A Kinase-Phosphatase Signaling Module with BSK8 and BSL2 Involved in Regulation of Sucrose-Phosphate Synthase

External supply of sucrose to carbon-starved Arabidopsis seedlings induced changes in phosphorylation of Brassinosteroid Signaling Kinase 8 (BSK8) at two different sites. Serine S20 lies within a phosphorylation hotspot at the N-terminal region of the protein, while S213 is located within the kinase domain of BSK8. Upon sucrose supply phosphorylation of BSK8S20 and BSK8S213 showed opposite behavior with increasing phosphorylation of S213 and decreased phosphorylation of S20 at 5 min after sucrose supply. Here we aim to systematically analyze the effects of BSK8 mutations on downstream cellular regulatory events and characterize molecular functions of BSK8 and its phosphorylation. Comparative phosphoproteomic profiling of a bsk8 knockout mutant and wild type revealed potential targets in sucrose metabolism. Activity of sucrose-phosphate synthase (SPS) was decreased by phosphorylation at S152, and SPS phosphorylation inversely correlated with sucrose-induced BSK8 activity. Furthermore, BSK8 was found to int...