Mitochondrial electron transfer in the wheat pathogenic fungus Septoria tritici: on the role of alternative respiratory enzymes in fungicide resistance

Certain phytopathogenic fungi are able to express alternative NADH- and qui nol-oxidising enzymes that are insensitive to inhibitors of the mitochondri al respiratory Complexes I and III. To assess the extent to which such enzy mes confer tolerance to respiration-targeted fungicides, an understanding o f mitochondrial electron transfer in these species is required. An isolatio n procedure has been developed which results in intact, active and coupled mitochondria from the wheat pathogen Septoria tritici, as evidenced by morp hological and kinetic data. Exogenous NADH, succinate and malate/glutamate are readily oxidised, the latter activity being only partly (approx. 70%) s ensitive to rotenone. Of particular importance was the finding that azoxyst robin (a strobilurin fungicide) potently inhibits fungal respiration at the level of Complex III. In some S. tritici strains investigated, a small but significant part of the respiratory activity (approx. 10%) is insensitive to antimycin A and azoxystrobin. Such resistant activity is sensitive to oc tyl gallate, a specific inhibitor of the plant alternative oxidase, This en zyme, however, could not be detected immunologically. On the basis of the a bove findings, a conceptual mitochondrial electron transfer chain is presen ted. Data are discussed in terms of developmental and environmental regulat ion of the composition of this chain. (C) 2000 Elsevier Science B.V. All ri ghts reserved.