Polyamines in plasma membrane function in melatonin-mediated tolerance of apricot fruit to chilling stress

Polyamines are closely associated with environmental stresses and melatonin pretreatment enhances the resistance of fruit to chilling stress. However, a mechanism underlying melatonin-mediated chilling resistance remains to be answered. This research aimed to illuminate whether polyamines would be involved in melatonin-mediated chilling resistance. Therefore, in the experiment, the polyamines conjugated to the plasma membrane from the melatonin-pretreated apricot flesh cell were examined under chilling stress. Chilling resistance was judged by four parameters: fruit browning degree, plasma membrane permeability, malondialdehyde content and plasma membrane protein sulfhydryl level. Results showed melatonin pretreatment led to obvious rises in the levels of non-covalently conjugated spermine and spermidine, and covalently conjugated putrescine and spermidine in the plasma membrane. Methylglyoxyl-bis (guanylhydrazone) pretreatment could inhibit the melatonin-induced increases of non-covalently conjugated spermidine and spermine by inhibiting S-adenosylmethionine decarboxylase (SAMDC) activity and free spermidine and spermine contents in flesh, coupled with the decrease in chilling resistance. Similarly, phenanthroline pretreatment could inhibit the melatonin-induced increases in covalently conjugated putrescine and spermidine in the plasma membrane through inhibiting transglutaminase (TGase) activity and simultaneously could aggravate chilling damage. The results suggested melatonin pretreatment could enhance chilling resistance by increasing non-covalently conjugated spermidine and spermine, as well as covalently conjugated putrescine and spermidine in the plasma membrane of apricot fruit.