Rice Genetics IV - IRRI books - International Rice Research Institute

product, or they might encode enzymes involved in the production of small moleculeligands that serve as recognition factors. The efficacy of any particular R gene in thefield depends on the population biology of the corresponding AVR gene from thefungus. In particular, it depends on the role the AVR gene plays in the pathogenicityprocess itself, and on the potential for mutation of the AVR gene to avoid recognitionwithout affecting pathogen fitness in the field.Individual R genes can be distinguished only by the resistance they confer tocorresponding avirulent strains of the pathogen (Flor 1971). It has been difficult tocompare rice blast R genes identified in different countries due to regulatory prohibitionsagainst sharing pathogen strains between geographically separated rice productionregions. R gene identification is also complicated by the common occurrence ofmultiple AVR genes in single field isolates of the pathogen, any one of which is sufficientto trigger resistance in the presence of its corresponding R gene. Thus, the numberof distinct blast R genes is uncertain and strategies for pyramiding R genes forblast control are hampered by imprecise R gene characterization. Molecular cloningand characterization of R genes and their corresponding AVR genes will provide thenecessary tools to determine the number and nature of R genes available for blastdisease control.We describe the first molecular characterization of an R gene/AVR gene pair fromthe rice blast system. The resistance gene Pi-ta, previously referred to as Pi-62(t) (Wuet al 1996), encodes a putative cytoplasmic receptor (Bryan et al 2000), and theavirulence gene AVR-Pita, formerly known as AVR2-YAMO, encodes a putativemetalloprotease (Orbach et al 2000). We report progress in understanding the molecularevents by which the Pi-ta protein and the AVR-Pita protein act together totrigger resistance and block disease (Jia et al 2000). An understanding of R geneaction in pathogen recognition is a first step toward targeted evolution of broad-spectrumR genes that will provide durable resistance. Cloning and sequencing of Pi-tahave identified ideal molecular markers for marker-assisted selection, and have enabledtransgenic strategies for pyramiding Pi-ta with other R genes. Understandingthe population biology of the AVR-Pita gene will aid in deploying Pi-ta using lineageexclusion strategies.Molecular characterization of Pi-ta and AVR-PitaGenetic identification of an R gene/AVR gene pairGenetic analysis identified an AVR gene derived from the Chinese field isolate O-137,collected in 1985 at the China National Rice Research Institute in Hangzhou. Thisgene, originally named AVR2-YAMO, determined avirulence toward rice varietyYashiro-mochi, a Japanese differential variety for Pi-ta (Yamada et al 1976). A fungalprogeny strain, 4360-R-62, from the O-137 pedigree contained the AVR2-YAMO genein isolation from other AVR genes segregating in these crosses, and it was used toidentify the corresponding R gene, originally named Pi-62(t), using doubled-haploidand F 2 mapping populations from reciprocal crosses between rice varieties Yashiro-310 Valent et al