Diverse roles for chloroplast stromal and thylakoid-bound ascorbate peroxidases in plant stress responses
Résumé
Photosynthetic light reactions comprise a significant source of hydrogen peroxide (H2O2) in illuminated leaves. Ascorbate peroxidases (APXs) reduce H2O2 to water, and play an important role in the antioxidant system of plants. We addressed the significance of chloroplast APXs in stress tolerance and signaling in Arabidopsis thaliana. To this end, T-DNA insertion mutants tapx, sapx and tapx sapx, lacking the thylakoid-bound APX (tAPX), stromal APX (sAPX) or both, respectively, were characterized. Photo-oxidative stress during germination lead to bleaching of chloroplasts in sapx single mutant and particularly in the tapx sapx double mutant plants, whereas the greening process of wild type and tapx plants was only partially impaired. Mature leaves of tapx sapx double mutants were also susceptible to short-term photo-oxidative stress induced by high light or methyl viologen treatments. After a two-week acclimation period under high light or under low temperature, none of the mutants exhibited enhanced stress symptoms. Immunoblot analysis revealed that high-light-stress-acclimated tapx sapx double mutants compensated the absence of tAPX and sAPX by increasing the level of 2-cys peroxiredoxin. Furthermore, the absence of tAPX and sAPX induced alterations in the transcriptomic profile of tapx sapx double mutant plants already under quite optimal growth conditions. We conclude that sAPX is particularly important for photoprotection during the early greening process. In mature leaves, tAPX and sAPX are functionally redundant, and crucial upon sudden onset of oxidative stress. Moreover, chloroplast APXs contribute to the fine-tuning of chloroplast signaling pathways via controlling the accumulation of H2O2 in chloroplasts.
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