Light involved regulation of BZR1 stability and phosphorylation status to coordinate plant growth in Arabidopsis

Light and brassinosteroid (BR) are master environmental stimulus and endogenous cue for plant growth and development respectively. Great progress has been made in elucidating the molecular mechanisms on the cross-talk between light and BR. However, little is known about how BZR1, the pivotal integration node, is regulated by light and dark. Here, we demonstrated that an intact BR signaling pathway is essential for dark-induced hypocotyl elongation. Consequent expression assay showed that light-dark switch affected BZR1 phosphorylation and accumulation. Moreover, blocking the 26S proteasome pathway promoted the accumulation of both phosphorylated and dephosphorylated BZR1 proteins. Restriction of new protein biosynthesis had multiple effects on BZR1 phosphorylation status and stability, relying on the availability of light and the 26S proteasome pathways. Furthermore, sugar treatment strikingly enhanced the accumulation of total BZR1 under either light or dark conditions, likely by repressing transcript abundance of MAX2, a gene encoding an E3 ligase for BZR1. Finally, light-regulated phosphorylation change of BZR1 requires the existence of endogenous BR as well as functional BIN2 and protein phosphatase 2A (PP2A). Taken together, our results depicted a light-involved complex regulation network of BZR1 stability and phosphorylation status.