Rice Genetics IV - IRRI books - International Rice Research Institute

(Inukai et al 1994, Yu et al 1996, Zheng et al 1996). The Pi-ta 2 -containing varietyKaty, from the southern United States, has been reported to contain a tightly linkedcluster of at least seven R genes that map in the same region as Pi-ta and Pi-ta 2 (Chaoet al 1999, Moldenhauer et al 1992). Genome sequencing now under way in rice willfacilitate the cloning and characterization of all R genes in this complex chromosomalregion.Future prospectsCloning Pi-ta will facilitate its incorporation into agronomically useful crop varieties,either through breeding using marker-assisted selection or through transgenicstrategies. Transgenic strategies are especially valuable for R genes linked to centromeres,since reduced recombination near centromeres will exacerbate linkage dragassociated with R gene introgression by classical breeding. Understanding the fieldbiology of the AVR-Pita resistance trigger within lineages of the rice blast fungus isalready identifying germplasm that would benefit from pyramiding Pi-ta with nativeR genes. The conservation of the AVR-Pita structure among field isolates of the pathogenfrom China and Colombia contrasts with the genetic instability we see for thetelomeric AVR-Pita locus in laboratory studies (Orbach et al 2000). The full impact oftelomeric location of AVR genes for the dynamics of rice blast disease in the fieldremains to be determined.This research provides the first direct evidence concerning molecular interactionsbetween an AVR gene and its corresponding R gene in the rice blast system. Thecurrent hypothesis is that a mature AVR-Pita protease triggers Pi-ta-mediated resistanceby binding directly to the cytoplasmic Pi-ta receptor protein. Future directionsinclude biochemical demonstration of protease activity for AVR-Pita, defining itsnatural substrate, and understanding its role in the infection process. If protease activityis confirmed for AVR-Pita, it is important to determine what relationship this activityhas to the protein’s avirulence function in triggering Pi-ta-mediated resistance.The AVR-Pita system presents an opportunity for understanding the presumed dualfunctions an AVR gene plays in the lifestyle of the pathogen, in contributing to thepathogenicity process and in triggering R-gene-mediated defense responses in thepresence of its corresponding R gene. It is also of interest to know when and how theAVR-Pita protein enters the plant cell during the infection process, and where andhow this protein gets processed. Important challenges lie in defining the completesignal transduction pathway downstream of the Pi-ta protein. Understanding the molecularbasis for AVR gene triggering of resistance will lead to the development ofnovel strategies for broad-spectrum, durable disease control using the native plantdefense systems.Enhancing deployment of genes for blast resistance: . . . 319