This is the submitted version of the journal article: Osnato, Michela. «Novel dwarfing alleles for the next green revolution : Mu- tations in DTL and OSH15 alter internode elongation and grain size in rice». The Plant cell, Vol. 34, Issue 10 (October 2022), p. 3499-3500. DOI 10.1093/pl- cell/koac186 This version is available at https://ddd.uab.cat/record/269313 under the terms of the license IN BRIEF Novel dwarfing alleles for the next green revolution: mutations in DTL and OSH15 alter internode elonga- tion and grain size in rice The green revolution that transformed agriculture in the 1960s encompassed an array of new technolo- gies that doubled the productivity of cereal crops mainly in Mexico and Asia. Together with innovative agri- cultural practices (e.g., use of agrochemicals, irrigation, mechanization), the development of semi-dwarf high-yielding varieties greatly reduced harvest loss due to increased resistance to lodging (i.e., stem bend- ing towards the soil) and improved the harvest index (i.e., grain to straw ratio) due to efficient use of nutri- ents for the development of grains rather than vegetative organs. In contrast to semi-dwarf cultivars, taller plants are more prone to lodging while shorter plants produce less seeds. Thus, the success of modern cul- tivation methods also relies on genetic improvement of plant architecture through optimization of plant height. In rice (Oryza sativa), dwarfism is caused by mutations in genes involved in the metabolism or signal transduction of the phytohormones gibberellins (GAs) and brassinosteroids (BRs), which regulate the elon- gation of internodes via complex feedback mechanisms (Tong et al., 2014). In the last decades, GA-related genes have been extensively characterized and shown to control the favora- ble traits selected during the green revolution, including the shortening of the upper internode (IN1). How- ever, little is known about components of the BR pathway controlling stem elongation. Among few mutations affecting BR-signaling described so far, a deletion in the coding region of a member of the GRAS family of transcription factors underlies the dwarf and low tillering (dlt, also known as d62) phenotype (Tong et al., 2009). Functional DLT is involved in feedback inhibition of BR biosynthesis genes and its loss of function causes impaired BR sensitivity, leading to the shortening of internodes. In this issue, Mei Niu and colleagues (Niu et al., 2022) screened a population obtained by large-scale mutagenesis of the dlt genotype and identified enhancers of the semidwarf phenotype. Specifically, two mutants in the dlt background (named d76 and d140) displayed shortening of all internodes below IN1. Genetic analysis of backcross generations revealed that the dwarf phenotype of single mutants (named s76 and s140) was caused by novel defective alleles of Oryza sativa homeobox 15 (OSH15, also known as the causal gene of d6), a KNOTTED-like gene previously described as a key regulator of internode elongation (Sato et al., 1999). Phenotypic analysis of single (s76 and s140) and double (d76 and d140) mutants sug- gested a complex genetic interaction between DLT and OSH15 that also depends on the tissue including different internodes. The authors proposed that these regulators have synergistic/additive effects on stem elongation and grain width (Figure 1) but an antagonistic effect on tiller formation. In internodes, DLT and OSH15 showed opposite expression patterns: the promoter activity of DLT gradu- ally decreased from IN1 to IN3 whereas that of OSH15 increased in the division zone of IN2. Interestingly, transcriptome analyses of single and double mutants revealed that more genes are mis-regulated in IN1 of dlt and in IN2 of s140. Nevertheless, several genes involved in BR metabolism and signaling were differen- tially expressed in all mutants examined, suggesting that DLT and OSH15 might cooperatively control BR- mediated stem elongation. Additional experiments demonstrated that OSH15 directly regulated several BR- related genes and interacted with DLT to form a complex that targeted Rice BRASSINOSTEROID INSENSI- TIVE1 (OsBRI1, also known as the causal gene of d61), an important gene encoding a BR receptor kinase. To conclude, this study indicates that BR plays a crucial role in the control of rice architecture, besides the known crosstalk with GA. Novel alleles of regulatory genes involved in the BR pathway might be em- ployed in breeding strategies to optimize not only plant height through modulating elongation of specific internodes but also grain morphology. Since the development of internodes begins during the vegetative phase but their elongation occurs during the reproductive phase, it will be interesting to further investigate the function of these regulators in coordinating stem growth and panicle development upon the floral tran- sition. Michela Osnato 1 Assistant Features Editor, The Plant Cell, American Society of Plant Biologists 2 Institut de Ciencia i Tecnologia Ambientals, Universitat Autónoma de Barcelona (ICTA-UAB) michela.osnato@uab.cat ORCID: 0000-0002-2439-8581 Figure 1. Synergistic/additive effect of DTL and OSH15 on plant architecture and seed morphology in rice Compared to wild-type plants (DLT OSH15), single and double mutants show a compact stature due to shortening of internodes (below IN1) and inflorescence branches before heading as well as increased seed width at ripening. Figure credits: M. Osnato (modified schematic model from Niu et al. [2022] Figures 3 and 6). REFERENCES Tong H, Xiao Y, Liu D, Gao S, Liu L, Yin Y, Jin Y, Qian Q, and Chu C (2014). Brassinosteroid regulates cell elongation by modulating gibberellin metabolism in rice. Plant Cell 26, 4376-4393. Tong HN, Jin Y, Liu WB, Li F, Fang J, Yin YH, Qian Q, Zhu LH, and Chu CC (2009). DWARF AND LOW-TILLER- ING, a new member of the GRAS family, plays positive roles in brassinosteroid signaling in rice. Plant J 58, 803-816. Niu M, Wang H, Yin W, Meng W, Xiao Y, Liu D, Zhang X, Dong N, Liu J, Yang Y, Zhang F, Chu C, and Tong H (2022). 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