8-Mercapto-Cyclic GMP Mediates Hydrogen Sulfide-Induced Stomatal Closure in Arabidopsis

Plants are exposed to hydrogen sulfide (H₂S) both exogenously, as it exists as a pollutant gas in the environment, and endogenously, as it is synthesized in cells. H₂S has recently been found to function as a gaseous signaling molecule, but its signaling cascade remains unknown. Here, we examined H₂S-mediated guard cell signaling in Arabidopsis. The H₂S donor GYY4137 (morpholin-4-ium-4-methoxyphenyl [morpholino] phosphinodithioate) induced stomatal closure, which peaked after 150 min at 1 μM or after 90 min at 10 and 100 μM. After reaching maximal closure, stomatal apertures gradually increased in size in response to further exposure to GYY4137. GYY4137 induced nitric oxide (NO) generation in guard cells, and GYY4137-induced stomatal closure was reduced by an NO scavenger and inhibitors of NO-producing enzymes. Mass spectrometry analyses showed that GYY4137 induces the synthesis of 8-nitro-cGMP and 8-mercapto-cGMP and that this synthesis is mediated by NO. In addition, 8-mercapto-cGMP triggered stomatal closure. Moreover, inhibitor and genetic studies showed that calcium, cADP ribose and slow anion channel 1 act downstream of 8-mercapto-cGMP. This study therefore demonstrates that 8-mercapto-cGMP mediates the H₂S signaling cascade in guard cells.