Strategies for successful infection of host plants are highly diverse in fungal pathogens, which range from biotrophs to necrotrophs. As more microbial genomes have been sequenced, more fungal genes have been identified as being involved in pathogenesis, as exemplified by those for biosynthesis of toxic secondary metabolites such as host-specific toxins. Filamentous fungi produce a diverse array of secondary metabolites—small molecules that are not necessary for normal growth or development. The role of host-specific toxins in plant–fungus interaction as well as the biochemistry and molecular basis of toxin biosynthesis are discussed. The availability of fungal genomic sequences has revealed a remarkably large number of biosynthetic gene clusters for secondary metabolites, e.g., polyketides and nonribosomal peptides including cyclic peptides, extremely large classes of natural products of fungal origin. The origin and evolutionary processes for these gene clusters are largely unknown. Analysis of the arrangement and sequences of genes in the clusters should shed light on how the clusters and abilities to produce toxic secondary metabolites evolved.
Strains of Ralstonia solanacearum from eggplant and tomato fields in Tochigi Prefecture separated into four distinct groups (I, II, IV and one unknown) based on differences in pathogenicity to four species of Solanum plants. In repetitive sequence-based polymerase chain reaction (rep-PCR) DNA fingerprinting to discriminate the strains belonging to each pathogenicity group, two characteristic bands were identified. One was universally amplified from all tested strains, and the other was amplified only from strains in groups I, II and V. Based on the DNA sequences of the bands, we designed two more PCR primer sets (RsUp-F and RsUp-R, and RsDwn-F and RsDwn-R) that discriminated the strains in group I, II and V from those in III and IV.