Evidence for a JA-responsive SNAC1–ASMT1 regulatory module contributing to melatonin-mediated salinity stress tolerance in Barley

Melatonin has emerged as a crucial mediator in plant responses to abiotic stresses, with its regulatory effects closely dependent on its endogenous levels and biosynthetic dynamics. However, in barley (Hordeum vulgare L.), the upstream regulatory mechanisms of melatonin biosynthesis under salinity stress, and their connection to hormonal signaling, remain largely unknown. In this study, we explore the potential regulatory modules of the key melatonin biosynthesis gene, Acetylserotonin O-Methyltransferase 1 (ASMT1), in barley. Promoter analysis identified putative Stress-responsive NAC1 (SNAC1) binding motifs within the ASMT1 promoter region, and transcriptomic data showed the differential expression of SNAC1 and ASMT1 in response to salinity exposure. To further investigate this regulatory relationship, we performed a controlled greenhouse experiment with six treatments: control, Jasmonic acid (JA), DIECA (a JA biosynthesis inhibitor), salinity (S), JA + S, and DIECA + S. Exogenous JA significantly increased SNAC1 and ASMT1 expression, boosted melatonin levels, activated antioxidant enzymes (SOD, CAT, APX), and reduced oxidative damage and photosynthetic decline under salinity. In contrast, inhibition of JA biosynthesis by DIECA attenuated these responses, supporting the involvement of JA signaling in this pathway. Additionally, we observed a statistically significant correlation between gene expression profiles and melatonin content. While further functional validation is needed, our results support a model in which JA signaling contributes to salinity-induced melatonin biosynthesis, possibly through the SNAC1–ASMT1 axis. These findings offer new insights into how hormones regulate melatonin during stress and provide a framework for future functional studies aimed at improving stress tolerance in barley.