The phytotoxic lichen metabolite, usnic acid, is a potent inhibitor of plant p-hydroxyphenylpyruvate dioxygenase

The lichen secondary metabolite usnic acid exists as a (-) and a (+) enanti omer, indicating a alpha or beta projection of the methyl group at position 9b, respectively, (-)-Usnic caused a dose-dependent bleaching of the cotyl edonary tissues associated with a decrease of both chlorophylls and caroten oids in treated plants whereas no bleaching was observed with the (+) enant iomer, (-)-Usnic acid inhibited protophorphyrinogen oxidase activity (I-50 = 3 mu M), but did not lead to protoporphyrin IX accumulation. Bleaching ap pears to be caused by irreversible inhibition of the enzyme 4-hydroxyphenyl pyruvate dioxygenase by (-)-usnic acid (apparent IC50 = 50 nM), (C) 2000 Fe deration of European Biochemical Societies. Published by Elsevier Science B .V. All rights reserved.