Exogenous tryptophan enhances cold resistance of soybean seedlings by promoting melatonin biosynthesis.

Given the global climate change, soybean production is highly susceptible to low temperature. Although tryptophan, the synthesis precursors of melatonin and auxin, exhibited a positive effect in regulating plant growth, it is still unclear whether tryptophan could improve the tolerance of soybean to low temperature stress through endogenous melatonin synthesis. Therefore, the effect of tryptophan on the resistance of two varieties of soybean seedlings to low temperature (4°C) was evaluated, and the main regulation pathway of tryptophan was verified with melatonin synthesis inhibitors. The results revealed that low temperature stress significantly inhibited the growth of soybean, while the application of exogenous tryptophan significantly enhanced the antioxidant activity of soybean seedlings to reduce the content of reactive oxygen species, including O₂ ^- (11.3%) and H₂O₂ (17.8%), and effectively protected the photosynthetic capacity of leaves, involving net photosynthetic rate (22.94%), transpiration rate (15.31%), stomatal conductance (20.27%). And the application of tryptophan significantly increased the leaf area (16.63%), plant height (7.14%), root surface area (24.37%), root volume (22.92%) and root tip number (29.67%) of seedlings at low temperature. However, p-chlorophenylalanine inhibited the synthesis of melatonin and eliminated the effect of tryptophan. In conclusion, tryptophan mainly improved the cold tolerance of soybean seedlings by promoting endogenous melatonin synthesis, which provided a theoretical basis for tryptophan to enhance the cold tolerance of soybean in field production.