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SUMMARY: Hydrogen sulfide (H2S) promotes plant tolerance against various environmental cues, and d‐cysteine desulfhydrase (DCD) is an enzymatic source of H2S to enhance abiotic stress resistance. However, the role of DCD‐mediated H2S production in root growth under abiotic stress remains to be further elucidated. Here, we report that DCD‐mediated H2S production alleviates osmotic stress‐mediated root growth inhibition by promoting auxin homeostasis. Osmotic stress up‐regulated DCD gene transcript and DCD protein levels and thus H2S production in roots. When subjected to osmotic stress, a dcd mutant showed more severe root growth inhibition, whereas the transgenic lines DCDox overexpressing DCD exhibited less sensitivity to osmotic stress in terms of longer root compared to the wild‐type. Moreover, osmotic stress inhibited root growth through repressing auxin signaling, whereas H2S treatment significantly alleviated osmotic stress‐mediated inhibition of auxin. Under osmotic stress, auxin accumulation was increased in DCDox but decreased in dcd mutant. H2S promoted auxin biosynthesis gene expression and auxin efflux carrier PIN‐FORMED 1 (PIN1) protein level under osmotic stress. Taken together, our results reveal that mannitol‐induced DCD and H2S in roots promote auxin homeostasis, contributing to alleviating the inhibition of root growth under osmotic stress. Significance Statement: Hydrogen sulfide (H2S) promotes plant tolerance against various environmental stress resistance. However, the role of H2S in root development under abiotic stress remains to be further elucidated. Here, we report that d‐cysteine desulfhydrase‐mediated H2S accumulation alleviates root growth inhibition under osmotic stress by promoting auxin homeostasis. [ABSTRACT FROM AUTHOR]

Summary: Osmotic stress influences root system architecture, and polar auxin transport (PAT) is well established to regulate root growth and development. However, how PAT responds to osmotic stress at the molecular level remains poorly understood. In this study, we explored whether and how the auxin efflux carrier PIN‐FORMED3 (PIN3) participates in osmotic stress‐induced root growth inhibition in Arabidopsis (Arabidopsis thaliana).We observed that osmotic stress induces a HD‐ZIP II transcription factor‐encoding gene HOMEODOMAIN ARABIDOPSIS THALIANA2 (HAT2) expression in roots. The hat2 loss‐of‐function mutant is less sensitive to osmotic stress in terms of root meristem growth. Consistent with this phenotype, whereas the auxin response is downregulated in wild‐type roots under osmotic stress, the inhibition of auxin response by osmotic stress was alleviated in hat2 roots. Conversely, transgenic lines overexpressing HAT2 (Pro35S::HAT2) had shorter roots and reduced auxin accumulation compared with wild‐type plants.PIN3 expression was significantly reduced in the Pro35S::HAT2 lines. We determined that osmotic stress‐mediated repression of PIN3 was alleviated in the hat2 mutant because HAT2 normally binds to the promoter of PIN3 and inhibits its expression.Taken together, our data revealed that osmotic stress inhibits root growth via HAT2, which regulates auxin activity by directly repressing PIN3 transcription. [ABSTRACT FROM AUTHOR]