Hydrogen-Modulated Stomatal Sensitivity to Abscisic Acid and Drought Tolerance Via the Regulation of Apoplastic pH in Medicago sativa

Abstract

Hydrogen gas (H2) was recently proposed as a novel gaseous signaling molecule. In our previous study, H2-mediated enhancement of plant tolerance to drought stress was preliminarily suggested. However, the detailed mechanisms of the action of H2 have not been fully explored. In this study, we observed that H2 production and hydrogenase activity were significantly induced by abscisic acid (ABA) and drought stress. Alfalfa seedlings pretreated with hydrogen-rich water (HRW) were hypersensitive to exogenous ABA. In response to ABA or water deficit, HRW-pretreated seedlings rapidly accumulated higher amounts of hydrogen peroxide (H2O2), and exhibited more tolerance to drought stress. By contrast, the inhibition or scavenging of H2O2 reduced HRW-induced drought tolerance. Further results showed that the apoplastic pH of leaves was significantly increased by HRW and/or drought stress. Cotreatment with the H+-ATPase inhibitor, however, could prevent the effects of H2 on the alkalinization of the apoplastic sap and stomatal sensitivity to exogenous ABA or water deficit. These responses were interpreted as an effect of H2 on sap pH and closure of stomata in alfalfa via an ABA-based mechanism. Overall, these results suggested a novel regulating mechanism of H2 in plant drought response.