Ascorbate peroxidase postcold regulation of chloroplast NADPH dehydrogenase activity controls cold memory.

Exposure of Arabidopsis (Arabidopsis thaliana) to 4°C imprints a cold memory that modulates gene expression in response to a second (triggering) stress stimulus applied several days later. Comparison of plastid transcriptomes of cold-primed and control plants directly before they were exposed to the triggering stimulus showed downregulation of several subunits of chloroplast NADPH dehydrogenase (NDH) and regulatory subunits of ATP synthase. NDH is, like proton gradient 5 (PGR5)-PGR5-like1 (PGRL1), a thylakoid-embedded, ferredoxin-dependent plastoquinone reductase that protects photosystem I and stabilizes ATP synthesis by cyclic electron transport (CET). Like PGRL1A and PGRL1B transcript levels, ndhA and ndhD transcript levels decreased during the 24-h long priming cold treatment. PGRL1 transcript levels were quickly reset in the postcold phase, but expression of ndhA remained low. The transcript abundances of other ndh genes decreased within the next days. Comparison of thylakoid-bound ascorbate peroxidase (tAPX)-free and transiently tAPX-overexpressing or tAPX-downregulating Arabidopsis lines demonstrated that ndh expression is suppressed by postcold induction of tAPX. Four days after cold priming, when tAPX protein accumulation was maximal, NDH activity was almost fully lost. Lack of the NdhH-folding chaperonin Crr27 (Cpn60ß4), but not lack of the NDH activity modulating subunits NdhM, NdhO, or photosynthetic NDH subcomplex B2 (PnsB2), strengthened priming regulation of zinc finger of A. thaliana 10, which is a nuclear-localized target gene of the tAPX-dependent cold-priming pathway. We conclude that cold-priming modifies chloroplast-to-nucleus stress signaling by tAPX-mediated suppression of NDH-dependent CET and that plastid-encoded NdhH, which controls subcomplex A assembly, is of special importance for memory stabilization.

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

The redox imbalanced 6 mutant (rimb6) of Arabidopsis thaliana 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. rimb6 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 rimb6 carries a single nucleotide polymorphism in the gene encoding CONSTITUTIVE EXPRESSER OF PATHOGENESIS RELATED (PR) GENES 1, CPR1 (At4g12560), leading to a premature stop codon. CPR1 is known as a repressor of pathogen signaling and regulator of microtubule organization. Allelism of rimb6 and cpr1 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.

Specificity versus redundancy in the RAP2.4 transcription factor family of Arabidopsis thaliana: transcriptional regulation of genes for chloroplast peroxidases.

BACKGROUND: The Arabidopsis ERFIb / RAP2.4 transcription factor family consists of eight members with highly conserved DNA binding domains. Selected members have been characterized individually, but a systematic comparison is pending. The redox-sensitive transcription factor RAP2.4a mediates chloroplast-to-nucleus redox signaling and controls induction of the three most prominent chloroplast peroxidases, namely 2-Cys peroxiredoxin A (2CPA) and thylakoid- and stromal ascorbate peroxidase (tAPx and sAPx). To test the specificity and redundancy of RAP2.4 transcription factors in the regulation of genes for chloroplast peroxidases, we compared the DNA-binding sites of the transcription factors in tertiary structure models, analyzed transcription factor and target gene regulation by qRT-PCR in RAP2.4, 2-Cys peroxiredoxin and ascorbate peroxidase T-DNA insertion lines and RAP2.4 overexpressing lines of Arabidopsis thaliana and performed promoter binding studies. RESULTS: All RAP2.4 proteins bound the tAPx promoter, but only the four RAP2.4 proteins with identical DNA contact sites, namely RAP2.4a, RAP2.4b, RAP2.4d and RAP2.4h, interacted stably with the redox-sensitive part of the 2CPA promoter. Gene expression analysis in RAP2.4 knockout lines revealed that RAP2.4a is the only one supporting 2CPA and chloroplast APx expression. Rap2.4h binds to the same promoter region as Rap2.4a and antagonizes 2CPA expression. Like the other six RAP2.4 proteins, Rap2.4 h promotes APx mRNA accumulation. Chloroplast ROS signals induced RAP2.4b and RAP2.4d expression, but these two transcription factor genes are (in contrast to RAP2.4a) insensitive to low 2CP availability, and their expression decreased in APx knockout lines. RAP2.4e and RAP2.4f gradually responded to chloroplast APx availability and activated specifically APx expression. These transcription factors bound, like RAP2.4c and RAP2.4g, the tAPx promoter, but hardly the 2CPA promoter. CONCLUSIONS: The RAP2.4 transcription factors form an environmentally and developmentally regulated transcription factor network, in which the various members affect the expression intensity of the others. Within the transcription factor family, RAP2.4a has a unique function as a general transcriptional activator of chloroplast peroxidase activity. The other RAP2.4 proteins mediate the fine-control and adjust the relative availability of 2CPA, sAPx and tAPx.