Effect of vulculic acid produced by Nimbya alternantherae on the photosynthetic apparatus of Alternanthera philoxeroides.

The effect of the toxin vulculic acid produced by Nimbya alternantherae, on the photosynthetic apparatus of Alternanthera philoxeroides, was investigated via the photochemical activity and SDS-PAGE of protein on thylakoid membranes, fast chlorophyll a fluorescence transient measurements and the JIP-test. The electron transport rate of photosystem II (PSII), non-cyclic photophosphorylation activity, as well as the activity of chloroplast ATPase and Rubisco reduced significantly after vulculic acid treatment. Vulculic acid affected the O-J-I-P fluorescence induction kinetics, showing an increase of the parameters FV/FO, VK and VJ and a decrease of FO, FM, PIABS, φPo, ψEo, φEo, φRo, δRo and PItotal. In addition, it significantly decreased the amounts of major photosystem I (PSI) and PSII proteins. It is concluded that vulculic acid is a photosynthetic inhibitor with multiple action sites. The main targets are the light harvesting complex (LHC) and the oxygen evolving complex (OEC) on the PSII donor side. Vulculic acid blocks electron transport beyond QA and on the PSI acceptor side by digesting major PSI and PSII proteins.