The Plant Immunity Regulating F-Box Protein CPR1 Supports Plastid Function in Absence of Pathogens

Abstract

The <i>redox imbalanced 6</i> mutant (<i>rimb6)</i> of <i>Arabidopsis thaliana</i> was isolated in a genetic screening approach for mutants with defects in chloroplast-to-nucleus redox signaling. It has an atypically low activation status of the 2-Cys peroxiredoxin-A promoter in the seedling stage. <i>rimb6</i> shows wildtype-like germination, seedling development and greening, but slower growth and reduced biomass in the rosette stage. Mapping of the casual mutation revealed that <i>rimb6</i> carries a single nucleotide polymorphism in the gene encoding <i>CONSTITUTIVE EXPRESSER OF PATHOGENESIS RELATED (PR) GENES 1, CPR1</i> (At4g12560), leading to a premature stop codon. CPR1 is known as a repressor of pathogen signaling and regulator of microtubule organization. Allelism of <i>rimb6</i> and <i>cpr1</i> revealed a function of CPR1 in chloroplast stress protection. Expression studies in pathogen signaling mutants demonstrated that CPR1-mediated activation of genes for photosynthesis and chloroplast antioxidant protection is, in contrast to activation of pathogen responses, regulated independently from PAD4-controlled salicylic acid (SA) accumulation. We conclude that the support of plastid function is a basic, SA-independent function of CPR1.