HAM1 negatively regulates drought tolerance in Brassica juncea

Drought stress poses a severe and growing threat to global crop production due to climate change. Brassica juncea is an important vegetable and oil crop with high potential for cultivation in arid and semi-arid regions, yet its genetic mechanisms underlying drought resilience remain poorly understood. Here, we evaluated the phenotypes of geographically diverse B. juncea accessions under drought stress and identified a candidate gene BjuB.HAM1 via genome-wide association study (GWAS) approach, a plant-specific GRAS-family transcription factor, as a key negative regulator of drought tolerance. We discovered two structural variations (SVs) within BjuB.HAM1 that significantly impair drought tolerance in B. juncea. Using heterologous expression of BjuB.HAM1 in Arabidopsis thaliana, combined with downstream gene validation, we demonstrated that this gene inhibits the expression of AtUGT76C2, a cytokinin glycosyltransferase, and reduces drought tolerance by binding to its GT cis-elements in the promoter. Collectively, these findings uncover a new insight into the functional gene module of drought resistance, and provide valuable genetic targets for drought resistance breeding in Brassica juncea.