Rice Genetics IV - IRRI books - International Rice Research Institute

M. griseaAVR-PitaPlant cellProcessing?Pi-taAVR-Pita 176Kinase?Signal transductionNucleusResistanceresponsesFig. 3. Model for resistance mediated by Pi-ta. We hypothesize thatthe predicted mature AVR-Pita 176 protein binds to the Pi-ta receptorin the cytoplasm of the invaded plant cell and initiates the defenseresponse. The timing and mechanism for processing of AVR-Pita andits delivery into the plant cell cytoplasm have not yet been addressed.genes. Differential rice varieties, especially those with single, mapped R genes, havehelped in some cases to define AVR gene composition within pathotypes (Atkins et al1967, Kiyosawa 1984, Yamada et al 1976). However, pathotype characterization hasbeen limited by the occurrence of multiple AVR genes within single field isolates ofthe pathogen, by the inability of researchers to share pathogen strains among differentrice-growing regions of the world, and by the imprecise knowledge of R gene compositionin individual rice varieties.DNA fingerprinting analyses using the dispersed, middle-repetitive DNA sequenceMGR586 have shown that blast fungal populations in major rice-growing areas arepredominantly asexual and composed of dozens of lineages, with each lineage apparentlyderived through clonal propagation from a common ancestor (Levy et al 1991,1993, Xia et al 1993, Zeigler et al 1994). Clearly, pathotypes have evolved independentlywithin different lineages—some lineages are associated with a single pathotypeand others show a more complex pathotype structure. All lineages show uniformavirulence toward some R gene(s), raising the possibility that members of a givenlineage may be limited in ability to modify AVR recognition corresponding to these Rgenes. Lineage exclusion breeding strategies based on pyramiding of “lineage-specific”R genes, effective against all strains of a particular clonal population, may316 Valent et al