PP2A activates brassinosteroid-responsive gene expression and plant growth by dephosphorylating BZR1

When brassinosteroid (BR) levels are low, the GSK3-like kinase BIN2 phosphorylates and inactivates the BZR1 transcription factor to inhibit growth in plants. BR promotes growth by inducing dephosphorylation of BZR1, but the phosphatase that dephosphorylates BZR1 has remained unknown. Here we identified protein phosphatase 2A (PP2A) as BZR1-interacting proteins using tandem affinity purification. Genetic analyses demonstrated a positive role of PP2A in BR signalling and BZR1 dephosphorylation. Members of the B'regulatory subunits of PP2A directly interact with BZR1's putative PEST domain containing the site of the bzr1-1D mutation. Interaction with and dephosphorylation by PP2A are enhanced by the bzr1-1D mutation, reduced by two intragenic bzr1-1D suppressor mutations, and abolished by deletion of the PEST domain. This study reveals a crucial function of PP2A in dephosphorylating and activating BZR1 and completes the set of core components of the BR-signalling cascade from cell surface receptor kinase to gene regulation in the nucleus.