Brassinosteroid Signaling Regulates Leaf Erectness in Oryza sativa via the Control of a Specific U-Type Cyclin and Cell Proliferation

• Abaxial sclerenchyma cell number in rice lamina joints affects leaf erectness
• Brassinosteroids (BRs) inhibit abaxial sclerenchyma cell proliferation
• CYC U4;1 is a U-type cyclin controlling abaxial sclerenchyma cell proliferation
• BR signaling regulates both CYC U4;1 gene expression and protein activity

SummaryLeaf erectness is key in determining plant architecture and yield, particularly in cereal crops. Brassinosteroids (BRs) play a unique role in controlling this trait in monocots, but the underlying cellular and molecular mechanisms remain big mysteries. Here we report that the abaxial sclerenchyma cell number of rice lamina joints (LJs) is closely related to leaf erectness, and BR signaling tightly regulates their proliferation. We identified a rice U-type cyclin CYC U4;1 enriched in rice LJs, with its expression accompanying LJ development. Genetic and biochemical studies demonstrated that CYC U4;1 plays a positive role in promoting leaf erectness by controlling the abaxial sclerenchyma cell proliferation. Furthermore, BR signaling inhibits the abaxial sclerenchyma cell division by coordinately regulating CYC U4;1 expression through BES1 and CYC U4;1 protein activity through GSK3 kinases. These results support a key role of the cyclin CYC U4;1 in mediating BR-regulated cell division to control leaf erectness.