Dimerization-activated PP2C-D2 and D5 phosphatases negatively regulate brassinosteroid signaling by antagonizing BRI1

Brassinosteroids (BRs) are key hormones that promote plant growth and development. While reversible phosphorylation of the BR receptor BRASSINOSTEROID-INSENSITIVE 1 (BRI1) is critical for BR signaling, much remains to be learned about the dephosphorylation mechanisms of BRI1. Here, we demonstrate that the D-clade type 2C protein phosphatases (PP2C-Ds) negatively regulate BR signaling by dephosphorylating key residues in the kinase activation loop of BRI1. The phosphatase activity of PP2C-D2 and D5 is activated through homo- or heterodimerization, a process antagonized by BR-induced BRI1 phosphorylation or SMALL AUXIN UP RNA15 (SAUR15) binding. BL treatment, BRI1-mediated phosphorylation, or SAUR15 binding disrupts dimerization, returning PPC2C-Ds to a monomeric, inactive state. These data not only reveal another phosphorylation/dephosphorylation cascade regulating BR signaling but also decode unrecognized regulatory mechanisms of PP2C-Ds in plants.