Negative regulator in brassinosteroid signal transduction pathway important for plant growth. May be also involved in auxin signaling pathway. Phosphorylates and increases the degradation of BZR1 and BZR2/BES1 by the proteasome. Phosphorylates BHLH150, beet curly top virus C4 and tomato golden mosaic virus AC4 on threonine and serine residues. Upon brassinosteroid signaling, inhibits stomatal development by phosphorylating and inhibiting the MAPKK kinase YDA and the MAPK kinases MKK4 and MKK5 (PubMed:11847343, PubMed:12114546, PubMed:12427989, PubMed:17280695, PubMed:22307275, PubMed:23341468). Phosphorylates BSK1, BSK3, BSK5, BSK6, BSK8 AND BSK11 in vitro (PubMed:23496207). Phoyphorylates and destabilizes WRKY46, WRKY54 and WRKY70 (PubMed:28576847). Mediates BASL nuclear exclusion; kinase activity is required for this function (PubMed:30429609). Required first at the cortical polarity site, to restrict MAPK signaling and promote asymmetric cell division (ACD), and second in the nucleus of stomatal lineage ground cells (SLGCs) or meristemoids, to limit cell division and to promote differentiation into pavement or stomatal guard cells, respectively, likely by initiating BASL polarization (PubMed:30429609). Phosphorylates BASL, YDA and SPCH in vitro and POLAR in vivo (PubMed:30429609). Phosphorylates and inhibits SPCH in the nucleus of SLGC undergoing ACD, thus negatively regulating stomatal development (PubMed:30429609, PubMed:22466366). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Inactivated by an unknown mechanism after binding of brassinosteroids to the brassinosteroid receptor complex (Probable). Inhibited by lithium. Inhibited by dephosphorylation at Tyr-200 by BSU1 (PubMed:21855796). Competitive inhibition by KIB1 that reduces substrate (e.g. BZR1) access (PubMed:28575660). Repressed by bikinin (PubMed:22466366). Interacts in vitro with the C-terminal fragment of BZR1 and with BES1/BZR2, but not through the kinase domain. Interacts with BHLH150, beet curly top virus AL4/C4 and tomato golden mosaic virus AL4/AC4. Interacts with YDA. Interacts with MKK4. Interacts with KIB1 and KIB2 in a brassinosteroid (BR)-dependent manner (PubMed:28575660). Interacts with BSK1, BSK6, BSK8 and BSK11 (PubMed:23496207). Binds to WRKY46, WRKY54 and WRKY70 (PubMed:28576847). Component of a complex made of POLAR, BASL, ASK7/BIN2 and ASK3/SK12 (PubMed:30429609). Binds to POLAR and BASL (PubMed:30429609). Spotty cytoplasmic as well as nuclear localization (PubMed:28652362). Recruited to the plasma membrane by OPS (PubMed:28652362). Localized throughout the plasma membrane in asymmetric cell division (ACD) precursors (PubMed:30429609). Accumulates transiently in a polarized pattern at the plasma membrane immediately before ACD, relocalizing to well-defined foci, in a POLAR-dependent and in the presence of BASL (PubMed:30429609). Mostly abundant and more polarized at the cell cortex of stomatal lineage cells with asymmetric cell division (ACD) potential (PubMed:30429609). A number of isoforms are produced. According to EST sequences. In the two outer cell layers of the developing seed coat and restricted to the suspensor cells in developing embryos (PubMed:10080716). Mostly expressed in stomatal lineage cells with asymmetric cell division (ACD) potential (PubMed:30429609). Observed in small cells of non-protruding hypocotyl cell files and of developing cotyledon epidermis (PubMed:22466366). Detected throughout the epidermis in young seedlings, and accumulates progressively during epidermal cell maturation, especially in stomatal lineage cells with asymmetric cell division (ACD) potential. Autophosphorylated mainly on threonine and serine residues. Ubiquitination and subsequent proteasomal degradation mediated by KIB1. Increased stability of SPCH and formation of excessive stomatal and non-stomatal cell. Unlike other GSK3 kinases, does not require a priming phosphorylation event or the presence of a scaffold protein to phosphorylate its substrates. Belongs to the protein kinase superfamily. CMGC Ser/Thr protein kinase family. GSK-3 subfamily.