Plants utilize plasma membrane-localized receptor-like kinases(RLKs)to sense extracellular signals to coordinate growth,development,and innate immune responses.BAK1 regulates multiple signaling pathways acting as a co-receptor of several distinct ligand-binding RLKs.It has been debated whether BAK1 serves as an essential regulatory component or only a signal amplifer without pathway specifcity.This issue has been clarifed recently.Genetic and structural analyses indicated that BAK1 and its homologs play indispensible roles in mediating brassinosteroid(BR)signaling pathway by directly perceiving the ligand BR and activating the receptor of BR,BRI1.The mechanism revealed by these studies now serves as a paradigm for how a pair of RLKs can function together in ligand binding and subsequent initiation of signaling.
Brassinosteroids (BRs) are a group of naturally-occurring steroidal phytohormones playing fundamental roles during normal plant growth and development.Using a combination of experimental approaches,including analytical chemistry,genetics,and biochem-istry,the major BR biosynthetic pathway has been largely eluci-dated.The least-understood knowledge in the BR research field is probably the molecular mechanisms controlling the bioactive levels of BRs in response to various developmental and envi-ronmental cues.In this review,we focus our discussion on a recently-proposed,8-step predominant BR biosynthetic pathway,several newly-identified transcription factors regulating the expres-sion of key enzymes that catalyze BR biosynthesis,and up-to-date information about the mechanisms that plants use to inactivate unnecessary BRs.
Our previous studies indicated that TCP1 is a positive regulator of the brassinosteroid(BR) biosynthesis pathway by mediating the transcription of DWF4,one of the key BR biosynthetic genes in Arabidopsis thaliana.Whether TCP1 can directly bind to the promoter region of DWF4,however,has not been experimentally demonstrated.Here we provide our biochemical and genetic evidence that TCP1 mediates the expression of DWF4 by directly associating with the two GGNCCC motifs in the promoter region of DWF4.The expression levels of DWF4 are positively correlated to TCP1 abundance in planta.Electrophoretic mobility shift assays(EMSAs) using various synthetic DNA fragments suggest that the GGNCCC core sequence is critical for TCP1 binding.DNA sequences flanking the GGNCCC motifs are less important for the association of TCP1.Using DWF4p-GUS transgenic plants as an assay system,it is clearly indicated that these motifs are required for the positive regulation of DWF4 transcription by TCP1.More significantly,whole genome microarray analyses indicate that TCP1 can directly or indirectly regulate the expression of many other genes known to be important for normal plant growth and development.
