BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis

Plant steroid hormones, brassinosteroids (BRs), are perceived by the plasma membrane-localized leucine-rich-repeat-receptor kinase BRI1. Based on sequence similarity, we have identified three members of the BRI1 family, named BRL1, BRL2 and BRL3. BRL1 and BRL3, but not BRL2, encode functional BR receptors that bind brassinolide, the most active BR, with high affinity. In agreement, only BRL1 and BRL3 can rescue bril mutants when expressed under the control of the BRIJ promoter. While BRI1 is ubiquitously expressed in growing cells, the expression of BRL1 and BRL3 is restricted to non- overlapping subsets of vascular cells. Loss-of-function of bri1 causes abnormal phloemxylem differentiation ratios and enhances the vascular defects of a weak bril mutant. brl1 brl1 brl3 triple mutants enhance bri1 dwarfism and also exhibit abnormal vascular differentiation. Thus, Arabidopsis contains a small number of BR receptors that have specific functions in cell growth and vascular differentiation.