Alternaria alternata Crofton-weed toxin: a natural inhibitor of photosystem II in Chlamydomonas reinhardtii thylakoids.

The action site of Alternaria alternata Crofton-weed toxin (AAC-toxin), isolated first from Alternaria alternata (Fr.) Keissler, was investigated in Chlamydomonas reinhardtii thylakoids. The results revealed that AAC-toxin inhibited photophosphorylation in a concentration-dependent pattern. Similarly, toxin inhibited uncoupled, basal electron flow and photosystem II (PSII) electron transport as well. However, toxin did not affect photosystem I (PSI) activity or the partial reaction of electron transport from H2O to silicomolybdic acid (SiMo). Therefore, the action site of toxin was located at QB level. In addition, the toxin may behave as an energy-transfer inhibitor at high concentrations by inhibiting phosphorylating electron transport and Mg2+ATPase activity. Chlorophyll a fluorescence induction and JIP test corroborated the inhibition at QB level. Through observations of the different sensitivity of toxin on D1 mutants of C. reinhardtii, evidence further confirmed that AAC-toxin inhibited electron transport by displacing the QB on the D1 protein, and the mode of action was similar to phenol-type PSII inhibitors.