Serine-threonine kinase that acts as a negative regulator of brassinosteroid (BR) signaling (PubMed:22685166, PubMed:27332964, PubMed:30920691). Phosphorylates DLT and BZR1, two positive regulators that mediates several BR responses. Phosphorylation of DLT and BZR1 inhibits their activities in BR signaling (PubMed:22685166). Phosphorylates OFP8, a positive regulator of BR responses. Phosphorylated OFP8 shuttles from the nucleus to the cytoplasm where it is degraded by the proteasome (PubMed:27332964). Phosphorylates the E3 ubiquitin-protein ligase PUB24, a negative regulator of BR signaling, which targets BZR1 and promotes its degradation via the 26S proteasome (PubMed:30920691). Phosphorylation of PUB24 increases its stability (PubMed:30920691). Phosphorylates the AP2-ERF transcription factor SMOS1, a positive regulator of BR signaling, which cooperatively functions in a transactivating complex with BZR1 to enhance the transcription of BR biosynthetic genes (PubMed:30920691). Phosphorylation of SMOS1 leads t