Arabidopsis Chloroplastic Ascorbate Peroxidase Isoenzymes Play a Dual Role in Photoprotection and Gene Regulation under Photooxidative Stress

Though two types of chloroplastic ascorbate peroxidase (APX) located in the thylakoid membrane (tAPX) and stroma (sAPX) have been thought to be key regulators of intracellular levels of H2O2, their physiological significance in the response to photooxidative stress is still under discussion. Here we characterized single mutants lacking either tAPX (KO-tAPX) or sAPX (KO-sAPX). Under exposure to high light or treatment with methylviologen under light, H2O2 and oxidized proteins accumulated to higher levels in both mutant plants than in the wild-type plants. On the other hand, the absence of sAPX and tAPX drastically suppressed the expression of H2O2-responsive genes under photooxidative stress. Interestingly, the most marked effect of photooxidative stress on the accumulation of H2O2 and oxidized protein and gene expression was observed in the KO-tAPX plants rather than the KO-sAPX plants. The present findings suggest that both chloroplastic APXs, but particularly tAPX, are important for photoprotection and gene regulation under photooxidative stress in Arabidopsis leaves.