Brassinosteroid-insensitive-1 is a ubiquitously expressed leucine-rich repeat receptor serine/threonine kinase

Brassinosteroid (BR) mutants of Arabidopsis have pleiotropic phenotypes and provide evidence that BRs function throughout the life of the plant from s eedling development to senescence. Screens for BR signaling mutants identif ied one locus, BRI1, which encodes a protein with homology to leucine-rich repeat receptor serine (Ser)/threonine (Thr) kinases. Twenty seven alleles of this putative BR receptor have been isolated to date, and we present her e the identification of the molecular lesions of 14 recessive alleles that represent five new mutations. BR-insensitive-l (BRI1) is expressed at high levels in the meristem, root, shoot, and hypocotyl of seedlings and at lowe r levels later in development. Confocal microscopy analysis of full-length BRI1 fused to green fluorescent protein indicates that BRI1 is localized in the plasma membrane, and an in vitro kinase assay indicates that BRI1 is a functional Ser/Thr kinase. Among the bri1 mutants identified are mutants i n the kinase domain, and we demonstrate that one of these mutations severel y impairs BRI1 kinase activity. Therefore, we conclude that BRI1 is a ubiqu itously expressed leucine-rich repeat receptor that plays a role in BR sign aling through Ser/Thr phosphorylation.