MODE OF ACTION OF FUNGICIDES IN REAL PLANT PATHOGENS
Cornell University, Department of Plant Pathology, New York State Agricultural Experiment Station, Geneva, NY 14458, USA
The best understood fungal organisms Saccharomyces cerevisiae, Aspergillus nidulans and Neurospora crassa are saprophytes lacking traits of pathogenicity. The recent cloning and sequencing of the entire S. cerevisiae genome in combination with tools provided by genomics and biomformatics offers an opportunity to evaluate the question, whether mode of fungicide actions identified for S. cerevisiae can be easily transferred to plant pathogens. The new class of strobilurin fungicides will serve as an example. Strobilurins have been introduced as broad-spectrum agricultural fungicides, and their mode of action was identified as the inhibition of respiration by binding to cytochrome b. The initial mode of action studies were conducted with liver mitochondria, and respective results were confirmed with yeast mitochondria and then with mitochondria isolated from plant pathogens such as Pyricularia oryzae and Septoria tritici. However, our own work with Venturia inaequalis and results provided for other pathogens clearly indicated that the primary strobilurin mode of action is not sufficient to explain the inhibition of fungal growth. The sometimes striking discrepancies between inhibitory in Vitro and in vivo effects of strobilurins was explained by the interference of alternative respiration with strobilurin action. Active oxygen radicals formed in the presence of a cytochrome b blocked by a strobilurin induce the expression of alternative oxidase. The protective enzyme is imported into mitochondria and circumvents the strobilurin inhibitor site. This circumvention mechanism is not active in S. cerevisiae, because this highly specialized fermentative yeast apparently lacks an alternative oxidase gene. The example illustrates that the primary mode of action of a fungicide can be determined with S. cerevisiae. It also suggests that nonpathogenic organisms such as S. cerevisiae can be of limited value in studies aimed at the understanding of modes of fungal growth inhibition, because metabolic events in addition to the inhibition of a primary target site are determinants of a more comprehensive scenario of inhibitor activities.