Hydrogen Sulfide’s Role in Stomatal Closure

Hydrogen sulfide (H₂S) is an important gaseous signaling molecule in plants that participates in stress responses, development, and stomatal closure. In plants, H₂S is enzymatically produced by cysteine desulfhydrase in the catalyzed conversion of cysteine to pyruvate, H₂S, and NH₃⁺. In Arabidopsis thaliana, the cytosolic protein DES1 was recently characterized as a novel L-cysteine desulfhydrase. DES1 is involved in plant response to heavy metal stress, immunity, autophagy, and stomatal closure. Although DES1 does not account for all L-cysteine desulfhydrase activity, previous studies have shown that DES1 activity is required for ABA-dependent stomatal closure, and DES1 acts early in ABA-mediated signal transduction. Scuffi et al. (10.1104/pp.17.01636) have combined genetic and pharmacological approaches to shed further light on the involvement of H₂S in stomatal closure in Arabidopsis. In particular, they have tested the interplay between H₂S and other second messengers of the guard cell signaling network, such as hydrogen peroxide (H₂O₂) and phospholipase D (PLD)-derived phosphatidic acid (PA). In plants, PA is generated via phospholipase C activity in concert with diacylglycerol kinase (DGK), or directly through hydrolysis of structural lipids by PLD. PA has emerged as a key player in plant signaling. The authors present evidence that H₂S stimulates H₂O₂ production in Arabidopsis guard cells and that two NADPH oxidase isoforms are required for H₂S-induced stomatal closure. Additionally, the authors studied the interplay between H₂S and PLD activity in the regulation of reactive oxygen species production and stomatal movement. The PLDα1 and PLDδ isoforms were required for H₂S-induced stomatal closure, and most of the H₂S-dependent H₂O₂ production required the activity of PLDα1. Finally, the authors also present evidence that H₂S induces increases in the PLDδ-derived levels of PA in guard cells. In toto, these results reveal the involvement of H₂S in the signaling network that controls stomatal closure, and suggest that H₂S regulates the activities NADPH oxidases and PLD in guard cells.