Melatonin Alleviates Cold Damage in Apple Fruit via Raising the Conjugated Polyamines in Tonoplast and Thereby Maintaining Vacuole Structure.

Treatment with exogenous melatonin can raise postharvest fruit cold tolerance and alleviate cold injury. However, the mechanism, by which melatonin mediate cold tolerance, is still to be explored. So in this paper, the conjugated polyamines in tonoplast and vacuole structure were illuminated in the flesh cells of apple fruit subjected to cold stress, which were pretreated with exogenous melatonin. The results showed that under cold condition, fruit firmness decreased and flesh browning area increased, and vacuole structure changed. The cold injury could be alleviated by pretreatment with exogenous melatonin, as judged by stabilization of the vacuole structure, increased fruit firmness and reduced flesh browning area. Furthermore, the cold alleviation was coupled with the increases of the levels of covalently conjugated putrescine and non-covalently conjugated spermidine and spermine in the tonoplast. The results suggested that these conjugated polyamines in tonoplast and vacuole structure might be involved in melatonin-mediated cold tolerance. The suggestion was further verified by applications with inhibitors, phenanthroline and methylglyoxal-bis (guanylhydrazone) (MGBG). Phenanthroline and MGBG could restrain the melatonin-induced increases of covalently and non-covalently conjugated polyamines mentioned above in the tonoplast, respectively, and increase flesh browning area, decrease fruit firmness and vacuole structure stabilization. So, we can conclude that exogenous melatonin alleviates cold injury in apple fruit via elevating the contents of the polyamines conjugated to the tonoplast and thereby maintaining vacuole structure. [ABSTRACT FROM AUTHOR]