Effect of a nonhost-selective toxin from Alternaria alternata on chloroplast-electron transfer activity in Eupatorium adenophorum

AAC-toxin, a putative nonhost-selective phytotoxin, was obtained from Alternaria alternata causing a brown leaf spot disease of Crofton weed (Eupatorium adenophorum). The effect of AAC-toxin on the electron transfer reaction of chloroplasts showed that the activity of photosystem II, but not photosystem I, was completely inhibited by the toxin. AAC-toxin affected the following chlorophyll fluorescence parameters: coefficient of photochemical quenching (q(P)), the half-time value of fluorescence rise, and the O-J-I-P fluorescence induction kinetics curve, but not the ratio values of F-v/F-m (the quantum yield of photosystem II) and the half-time value of fluorescence quenching. It was concluded that the toxin inhibited electron transfer from Q(A) to Q(B) (primary and secondary quinine acceptors of photosystem II) in photosystem II by competing with Q(B) for the binding site in D1 protein on the thylakoid membrane.