News and Views Another gun Dismantled: ABSCISIC ACID INSENSITIVE4 Is Not a Target of Retrograde Signaling The nuclear genome encodes the majority of chloroplast- How the retrograde signaling is perceived by the localized proteins and, in return, chloroplasts exert some nucleus is still enigmatic, and in this context, the control over nuclear gene expression via so-called retro- transcription factor ABSCISIC ACID INSENSITIVE4 grade signals. These signals derive from developing (ABI4) was proposed to act as a nuclear target of chloroplasts (referred to as biogenic signals) or from ma- the GUN1-mediated retrograde signals (Nott et al., ture chloroplasts in response to environmental constraints 2006; Koussevitzky et al., 2007; Sun et al., 2011). Several such as excessive light, drought, or heat (referred to as reports showed that abi4 mutants have a weak gun phe- operational signals; Crawford et al., 2018; Dubreuil et al., notype exhibiting elevated PhANG expression following 2018; Hernández-Verdeja and Strand, 2018). The major norflurazon treatment. In addition, many photosynthesis- nuclear targets of plastid signals are photosynthesis- related nuclear genes controlled by plastid signals con- associated nuclear genes (PhANGs); retrograde signals tain abscisic acid-responsive elements in their promoters also target developmental processes such as seedling (Koussevitzky et al., 2007). Surprisingly, several research photomorphogenesis (Martín et al., 2016). groups were unable to reproduce the gun phenotype Despite the plethora of research focusing on chloro- described for abi4 mutants. In this issue of Plant plast signaling, retrograde signals remain poorly under- Physiology, Kacprzak et al. (2019) address this by an- stood and probably consist of several signals including alyzing the gun phenotypes in multiple abi4 alleles reactive oxygen species and metabolites (Fig. 1). In most in different laboratories. The authors show that abi4 studies, biogenic retrograde signaling is induced by the mutants, unlike gun1, failed to restore the expression inhibition of carotenoid biosynthesis using norflurazon or of a set of PhANGs in the presence of norflurazon and by the inhibition of plastid translation using lincomycin, lincomycin. Thus, Kacprzak et al. (2019) convincingly which inhibit PhANG expression. Six Arabidopsis demonstrate that ABI4 is not a target of retrograde (Arabidopsis thaliana) genomes uncoupled (gun) mutants signaling and should be omitted from future models have been isolated that maintain PhANG expression of biogenic retrograde signaling. under these treatments (Susek et al., 1993). All six Interestingly, the same researchers last year dismantled original GUN proteins are localized in the chloroplast; another gun-like mutant. The plastid transmembrane- GUN2 to GUN6 act in the tetrapyrrole synthesis path- localized transcription factor PTM was once described way, and GUN1 is a pentatricopeptide repeat protein, as being activated by GUN1. After this activation, the responding to disruptions in plastid gene expression. cleaved PTM moved to the nucleus, where it promoted Figure 1. Updated model of biogenic retrograde signals. The current model omits ABI4 and PTM and highlights different GUN proteins and intermediates derived from the tetrapyrrole biosynthesis pathway. GUN1 is believed to be a signal integrator. GUN6 (FERROCHELATASE1), GUN2 (HEME OXYGENASE1), and GUN3 (PHYTOCHROMOBILIN SYNTHASE) are involved in the heme branch of tetrapyrrole biosynthesis. GUN5 (SUBUNIT OF MG-CHELATASE) and GUN4 (a porphyrin-binding protein that enhances the activity of MG- CHELATASE) are involved in the biosynthesis of Mg-protoporphyrin IX. Tetrapyrrole intermediates such as Mg-protoporphyrin IX and heme are suggested to be exported from plastids to the nucleus and to act as regulators of PhANGs. The transport mechanism involved in signaling remains unknown, and nuclear targets of retrograde signals, although via unknown mechanisms, are the transcription factors GOLDEN-LIKE1 (GLK1), GLK2, ELONGATED HYPOCOTYL5 (HY5), and the blue-light receptor CRYPTOCHROME1 (CRY1). Plant PhysiologyÒ, January 2019, Vol. 179, pp. 13–14, www.plantphysiol.org Ó 2019 American Society of Plant Biologists. All Rights Reserved. 13 Downloaded from on February 11, 2020 - Published by www.plantphysiol.org Copyright © 2019 American Society of Plant Biologists. All rights reserved. News and Views ABI4 expression and repressed PhANG expression. Charlotte M.M. Gommers However, with an approach similar to that presented ORCID ID: 0000-0002-0817-5134 here, Page et al. (2017) concluded that PTM should be b Plant Development and Signal Transduction excluded as a major player in retrograde signaling. Based Program, Center for Research in Agricultural on these results, it would be interesting to further inves- Genomics, Barcelona 08193, Spain tigate the factors that might explain discrepancies with the original studies, especially in the case of abi4, since several groups previously reported a gun-like phenotype LITERATURE CITED for abi4. Together, these studies finally resolved uncer- Crawford T, Lehotai N, Strand Å (2018) The role of retrograde signals tainties surrounding the roles of ABI4 and PTM in ret- during plant stress responses. J Exp Bot 69: 2783–2795 rograde signaling. They point to the need to have several Dubreuil C, Jin X, Barajas-López JD, Hewitt TH, Tanz S, Dobrenel T, lines of evidence supporting our conclusions and to Schröder W, Hanson J, Pesquet E, Grönlund A, et al (2018) Establish- carefully question the biological relevance of data. ment of photosynthesis through chloroplast development is controlled by two distinct regulatory phases. Plant Physiol 176: 1199–1214 The work of Kacprzak and colleagues proves that Hernández-Verdeja T, Strand Å (2018) Retrograde signals navigate the much work remains to be done to understand the path to chloroplast development. Plant Physiol 176: 967–976 complex organellar signaling events during develop- Kacprzak SM, Mochizuki N, Naranjo B, Xu D, Leister D, Kleine T, ment and stress responses. An important starting point Okamoto H, Terry MJ (2019) Plastid-to-nucleus retrograde signalling would be to unravel the function of GUN1, the main during chloroplast biogenesis does not require ABI4. Plant Physiol Koussevitzky S, Nott A, Mockler TC, Hong F, Sachetto-Martins G, mediator of retrograde signaling. Recent work has Surpin M, Lim J, Mittler R, Chory J (2007) Signals from chloroplasts shown that GUN1 is involved in chloroplast biogenesis converge to regulate nuclear gene expression. Science 316: 715–719 as well as retrograde signaling. Rapid protein turnover Martín G, Leivar P, Ludevid D, Tepperman JM, Quail PH, Monte E (2016) in green tissues helps to specify GUN1 accumulation Phytochrome and retrograde signalling pathways converge to antago- during plastid development and stress (Song et al., nistically regulate a light-induced transcriptional network. Nat Com- mun 7: 11431 2018; Wu et al., 2018; Zhao et al., 2018). Nevertheless, Nott A, Jung HS, Koussevitzky S, Chory J (2006) Plastid-to-nucleus ret- it remains unknown how GUN1 degradation is pre- rograde signaling. Annu Rev Plant Biol 57: 739–759 vented in the absence of functioning chloroplasts and Page MT, Kacprzak SM, Mochizuki N, Okamoto H, Smith AG, Terry MJ what type of signal this releases. Future endeavors will (2017) Seedlings lacking the PTM protein do not show a genomes un- coupled (gun) mutant phenotype. Plant Physiol 174: 21–26 help bridge our understanding of chloroplast damage, Song L, Chen Z, Larkin RM (2018) The genomes uncoupled mutants are GUN1 accumulation, nuclear gene expression, and ul- more sensitive to norflurazon than wild type. Plant Physiol 178: 965–971 timately plant performance. To achieve this, we need a Sun X, Feng P, Xu X, Guo H, Ma J, Chi W, Lin R, Lu C, Zhang L (2011) A multidisciplinary approach and possibly many more chloroplast envelope-bound PHD transcription factor mediates chloro- guns to be dismantled. plast signals to the nucleus. Nat Commun 2: 477 Susek RE, Ausubel FM, Chory J (1993) Signal transduction mutants of Arabidopsis uncouple nuclear CAB and RBCS gene expression from Amna Mhamdi1,2 chloroplast development. Cell 74: 787–799 ORCID ID: 0000-0001-9959-1362 Wu GZ, Chalvin C, Hoelscher M, Meyer EH, Wu XN, Bock R (2018) a Control of retrograde signaling by rapid turnover of GENOMES UN- Ghent University, Department of Plant COUPLED1. Plant Physiol 176: 2472–2495 Biotechnology and Bioinformatics, and VIB Center Zhao X, Huang J, Chory J (2018) genome uncoupled1 mutants are hyper- for Plant Systems Biology, 9052 Ghent, Belgium sensitive to norflurazon and lincomycin. Plant Physiol 178: 960–964 1 Author for contact: amna.mhamdi@psb.ugent.be. 2 Senior author. www.plantphysiol.org/cgi/doi/10.1104/pp.18.01421 14 Plant Physiol. Vol. 179, 2019 Downloaded from on February 11, 2020 - Published by www.plantphysiol.org Copyright © 2019 American Society of Plant Biologists. All rights reserved.