Effect of rust infection on rates of photochemical polyphenol oxidation and latent polyphenol oxidase activity of Vicia faba chloroplast membranes

Abstract

Changes in (i) the rate of light-dependent polyphenol oxidation (via the non-cyclic electron transport pathway) and (ii) the activity of latent membrane-bound polyphenol oxidase (PPO) (measured in the dark) were determined following infection with the rust fungus Uromyces fabae . The rate of photochemical polyphenol oxidation was consistently suppressed in chloroplast membranes isolated from infected leaves, relative to their healthy counterparts. This decrease seemed to be due to the block in the non-cyclic electron transport pathway that we described earlier for the photoreduction of NADP. This conclusion was supported by the finding that rust infection effected a reduction in the rate of the photo-oxidation of both water (NADP as acceptor) and dihydroxyphenylalanine (DOPA) (O 2 as acceptor). In contrast to light-dependent oxidation, enzyme-mediated polyphenol oxidation [assayed in the dark in either the absence or presence of sodium dodecyl sulfate (SDS)] was elevated in chloroplasts from infected leaves. In this respect, latent membrane-bound chloroplast PPO from Vicia faba resembled its soluble active counterpart from other plants.