Exogenous Melatonin Enhances the Cold Tolerance of Harvested Peach (Prunus persica Batsch) Fruit by Elevating SAMDC and Decreasing ACC Synthase Activities

Exogenous melatonin can alleviate cold injury in post-harvested fruit during fruit refrigeration. The mechanism of melatonin-mediated cold tolerance is increasingly being focused on in studies. However, in cold + melatonin-treated fruit, the changes in the activities of S-adenosylmethionine decarboxylase (SAMDC) and 1-aminocyclopropane-1-carboxylic acid (ACC) synthase, the key enzymes that catalyze polyamine and ethylene biosynthesis, respectively, are still unclear. In this study, the typical symptoms of cold injury, the contents of polyamines and ethylene, and the activities of SAMDC and ACC synthase are elucidated in melatonin-treated peach fruit under cold conditions. The results show that exogenous melatonin alleviated cold injury in peach fruit and elevated SAMDC activity and, thereby, the levels of spermidine and spermine. More importantly, ACC synthase activity, the ACC level and the ethylene emission rate decreased. These results hint that exogenous melatonin enhanced the tolerance of peach fruit to cold stress by elevating SAMDC and decreasing ACC activities. The results of the inhibitor experiments supplied additional evidence for this hypothesis. The inhibitor methylglyoxal-bis (guanylhydrazone) could inhibit SAMDC activity, decrease the levels of spermidine and spermine, increase the ACC level and ethylene emission rate, and thereby decrease the fruit’s cold tolerance. Meanwhile, the inhibitor aminoethoxyvinylglycine could decrease the ACC synthase activity, the ACC level, and the ethylene emission rate, and also increase the fruit’s cold tolerance. Furthermore, the proposed competitive relationship between SAMDC and ACC synthase for substrate S-adenosyl-L-Met is also discussed in this paper.