Regulatory Subunit B′ γ of Protein Phosphatase 2A Prevents Unnecessary Defense Reactions under Low Light in Arabidopsis

Light is an important environmental factor that modulates acclimation strategies and defense responses in plants. We explored the functional role of the regulatory subunit B' gamma (B' gamma) of protein phosphatase 2A (PP2A) in light-dependent stress responses of Arabidopsis (Arabidopsis thaliana). The predominant form of PP2A consists of catalytic subunit C, scaffold subunit A, and highly variable regulatory subunit B, which determines the substrate specificity of PP2A holoenzymes. Mutant leaves of knockdown pp2a-b' gamma plants show disintegration of chloroplasts and premature yellowing conditionally under moderate light intensity. The cell-death phenotype is accompanied by the accumulation of hydrogen peroxide through a pathway that requires CONSTITUTIVE EXPRESSION OF PR GENES5 (CPR5). Moreover, the pp2a-b' gamma cpr5 double mutant additionally displays growth suppression and malformed trichomes. Similar to cpr5, the pp2a-b' gamma mutant shows constitutive activation of both salicylic acid-and jasmonic acid-dependent defense pathways. In contrast to cpr5, however, pp2a-b' gamma leaves do not contain increased levels of salicylic acid or jasmonic acid. Rather, the constitutive defense response associates with hypomethylation of DNA and increased levels of methionine-salvage pathway components in pp2a-b' gamma leaves. We suggest that the specific B' gamma subunit of PP2A is functionally connected to CPR5 and operates in the basal repression of defense responses under low irradiance.