A Mechanistic View of the Fungal/Plant Interaction Based on Host-Specific Toxin Studies

Abstract

Studies of host-specific toxins in the United States have been initiated in several laboratories, but the primary proponent of their utility to probe molecular mechanisms of fungal pathogenesis was Robert P. Scheffer. Under his influence, advances have been made in the discovery of host-specific toxins, their chemistry, their sites of action in and physiological effects on host plants, their roles in fungal pathogenesis, and the genetics of toxin production by fungi. One of his most significant contributions was to identify single genetic elements, each of which controls biosynthesis of a particular host-specific toxin. This led several investigators, after the advent of molecular genetics, to clone these elements and discover that they are large complex regions of DNA containing genes essential for toxin biosynthesis and transport. Moreover, each of these regions is unique to the genome of a particular toxin producing strain, prompting speculation that toxin biosynthesis in fungi arises as a result of horizontal gene transfer. Although it is well established that host-specific toxins contribute to the determination of fungal host range, other factors are required for general pathogenesis to plants. We have recently cloned a gene encoding a cyclic peptide synthetase, which is found in a wide array of fungal plant pathogens, and have proven by gene disruption that the hypothetical cyclic peptide is required for pathogenesis by at least two of them. Thus, it appears that some fungi exploit their capacity for production of multiple secondary metabolites to mediate their interactions with host plants.