Tenuazonic acid, toxin of rice blast fungus, induces disease resistance and reactive oxygen production in plants

Effects of tenuazonic acid (TA) on rice leaf segments and on their interaction with compatible races of the blast fungus (Magnaporthe grisea, former name is Pyricularia oryzae) were studied. TA induced small brown necrotic spots on leaves Application of TA (1 or 5 mM) to leaves in mixtures with M. grisea spores induced a local disease resistance, which reduced the frequency of compatible lesions. TA was not fungitoxic but, in contact with the leaf, increased the capability of leaf diffusates to inhibit germination of M. grisea spores. In the infected leaves, the diffusate fungitoxicity was higher than in the healthy ones. Antioxidant enzymes, superoxide dismutase and catalase, and scavengers of hydroxyl radical, mannitol and formate, strongly inhibited the TA-induced diffusate fungitoxicity. It is suggested that the disease resistance induced by TA is mediated, at least partially, by generation of reactive oxygen species by rice leaves, which inhibit the development of the fungus directly or indirectly.