POLYGALACTURONASE ISOZYMES PRODUCED BY PHOMOPSIS-CUCURBITAE IN RELATION TO POSTHARVEST DECAY OF CANTALOUPE FRUIT

Production of polygalacturonase (PG), a cell wall-degrading enzyme, by Phomopsis cucurbitae (latent infection fungus) was studied in relatio n to different carbon sources and various stages of cantaloupe fruit d evelopment. P. cucurbitae produced multiple PG isozymes both in vitro and in vivo. The fungus produced the highest PG activity and the great est number of isozymes on pectin compared with those produced on gluco se, galactose, and sucrose. Eight P. cucurbitae PG isozymes (pIs 3.7, 4.2, 6.6, 7.0, 7.3, 7.5, 7.8, and 8.6) were detected in extract from i noculated mature fruit (40 days after anthesis) by isoelectric focusin g. Isozyme bands with pIs of 4.2, 7.3, and 7.8 were the most prominent . A similar set of PG isozymes was produced by P. cucurbitae in autocl aved mature fruit tissue (mesocarp). When tissue discs taken from 20-, 30-, 40-, and 50-day postanthesis fruit were inoculated with P. cucur bitae. PG activity and the number of PG isozymes extracted from the ma cerated fruit tissue discs increased with the degree of fruit maturity and ripening. Increases in PG activity and PG isozymes were also corr elated with reactivation of latent infections and the beginning of tis sue maceration. An anionic PG isozyme (pI 4.2) was only visualized on decayed 50-day-old fruit exocarp, as well as 40- and 50-day-old Fruit mesocarp. The experimental results support the hypotheses that P. cucu rbitae PG isozymes play an important role in fruit decay once latent i nfection becomes active following harvest.