The phytohormone abscisic acid (ABA) plays pivotal roles in triggering stomatal closure and facilitating adaptation of plants to drought stress. Hydrogen sulfide (H 2 S), a small signaling gas molecule, is involved in ABA-dependent stomatal closure. However, how H 2 S regulates ABA signaling remains largely unclear.Here, we show that ABA induces the production of H 2 S catalyzed by L-CYSTEINE DESULFHYDRASE1 (DES1) in guard cells, and H 2 S in turn positively regulates ABA signaling through persulfidation of Open Stomata 1 (OST1)/SNF1-RELATED PROTEIN KINASE2.6 (SnRK2.6). Two cysteine (Cys) sites, Cys131 and Cys137, which are exposed on the surface of SnRK2.6 and close to the activation loop, were identified to be persulfidated, which promotes the activity of SnRK2.6 and its interaction with ABA response element-binding factor 2 (ABF2), a transcription factor acting downstream of ABA signaling. When Cys131, Cys137, or both residues in SnRK2.6 were substituted with serine (S), H 2 S-induced SnRK2.6 activity and SnRK2.6-ABF2 interaction were partially (SnRK2.6 C131S and SnRK2.6 C137S ) or completely (SnRK2.6 C131SC137S ) compromised. Introduction of SnRK2.6 C131S , SnRK2.6 C137S , or SnRK2.6 C131SC137S into the ost1-3 mutant could not rescue the mutant phenotype: less sensitivity to ABA-and H 2 S-induced stomatal closure and Ca 2+ influx as well as increased water loss and decreased drought tolerance. Taken together, our study reveals a novel post-translational regulatory mechanism of ABA signaling whereby H 2 S persulfidates SnRK2.6 to promote ABA signaling and ABA-induced stomatal closure.

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Abstract Hydrogen sulfide (H2S) displays a positive physiological function in plant growth. Limited data are available on the influence of H2S on cell wall growth in plants. Here, we outline a novel mechanism to show how H2S regulates cell wall cellulose biosynthesis and deposition in Alfalfa. NaHS treatment increased the length of epidermal cells in the hypocotyl of Alfalfa. Transcriptome analysis revealed that NaHS treatment caused the differential expression of a mass of cell wall-related genes. These differentially expressed genes (DEGs) directly associated with cellulose and hemicellulose biosynthesis, and pectin degradation. Cell wall composition analysis found that NaHS treatment increased the contents of cellulose and hemicellulose, but decreased pectin content. Atomic force microscopy (AFM) was used to observe cellulose microfibrils. The diameter of cellulose fibril was decreased, the arrangement of fibrillar bundles was altered, and NaHS increased the interspace of fibrillar bundles. Cellulose synthase complexes (CSCs) dynamics closely related to cellulose synthesis. We found that NaHS increased the mobile rates of CSC particles. These data suggest that the H2S signal enhanced the plasticity of the cell wall by regulating cellulose fibrils deposition and decreasing the pectin content. The increase of cellulose and hemicellulose content resulted in cell wall expansion and cell elongation.