Rice blast caused by the fungus Magnaporthe grisea is one of the most destructive diseases of rice. The development of resistant cultivars is potentially the most effective and economical method of controlling blast disease. Although, many rice cultivars with complete resistance have been developed, most of resistance were broken within a few years after cultivar release as a result of the differentiation of new races of rice blast fungus. The use of multilines was suggested as a means to prevent the breakdown of the resistance to disease. Nearisogenic lines with different complete resistance genes to blast have been reported by many researchers in Japan.

Isogenic lines with diverse blast-resistant genes have been developed in Korea. Two multiline cultivars, Saechucheongbyeo (Suweon433-I and Suweon433-II) and Anseongbyeo (Suweon434-I and Suweon434-II), were formed by combining in equal proportions by seed weight three different blast-resistant isogenic lines in which the recurrent parent was either Chucheongbyeo or Suweon345, respectively. Donor parents of the Chucheongbyeo isogenic lines were Seolagbyeo, Daeseongbyeo and Bongkwangbyeo, while those of the Suweon345 isogenic lines were Daesongbyeo, 54BC-68, Suweon365 and a F₄ line of the cross Seolagbyeo/54BC-68. The component isogenic lines of these Korean multilines differed from one another in their reactions to the blast fungus (Choi et al., 2000a) but the operative resistant genes of these isogenic lines remain to be identified. This experiment was undertaken to clarify the origins of these genes based on pedigree analysis, use of SSR, STS and CAPS markers linked with known resistant genes (Table 1), and comparisons with monogenic lines developed in IRRI.

The recurrent parent Chucheongbyeo was known to have the Pia gene for blast resistance, but the effectiveness of this gene to leaf blast was low in the field in Korea. Two resistant genes Pia and Pik-m were identified in donor parent Bongkwangbyeo. Seolagbyeo was developed from a cross between BL1 (having Pib and Pish genes) and Fuji280 (having Pia and Pita-2). The third donor parent Daeseongbyeo was developed from the cross Bongkwangbyeo x Fuji280. All isogenic lines in the Chucheongbyeo genetic background are likely to have the Pia gene from Chucheongbyeo, because the chromosomal region containing this gene was monomorphic for the flanking markers RM441 and RM552 in the isogenic lines and the parent. Isogenic line M7 was shown to have genes Pita and Pi25(t), M28 to have Pi5(t) and Pi44(t), M64 to have Pi44(t), and M94 to have Pi10(t) with Pia. M85 and M88 developed from donor Daeseongbyeo would contain Pi7(t) and Pi25(t). The region on chromosome 12 in which Pita and Pi25(t) are located is also the site for the clustering of many other resistance genes such as Pita-2, Pi4 and Pi12(t). The Pi5(t) gene of isogenic line M28 may be the same gene as Pii, because Pi5(t) is in the same 170kb genomic segment as Pi3(t) (Jeon et al., 2003), which is allelic or closely linked with Pii gene. In summary, the resistance genes carried by the isogenic lines in the Chucheongbyeo background would be Pia, Pita, Pi5(t) and Pi25(t); there is also a need to screen more markers flanking Pi7(t) and Pi44(t).

The recurrent parent Suweon345 is an elite japonica line of high eating quality and resistance to leaf blast, but it is susceptible to neck blast in the field in Korea. The donor parents for blast resistance were Daeseongbyeo, Suweon365, F₄ line of Seolagbyeo/54BC-68, and 54BC-68. Suweon365 has genes Pi18 on chromosome 11 and Pi25(t) on chromosome 12 (Kwon et al., 2002) and has shown resistance of degree 1-4 at the blast nursery in eleven Korean regions since 1989. Based on pedigree analysis Suweon345 could contain genes Pib, Pii, Pita-2 and others. However, based on screening with CAPS markers, Suweon 345 does not contain genes Pita, Pi1, Pi2, Pi5(t), and Pi9(t). All isogenic lines of Suweon345 genetic background were identified to have Pib gene on chromosome 2 by screening CAPS marker Pib-1/2. Isogenic lines M130, M133 and M136 developed from the common donor 54BC-68 contain Pi25(t) on chromosome 12 (Table 2). M130 and M133 from a donor line F₄ of Seolagbyeo/54BC-68 contain Pitq-5 and Pib, two genes located closely together distally on the long arm of chromosome 2. M151 and M193 lines developed from Daeseongbyeo have Pi15(t) on chromosome 9 with Pib. Four isogenic lines M133, M136, M151 and M193 were identified to have Pita gene on chromosome 12 based on screening of CAPS markers.

As mentioned above, isogenic lines developed from Seolagbyeo as donor parent would have Pita and/or Pi25(t) genes on chromosome 12. Although isogenic lines developed from Daeseongbyeo were inferred to have five blast resistance genes on four chromosomes, there is a need to analyze more markers and to test the inferred allelic relationships with known genes. Six isogenic lines of Chucheongbyeo were each resistant against 4-5 out of ten isolates screened (data not shown). On the other hand, six isogenic lines of Suweon345 showed resistance for all but 1-3 isolates. Multilines Suweon433-I mixed with M85, M88 and M94, Suweon433-II mixed with M7, M28 and M64 in the Chucheongbyeo genetic background, and Suweon443-I mixed with M133, M151 and M193, and Suweon443-II mixed with M130, M136 and M235 in the Suweon345 genetic background were screened to leaf and neck blast in the

field (Table 4). In the case of leaf blast, four multilines had the effect of reducing the incidence of disease when compared with their recurrent parents. In the case of neck blast, the incidence of disease was reduced ten-fold in Suweon433-I and two-fold in Suweon443-I. Although multiline Suweon443-I had the effect to reduce the incidence to neck blast, it was over 10% on the average. Because neck blast causes an immediate loss of yield, it is important to develop rice varieties with resistance to this disease. It is interesting that isogenic lines of Chucheongbyeo containing the Pia gene were more effective in field resistance, especially to neck blast, than those isogenic lines of Suweon345 containing Pib gene. We infer that it is more important to combine the right kinds of genes than a particular number of genes. In summary, there is a need to test the allelic relationships for the inferred genes, and to identify those combinations of genes that are effective in enhancing for broad-spectrum and stable resistance for blast.

Acknowledgments

The authors are thankful to Dr. J.S. Jeon (Kyonghee University, Korea) and Dr. N.S. Jwa (Sejong University, Korea) for the information on CAPS markers related with Pi5(t) and Pib, respectively, and for their kind comments.

References

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