The Ascorbic Acid Redox State Controls Guard Cell Signaling and Stomatal Movement

Abstract

H₂O₂ serves an important stress signaling function and promotes stomatal closure, whereas ascorbic acid (Asc) is the major antioxidant that scavenges H₂O₂. Dehydroascorbate reductase (DHAR) catalyzes the reduction of dehydroascorbate (oxidized ascorbate) to Asc and thus contributes to the regulation of the Asc redox state. In this study, we observed that the level of H₂O₂ and the Asc redox state in guard cells and whole leaves are diurnally regulated such that the former increases during the afternoon, whereas the latter decreases. Plants with an increased guard cell Asc redox state were generated by increasing DHAR expression, and these exhibited a reduction in the level of guard cell H₂O₂. In addition, a higher percentage of open stomata, an increase in total open stomatal area, increased stomatal conductance, and increased transpiration were observed. Guard cells with an increase in Asc redox state were less responsive to H₂O₂ or abscisic acid signaling, and the plants exhibited greater water loss under drought conditions, whereas suppressing DHAR expression conferred increased drought tolerance. Our analyses suggest that DHAR serves to maintain a basal level of Asc recycling in guard cells that is insufficient to scavenge the high rate of H₂O₂ produced in the afternoon, thus resulting in stomatal closure.