Knock-down of protein phosphatase 2A subunit B'gamma 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'gamma regulatory subunit of PP2A mediates basal repression of immune reactions in Arabidopsis thaliana. Knock-down pp2a-b'gamma mutants display constitutive defense reactions and premature yellowing conditionally under moderate light intensity. Here we show that knock-down of PP2A-B'gamma renders CALRETICULIN 1 (CRT1) 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'gamma leaves. We speculate that in dephosphorylated state, CRT1 promotes the degradation of unfolded proteins in ER. Our findings suggest that in wild type plants, dephosphorylation of CRT1 is mediated by PP2A-B'gamma dependent signaling effects. In pp2a-b'gamma, strong phosphorylation of CRT1 may partially imbalance the quality control of protein folding, thereby eliciting ER-stress and premature yellowing in leaves.