BRASSINOSTEROID-SIGNALING KINASE 3, a plasma membrane-associated scaffold protein involved in early brassinosteroid signaling

Brassinosteroids (BRs) are steroid hormones essential for plant growth and development. The BR signaling pathway has been studied in some detail, however, the functions of the BRASSINOSTEROID-SIGNALING KINASE (BSK) family proteins in the pathway have remained elusive. Through forward genetics, we identified five semi-dominant mutations in the BSK3 gene causing BSK3 loss-of-function and decreased BR responses. We therefore investigated the function of BSK3, a receptor-like cytoplasmic kinase, in BR signaling and plant growth and development. We find that BSK3 is anchored to the plasma membrane via N-myristoylation, which is required for its function in BR signaling. The N-terminal kinase domain is crucial for BSK3 function, and the C-terminal three tandem TPR motifs contribute to BSK3/BSK3 homodimer and BSK3/BSK1 heterodimer formation. Interestingly, the effects of BSK3 on BR responses are dose-dependent, depending on its protein levels. Our genetic studies indicate that kinase dead BSK3(K86R) protein partially rescues the bsk3-1 mutant phenotypes. BSK3 directly interacts with the BSK family proteins (BSK3 and BSK1), BRI1 receptor kinase, BSU1 phosphatase, and BIN2 kinase. BIN2 phosphorylation of BSK3 enhances BSK3/BSK3 homodimer and BSK3/BSK1 heterodimer formation, BSK3/BRI1 interaction, and BSK3/BSU1 interaction. Furthermore, we find that BSK3 upregulates BSU1 transcript and protein levels to activate BR signaling. BSK3 is broadly expressed and plays an important role in BR-mediated root growth, shoot growth, and organ separation. Together, our findings suggest that BSK3 may function as a scaffold protein to regulate BR signaling. The results of our studies provide new insights into early BR signaling mechanisms. Author summary Steroid hormones exist in both animals and plants. Brassinosteroids (BRs) are steroid hormones that regulate numerous growth and developmental processes throughout the plant life cycle. Discovered in 2008, the BRASSINOSTEROID-SIGNALING KINASE (BSK) protein family is composed of twelve members. However, the functions of these receptor-like cytoplasmic kinases in the BR signaling pathway are poorly understood. Here, we focus on BSK3 and investigate its function in BR signaling and plant growth and development. Our genetic and biochemical studies suggest that BSK3 may function as a scaffold protein to regulate BR signaling. BSK3 is important for BR-mediated root growth, shoot growth, and organ separation. BSK3 activates BR signaling via upregulating BSU1 transcript levels, and BIN2 phosphorylation of BSK3 influences BSK3 interactions with other signaling components, revealing new layers of regulation in BR signaling. Together, our findings elucidate the function of BSK3 in BR signaling and plant growth and development, and provide new insights into early BR signaling mechanisms.