Plant Genome IV Conference Plant Genome IV Conference
Although rice germplasm is rich in sources of resistance to Magnaporthe grisea, the causal agent of blast disease, breeding for stable blast resistance has been difficult. Although the "breakdown" of resistance has been common, durably resistant varieties are known (e.g., Moroberekan and IAC165). We have used a molecular marker-based approach to analyze both the structure of pathogen populations and the genetics of blast resistance. Based on the former, we developed a well-characterized collection of isolates that can be used for analyzing resistance, and a healthy respect for the challenge of resistance breeding. We used our isolate collection to characterize the resistance spectra of the major genes carried in near-isogenic lines (NILs) for blast resistance, and to determine which combinations of genes could be most useful for rice improvement. Our resistance gene mapping efforts have focused on recombinant inbred (RI) populations derived from the crosses Moroberekan/CO39 and IAC165/CO39. For both populations, both major and minor (QTL) genes were identified, supporting the hypothesis that durable resistance is associated with complexity. Chromosomal segments consistently associated with significant but incomplete reductions in disease in greenhouse and field experiments were identified. We selected two RI lines believed to carry the putative Moroberekan QTL near marker RG64 on chr. 6, and having relatively few other Moroberekan alleles. These "pre-isogenic lines" for the putative QTL were used, along with four NILs carrying individual major genes, in quantitative inoculation assays using diverse isolates compatible with one or more of the major genes. The lines carrying the putative QTL showed consistently fewer compatible lesions than did CO39. In contrast, the lines carrying major genes showed erratic behavior, with some compatible isolates showing consistently more lesions on the major-gene lines than on the compatible recurrent parent. We are now trying to fine-map the putative QTL, and to develop methods for "diagnosis" of resistance genes in breeding lines, varieties and germplasm.