Rice Genetics IV - IRRI books - International Rice Research Institute

The relationship between Pi-ta and Pi-ta 2The Pi-ta 2 gene, reported as being allelic or tightly linked to Pi-ta (Kiyosawa 1971,Rybka et al 1997), has a broader disease control spectrum than Pi-ta. We sequencedthe Pi-ta allele from Pi-ta 2 -containing varieties Tetep, Reiho, and Katy, and foundthat these varieties all contain the resistant Pi-ta allele (Table 1). This is interesting inview of reports that Pi-ta is required for the function of Pi-ta 2 (Kiyosawa 1967, 1971,Rybka et al 1997, Silué et al 1993). A viable working hypothesis is that Pi-ta 2 consistsof Pi-ta plus another R gene, presumably located in the chromosome 12 centromericregion.Infection assays summarized in Table 2 support our sequencing results that Pi-ta 2varieties contain Pi-ta (Table 1). Strain O-137 with AVR-Pita is avirulent on both Pitaand Pi-ta 2 rice varieties, but the spontaneous O-137 avr-pita – mutant CP3337 hasgained the ability to infect both types of rice. This result suggests that AVR-Pita instrain O-137 is responsible for triggering resistance in Pi-ta 2 and in Pi-ta rice, andthat O-137 lacks other AVR genes effective toward the tested Pi-ta 2 varieties. AnotherChinese field isolate, O-135, lacks homology to the AVR-Pita gene and fails to triggerPi-ta-mediated resistance in Yashiro-mochi and K1. However, O-135 is avirulent onPi-ta 2 varieties, suggesting that it contains an AVR gene that does not occur in strainO-137. A spontaneous mutant of O-135, named CP753, was selected for virulence onPi-ta 2 -containing variety Reiho. This mutant had gained the ability to infect all Pi-ta 2varieties tested, while retaining avirulence toward C101A51 with a different R gene.Taken together, these results suggest that the broader spectrum Pi-ta 2 gene is a combinationof at least two R genes: Pi-ta plus a second, presumably linked, R gene.These results underscore the difficulty in precisely defining the R gene compositionof commonly used rice varieties using uncharacterized fungal pathogen strains.Hybridization studies indicate that Pi-ta is single-copy gene in rice and does notidentify homologues as candidate genes for Pi-ta 2 . Although sample genomic sequencingof 750–800 kb of the assembled Pi-ta-chromosomal region has not identifieda Pi-ta 2 candidate, genetic distance will give a misleading indication of physicaldistance in the region of the centromere. Other R genes, including Pi-4 a (t), Pi-4 b (t),Pi-6(t), and Pi-12(t), have been mapped to the centromeric region of chromosome 12Table 2. Avirulent fungus/mutant pairs identify Pi-ta and a second R gene in rice varietieswith Pi-ta 2 .Variety O-137 CP3337 a Pi-ta O-135 CP753 b 2nd R geneAVR-Pita avr-pita – present AVR-Reih avr-reih – presentYashiro-mochi c AVR vir + vir vir –Tetep c AVR vir + AVR vir +C101A51 vir vir – AVR AVR –YT16 vir vir – vir vir –aCP3337 is a spontaneous mutant of O-137 that has lost the AVR-Pita function. b CP753 is a spontaneousmutant of O-135 recovered as a rare lesion on Pi-ta 2 -containing variety Reiho. c Additional Pi-tacontainingvariety K1 has the same specificity as Yashiro-mochi and additional Pi-ta 2 -containing varietiesTadukan, Reiho, and Katy show the same specificity as Tetep.318 Valent et al