Interplay between phosphorylation and SUMOylation events determines CESTA protein fate in brassinosteroid signalling

Abstract

Brassinosteroids are steroid hormones that are essential for plant growth. Responses to these hormones are mediated by transcription factors of the BES1/BZR1 subfamily, and brassinosteroids activate these factors by impairing their inhibitory phosphorylation by GSK3/shaggy-like kinases. Here we show that brassinosteroids induce nuclear compartmentalization of CESTA (CES), a bHLH transcription factor that regulates brassinosteroid responses, and reveal that this process is regulated by CES SUMOylation. We demonstrate that CES contains an extended SUMOylation motif, and that SUMOylation of this motif is antagonized by phosphorylation to control CES subnuclear localization. Moreover, we provide evidence that phosphorylation regulates CES transcriptional activity and protein turnover by the proteasome. A coordinated modification model is proposed in which, in a brassinosteroid-deficient situation, CES is phosphorylated to activate target gene transcription and enable further posttranslational modification that controls CES protein stability.

Brassinosteroids participate in the control of basal and acquired freezing tolerance of plants

“…The overall SUMOylation of proteins drastically increases in response to cold stress (6), and the SUMOylation of ICE1 enhances its activity in CBF3 transcription. The manner by which ICE1 SUMOylation is induced is currently unknown, although a phosphodeficient mutant of ICE1 is more readily SUMOylated (30), and also in CES, dephosphorylation promotes SUMOylation (17). It is thus possible that in response to cold, BR signaling is activated to alter the phosphorylation state of TFs, inducing SUMOylation and activating them in COR gene expression.…”

“…The current postulation is that in response to BR, CES phosphorylation by BIN2 is inhibited, CES accumulates in an unphosphorylated state, promoting CES SUMOylation and nuclear compartmentalization (17). This may restrict CES activity on one type of promoters, for example those of BR biosynthesis genes (14), which are repressed when BR levels become high (23,27), but may facilitate activity on other types such as CBFs.…”

“…CES activity is altered by SUMOylation, which is induced in response to an activation of BR signaling (17). Because protein SUMOylation plays a central role in cold stress responses (6), we addressed whether the SUMOylation state impacts on CES activity in COR15A expression and freezing tolerance.…”

“…Luciferase assays were performed using a Dual-Luciferase Reporter Assay System (Promega) and a Lumat LB9501 luminometer (Berthold) as described previously (17).…”

“…CES is the closest homolog of BRASSINOSTEROID ENHANCED EXPRESSION 1 (BEE1) and BEE3 (15) and is required for elongation growth in both vegetative (14) and reproductive tissues (16). BRs control CES protein activity, abundance, and subnuclear localization via BIN2-mediated phosphorylation and phosphorylation-repressed SUMOylation (17).…”