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. Phosphorylates BSK1, BSK3, BSK5, BSK6, BSK8 AND BSK11 in vitro. Phoyphorylates and destabilizes WRKY46, WRKY54 and WRKY70. Mediates BASL nuclear exclusion; kinase activity is required for this function. 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. Phosphorylates BASL, YDA and SPCH in vitro and POLAR in vivo. Phosphorylates and inhibits SPCH in the nucleus of SLGC undergoing ACD, thus negatively regulating stomatal development.
