LAUSR

Brassinosteroids (BRs) are a crucial class of plant hormones that regulates plant growth and development, thus imparting on many important agronomic traits in crops. However, there are still significant gaps in our understanding of the BR signaling pathway in rice. In this study, we collected multiple lines of evidence to indicate that OsBSK1-1 likely represents a missing component in the BR signaling pathway in rice. We showed that knockout mutants of OsBSK1-1 are less sensitive to BR and exhibit a pleiotropic phenotype, including lower plant height, less tiller number and shortened grain length, whereas transgenic plants overexpressing a gain-of-function dominant mutant form of OsBSK1-1 (OsBSK1-1 A295V) are hypersensitive to BR and exhibit some enhanced BR-responsive phenotypes. We found that OsBSK1-1 physically interacts with the BR receptor OsBRI1, and OsGSK2, a downstream component crucial for BR signaling. Moreover, we showed that OsBSK1-1 can be phosphorylated by OsBRI1 and can inhibit OsGSK2-mediated phosphorylation of OsBZR1. We further demonstrated that OsBSK1-1 genetically acts downstream of OsBRI1, but upstream of OsGSK2. Together, our results suggest that OsBSK1-1 likely serves as a scaffold protein directly bridging OsBRI1 and OsGSK2 to positively regulate BR signaling, thus affecting plant architecture and grain size in rice.