Molecular analysis of species-specific parasitism in Magnaporthe grisea

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Mechanisms of the species-specific parasitism in Magnaporthe grisea were investigated. An F1 population derived from a cross, a Setaria isolate x a Tritucum isolates, showed bifactorial segregation in the pathogenicity on wheat. These genes were designated as Pwt1 and Pwt2. Another F1 population derived from a cross, an Avena isolate and a Triticum isolate, showed monofactorial segregation on oat. This gene was designated as Pat1. Closely linked AFLP markers were found in bulked segregant analyses. Chromosome walking is under way toward cloning of those genes.A phytotoxin was extracted from a Digitaria isolate. The toxin was revealed to be Pyrichalasin H.Among various gramineous plants, crabgrass (the host of Digtaria isolates) showed the highest susceptibility to this toxin. Pyrichalasin H was produced in a large quantity only by Digitaria isolates, and Pyrichalasin H-defidient mutants lost their pathogenicity on crabgrass. Crabgrass leaves treated with Pyrichalasin H became accessible to nonpathogens. These results suggested that Pyrichalasis H is a determinant of the species-specific parasitism of Digitaria isolates.Retrotransposon MAGGY was the most active element among transposable elements that have been found in the genome of Magnaporthe grisea. MAGGY was present in Oryza isolates and Setaria isolates, but absent in other isolates. We assuned that this element may have been involved in the evolution of the race- cultivar specificity in those isolates.