Rice, as a widely and intensively cultivated crop, should be a target for parasite host shifts and a source for shifts to co-occurring weeds. Magnaporthe oryzae, of the M. grisea species complex, is the most important fungal pathogen of rice, with a high degree of host specificity. On the basis of 10 loci from six of its seven linkage groups, 37 multilocus haplotypes among 497 isolates of M. oryzae from rice and other grasses were identified. Phylogenetic relationships among isolates from rice (Oryza sativa), millet (Setaria spp.), cutgrass (Leersia hexandra), and torpedo grass (Panicum repens) were predominantly tree like, consistent with a lack of recombination, but from other hosts were reticulate, consistent with recombination. The single origin of rice-infecting M. oryzae followed a host shift from a Setaria millet and was closely followed by additional shifts to weeds of rice, cutgrass, and torpedo grass. Two independent estimators of divergence time indicate that these host shifts predate the Green Revolution and could be associated with rice domestication. The rice-infecting lineage is characterized by high copy number of the transposable element MGR586 (Pot3) and, except in two haplotypes, by a loss of AVR-Co39. Both mating types have been retained in ancestral, well-distributed rice-infecting haplotypes 10 (mainly temperate) and 14 (mainly tropical), but only one mating type was recovered from several derived, geographically restricted haplotypes. There is evidence of a common origin of both ACE1 virulence genotypes in haplotype 14. Host-haplotype association is evidenced by low pathogenicity on hosts associated with other haplotypes. W IDELY distributed, high-density populations, such commonly follows (Tibayrenc et al. 1991; Maynard as those of cultivated crops or humans, are large Smith et al. 1993; Brasier and Buck 2001). While we targets for host shifts by parasites. After crossing host would expect that virulence determinants in a pandemic species boundaries, successful shifts culminate with speclone would remain associated and modified only cialization and genetic divergence on the new host through mutation, any genetic exchange with recombi- (Antonovics et al. 2002). If the parasite crosses a species nation would shuffle these determinants into novel combarrier only once, virulence determinants would derive binations across the parasite population. Within large from this common origin. In contrast, if the target host pathogen populations, recombination would increase species is invaded in multiple, independent host shifts, the rate at which multiple virulence factors could comvirulence could have multiple, fundamentally different bine to generate "super parasite" genotypes (Crow and determinants among host populations (MacLeod et al. Kimura 1970). 2001.The rice blast pathosystem is a model system not only After successful establishment on a new host, espefor plant-pathogen interactions (Valent 1990; Talbot cially one that is abundant and relatively genetically 2003), but also for host ...

Isolates of the rice blast fungus Magnaporthe grisea that carry the gene encoding Avirulence Conferring Enzyme1 (ACE1) are specifically recognized by rice (Oryza sativa) cultivars carrying the resistance gene Pi33. This recognition enables resistant plants to activate a defense response. ACE1 was isolated by map-based cloning and encodes a putative hybrid between a polyketide synthase and a nonribosomal peptide synthetase, enzymes involved in microbial secondary metabolism. ACE1 is expressed exclusively during fungal penetration of host leaves, the time point at which plant defense reactions are triggered. Ace1 appears to be localized in the cytoplasm of the appressorium. Mutation of the putative catalytic site of the b-ketoacyl synthase domain of Ace1 abolishes recognition of the fungus by resistant rice. This suggests that Ace1 biosynthetic activity is required for avirulence. Our results are consistent with the hypothesis that the fungal signal recognized by resistant rice plants is the secondary metabolite whose synthesis depends on Ace1.

The completion of the genome sequences of both rice and Magnaporthe oryzae has strengthened the position of rice blast disease as a model to study plant-pathogen interactions in monocotyledons. Genetic studies of blast resistance in rice were established in Japan as early as 1917. Despite such long-term study, examples of cultivars with durable resistance are rare, partly due to our limited knowledge of resistance mechanisms. A rising number of blast resistance genes and quantitative trait loci (QTL) have been genetically described, and some have been characterized during the last 20 years. Using the rice genome sequence, can we now go a step further toward a better understanding of the genetics of blast resistance by combining all these results? Is such knowledge appropriate and sufficient to improve breeding for durable resistance? A review of bibliographic references identified 85 blast resistance genes and approximately 350 QTL, which we mapped on the rice genome. These data provide a useful update on blast resistance genes as well as new insights to help formulate hypotheses about the molecular function of blast QTL, with special emphasis on QTL for partial resistance. All these data are available from the OrygenesDB database.