Abstract
Brassinosteroids (BRs) are a class of steroid molecules perceived at the cell surface that act as plant hormones. The BR LRR-RLK receptor BRI1 offers a model to understand receptor-mediated signaling in plants and the role of post-translational modifications. We previously reported that undergoes Ub-mediated endocytosis following its massive decoration with K63-linked polyUb chains. A non-ubiquitinatable BRI1 variant lacking 25 intracellular lysine residues indeed shows slower internalization kinetics and inability to reach the lytic vacuole. Interestingly, a slight increase in ambient temperature triggers BRI1 destabilization, presumably through Ub-mediated endocytosis since the BRI125KR variant fails to respond. Here we identify SUMOylation as a new modification, targeting BRI1 to regulate its activity. We show that BRI1 is SUMOylated in planta on two lysine residues, which are also Ub targets, and that the levels of BRI1-SUMO conjugates are controlled by the Desi3a SUMO protease. We demonstrate that BRI1 is deSUMOylated at elevated temperature by Desi3a, leading to increased BRI1 interaction with the negative regulator of BR signaling BIK1 and enhanced BRI1 endocytosis. Loss of Desi3a or BIK1 results in increased response to temperature elevation, indicating that BRI1 deSUMOylation acts as a safety mechanism necessary to keep temperature responses in check. Altogether, our work establishes BRI1 deSUMOylation as a molecular crosstalk mechanism between temperature and BR signaling, allowing plants to translate environmental inputs into growth response. Besides, it sheds light on the possible interplay between SUMO and Ub at the same target lysine residues to control BRI1 stability.
Published Version
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