Gene expression of cell-wall degrading enzymes in sapodilla (Manilkara zapota) fruit

Sapodilla (Manilkara zapota) is a tropical fruit crop grown commercially in Thailand which undergoes a rapid change after harvest. Information on these postharvest changes is rare. In this work, firmness of the fruit flesh in sapodilla cvv. Makok-Yai and Kra-Suay was observed to sharply decrease and was low by day 5 after harvest. The decrease in fruit firmness was hastened after ethylene treatment, and prevented after treatment with 1-methylcyclopropene. Three genes encoding cell wall-degrading enzymes, an endo-β-l,4-glucanase (MzEG), a pectate lyase (MzPL), and a polygalacturonase (MzPG), were isolated. In both cultivars tested, the transcript abundance of the isolated MzEG was correlated with fruit growth and not with loss of fruit firmness after harvest. In contrast, the mRNA of the isolated MzPL and MzPG accumulated during postharvest ripening. Ethylene treatment increased the transcript abundance in both genes. Throughout the treatments the expression of MzPG was well correlated with the decrease of fruit firmness, whereas the expression of MzPL was not. The PG activity in the fruit flesh was also well correlated with the decrease of flesh firmness. Hence the expression of the isolated MzPG was correlated both with PG activity and with firmness. Our data indicated that MzPG plays an important role in the rapid softening of sapodilla fruits during ripening.