OsBSK3 Positively Regulates Grain Length and Weight by Inhibiting the Phosphatase Activity of OsPPKL1.

Brassinosteroids (BRs) are a crucial class of plant hormones that regulate many important agronomic traits in rice ( L.); thus, the BR signaling pathway is a very important tool for breeders to improve the grain yield and quantity of rice. Contrary to the well-established BR signaling pathway in , there are significant gaps in the rice BR signaling pathway, especially the regulation mechanism from OsBSK3 to OsPPKLs and OsGSKs. In this study, we report how knockout mutants confer shorter and lighter grains and exhibit a typical BR-insensitive phenotype, suggesting plays a positive role in BR signaling without genetic redundancy with homologs. Furthermore, OsBSK3 could physically interact with OsPPKL1 and OsGSK3, the downstream components in BR signaling, as a scaffold protein, and inhibit the phosphatase activity of OsPPKL1 on the dephosphorylation of OsGSK3. In addition, the genetic evidence showed acts upstream of in regulating grain length and weight. Our results clarify the role of and provide new insights into BR-signaling mechanisms, leading to potential new targets for the genetic improvement of rice.