| Abstract
| - The action site of Alternaria alternata Crofton-weed toxin (AAC-toxin), isolated first from Alternariaalternata (Fr.) Keissler, was investigated in Chlamydomonas reinhardtii thylakoids. The results revealedthat AAC-toxin inhibited photophosphorylation in a concentration-dependent pattern. Similarly, toxininhibited 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 H2Oto 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 JIPtest corroborated the inhibition at QB level. Through observations of the different sensitivity of toxinon D1 mutants of C. reinhardtii, evidence further confirmed that AAC-toxin inhibited electron transportby displacing the QB on the D1 protein, and the mode of action was similar to phenol-type PSIIinhibitors. Keywords: AAC-toxin; Chlamydomonas reinhardtii; electron transport; mutants; PSII inhibitor
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