Inhibition of photosynthetic electron transport by metabolites produced by Phialophora gregata

In soybeans. adzuki beans, and mung beans the disease brown stem rot, which is cawed by Pkialopnqril grtg~ is characterized by vascu.lar browning of the 'tem and, in some cases, by chlorosis and necrosis of the leave3. We investigated the effect of compounds produced by P. grtgala on light--d.riven dectron transpon in thyJakoid membranes. The compounds investigated were contained in three different extracts isolated from the fungal culture: crude extract, gregatin A and the mother liquor from the crystallization of gregatin A. Electron tramport rates were measured potentiometrically wing a Clark"type oxygen electrode and spectropholometricaUy by monitoring light.induced absorbance changes due to cytochrome.f. The data show that at least one metabolite produced by P. gregaliJ can completely inhibit photosynthetic electron trawport. The rn03t potent inhibitor was the mother liquor which inhibited the reaction from H:t0 to methylviologen by 50% at a concentration of 20J,Lgml1. Comparison of the efficacy of the different extracts indicateS that gregatin A is a IC3S effective inhibitor. The effect of the extracts an electron tramport reactions that include limited segments of the photosynthetic apparatus indi· cato that there are at least two sito ofinhibition, the Phot03ynem II complex and the cytochrome hifcomplex. The results raise the pmsibility that the leafsymptoms observed in brown.rot may be due to inhibition ofphotosynthetic electron tramport ~y a metabolite produced by P. grtgaliJ.