Genetic Dissection and Mapping of Genes Conferring Field Resistance to Rice Blast in Japanese Upland Rice

Field resistance to rice blast in Japanese upland was analyzed genetically. Chromosomal regions that are involved in field resistance were detected by quantitative trait loci (QTL) analysis using F4 lines of a cross between resistant upland rice variety Owarihatamochi and susceptible irrigated rice variety Nipponbare. At two QTLs on chromosome 4 and one on chromosome 12, alleles from Owarihatamochi were resistant, while an allele from Nipponbare was resistant at one QTL on chromosome 9. Backcrossed progeny lines with each of the resistant alleles from upland rice were developed using the highly susceptible irrigated rice cultivar Aichiasahi as the recurrent parent. At all three QTLs, the lines with the Owarihatamochi allele showed a significantly higher level of field resistance than did those with the Aichiasahi allele. The differences in resistance between two genotypes were largest at the QTL on chromosome 4 and smallest at the QTL on chromosome12, in good accordance with the result of QTL analysis. The QTL of largest effect on chromosome 4 was mapped as a single recessive gene designated as pi21 by genetic linkage analysis. Using a mapping population consisting of 82 lines, we located pi21 between marker loci G271 and G317 at a distance of 5.0 cM and 8.5 cM, respectively. High-resolution genetic linkage analysis using 1104 individuals identified several DNA markers tightly linked with the pi21locus. These markers are being used to identify P1 artificial chromosome (PAC) clones containing this locus for positional cloning of pi21.