Promoter-Level Regulation of Melatonin Biosynthesis in Plant Stress Responses: A Framework for Climate-Resilient Crop Improvement.

Climate change increasingly exposes crops to overlapping abiotic and biotic stresses, creating a need for regulatory strategies that improve stress tolerance without imposing unnecessary fitness costs under favorable conditions. Melatonin has been widely associated with plant responses to drought, salinity, temperature extremes, oxidative stress, and pathogen challenge, where it contributes to redox balance, hormone crosstalk, and stress-responsive gene regulation. However, the benefits of melatonin appear to depend strongly on when, where, and to what extent it is produced. In this review, we examine melatonin biosynthesis and function from a promoter-centered perspective, focusing on how stress-associated signals may regulate the core biosynthetic genes TDC, T5H, SNAT, and ASMT/COMT across tissues and stress contexts. Because direct functional validation of specific promoter architectures in plant melatonin biosynthesis genes remains limited, this review presents the promoter-centered model as a hypothesis-generating framework rather than a fully established regulatory mechanism. Here, we argue that the melatonin-mediated stress tolerance depends primarily on regulated, context-dependent pathway activation rather than constitutive pathway enhancement. We therefore discuss how current knowledge of stress signaling, cis-regulatory organization, and genome editing can be used to frame future efforts in promoter engineering of melatonin biosynthesis genes. Throughout, we distinguish established findings from forward-looking hypotheses and highlight key experimental questions that must be addressed before these concepts can be translated into crop improvement.