Polyphenoloxidase activity in soybean hypocotyls at sites inoculated with Phytophthora megasperma f. sp. glycinea

Abstract

Etiolated 6-day-old soybean hypocotyls were inoculated with Phytophthora megasperma f. sp. glycinea by placing drops of zoospore suspensions onto the hypocotyl surface. Polyphenoloxidase (PPO) activity at the inoculated sites was determined after various periods of incubation by replacing the inoculum drops with l-β-3,4-dihydroxyphenylalanine (l-Dopa) and measuring the formation of dopaquinone spectrophotometrically (λ480 n m). Activity was detected at incompatibly-reacting sites within 2 to 3 h following inoculation. No activity was detected at compatibly-reacting sites, although these sites developed activity if subsequently re-inoculated with an incompatible race. Activity was also demonstrated in hypocotyl tissues injured by abrasion or localized freezing. The results indicate that the development of PPO activity is one of the earliest recognizable events in incompatible interactions and suggest that it is a consequence of cell damage that is induced by incompatible but not by compatible races.