Knock-down of protein phosphatase 2A subunit B'γ promotes phosphorylation of CALRETICULIN 1 in Arabidopsis thaliana.

Controlled protein dephosphorylation by protein phosphatase 2A (PP2A) regulates diverse signaling events in plants. Recently, we showed that a specific B’γ regulatory subunit of PP2A mediates basal repression of immune reactions in Arabidopsis thaliana. Knock-down pp2a-b’γ mutants display constitutive defense reactions and premature yellowing conditionally under moderate light intensity. Here we show that knock-down of PP2A-B’γ renders CALRETI CULIN 1 (CRT 1) highly phosphorylated. Calreticulins are ER-resident chaperonins that operate in the unfolded protein response to prevent ER-stress, components of which are differentially regulated at mRNA level in pp2a-b’γ leaves. We speculate that in dephosphorylated state, CRT 1 promotes the degradation of unfolded proteins in ER. Our findings suggest that in wild type plants, dephosphorylation of CRT 1 is mediated by PP2A-B’γ dependent signaling effects. In pp2a-b’γ, strong phosphorylation of CRT 1 may partially imbalance the quality control of protein folding, thereby eliciting ER-stress and premature yellowing in leaves.