Dual functions of OsCesAs in modulating grain size and culm mechanical strength in rice

Grain size and lodging resistance determine rice yield by affecting grain weight, and are also associated with secondary cell wall (SCW) thickness. In this study, we demonstrate the dual roles of OsCesAs, which encode cellulose synthases, in the regulation of grain size and culm mechanical strength through the brassinosteroid (BR) signaling pathway. RT-qPCR analysis revealed that the transcript levels of all rice CesA gene family members were induced by exogenous brassinolide (BL) treatment. Moreover, OsCesA4/7/9, which are involved in secondary cell wall synthesis, exhibited similar expression patterns during rice culm and grain development. Overexpression of OsCesA4/9 not only enhanced secondary cell wall thickness and cellulose content in culms, thereby improving mechanical strength, but also increased grain size. OsBZR1 directly binds to the promoters of OsCesA4/7/9 through a regulatory mechanism distinct from that in Arabidopsis, thus negatively regulating their expression. This ultimately results in increased culm cellulose content and mechanical strength in the cr-osbzr1 mutant. However, analysis of culm mechanical strength and grain size in mutants of qGL3, OsGSK3, and OsBZR1, which are key components of the BR signaling pathway, revealed a weak correlation between these two traits. This work helps breeders better understand the important functions of BR signaling in plant growth and development, enabling targeted molecular design breeding of rice for lodging tolerance and grain size.