varieties Barke (dark grey) and Bojos (light grey). Additional details are in the Figure 1 legend. essential for the assimilation of inorganic N into amino acids, in the presence of other wavelengths (Hogewoning et al., 2010; proteins, and nucleic acids and vice versa. On the other hand, Hernández and Kubota, 2016). The opposite effects of blue light N-deficiency reduces the rates of photosynthetic carbon uptake on growth and photosynthesis can be the reason for observed and accumulation of sugars (Wang and Ruan, 2016). discrepancies in biomass formation under blue light conditions. Several studies, however, reported that photosynthetic The role of blue light signaling in RSA is still not well understood. performance increased with increasing blue light intensity only However, under blue light, cry mutants of Arabidopsis have Frontiers in Plant Science | www.frontiersin.org 14 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley substantial differences in primary root length relative to wild however, needed to further elucidate the effect of blue light on type (Canamero et al., 2006). this regulatory pathway. In our study, the R-B regimen (relative to the R regimen) led to a reduced content of ABA and an increased content of JA in leaves. This is in accordance with stomatal responses to the R-B regimen. Responses to UV-A Radiation Boccalandro et al. (2012) examined the roles of CRY1, CRY2, and PHOTs and CRYs both percept UV-A and blue light (Gyula et al., ABA in stomatal regulation and reported that blue light led to a 2003), so activation of these photoreceptors may be expected to have substantial increase of stomatal conductance and a reduced ABA similar effects on hormone production. Our results, however, show content, but this did not occur in CRY1 or CRY2 mutants. that UV-A radiation did not have the same effect as the blue light on Some studies suggested a close relationship between the CO2 assimilation and stomatal opening. Also, the R-UVA regimen had biosynthesis of ABA and JA (Brossa et al., 2011). However, a weaker effect on TLA and root parameters than the R-B regimen. these results may simply indicate that reduced water availability Recently, we have also shown that UV radiation alone, and independently stimulates JA and ABA accumulation. Previous in combination with elevated atmospheric CO2 concentration, studies have examined the nature of this relationship using alters N allocation between leaves and roots of European beech mutants. In particular, although the blue light–induced reduction saplings, leading to changes in leaf and root C:N stoichiometry and of ABA content is mediated by CRY1 and CRY2, blue light also morphological traits (Uchytilová et al., 2019). C:N stoichiometry can increases JA biosynthesis (Svyatyna and Riemann, 2012). Blue be considered as one of the most relevant indicators of source:sink light stimulates the biosynthesis of JA by activating the expression balance and can regulate expression of several genes involved in of genes involved in JA biosynthesis, although phytochrome A N metabolism (Nunes-Nesi et al., 2010), secondary metabolism signaling has a stronger effect. Although mechanistic studies with (Royer et al., 2013), photosynthesis (Ruiz-Vera et al., 2017; Foyer et phytochrome mutants reported a crucial role of the R:FR ratio in al., 2018), and plant biomass allocation (Kruse et al., 2010). regulation of the JA biosynthesis and resistance to fungal pathogens Several studies reported that supplemental UV-A radiation (Cerrudo et al., 2012), our results showed that supplementary increases leaf area (Biswas and Jansen, 2012; Zhang et al., 2014). blue light significantly increased JA accumulation, and this could However, UV-A radiation probably acts with other signaling provide increased resistance to environmental stressors. Blue light pathways to increase the elongation of different leaf parts, also induced the accumulation of several secondary metabolites suggesting the presence of intercellular signaling (Verdaguer in barley varieties studied, such as p-coumaric acid, chlorogenic et al., 2017). Some studies have also shown the higher allocation acid, aucubin, and homoorientin (but had no impact on isovitexin of resources to root biomass under UV-A radiation (Zhang et al., or kaempferol), which increased in plants receiving the R-B- 2014), but others reported that UV-A radiation had a negative UVA-FR regimen. Generally, these secondary metabolites play effect or no effect on root biomass (Cooley et al., 2001). important roles as antioxidants scavenging ROS, UV screening Although UV-B radiation has a major role in the induction molecules, or anti-infection agents (reviewed by Falcone Ferreyra of flavonoid biosynthesis, UV-A radiation also promotes the et al., 2012). Their role also depends on location within the leaf, accumulation of flavonoids in many species (Morales et al., having screening functions when located in the adaxial epidermis 2010). However, rather than total phenolics, UV-A radiation and antioxidant functions when located in the mesophyll. This regulates the accumulation of specific phenolic compounds is particularly true for dihydroxy B-ring–substituted flavonols (Verdaguer et al., 2017). While UV-A radiation had only a minor such as kaempferol or quercetin (Agati et al., 2012). Soluble effect on the accumulation of flavonoids in our study, the R-B- hydroxycinnamates such as p-coumaric acid or chlorogenic acid UVA-FR regimen induced accumulation of most flavonoids, and mesophyll flavonoids are constitutively present in young indicating that various spectral bands may contribute to leaves, but these are gradually replaced by epidermal flavonols in flavonoid biosynthesis. Similarly, our earlier study has proven mature leaves acclimated to light conditions (Burchard et al., 2000; the fundamental importance of the combined effect of PAR Agati et al., 2013). Walter et al. (2010) summarized the role of cell and UV on the accumulation of phenolics such as lutonarin wall–associated plant secondary metabolites, such as p-coumaric and 3-feruloylquinic acid, xanthophyll cycle pigments, and acid and other hydroxycinnamic acids, in the inhibition of fungal subsequent tolerance to high radiation stress (Klem et al., 2015). growth and resistance to Fusarium head blight infection. Their UV-A radiation can also ameliorate the UV-B radiation– results indicated that the blue light–induced accumulation of JA induced damage to DNA by promoting DNA repair (Krizek, and some phenolic compounds had an important role in the blue 2004). On the other hand, several studies reported that UV-A light–induced resistance to fungal infection. radiation damages PSII (Nawkar et al., 2013), although we We found that blue light strongly stimulated the accumulation observed no such effect. Under natural conditions, UV-A of the sesquiterpene sorgolactone, a strigolactone that stimulates radiation accounts for most of the sunlight-induced damage to germination of parasitic weeds. However, strigolactones also photosynthesis because there is much more UV-A than UV-B act as endogenous hormones that regulate shoot branching, radiation in the solar spectrum (Verdaguer et al., 2017). secondary growth of stems, leaf senescence, primary root One of the compounds whose level increased the most by UV-A growth, adventitious root formation, and root hair elongation radiation in our study was GABA (a non-protein amino acid). This (Seto et al., 2012). Strigolactones also increase plant tolerance compound rapidly accumulates in plant tissues in response to biotic to drought and salinity, mainly through its effect on stomatal and abiotic stress and is a putative signaling molecule (Roberts, density and responsivity (Ha et al., 2014). Additional studies are, 2007). GABA may also have possible roles in the interfacing of C and Frontiers in Plant Science | www.frontiersin.org 15 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley N metabolism (Michaeli and Fromm, 2015) and protection against CONCLUSION oxidative stress and herbivorous pests (Bouché and Fromm, 2004). Other studies reported that increased GABA levels correlated with leaf Our findings suggest that light quality has a different impact on senescence (Diaz et al., 2005) and that GABA responses depend on the shoot and root morphology, physiology, and biochemistry, ethylene and ABA signaling (Lancien and Roberts, 2006). Mekonnen which may have major implications for plant performance under et al. (2016) used a GABA-depleted mutant of Arabidopsis thaliana stress conditions. While the growth of above-ground biomass to demonstrate the role of this compound in stomatal regulation and and photosynthetic performance were enhanced mainly by sensitivity to drought stress. Li et al. (2013) suggested that the effect of the combined action of red, blue, far-red, and UV-A light, the light on GABA accumulation is due to light-induced upregulation of root growth was stimulated particularly by supplementary glutamate decarboxylase or glutamic acid decarboxylase (GAD). To far-red light to red light. These findings may have significant our best knowledge, the present study is the first to show that UV-A implications for improving root development and, consequently, radiation increases the biosynthesis of GABA. the drought resistance, e.g., by adjusting sowing density. Exposition of plants to the full light spectrum stimulates the Interactions of UV-A, Blue, and Far-Red accumulation of defense compounds such as proline, secondary Light metabolites with antioxidative functions, or JA, and reduces The R-B-UVA-FR regimen had the strongest effects on increasing the accumulation of ABA, which allows to assume increased leaf area and photosynthetic parameters. This suggests the stress tolerance. In addition, blue light induced accumulation of importance of simultaneous irradiation with multiple regions GABA, sorgolactone, or several secondary metabolites. As these of the spectrum. The effect of the R-B-UVA-FR regimen on compounds play important roles as osmolytes, antioxidants, UV photosynthetic parameters was comparable to the R-B regimen, screening compounds, or growth regulators, the importance of suggesting that blue light had a greater effect than FR or UV-A light quality in the induction of protective mechanisms against light on this variable. Relative to the R regimen, the R-B-UVA-FR abiotic and biotic stress factors is unequivocal. Application of a regimen strongly increased the above-ground biomass but had suitable spectrum of light wavelengths by specific plastic filters, negligible effects on root biomass and root architecture parameters. monochromatic LED light sources, and/or crop management This suggests that the addition of blue light, UV-A radiation, measures may thus provide important tools for adjusting plant or both impaired the positive effect of FR light on the growth of tolerance to abiotic and biotic stress. seminal and lateral roots. Relative to the R regimen, the R-B-UVA-FR regimen also increased DATA AVAILABILITY the accumulation of specific amino acids (proline, valine, leucine), phenolics (isovitexin, p-coumaric acid, kaempferol), and saccharides The datasets generated for this study are available on request to (fructose) in leaves (Figures 6, 7, and 9). Moreover, such changes the corresponding author. under the R-B-UVA-FR regimen resulted in significant increases in leaf:root ratio of saccharides (particularly fructose), which were low in AUTHOR CONTRIBUTIONS other regimens. This important result indicates that light can induce KK, OU, and WR designed the experiments. KK, WR, BV, and changes in the transport of saccharides from the shoots to the roots. PH conducted the experiment, MO conducted the metabolomic A previous study showed that sucrose activated and promoted analyses, WR conducted the root system architecture analysis, root elongation (Kircher and Schopfer, 2012), and coordinated root and AG-G and KK conducted the statistical analyses of data. KK, and shoot growth with light intensity and spectral quality (Wang AG-G, PH, JS, JP, and OU wrote the manuscript. All authors read and Ruan, 2016). Sugars also provide crosstalk with other signaling and approved the final manuscript. and metabolic processes to regulate local and systemic signaling pathways (Ljung et al., 2015), which is integrated via PIF (Stewart et al., 2011). However, as discussed earlier, the roles of individual FUNDING sugars can be different as glucose and sucrose interact mainly with This research was supported by SustES—Adaptation strategies auxin and promote plant growth, while fructose interacts with for sustainable ecosystem services and food security under inhibiting plant hormones such as ABA or ethylene. adverse environmental conditions, project no. CZ.02.1.01/0. It has been demonstrated by Ahanger et al. (2018) that 0/0.0/16_019/0000797. accumulation of osmolytes, antioxidants, carotenoids, and other defense compounds under environmental stress exhibits crosstalk with the cascade of phytohormone signaling, SUPPLEMENTARY MATERIAL which is mediated by brassinosteroids. Brassinosteroids can have synergistic or antagonistic interactions with other The Supplementary Material for this article can be found online phytohormones for elicitation of stress response involving at: https://www.frontiersin.org/articles/10.3389/fpls.2019.01026/ the antioxidant system, synthesis osmotic constituents, or full#supplementary-material other mechanisms. Thus, brassinosteroids play a potentially FIGURE S1 | Scheme of the growth unit layout (upper right), growth unit with integrating role in responding to light quality and inducing the growing plant (upper left), and arrangement of growth units in a plastic plant protection mechanisms for stress. bin with micro-irrigation pump inside. Frontiers in Plant Science | www.frontiersin.org 16 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley REFERENCES Chen, M., Chory, J., and Fankhauser, C. (2004). Light signal transduction Agati, G., Azzarello, E., Pollastri, S., and Tattini, M. (2012). Flavonoids as in higher plants. Annu. Rev. Genet. 38, 87–117. doi: 10.1146/annurev. antioxidants in plants: location and functional significance. Plant Sci. 196, genet.38.072902.092259 67–76. doi: 10.1016/j.plantsci.2012.07.014 Cho, Y.-H., and Yoo, S.-D. (2011). Signaling role of fructose mediated by FINS1/ Agati, G., Brunetti, C., Di Ferdinando, M., Ferrini, F., Pollastri, S., and Tattini, M. FBP in Arabidopsis thaliana. PLoS Genet. 7, e1001263. doi: 10.1371/journal. (2013). Functional roles of flavonoids in photoprotection: new evidence, pgen.1001263 lessons from the past. Plant Physiol. Biochem. 72, 35–45. doi: 10.1016/j. Cooley, N. M., Higgins, J. T., Holmes, M. G., and Attridge, T. H. (2001). Ecotypic plaphy.2013.03.014 differences in responses of Arabidopsis thaliana L. to elevated polychromatic Ahanger, M. A., Ashraf, M., Bajguz, A., and Ahmad, P. (2018). Brassinosteroids UV-A and UV-B+A radiation in the natural environment: a positive correlation regulate growth in plants under stressful environments and crosstalk with between UV-B+A inhibition and growth rate. J. Photochem. Photobiol. B, Biol. other potential phytohormones. J. Plant Growth Regul. 37, 1007–1024. doi: 60, 143–150. doi: 10.1016/S1011-1344(01)00140-3 10.1007/s00344-018-9855-2 Das, P. K., Shin, D. H., Choi, S.-B., and Park, Y.-I. (2012). Sugar-hormone cross- Anderson, M., Gorley, R. N., and Clarke, R. K. (2008). Permanova+ for primer: talk in anthocyanin biosynthesis. Mol. Cells 34, 501–507. doi: 10.1007/ guide to software and statistical methods. Primer-E Limited. Lutton, UK. s10059-012-0151-x Aphalo, P. J., and Lehto, T. (2001). Effect of lateral far-red light supplementation on de Lucas, M., and Prat, S. (2014). PIFs get BRright: PHYTOCHROME the growth and morphology of birch seedlings and its interaction with mineral INTERACTING FACTORs as integrators of light and hormonal signals. New nutrition. Trees 15, 297–303. doi: 10.1007/s004680100100 Phytol. 202, 1126–1141. doi: 10.1111/nph.12725 Ballaré, C. L. (2009). Illuminated behaviour: phytochrome as a key regulator of de Lucas, M., Davière, J.-M., Rodríguez-Falcón, M., Pontin, M., Iglesias- light foraging and plant anti-herbivore defence. Plant Cell Environ. 32, 713– Pedraz, J. M., Lorrain, S., et al. (2008). A molecular framework for light and 725. doi: 10.1111/j.1365-3040.2009.01958.x gibberellin control of cell elongation. Nature 451, 480–484. doi: 10.1038/ Bian, Z. H., Yang, Q. C., and Liu, W. K. (2015). Effects of light quality on the nature06520 accumulation of phytochemicals in vegetables produced in controlled de Simone, S., Oka, Y., and Inoue, Y. (2000). Effect of light on root hair formation environments: a review. J. Sci. Food Agric. 95, 869–877. doi: 10.1002/jsfa.6789 in Arabidopsis thaliana phytochrome-deficient mutants. J. Plant Res. 113, Biswas, D. K., and Jansen, M. A. K. (2012). Natural variation in UV-B protection 63–69. doi: 10.1007/PL00013917 amongst Arabidopsis thaliana accessions. Emir. J. Food Agric. 246, 621–631. doi: Demmig-Adams, B., Adams, W. W., III, Barker, D. H., Logan, B. A., Bowling, D. R., 10.9755/ejfa.v24i6.14681 and Verhoeven, A. S. (1996). Using chlorophyll fluorescence to assess the Blilou, I., Xu, J., Wildwater, M., Willemsen, V., Paponov, I., Friml, J., et al. (2005). fraction of absorbed light allocated to thermal dissipation of excess excitation. The PIN auxin efflux facilitator network controls growth and patterning in Physiol. Plant. 98, 253–264. doi: 10.1034/j.1399-3054.1996.980206.x Arabidopsis roots. Nature 433, 39. doi: 10.1038/nature03184 Diaz, C., Purdy, S., Christ, A., Morot-Gaudry, J.-F., Wingler, A., and Masclaux- Boccalandro, H. E., Giordano, C. V., Ploschuk, E. L., Piccoli, P. N., Bottini, R., Daubresse, C. (2005). Characterization of markers to determine the extent and and Casal, J. J. (2012). Phototropins but not cryptochromes mediate the variability of leaf senescence in Arabidopsis. A metabolic profiling approach. blue light-specific promotion of stomatal conductance, while both enhance Plant Physiol. 138, 898–908. doi: 10.1104/pp.105.060764 photosynthesis and transpiration under full sunlight. Plant Physiol. 158, 1475– Falcone Ferreyra, M. L., Rius, S. P., and Casati, P. (2012). Flavonoids: biosynthesis, 1484. doi: 10.1104/pp.111.187237 biological functions, and biotechnological applications. Front. Plant Sci. 3(222), Bolouri-Moghaddam, M. R., Roy, K. L., Xiang, L., Rolland, F., and Van den 1–15. doi: 10.3389/fpls.2012.00222 Ende, W. (2010). Sugar signalling and antioxidant network connections in Farré, E. M. (2012). The regulation of plant growth by the circadian clock. Plant plant cells. FEBS J. 277, 2022–2037. doi: 10.1111/j.1742-4658.2010.07633.x Biol. 14, 401–410. doi: 10.1111/j.1438-8677.2011.00548.x Bouché, N., and Fromm, H. (2004). GABA in plants: just a metabolite? Trends Folta, K. M., and Childers, K. S. (2008). Light as a growth regulator: controlling Plant Sci. 9, 110–115. doi: 10.1016/j.tplants.2004.01.006 plant biology with narrow-bandwidth solid-state lighting systems. Hortscience Brossa, R., López-Carbonell, M., Jubany-Marí, T., and Alegre, L. (2011). Interplay 43, 1957–1964. doi: 10.21273/HORTSCI.43.7.1957 between abscisic acid and jasmonic acid and its role in water-oxidative stress Folta, K. M., Pontin, M. A., Karlin-Neumann, G., Bottini, R., and in wild-type, ABA-deficient, JA-deficient, and ascorbate-deficient Arabidopsis Spalding, E. P. (2003). Genomic and physiological studies of early plants. J. Plant Growth Regul. 30, 322–333. doi: 10.1007/s00344-011-9194-z cryptochrome 1 action demonstrate roles for auxin and gibberellin in Burchard, P., Bilger, W., and Weissenbock, G. (2000). Contribution of the control of hypocotyl growth by blue light. Plant J. 36, 203–214. doi: hydroxycinnamates and flavonoids to epidermal shielding of UV-A and UV-B 10.1046/j.1365-313X.2003.01870.x radiation in developing rye primary leaves as assessed by ultraviolet-induced Foyer, C. H., Noctor, G., and Verrier, P. (2018). Photosynthetic carbon–nitrogen chlorophyll fluorescence measurements. Plant Cell Environ. 2312, 1373–1380. interactions: modelling inter-pathway control and signalling. Annu. Rev. Plant doi: 10.1046/j.1365-3040.2000.00633.x Biol. 22, 325–347. doi: 10.1002/9780470988640 Campos, M. L., Yoshida, Y., Major, I. T., de Oliveira Ferreira, D., Weraduwage, S. M., Franklin, K. A. (2008). Shade avoidance. New Phytol. 179, 930–944. doi: Froehlich, J. E., et al. (2016). Rewiring of jasmonate and phytochrome B 10.1111/j.1469-8137.2008.02507.x signalling uncouples plant growth–defense tradeoffs. Nat. Commun. 7, 12570. Galvão, V. C., and Fankhauser, C. (2015). Sensing the light environment in plants: doi: 10.1038/ncomms12570 photoreceptors and early signaling steps. Curr. Opin. Neurobiol. 34, 46–53. doi: Canamero, R. C., Bakrim, N., Bouly, J.-P., Garay, A., Dudkin, E. E., Habricot, Y., 10.1016/j.conb.2015.01.013 et al. (2006). Cryptochrome photoreceptors cry1 and cry2 antagonistically Gelderen van, K., Kang, C., Paalman, R., Keuskamp, D., Hayes, S., and Pierik, R. regulate primary root elongation in Arabidopsis thaliana. Planta 224, 995– (2018a). Far-red light detection in the shoot regulates lateral root development 1003. doi: 10.1007/s00425-006-0280-6 through the HY5 transcription factor. Plant Cell 30, 101–116. doi: 10.1105/ Casal, J. J., Sánchez, R. A., and Botto, J. F. (1998). Modes of action of phytochromes. tpc.17.00771 J. Exp. Bot. 49, 127–138. doi: 10.1093/jxb/49.319.127 Gelderen van, K., Kang, C., and Pierik, R. (2018b). Light signaling, root Catalá, R., Medina, J., and Salinas, J. (2011). Integration of low temperature and development, and plasticity. Plant Physiol. 176, 1049–1060. doi: 10.1104/ light signaling during cold acclimation response in Arabidopsis. Proc. Natl. pp.17.01079 Acad. Sci. U. S. A. 108, 16475–16480. doi: 10.1073/pnas.1107161108 Goins, G. D., Yorio, N. C., Sanwo, M. M., and Brown, C. S. (1997). Cerrudo, I., Keller, M. M., Cargnel, M. D., Demkura, P. V., de Wit, M., Photomorphogenesis, photosynthesis, and seed yield of wheat plants grown Patitucci, M. S., et al. (2012). Low red/far-red ratios reduce Arabidopsis under red light-emitting diodes (LEDs) with and without supplemental blue resistance to Botrytis cinerea and jasmonate responses via a COI1-JAZ10– lighting. J. Exp. Bot. 48, 1407–1413. doi: 10.1093/jxb/48.7.1407 dependent, salicylic acid–independent mechanism. Plant Physiol. 158, 2042– Granot, D., and Kelly, G. (2019). Evolution of guard-cell theories: the story of 2052. doi: 10.1104/pp.112.193359 sugars. Trends Plant Sci. 24, 507–518. doi: 10.1016/j.tplants.2019.02.009 Frontiers in Plant Science | www.frontiersin.org 17 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley Granot, D., Kelly, G., Stein, O., and David-Schwartz, R. (2014). Substantial roles Klem, K., Ač, A., Holub, P., Kováč, D., Špunda, V., Robson, T. M., et al. (2012). of hexokinase and fructokinase in the effects of sugars on plant physiology and Interactive effects of PAR and UV radiation on the physiology, morphology development. J. Exp. Bot. 65, 809–819. doi: 10.1093/jxb/ert400 and leaf optical properties of two barley varieties. Environ. Exp. Bot. 75, 52–64. Green-Tracewicz, E., Page, E. R., and Swanton, C. J. (2012). Light quality and the doi: 10.1016/j.envexpbot.2011.08.008 critical period for weed control in soybean. Weed Sci. 60, 86–91. doi: 10.1614/ Klem, K., Holub, P., Štroch, M., Nezval, J., Špunda, V., Tříska, J., et al. (2015). WS-D-11-00072.1 Ultraviolet and photosynthetically active radiation can both induce Gundel, P. E., Pierik, R., Mommer, L., and Ballaré, C. L. (2014). Competing photoprotective capacity allowing barley to overcome high radiation stress. neighbors: light perception and root function. Oecologia 176, 1–10. doi: Plant Physiol. Biochem. 93, 74–83. doi: 10.1016/j.plaphy.2015.01.001 10.1007/s00442-014-2983-x Kopsell, D. A., and Sams, C. E. (2013). Increases in shoot tissue pigments, Gyula, P., Schäfer, E., and Nagy, F. (2003). Light perception and signalling in higher glucosinolates, and mineral elements in sprouting broccoli after exposure to plants. Curr. Opin. Plant Biol. 6, 446–452. doi: 10.1016/S1369-5266(03)00082-7 short-duration blue light from light emitting diodes. J. Am. Soc. Hortic. Sci. 138, Ha, C. V., Leyva-González, M. A., Osakabe, Y., Tran, U. T., Nishiyama, R., 31–37. doi: 10.21273/JASHS.138.1.31 Watanabe, Y., et al. (2014). Positive regulatory role of strigolactone in plant Krizek, D. T. (2004). UV radiation effects on pathogens and insect pests of responses to drought and salt stress. Proc. Natl. Acad. Sci. U. S. A. 111, 851–856. greenhouse-grown crops. Photochem. Photobiol. 79, 217. doi: 10.1111/j.1751- doi: 10.1073/pnas.1322135111 1097.2004.tb00400.x Hernández, R., and Kubota, C. (2016). Physiological responses of cucumber Kruse, J., Hänsch, R., Mendel, R. R., and Rennenberg, H. (2010). The role of root seedlings under different blue and red photon flux ratios using LEDs. Environ. nitrate reduction in the systemic control of biomass partitioning between Exp. Bot. 121, 66–74. doi: 10.1016/j.envexpbot.2015.04.001 leaves and roots in accordance to the C/N-status of tobacco plants. Plant Soil Hideg, E., Jansen, M. A. K., and Strid, A. (2013). UV-B exposure, ROS, and stress: 332, 387–403. doi: 10.1007/s11104-010-0305-6 inseparable companions or loosely linked associates? Trends Plant Sci. 182, Kurepin, L. V., Walton, L. J., Yeung, E. C., Chinnappa, C. C., and Reid, D. M. 107–115. doi: 10.1016/j.tplants.2012.09.003 (2010). The interaction of light irradiance with ethylene in regulating growth of Hogewoning, S. W., Trouwborst, G., Maljaars, H., Poorter, H., van Leperen, W., Helianthus annuus shoot tissues. Plant Growth Regul. 62, 43–50. doi: 10.1007/ and Harbinson, J. (2010). Blue light dose–responses of leaf photosynthesis, s10725-010-9483-8 morphology, and chemical composition of Cucumis sativus grown under Lancien, M., and Roberts, M. R. (2006). Regulation of Arabidopsis thaliana 14-3-3 different combinations of red and blue light. J. Exp. Bot. 61, 3107–3117. doi: gene expression by γ-aminobutyric acid. Plant Cell Environ. 29, 1430–1436. 10.1093/jxb/erq132 doi: 10.1111/j.1365-3040.2006.01526.x Horacio, P., and Martinez-Noel, G. (2013). Sucrose signaling in plants: a world yet Lanoue, J., Leonardos, E. D., and Grodzinski, B. (2018). Effects of light quality and to be explored. Plant Signal. Behav. 8, e23316. doi: 10.4161/psb.23316 intensity on diurnal patterns and rates of photo-assimilate translocation and Horai, H., Arita, M., Kanaya, S., Nihei, Y., Ikeda, T., Suwa, K., et al. (2010). transpiration in tomato leaves. Front. Plant Sci. 9 (756), 1–14. doi: 10.3389/ MassBank: a public repository for sharing mass spectral data for life sciences. fpls.2018.00756 J. Mass Spectrom. 45, 703–714. doi: 10.1002/jms.1777 Lastdrager, J., Hanson, J., and Smeekens, S. (2014). Sugar signals and the control of Huché-Thélier, L., Crespel, L., Gourrierec, J. L., Morel, P., Sakr, S., and Leduc, plant growth and development. J. Exp. Bot. 65, 799–807. doi: 10.1093/jxb/ert474 N. (2016). Light signaling and plant responses to blue and UV radiations— Lau, O. S., and Deng, X. W. (2010). Plant hormone signaling lightens up: perspectives for applications in horticulture. Environ. Exp. Bot. 121, 22–38. doi: integrators of light and hormones. Curr. Opin. Plant Biol. 135, 571–577. doi: 10.1016/j.envexpbot.2015.06.009 10.1016/j.pbi.2010.07.001 Kami, C., Lorrain, S., Hornitschek, P., and Fankhauser, C. (2010). “Chapter Lavenus, J., Goh, T., Roberts, I., Guyomarc’h, S., de Lucas, M., Smet, I., et al. two—light-regulated plant growth and development,” in Current Topics (2013). Lateral root development in Arabidopsis: fifty shades of auxin. Trends in Developmental Biology Plant Development. Ed. M. C. P. Timmermans Plant Sci. 18, 450–458. doi: 10.1016/j.tplants.2013.04.006 (Academic Press), 29–66. doi: 10.1016/S0070-2153(10)91002-8 Lee, M.-J., Park, S.-Y., and Oh, M.-M. (2015). Growth and cell division of lettuce Kanehisa, M., and Goto, S. (2000). KEGG: Kyoto encyclopedia of genes and plants under various ratios of red to far-red light-emitting diodes. Hortic. genomes. Nucleic Acids Res. 28, 27–30. doi: 10.1093/nar/28.1.27 Environ. Biotechnol. 56, 186–194. doi: 10.1007/s13580-015-0130-1 Kang, W. H., Park, J. S., Park, K. S., and Son, J. E. (2016). Leaf photosynthetic rate, Li, X., Kim, Y. B., Uddin, M. R., Lee, S., Kim, S.-J., and Park, S. U. (2013). Influence growth, and morphology of lettuce under different fractions of red, blue, and of light on the free amino acid content and γ-aminobutyric acid synthesis green light from light-emitting diodes (LEDs). Hortic. Environ. Biotechnol. 57, in Brassica juncea seedlings. J. Agric. Food Chem. 61 (36), 8624–8631. doi: 573–579. doi: 10.1007/s13580-016-0093-x 10.1021/jf401956v Kangasjärvi, S., Neukermans, J., Li, S., Aro, E.-M., and Noctor, G. (2012). Li, Y., Xin, G., Wei, M., Shi, Q., Yang, F., and Wang, X. (2017). Carbohydrate Photosynthesis, photorespiration, and light signalling in defence responses. accumulation and sucrose metabolism responses in tomato seedling leaves J. Exp. Bot. 63, 1619–1636. doi: 10.1093/jxb/err402 when subjected to different light qualities. Sci. Hortic. 225, 490–497. doi: Karapetyan, S., and Dong, X. (2018). Redox and the circadian clock in plant 10.1016/j.scienta.2017.07.053 immunity: a balancing act. Free Radic. Biol. Med. 119, 56–61. doi: 10.1016/j. Ljung, K., Nemhauser, J. L., and Perata, P. (2015). New mechanistic links between freeradbiomed.2017.12.024 sugar and hormone signalling networks. Curr. Opin. Plant Biol. 25, 130–137. Kasperbauer, M. J. (1987). Far-red light reflection from green leaves and effects doi: 10.1016/j.pbi.2015.05.022 on phytochrome-mediated assimilate partitioning under field conditions. Plant McLaren, J. S., and Smith, H. (1978). Phytochrome control of the growth and Physiol. 85, 350–354. doi: 10.1104/pp.85.2.350 development of Rumex obtusifolius under simulated canopy light environments. Kebrom, T. H., and Brutnell, T. P. (2007). The molecular analysis of the shade Plant Cell Environ. 1, 61–67. doi: 10.1111/j.1365-3040.1978.tb00748.x avoidance syndrome in the grasses has begun. J. Exp. Bot. 58, 3079–3089. doi: Mekonnen, D. W., Flügge, U.-I., and Ludewig, F. (2016). Gamma-aminobutyric 10.1093/jxb/erm205 acid depletion affects stomata closure and drought tolerance of Arabidopsis Keller, M. M., Jaillais, Y., Pedmale, U. V., Moreno, J. E., Chory, J., and Ballaré, C. L. thaliana. Plant Sci. 245, 25–34. doi: 10.1016/j.plantsci.2016.01.005 (2011). Cryptochrome 1 and phytochrome B control shade-avoidance Michaeli, S., and Fromm, H. (2015). Closing the loop on the GABA shunt in responses in Arabidopsis via partially independent hormonal cascades. Plant J. plants: are GABA metabolism and signaling entwined? Front. Plant Sci. 6 (419), 67, 195–207. doi: 10.1111/j.1365-313X.2011.04598.x 1–7. doi: 10.3389/fpls.2015.00419 Kinoshita, T., Doi, M., Suetsugu, N., Kagawa, T., Wada, M., and Shimazaki, K. Mishra, B. S., Singh, M., Aggrawal, P., and Laxmi, A. (2009). Glucose and auxin (2001). phot1 and phot2 mediate blue light regulation of stomatal opening. signaling interaction in controlling Arabidopsis thaliana seedlings root growth Nature 414, 656. doi: 10.1038/414656a and development. PLoS One 4, e4502. doi: 10.1371/journal.pone.0004502 Kircher, S., and Schopfer, P. (2012). Photosynthetic sucrose acts as cotyledon- Morales, L. O., Tegelberg, R., Brosché, M., Keinänen, M., Lindfors, A., and derived long-distance signal to control root growth during early seedling Aphalo, P. J. (2010). Effects of solar UV-A and UV-B radiation on gene development in Arabidopsis. Proc. Natl. Acad. Sci. U. S. A. 109, 11217–11221. expression and phenolic accumulation in Betula pendula leaves. Tree Physiol. doi: 10.1073/pnas.1203746109 30, 923–934. doi: 10.1093/treephys/tpq051 Frontiers in Plant Science | www.frontiersin.org 18 August 2019 | Volume 10 | Article 1026 Klem et al. Distinct Effects of Light Quality in Barley Murphy, G. P., and Dudley, S. A. (2009). Kin recognition: competition and Seto, Y., Kameoka, H., Yamaguchi, S., and Kyozuka, J. (2012). Recent advances in cooperation in Impatiens (Balsaminaceae). Am. J. Bot. 96, 1990–1996. doi: strigolactone research: chemical and biological aspects. Plant Cell Physiol. 53, 10.3732/ajb.0900006 1843–1853. doi: 10.1093/pcp/pcs142 Nawkar, G. M., Maibam, P., Park, J. H., Sahi, V. P., Lee, S. Y., and Kang, C. H. Smith, C. A., Want, E. J., O’Maille, G., Abagyan, R., and Siuzdak, G. (2006). XCMS: (2013). UV-induced cell death in plants. Int. J. Mol. Sci. 14, 1608–1628. doi: processing mass spectrometry data for metabolite profiling using nonlinear 10.3390/ijms14011608 peak alignment, matching, and identification. Anal. Chem. 78, 779–787. doi: Nunes-Nesi, A., Fernie, A. R., and Stitt, M. (2010). Metabolic and signaling aspects 10.1021/ac051437y underpinning the regulation of plant carbon nitrogen interactions. Mol. Plant Stewart, J. L., Maloof, J. N., and Nemhauser, J. L. (2011). PIF genes mediate the 36, 973–996. doi: 10.1093/mp/ssq049 effect of sucrose on seedling growth dynamics. PLoS One 6, e19894. doi: Obata, T., and Fernie, A. R. (2012). The use of metabolomics to dissect plant 10.1371/journal.pone.0019894 responses to abiotic stresses. Cell. Mol. Life Sci. 69, 3225–3243. doi: 10.1007/ Suzuki, A., Suriyagoda, L., Shigeyama, T., Tominaga, A., Sasaki, M., Hiratsuka, Y., s00018-012-1091-5 et al. (2011). Lotus japonicus nodulation is photomorphogenetically controlled Ohashi-Kaneko, K., Matsuda, R., Goto, E., Fujiwara, K., and Kurata, K. (2006). by sensing the red/far red (R/FR) ratio through jasmonic acid (JA) signaling. Growth of rice plants under red light with or without supplemental blue light. Proc. Natl. Acad. Sci. U. S. A. 108, 16837–16842. doi: 10.1073/pnas.1105892108 Soil Sci. Plant Nutr. 52, 444–452. doi: 10.1111/j.1747-0765.2006.00063.x Svyatyna, K., and Riemann, M. (2012). Light-dependent regulation of the jasmonate Ouzounis, T., Rosenqvist, E., and Ottosen, C.-O. (2015). Spectral effects of artificial pathway. Protoplasma 249, 137–145. doi: 10.1007/s00709-012-0409-3 light on plant physiology and secondary metabolism: a review. Hortscience 50, Tilbrook, K., Arongaus, A. B., Binkert, M., Heijde, M., Yin, R., and Ulm, R. 1128–1135. doi: 10.21273/HORTSCI.50.8.1128 (2013). The UVR8 UV-B photoreceptor: perception, signaling and response. Park, Y., and Runkle, E. S. (2017). Far-red radiation promotes growth of seedlings Arabidopsis Book 11 (e0164), 1–21. doi: 10.1199/tab.0164 by increasing leaf expansion and whole-plant net assimilation. Environ. Exp. Uchytilová, T., Krejza, J., Veselá, B., Holub, P., Urban, O., Horáček, P., et al. (2019). Bot. 136, 41–49. doi: 10.1016/j.envexpbot.2016.12.013 Ultraviolet radiation modulates C:N stoichiometry and biomass allocation in Péret, B., Larrieu, A., and Bennett, M. J. (2009). Lateral root emergence: a difficult Fagus sylvatica saplings cultivated under elevated CO2 concentration. Plant birth. J. Exp. Bot. 60, 3637–3643. doi: 10.1093/jxb/erp232 Physiol. Biochem. 134, 103–112. doi: 10.1016/j.plaphy.2018.07.038 Poorter, H., Niklas, K. J., Reich, P. B., Oleksyn, J., Poot, P., and Mommer, L. Verdaguer, D., Jansen, M. A. K., Llorens, L., Morales, L. O., and Neugart, S. (2017). (2012). Biomass allocation to leaves, stems and roots: meta-analyses of UV-A radiation effects on higher plants: exploring the known unknown. Plant interspecific variation and environmental control. New Phytol. 193, 30–50. doi: Sci. 255, 72–81. doi: 10.1016/j.plantsci.2016.11.014 10.1111/j.1469-8137.2011.03952.x Walter, S., Nicholson, P., and Doohan, F. M. (2010). Action and reaction of host Procko, C., Crenshaw, C. M., Ljung, K., Noel, J. P., and Chory, J. (2014). Cotyledon- and pathogen during Fusarium head blight disease. New Phytol., 185, 54–66. generated auxin is required for shade-induced hypocotyl growth in Brassica doi: 10.1111/j.1469-8137.2009.03041.x rapa. Plant Physiol. 165, 1285–1301. doi: 10.1104/pp.114.241844 Wang, J., Lu, W., Tong, Y., and Yang, Q. (2016). Leaf morphology, photosynthetic Rattanapichai, W., and Klem, K. (2016). Two-dimensional root phenotyping system performance, chlorophyll fluorescence, stomatal development of lettuce based on root growth on black filter paper and recirculation micro-irrigation. (Lactuca sativa L.) exposed to different ratios of red light to blue light. Front. Czech J. Genet. Plant Breed. 52, 64–70. doi: 10.17221/121/2015-CJGPB Plant Sci. 7 (250), 1–10 . doi: 10.3389/fpls.2016.00250 Roberts, M. R. (2007). Does GABA act as a signal in plants? Hints from molecular Wang, L., and Ruan, Y.-L. (2016). Shoot–root carbon allocation, sugar signalling studies. Plant Signal. Behav. 2, 408–409. doi: 10.4161/psb.2.5.4335 and their coupling with nitrogen uptake and assimilation. Funct. Plant Biol. 43, Robson, T. M., Klem, K., Urban, O., and Jansen, M. A. K. (2015). Re-interpreting 105–113. doi: 10.1071/FP15249 plant morphological responses to UV-B radiation. Plant Cell Environ. 385, Yang, C., and Li, L. (2017). Hormonal regulation in shade avoidance. Front. Plant 856–866. doi: 10.1111/pce.12374 Sci. 8 (1527), 1–8. doi: 10.3389/fpls.2017.01527 Royer, M., Larbat, R., Le Bot, J., Adamowicz, S., and Robin, C. (2013). Is the Zhang, L., Allen, L. H., Vaughan, M. M., Hauser, B. A., and Boote, K. J. (2014). Solar C:N ratio a reliable indicator of C allocation to primary and defence- ultraviolet radiation exclusion increases soybean internode lengths and plant related metabolisms in tomato? Phytochemistry 88, 25–33. doi: 10.1016/j. height. Agric. For. Meteorol. 184, 170–178. doi: 10.1016/j.agrformet.2013.09.011 phytochem.2012.12.003 Ruiz-Vera, U. M., De Souza, A. P., Long, S. P., and Ort, D. R. (2017). The role of sink Conflict of Interest Statement: The authors declare that the research was strength and nitrogen availability in the down-regulation of photosynthetic conducted in the absence of any commercial or financial relationships that could capacity in field-grown Nicotiana tabacum L. at elevated CO2 concentration. be construed as a potential conflict of interest. Front. Plant Sci. 8 (998), 1–12. doi: 10.3389/fpls.2017.00998 Sadras, V. O., Hall, A. J., Trapani, N., and Vilella, F. (1989). Dynamics Copyright © 2019 Klem, Gargallo-Garriga, Rattanapichai, Oravec, Holub, Veselá, of rooting and root-length: leaf-area relationships as affected by Sardans, Peñuelas and Urban. This is an open-access article distributed under the plant population in sunflower crops. Field Crops Res. 22, 45–57. doi: terms of the Creative Commons Attribution License (CC BY). The use, distribution 10.1016/0378-4290(89)90088-9 or reproduction in other forums is permitted, provided the original author(s) and Saini, S., Sharma, I., Kaur, N., and Pati, P. K. (2013). Auxin: a master regulator the copyright owner(s) are credited and that the original publication in this journal in plant root development. Plant Cell Rep. 32, 741–757. doi: 10.1007/ is cited, in accordance with accepted academic practice. No use, distribution or s00299-013-1430-5 reproduction is permitted which does not comply with these terms. Frontiers in Plant Science | www.frontiersin.org 19 August 2019 | Volume 10 | Article 1026