Objective   Polyamine metabolism and expression of cold tolerance genes associated with the mechanism of melatonin in alleviating low temperature stress in tomato were investigated.

  Method  Solutions of melatonin in the concentrations of 50, 100, 200, and 300 µmol·L⁻¹ were sprayed on the leaves of Dongnong 708 tomato plants (Solanum lycopersicum). Under low temperature, optimal concentration of melatonin for the stress alleviation was determined according to the malondialdehyde content and electrolyte leakage rate in the leaves. Effects of the treatment on the growth, polyamine metabolism, and cold tolerance gene expression, as well as those of putrescine (Put) synthesis inhibitor and melatonin pretreatment on polyamine content and antioxidant characteristics, of the plants were determined.

  Result  Pretreating the tomato plans with melatonin prior to low temperature exposure lessened the seedling growth retardation induced by the stress. The plant height and fresh/dry weights as well as the Put in leaf at all stages of the stress imposition were significantly increased. The contents of spermidine (Spd) and spermine (Spm) rose significantly on the 2^nd day after low temperature treatment before tapering off. The arginine (Arg) content, arginine decarboxylase (ADC) activity, and ADC synthesis-related gene expressions became significantly higher with the melatonin pretreatment, while the Put catabolic enzyme diamine oxidase (DAO) activity and gene expression significantly declined. A challenge test confirmed that, under low temperature stress, the Put synthesis inhibitor in the arginine pathway significantly reduced the melatonin-induced enhancements on Put accumulation and antioxidant enzyme activity, which would otherwise provide a protective effect on the cell membrane. Additionally, the expression of cold tolerance genes related to Put in tomato leaves was upregulated with the presence of melatonin.

  Conclusion  Melatonin positively regulated the accumulation, but inhibited the degradation, of Put mediated by arginine. The arginine mediated Put synthesis pathway played an important role in the antioxidant defense of the tomato plants improved by melatonin. In addition, melatonin might also increase the expression of cold tolerance genes by way of promoting the Put synthesis to further boost the cold tolerance of tomato seedlings.