ASKθ, a group-III Arabidopsis GSK3, functions in the brassinosteroid signalling pathway

P>Brassinosteroids (BRs) are plant hormones that regulate many processes including cell elongation, leaf development, pollen tube growth and xylem differentiation. GSK3/shaggy-like kinases (GSK) are critical regulators of intracellular signalling initiated by the binding of BR to the BRI1 receptor complex. Three GSKs have already been shown to relay BR responses, including phosphorylation of the transcriptional regulator BES1. However, recent studies indicate that one or more yet unidentified protein kinases are involved in BR signalling. Here, we show that the in vivo protein kinase activity of the group-III GSK, ASK theta, was negatively regulated by BRI1. Arabidopsis thaliana plants with enhanced ASK theta activity displayed a bri1-like phenotype. ASK theta overexpressors accumulated high levels of brassinolide, castasterone and typhasterol, and were insensitive to BR. ASK theta localized to the nucleus and directly phosphorylated BES1 and BZR1. Moreover, the BES1/BZR1-like transcription factor BEH2 was isolated as an ASK theta interaction partner in a yeast two-hybrid screen. ASK theta phosphorylated BEH2 both in vitro and in vivo. Overall, these data provide strong evidence that ASK theta is a novel component of the BR signalling cascade, targeting the transcription factors BES1, BZR1 and BEH2.