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

Andrea Trotta,Maija Holmström,Saijaliisa Kangasjärvi,Nina Sipari,Engelbert Weis,Judith Scharte,Michael Wrzaczek,Eva-Mari Aro,Moona Rahikainen,Mikko Tikkanen,Antje Von Schaewen,Paula Mulo,Grzegorz Konert,Hanna-Maija Hiltunen,Stephan Rips

doi:10.1104/pp.111.178442

Abstract

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'γ (B'γ) 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'γ 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'γ cpr5 double mutant additionally displays growth suppression and malformed trichomes. Similar to cpr5, the pp2a-b'γ mutant shows constitutive activation of both salicylic acid- and jasmonic acid-dependent defense pathways. In contrast to cpr5, however, pp2a-b'γ 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'γ leaves. We suggest that the specific B'γ subunit of PP2A is functionally connected to CPR5 and operates in the basal repression of defense responses under low irradiance.

Highlights

Light is an important environmental factor that modulates acclimation strategies and defense responses in plants
The pp2a-b#g mutant, deficient in a specific regulatory B#g subunit of phosphatase 2A (PP2A), was chosen for further analysis, since it showed an intriguing phenotype with wrinkled leaves, stunted growth, delayed flowering, and agedependent formation of yellowing lesions when grown under a moderate growth light intensity of 130 mmol photons m22 s21 (Fig. 1A; Table I)
We have demonstrated that a specific B#g subunit of PP2A is functionally connected with CONSTITUTIVE EXPRESSION OF PR GENES5 (CPR5) and mediates the negative regulation of defense reactions and senescence under low irradiances in Arabidopsis

Summary

Introduction

Light is an important environmental factor that modulates acclimation strategies and defense responses in plants. Subsequent analysis of rcn null mutants demonstrated that the absence of RCN1 renders plants insensitive to blue light, abscisic acid, and jasmonic acid (JA) signaling in stomatal closure (Kwak et al, 2002; Saito et al, 2008; Tseng and Briggs, 2010) and blocks the biosynthesis of ethylene (ET; Larsen and Cancel, 2003). Such multiple phenotypic effects of rcn suggest that, in wild-type plants, the specific targets for RCN1-containing PP2A complexes may be determined by the identity of the regulatory B subunit in the PP2A holoenzyme

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Discussion

Conclusion