Role of polygalacturonase in bean leaf abscission

The role of polygalacturonase in leaf abscission was studied in explants of Phaseolus vulgaris L. cv. Red Kidney. Bean polygalacturonase was partially characterized and comparisons were made between the bean enzyme and previously reported higher plant polygalacturonases. Polygalacturonase isolated from bean leaf abscission zones has a pH optimum between 4.5 and 5.0 and hydrolyzed polygalacturonides in an exo-fashion. Activity was found to be higher with a deesterified substrate than with an esterified pectin. No correlation between polygalacturonase activity and abscission was observed. Activity remained virtually constant over the course of abscission in explants aged either in air or in ethylene. The enzyme was primarily localized in the abscission zone, however, indicating a possible involvement in the abscission process. A theoretical model which could explain the relationship between polygalacturonase and bean leaf abscission is discussed.