Putrescine alleviates cold injury in peach fruit via elevating the conjugated polyamines in tonoplast and thereby maintaining vacuole conformation

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