BRASSINOSTEROID INSENSITIVE2 negatively regulates cellulose synthesis in <i>Arabidopsis</i> by phosphorylating cellulose synthase 1

Significance Cellulose is the most abundant biopolymer on Earth and is a critical component for plants to grow and develop. Cellulose is synthesized by large cellulose synthase complexes containing multiple cellulose synthase A (CESA) subunits; however, how cellulose synthesis is regulated remains unclear. In this study, we identify BRASSINOSTEROID INSENSITIVE2 (BIN2) as a protein kinase that directly phosphorylates Arabidopsis CESA1 and further demonstrate that this phosphorylation event negatively regulates CESA activity, and thus cellulose biosynthesis, in Arabidopsis . Therefore, this study provides a clear link between cell wall biosynthesis and hormonal signal transduction pathways that regulate plant growth and development.