Revealing the significance of melatonin in postharvest quality of tomato fruit, especially in sugar metabolism and transport

As a natural product, melatonin (MT) plays an important role in regulating postharvest fruit quality. However, the evaluation of MT’s effects on fruit quality remains underexplored. Here three tomato cultivars ‘Ailsa Craig (AC)’, ‘Fen Beibei (FBB)’, and ‘Provence (PV)’ were treated with 0, 50, 100, 200, and 500 μM MT, and measuring quality indicators combined with the comprehensive evaluation were conducted. Results showed that different concentrations of MT promoted fruit coloring and fruit softening, as well as elevated the sugar-to-acid ratio compared to control. The principal component analysis-weighted membership function (PCA-MF) indicated that 100 μM MT exhibited the best effect on improving postharvest fruit quality. At this concentration, MT enhanced lycopene accumulation and reduced chlorophyll content. Concurrently, the pectin, cellulose, and hemicellulose contents were decreased by MT. Glucose, fructose, and sucrose were increased, while citric acid and malic acid were decreased under MT treatment. According to PCA-MF, MT had differential effects on postharvest fruit quality among the three tomato cultivars, following the order PV>AC>FBB. Grey relational analysis revealed that the sugar metabolism-related indicators played a significant role in quality evaluation. MT affected fruit sugar metabolism and transport by upregulating the expression of SlTIV1, SlSWEET15, SlLIN5 and SlTMT2 , SlAMY2, SlBAM3 , downregulating the expression of SlSUT4, SlFRK2 and SlCIF1 . Meanwhile, the expression of sugar metabolism-related transcription factor genes SlAREB1 , SlHY5 , SlARF10 , SlbZIP1 , and SlCDPK27 was regulated by MT. Therefore, MT significantly affected sugar metabolism and transport of tomato fruit, thereby improving postharvest quality. • Melatonin promotes fruit coloring and softening, elevates sugar-acid ratio of tomato. • Melatonin at 100 μM exhibits a more powerful effect on improving fruit quality. • Melatonin shows the greatest impact on PV tomato, followed by AC and FBB tomato. • Melatonin significantly affects sugar metabolism and transport of tomato fruit.