Studies on pathogenicity genes of Alternaria alternata

Abstract

This article is an abstract of the paper presented by a winner of the Society Fellowship at the 2003 Annual Meeting of the Phytopathological Society of Japan in Tokyo There are seven known host–parasite combinations in which host-specific toxins produced by Alternaria alternata are responsible for the plant disease. Host-specific toxins are low-molecular-weight metabolites of diverse structure. The participation of host-specific toxins in the establishment of plant disease is one of the most clearly understood mechanisms of host-selective pathogenesis. Previously, species names were adopted for Alternaria pathogens that produce host-specific toxins. However, Syoyo Nishimura and his colleagues at Tottori University found that measurements of the conidial size of these pathogens fall statistically within the range described for A. alternata, a ubiquitous, generally saprophytic fungus. Because each pathogen has a distinct host range due to the production of a particular host-specific toxin, it is possible to distinguish one type of pathogen from another. Based on these features, Nishimura and his colleagues proposed that the Alternaria pathogens should be defined as distinct pathotypes of A. alternata. These pathogens provide an interesting example of intraspecific variation and evolution of pathogenicity in plant pathogenic fungi. Our efforts have focused on cloning genes required for the biosynthesis of host-specific toxins as a first step toward understanding the molecular basis for pathogenic specialization in A. alternata.