QTL analysis and mapping of pi21, a recessive gene for field resistance torice blast in Japanese upland rice

Field resistance is defined as the resistance that allows effective control of a parasite under natural field conditions and is durable when exposed t o new races of that parasite. To identify the genes for field resistance to rice blast, quantitative trait loci (QTLs) conferring field resistance to rice blast in Japanese upland rice were detected and mapped using RFLP and SSR markers. QTL analysis was carried out in F-4 progeny lines from the cro ss between Nipponbare (moderately susceptible, lowland) and Owarihatamochi (resistant, upland). Two QTLs were detected on chromosome 4 and one QTL was detected on each of chromosomes 9 and 12. The phenotypic variation explain ed by each QTL ranged from 7.9 to 45.7% and the four QTLs explained 66.3% o f the total phenotypic variation. Backcrossed progeny lines were developed to transfer the QTL with largest effect using the susceptible cultivar Aich iasahi as a recurrent parent. Among 82 F-3 lines derived from the backcross , resistance segregated in the expected ratio of resistant 1 : heterozygous 2 : susceptible 1. The average score for blast resistance measured in the field was 4.2 +/- 0.67, 7.5 +/- 0.51 and 8.2 +/- 0.66, for resistant, heter ozygous and susceptible groups, respectively. The resistance gene, designat ed pi21, was mapped on chromosome 4 as a single recessive gene between RFLP marker loci G271 and G317 at a distance of 5.0 cM and 8.5 cM, respectively . The relationship to previously reported major genes and QTLs conferring r esistance to blasts, and the significance of marker-assisted selection to i mprove field resistance, are discussed.