Characterization of rice blast resistance genes in the Pik cluster and fine mapping of the Pik-p locus.

Pik-p is carried by cv. K60, which is one of the Japanese differentials widely used in both Japan and China since the 1980s. Its utility and specificity was evaluated with a total of 612 isolates of Magnaporthe oryzae collected from various regions in China in combination with 16 main resistance genes being used in the breeding programs. Pik-p is an independently and dominantly acting gene in the Pik cluster, which conditions differential reactions against many isolates and contains higher resistance in Guangdong, Jiangsu, and Sichuan provinces, China, indicating that this gene could be still used in these regions. A high-resolution genetic map of Pik-p was constructed using genomic position-ready markers. A set of 47 recombinants out of 681 F(2) plants derived from the crosses cv. K60 (resistant) x cv. AS20-1 (susceptible) and x cv. Kasalath (susceptible) was identified in the genetic interval defined by the markers RM5926 and K37 which flank the Pik gene cluster. This set was then genotyped with seven markers known to reside within the interval. The closest markers to Pik-p were K28 (approximately 0.60 centimorgans [cM]) and K39 (approximately 0.07 cM). A further four markers in the K28-K39 interval were developed from an in silico analysis based on the cv. Nipponbare genome sequence, and these all co-segregated with Pik-p. This 0.67-cM region is equivalent to a physical separation in cv. Nipponbare of approximately 126 kb, plus an as-yet-unfilled genomic gap of unknown length. Four nucleotide-binding site leucine-rich repeat-type resistance genes are present in this interval, and these represent good candidates for Pik-p.