SlSNAT Interacts with HSP40, a Molecular Chaperone, to Regulate Melatonin Biosynthesis and Promote Thermotolerance in Tomato

The SNAT enzyme participates in the biosynthesis of melatonin, which is reported to regulate thermotolerance in many plants. However, the mechanistic basis of this regulation remains unclear. In this study, we identified the SlSNAT gene, which is responsible for melatonin biosynthesis in tomato. SlSNAT expression levels were 3- and 5-fold higher in SlSNAT overexpression lines OX-2 and OX-6, respectively. The melatonin levels were 3- and 4-fold higher than those in wild type. The melatonin levels decreased by 50% when the expression of SlSNAT was downregulated to 40%. Overexpression of SlSNAT in tomato plants provided significantly enhanced thermotolerance with better growth performance in Photosystem II (PSII) maximum photochemical quantum yield (Fv/Fm) and alleviated heat injury. Both exogenous melatonin treatment and endogenous melatonin manipulation by SlSNAT overexpression decreased the levels of reactive oxygen speciesaccumulation and Fv/Fm. The SlSNAT overexpression line showed protected ribulose bisphosphate carboxylase oxygenase proteins and upregulated response of heat transcription factors and heat shock proteins under heat stress. HSP40, a DnaJ-type chaperone, was found to interact with SlSNAT in the chloroplast. Downregulation of HSP40 showed lower melatonin synthesis under heat stress. HSP40 functions as a chaperone to protect the SNAT enzyme during melatonin synthesis under heat stress. HSP40 interacted with SlSNAT and together participated in melatonin-related thermotolerance regulation in tomato.