Untargeted mutagenesis of brassinosteroid receptor SbBRI1 confers drought tolerance by altering phenylpropanoid metabolism in Sorghum bicolor
Drought is a critical issue in modern agriculture, therefore there is a need to create crops with drought resilience. The complexity of plant responses to abiotic stresses, particularly in the field of brassinosteroid (BR) signaling, has been the subject of extensive research. In this study, we unveil compelling insights indicating that the BRASSINOSTEROID INSENSITIVE 1 (BRI1) receptor in Arabidopsis and Sorghum plays a critical role as a negative regulator of drought responses. Introducing untargeted mutation in the sorghum BRI1 receptor (SbBRI1) effectively enhances the plant ability to withstand osmotic and drought stress. Through DNA Affinity Purification sequencing (DAP-Seq) we show that the sorghum BRI1-EMS-SUPPRESSOR 1 (SbBES1) transcription factor, a downstream player of the BR signaling, binds to a conserved G-box binding motif, and it is responsible for regulating BR homeostasis, as its Arabidopsis ortholog AtBES1. We further characterized the drought tolerance of sorghum bri1 mutants and decipher SbBES1-mediated regulation of phenylpropanoid pathway. Our findings suggest that SbBRI1 signaling serves as a dual purpose: under normal conditions, it regulates lignin biosynthesis by SbBES1, but during drought conditions, BES1 becomes less active, allowing the activation of the flavonoid pathway. This adaptive shift improves the photosynthetic rate and photoprotection, reinforcing crop adaptation to drought. Overall design: To identify the early transcriptional targets of SbBES1 in drought sensing, we performed RNA sequencing (RNA-Seq) experiments of sorghum plants (root tissue) in control and drought stress conditions (30% field capacity). BC2F3 EMS-mutagenized plants carrying or lacking the SNP in the extracellular region of SbBRI1,  assuming  they are near isogenic lines(NIL-bri1-72 and NIL-WT-72), were used. RNA was extracted with Maxwell® 16 LEV Plant RNA Kit (Promega) and quality checked using Bioanalyzer.  Three biological replicates were obtained per genotype and condition.