<i>Arabidopsis</i> dehydroascorbate reductase 1 and 2 modulate redox states of ascorbate-glutathione cycle in the cytosol in response to photooxidative stress

<p>Ascorbate and glutathione are indispensable cellular redox buffers and allow plants to acclimate stressful conditions. <i>Arabidopsis</i> contains three functional dehydroascorbate reductases (DHAR1-3), which catalyzes the conversion of dehydroascorbate into its reduced form using glutathione as a reductant. We herein attempted to elucidate the physiological role in DHAR1 and DHAR2 in stress responses. The total DHAR activities in DHAR knockout <i>Arabidopsis</i> plants, <i>dhar1</i> and <i>dhar2</i>, were 22 and 92%, respectively, that in wild-type leaves. Under high light (HL), the levels of total ascorbate and dehydroascorbate were only reduced and increased, respectively, in <i>dhar1</i>. The oxidation of glutathione under HL was significantly inhibited in both <i>dhar1</i> and <i>dhar2</i>, while glutathione contents were only enhanced in <i>dhar1</i>. The <i>dhar1</i> showed stronger visible symptoms than the <i>dhar2</i> under photooxidative stress conditions. Our results demonstrated a pivotal role of DHAR1 in the modulation of cellular redox states under photooxidative stress.</p

Redox regulation of ascorbate and glutathione by a chloroplastic dehydroascorbate reductase is required for high-light stress tolerance in <i>Arabidopsis</i>

<p>Chloroplasts are a significant site for reactive oxygen species production under illumination and, thus, possess a well-organized antioxidant system involving ascorbate. Ascorbate recycling occurs in different manners in this system, including a dehydroascorbate reductase (DHAR) reaction. We herein investigated the physiological significance of DHAR3 in photo-oxidative stress tolerance in <i>Arabidopsis</i>. GFP-fused DHAR3 protein was targeted to chloroplasts in <i>Arabidopsis</i> leaves. A DHAR3 knockout mutant exhibited sensitivity to high light (HL). Under HL, the ascorbate redox states were similar in mutant and wild-type plants, while total ascorbate content was significantly lower in the mutant, suggesting that DHAR3 contributes, at least to some extent, to ascorbate recycling. Activation of monodehydroascorbate reductase occurred in <i>dhar3</i> mutant, which might compensate for the lack of DHAR3. Interestingly, glutathione oxidation was consistently inhibited in <i>dhar3</i> mutant. These findings indicate that DHAR3 regulates both ascorbate and glutathione redox states to acclimate to HL.</p> <p>The redox regulation of ascorbate and glutathione by a chloroplastic dehydroascorbate reductase (DHAR3) is essential for photo-oxidative stress tolerance in <i>Arabidopsis</i>.</p