Secondary metabolites related to host selection by plant pathogenic fungi

Abstract

Role of host-selective toxins and phytoalexins in host-parasite interaction was investigated from chemical as well as phytopathological point of view. As the results of phytopathological studies on host selection by A1ternaria mali and A. kikuchiana, the causal agents of A1ternaria leaf spot disease of apple and black spot disease of Japanese pear respec-tively, AM-toxin I, II and III, and AK-toxin I and II were isolated as the primary determinants of these diseases, chemical structure of which were elucidated to be (la), (lb), (lc), (2a) and (2b) respectively, and the proposed structures were confirmed by total syntheses. The isolated crystalline AM- and AK-toxins showed not only extremely potent host-selective toxicity, but dilute solutions of these toxins also induced the same necrotic symptoms only on their host plant tissues as those observed on inoculation with the spores of the causal fungi. Structural features of the toxins that express the host selective toxicity were also analyzed using synthetic analogs of AM- and AK-toxins. On the other hand, in the course of the studies on the host-selection mechanism by obligate parasites, specific accumulation of the phytoalexins, avenalumin I, II and III, were confirmed in oat leaves infected with incompatible strains of crown rust fungus, Puccinia coronata f. sp. avenae and their structures were characterized as (6a), (6b) and (6c). These structures were also confirmed-by total synthesis. © 1990 IUPAC