Ligand Perception,Activation,and Early Signaling of Plant Steroid Receptor Brassinosteroid Insensitive 1

Leucine-rich repeat receptor-like kinases(LRR-RLKs)belong to a large group of cell surface proteins involved in many aspects of plant development and environmental responses in both monocots and dicots.Brassinosteroid insensitive 1(BRI1),a member of the LRR X subfamily,was frst identifed through several forward genetic screenings for mutants insensitive to brassinosteroids(BRs),which are a class of plant-specifc steroid hormones.Since its identifcation,BRI1 and its homologs had been proved as receptors perceiving BRs and initiating BR signaling.The co-receptor BRI1-associated kinase 1and its homologs,and other BRI1 interacting proteins such as its inhibitor BRI1 kinase inhibitor 1(BKI1)were identifed by genetic and biochemical approaches.The detailed mechanisms of BR perception by BRI1 and the activation of BRI1receptor complex have also been elucidated.Moreover,several mechanisms for termination of the activated BRI1 signaling were also discovered.In this review,we will focus on the recent advances on the mechanism of BRI1 phosphorylation and activation,the regulation of its receptor complex,the structure basis of BRI1ectodomain and BR recognition,its direct substrates,and the termination of the activated BRI1 receptor complex.