Rice Genetics IV - IRRI books - International Rice Research Institute

mochi and Tsuyuake (Wu et al 1996). This AVR gene/R gene pair was shown to correspondto the previously identified Pi-ta gene, rather than to an undetected R gene inYashiro-mochi, by the following results: (1) the R gene mapped to the centromericregion of chromosome 12 in the expected location for Pi-ta; (2) the second Pi-tadifferential rice variety, K1 (Kiyosawa 1984), showed an infection spectrum identicalto that of Yashiro-mochi in inoculations with fungal progeny within the O-137 pedigree;and (3) Japanese pathogen strains (avirulent strains Ken54-20, Ina72, andKen54-04; virulent strains Ina168, Ken53-33, and P-2b) used in the original identificationof Pi-ta (Kiyosawa 1971) gave consistent infection results in our rice mappingpopulations. We concluded that we are analyzing the AVR-Pita and Pi-ta genes definedby Kiyosawa and colleagues (Kiyosawa 1967, 1971), and we have adoptedthese gene names for consistency.Pi-ta encodes a putative cytoplasmic receptorRandom amplified polymorphic DNA (RAPD) markers and bulked segregant analysiswere used to identify molecular markers linked to Pi-ta (Wu et al 1996). The Pi-tagene resides on chromosome 12 between flanking RAPD markers designated SP4B9and SP9F3, both of which show one recombination event relative to Pi-ta in a mappingpopulation of 990 individuals. The cosegregating restriction fragment lengthpolymorphism (RFLP) marker p7C3 was used to initiate chromosome walking (Wuet al 1996, Bryan et al 2000). We assembled an overlapping set of rice bacterial artificialchromosome (BAC) clones covering approximately 850 kb. The left bordermarker SP4B9 was identified in the assembled BAC contig. However, SP9F3 has notyet been identified, possibly due to decreased recombination in the vicinity of a centromere.We adopted a sample sequencing strategy in order to identify R gene candidatesin the BAC contig. BAC clones in this region are relatively high in repetitiveDNA sequences, including the centromere-specific sequence RCE1 (Dong et al 1998,Singh et al 1996). This result is consistent with a previous report that the region nearPi-ta is high in repetitive DNA sequences (Nakamura et al 1997).The Pi-ta candidate gene was identified by sequences from BAC142E8 showinghomology to the nucleotide-binding site (NBS) region of R genes cloned from othersystems, and this candidate R gene was subsequently confirmed as Pi-ta by stabletransformation of rice (Bryan et al 2000). Pi-ta encodes a predicted cytoplasmic protein(Fig. 1A) with an NBS region characteristic of the largest class of plant R genes(Bent 1996, Hammond-Kosack and Jones 1997). Pi-ta lacks either a leucine zippermotif or the Toll/Interleukin-1 receptor homology reported as key features of dicot Rgenes. The C-terminal portion of the Pi-ta protein contains a leucine-rich domain(LRD), which also lacks features of the leucine-rich repeats reported for other R geneproducts. Pi-ta is a single-copy gene that is constitutively expressed in both resistantand sensitive varieties.We compared Pi-ta sequences from several resistant and sensitive rice varieties(Bryan et al 2000). Differential rice varieties Yashiro-mochi (Yamada et al 1976) andEnhancing deployment of genes for blast resistance: . . . 311