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 to its degradation by an unknown mechanism (PubMed:30920691). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with DLT (PubMed:22685166). Interacts with OFP8 (PubMed:27332964). Interacts with GRF4 (PubMed:27250747). Interacts with PUB24 (PubMed:30920691). Interacts with SMOS1 (PubMed:28100707). Expressed in lamina joints, vascular tissue and nodes. Autophosphorylated. Plants over-expressing GSK2 display typical brassinosteroid (BR) loss-of-function phenotypes, and plants silencing GSK2 have enhanced BR signaling phenotypes. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. GSK-3 subfamily.