Polyamines conjugated to the bio-membranes and membrane conformations are involved in the melatonin-mediated resistance of harvested plum fruit to cold stress

The mechanism underlying melatonin-mediated tolerance of postharvest fruit to cold is well explored. However, the relationship among cold tolerance, polyamines (PAs) conjugated to bio-membrane and membrane conformations remains to be elucidated in the melatonin-pretreated fruit. Therefore in this study, the polyamines conjugated to the bio-membranes (plasma membrane, mitochondrial membrane and tonoplast) and corresponding membrane conformations were detected in the melatonin-pretreated fruit under cold condition. The results demonstrated that cold stress caused damage to the normal conformations of the aforementioned bio-membranes and led to cold injury, which was indicated by the increased flesh browning area and plasma membrane permeability, and the decreased fruit firmness and sulphydryl content. However, melatonin pretreatment mitigated fruit cold injury and stabilized the conformations of the bio-membranes mentioned above, which was comitant with increased contents of non-covalently (NC)-conjugated spermidine (Spd) and spermine (Spm), rising by nearly 2.5 times, as well as covalently (C-) conjugated putrescine (Put) and Spd in the bio-membranes, which rose by over 3.5 and 4, respectively, compared to the cold-treated. From the results, it could be inferred that the polyamines conjugated to the bio-membranes and membrane conformation stabilization were involved in the melatonin-mediated resistance of harvested fruit to cold stress. This inference was confirmed by the complementary experiments with methylglyoxal-bis (guanylhydrazone) (MGBG) and phenanthroline, which inhibited S-adenosylmethionine decarboxylase and transglutaminase activities, respectively. MGBG and phenanthroline inhibited the melatonin-induced increases in the levels of NC-conjugated Spd and Spm, and C-conjugated PAs in the bio-membranes, respectively. Furthermore, both MGBG and phenanthroline reversed the mitigation effects of melatonin on fruit cold injury, in parallel with the decrease of the membrane conformation stabilization. Collectively, it could be concluded that melatonin-mediated resistance of harvested plum fruit to cold stress might involve in the increases in the PAs conjugated to the bio-membranes, as well as the maintenance of membrane conformation stabilization. The relationship between conjugated PAs in the bio-membranes and membrane conformation stabilization was discussed in the research.