Light and Temperature Coordinately Regulate Phytomelatonin Synthesis to Maintain Plant Morphogenesis via the COP1‐HY5 Module

Light and temperature change constantly under natural conditions and play vital roles in coordinating plant morphogenesis. However, how these two signals are integrated with endogenous signals to fine-tune plant morphology requires further investigation. Given that phytomelatonin is a multifunctional regulator connecting environmental signals and plant development, here we propose that phytomelatonin is involved in the integration of light and temperature signals. When co-treated with darkness and warm ambient temperature, the light-temperature signal showed synergistic upregulation of phytomelatonin synthesis and thus hypocotyl growth. Phytomelatonin synthesis gene SEROTONIN N-ACETYLTRANSFERASE (SNAT) was induced under constant darkness or warm temperature, reaching its peak level under the combined treatment. The snat mutant, with reduced phytomelatonin content and hypocotyl length, was less sensitive to darkness and warm temperature, whereas 35S::SNAT-GFP had more phytomelatonin and longer hypocotyls than the wild type, indicating that SNAT is needed for light-temperature morphogenesis. Furthermore, SNAT expression and phytomelatonin content were reduced in cop1 but increased in hy5. HY5 inhibits SNAT expression by binding to its promoter. The hy5 snat seedlings had less phytomelatonin and shorter hypocotyls than the hy5 seedlings, along with the SNAT mutation in 35S::COP1 snat seedlings reversed the phenotype of 35S::COP1, further verifying that SNAT acts downstream of COP1-HY5 module. Moreover, RNA-Seq revealed that phytomelatonin is associated with light-temperature signal in controlling hypocotyl elongation-related genes. Taken together, our results showed that the light-temperature signal regulates SNAT-mediated phytomelatonin synthesis through COP1-HY5 module to coordinate plant morphogenesis.