POLYGALACTURONASES OF A LATENT AND WOUND POSTHARVEST FUNGAL PATHOGEN OF MUSKMELON FRUIT

Partially purified endo- and exo-polygalacturonases (PG) from two fung al pathogens (Phomopsis cucurbitae and Rhizopus stolonifer) were compa red in relation to their ability to macerate netted muskmelon tissue a t different stages of fruit development. PG extracts from P. cucurbita e, a latent infection pathogen, produced little maceration until fruit were 50 days post-anthesis (10 days postharvest). In contrast, PGs fr om R. stolonifer, a wound pathogen, produced high levels of maceration at all stages of fruit development from 20 to 50 days post-anthesis. Both pathogens demonstrated highest levels of total PG activity in mes ocarp and lowest levels in exocarp (peel) tissues. Isoelectrofocusing - polyacrylamide gel electrophoresis indicated two prominent PG isozym es in R. stolonifer and nine isozymes in P. cucurbitae. Cell wall carb ohydrate analysis showed an approximately 6-fold decrease in galactosy l residue content between 10 and 50 days post-anthesis in uninfected f ruit. Infected fruit showed approximately 7- and 8-fold decreases in g alacturonic acid content when infected with P, cucurbitae and R. stolo nifer, respectively. Significant decreases in cell wall rhamnosyl and arabinosyl residues occurred during infection of fruit with both patho gens. These results support a role for cell wall pectin degradation du ring the decay process of muskmelon by these pathogens. The ability to macerate fruit tissue, as related to the latent infection phenomenon, may be due to substrate specificity or inhibitors present in muskmelo n fruit tissue. (C) 1998 Elsevier Science B.V. All rights reserved.