Mapping QTL controlling durable resistance to rice blast in the cultivar Oryzica Llanos 5

Abstract

The rice cultivar Oryzica Llanos 5 (OL5) possesses a high level of resistance to the fungus Magnaporthe grisea. The number and chromosomal location of quantitative trait loci (QTL) conferring resistance against eight isolates of the blast fungus were tested in two different populations of recombinant inbred lines from the cross Fanny x OL5. Twenty one QTL were detected and associated with the resistance traits, disease leaf area and lesion type, on 9 rice chromosomes. Eight of these 21 resistance loci had significant resistance effects in both experiments, while the others had effects that were only statistically significant in one experiment. Most, but not all, of the QTL occurred in the same genomic regions as either genes with major race-specific effects or other resistance QTL that had been described in previous experiments. Most of the QTL appeared to be race-specific in their effects but it is possible some of the QTL with smaller effects were nonspecific. One of the blast isolates used was FL440, which causes limited disease on OL5 and was probably virulent on most or all of the major genes from OL5. Three QTL affected resistance to FL440 in both experiments, one of which mapped to a region on chromosome 9 where no blast resistance genes have yet been mapped. An advanced backcross strategy with marker-assisted selection for OL5 alleles in QTL regions was used to generate five BC2F3 populations carrying five different target regions associated with partial resistance to rice blast disease. Three of five of these populations were analyzed for segregation for resistance to the M. grisea isolate FL440. One QTL designated qrbr-11.3 near the bottom of rice chromosome 11 was found to be significantly associated with partial blast resistance in 120 lines of a BC2F3 population (P< 0.01). This QTL accounted for 12.4% and 8.0% of the phenotypic variation in diseased leaf area and lesion type observed under greenhouse inoculation. Examination of the genomic sequence at the qrbr-11.3 locus showed that twenty-nine candidate resistance genes are present at that locus (~1.8 Mb), twenty-seven of which are predicted NBS-LRR genes. Ultimately, the information from this study can be integrated into the development of improved lines with OL5-derived QTL for resistance.