Sinapate Esters Mediate UV-B-Induced Stomatal Closure by Regulating Nitric Oxide, Hydrogen Peroxide, and Malate Accumulation in Arabidopsis thaliana

Sinapate esters, which are induced in plants under ultraviolet-B (UV-B) irradiation, have important roles not only in the protection against UV-B irradiation but also in the regulation of stomatal closure. Here, we speculated that sinapate esters would function in the stomatal closure of Arabidopsis thaliana in response to UV-B. We measured the stomatal aperture size of the wild-type (WT) and bright trichomes 1 (brt1) and sinapoylglucose accumulator 1 (sng1) mutants under UV-B irradiation; the latter two mutants are deficient in the conversion of sinapic acid to sinapoylglucose (SG) and SG to sinapoylmalate (SM), respectively. Both the brt1 and sng1 plants showed smaller stomatal apertures than the WT under normal light and UV-B irradiation conditions. The accumulation of SM and malate were induced by UV-B irradiation in WT and brt1 plants but not in sng1 plants. Consistently, exogenous malate application reduced UV-B-induced stomatal closure in WT, brt1 and sng1 plants. Nonetheless, levels of reactive oxygen species (ROS), nitric oxide (NO) and cytosolic Ca2+ were higher in guard cells of the sng1 mutant than in those of the WT under normal white light and UV-B irradiation, suggesting that disturbance of sinapate metabolism induced the accumulation of these signaling molecules that promote stomatal closure. Unexpectedly, exogenous sinapic acid application prevented stomatal closure of WT, brt1 and sng1 plants. In summary, we hypothesize that SG or other sinapate esters may promote the UV-B-induced malate accumulation and stomatal closure, whereas sinapic acid inhibits the ROS-NO pathway that regulates UV-B-induced cytosolic Ca2+ accumulation and stomatal closure.