Blast in a Durably Resistant Rice Cultivar

Moroberekan, ajaponica rice cultivar with durable resistance to blast disease in Asia, was crossed to the highly susceptible indica cultivar, C039, and 281 F, recombinant inbred (RI) lines were produced by single seed descent. The population was evaluated for blast resistance in the greenhouse and the field, and was analyzed with 127 restriction fragment length polymorphism (RFLP) markers. Two dominant loci associated with qualitative resistance to five isolates of the fungus were tentatively named Pi-S(t) and P2”7(t). They were mapped on chromosomes 4 and 1 1 , respectively. To identify quantitative trait loci (QTLs) affecting partial resistance, RI lines were inoculated with isolate PO66 of Pyricularia oryzae in polycyclic tests. Ten chromosomal segments were found to be associated with effects on lesion number ( P < 0.0001 and LOD > 6.0). Three of the markers associated with QTLs for partial resistance had been reported to be linked to complete blast resistance in previous studies. QTLs identified in greenhouse tests were good predictors of blast resistance at two field sites. This study illustrates the usefulness of RI lines for mapping a complex trait such as blast resistance and suggests that durable resistance in the traditional variety, Moroberekan, involves a complex of genes associated with both partial and complete resistance. T HE use of resistant cultivars is the most economical and effective way of controlling rice blast, an often devastating disease that occurs in most ricegrowing areas worldwide (Ou 1985). However, the useful life span of many cultivars is only one or a few years in disease-conducive environments (e .g . , LEE and CHO 1990; KIYOSAWA 1982) due to the breakdown of resistance in the face of high pathogenic variability of Pyricularia oryzae Cavara (Ou 1979; BONMAN et al. 1986). Breeding for more durably resistant cultivars, therefore, has become a priority in rice improvement. Resistance is considered durable when it remains effective in a cultivar despite widespread cultivation in an environment favoring the disease. In different pathosystems, durable resistance is variously controlled by single genes, multiple genes with cumulative effects, polygenes, and the resistance may be either complete or incomplete (partial) (JOHNSON 1983; PARLEVLIET 1988). Several rice cultivars with durable blast resistance have been identified (LEE et al. 1989; BONMAN and MACKILL 1988). For example, some upland cultivars such as the Moroberekan RI population will be accessible in the Rice Genome Database The results of this analysis and the RFLP data set associated with the C039/ or through Gopher. (“RiceCenes”) through the National Agricultural Librruy in Washington, D.C., Davis, California 95616. ’ Current address: Department of Plant Pathology, University of California, University of California, Davis, California 95616. Current address: USDA-ARS, Department of Agronomy & Range Science, Delaware 19714. ’ Current address: Du Pont Agricultural Products, P.O. Box 30, Newark, New York 14853. Current address: Plant Breeding Department, Cornell University, Ithaca, To whom reprint requests should be addressed. Genetics 136 1421-1434 (April, 1994) traditional African cultivars Moroberekan and OS6 have been cultivated for many years in large areas in West Africa without high losses from blast (NO-ITEGHEM 1985; BONMAN and MACKILL 1988). These cultivars have been widely used as resistance donors in breeding programs. Blast resistance is generally classified into two types based on the way the gene(s) affect pathogen reproduction: qualitative (complete) and quantitative (partial). Qualitative resistance conditions incompatibility of the host and pathogen strain, preventing reproduction of the fungus, while partial resistance reduces the extent of pathogen reproduction within the context of a compatible interaction. Genetic studies of qualitative resistance to rice blast were started when GOTO established the differential system for races of P. oryzae in Japan in the early 1960s (OU 1985). Since then, the inheritance of resistance has been extensively studied (KIYOSAWA 1981; ATKINS andJOHNsToN 1965; MACKILL and BONMAN 1992; MACKILL et al. 1988) and several genes for complete resistance have been mapped relative to restriction fragment length polymorphism (RFLP) markers. Yu et al. (1991) identified FWLP markers linked to Pi-2(t) and Pi-4(t ) ; TOHME et al. (1991) identified markers linked to a resistance gene from IRAT13; and L. ZHU, (personal communication, Academia Sinica, Beijing, 1991) identified markers linked to Pi-zh(t) . Aside from genes for qualitative resistance to Pyricularia oryzae, varieties with durable resistance also have a high level of partial resistance. In 1968, TOIWAMA et al. reported high levels of field resistance in the variety 1422 G.-L. Wang et at. Chugoku 31. Inheritance of partial resistance was found to be polygenic in studies involving the cultivars IRAT13, IAC25, IAC47 and Dourado Precose (NOTTEGHEM 1985) and Moroberekan (LOUVEL 1985). Similar results were obtained by LIN (1986), whose study showed that inheritance of field resistance to rice blast was quantitative, with additive and partially dominant effects of minor genes. The broad-sense heritabilities ranged from 38 to 83% depending on how resistance was measured. WANG et al. (1989) found that inheritance of partial resistance in IR36 was most likely polygenic with very low narrowsense heritability. ROUMEN (1993) observed a consistent and high level of partial resistance in IR36 and IR64 in both greenhouse and field tests. Though considerable progress has been made, the genetic basis of resistance in durably resistant cultivars is still not well understood and attempts to transfer the character into different genetic backgrounds have not been widely successful due to the complexity of the trait and limitations of the research methodologies used. The recent development of RFLP techniques makes it possible to investigate the inheritance of complex traits and to locate and manipulate individual genetic factors associated with these traits (TANKSLEY 1993). Recent studies in tomato (PATERSON et al. 1988,1990,1991; TANKSLEY and HEWITT 1988; TANKSLEY et al. 1989), maize (BURR et al. 1988; GRANT et al. 1989; BEAVIS et al. 1991; STUBER et al. 1992), and soybean (KEIM et al. 1990) have demonstrated that RFLP mapping is a powerful approach for identifymg quantitative trait loci (QTLs) controlling agronomically important characters. Although several single-gene characters have been located via linkage to mapped RFLP markers in rice (WCKILL et al. 1993; RONALD et al. 1992; MCCOUCH and TANKSLEY 1991; MCCOUCH et al. 1991; Yu et al. 1991; YOSHIMURA et al. 1992), this is the first published report of a QTL analysis