Rice stripe, caused by rice stripe virus (RSV), is one of the most damaging diseases in temperate regions of East Asia. RSV is transmitted by the small brown planthopper (SPBH, Laodelphax striatellus Fallen). Utilization of genetic resistance is the most effective and economical way of controlling RSV.

In the preliminary experiment, an indica rice cultivar, 'IR24' was found to be moderately resistant to RSV. Therefore,71 recombinant inbred lines (RILs) and 66 chromosome segment substitution lines (CSSLs) derived from the cross of Asominori (japonica) x IR24 (indica), were used for QTL mapping for RSV resistance. A proportion of 35% and 39% viruliferous SBPH were used in the individual inoculation test and the field test, respectively. RSV resistance for each plant was evaluated according to the methods by Washio et al. (1968). QTL mapping in RILs was performed with 375 RFLP markers using QTL Cartographer software (version 1.13). CSSLs having IR24 segments in the genetic background of Asominori were used to estimate the gene locations by the graphical genotype comparison method of Eshed and Zamir (1995).

The putative QTLs detected in RILs are presented in Table 1. In the artificial inoculation test, a total of four loci (qSTV-3a, qSTV-8, qSTV-11a, qSTV-11b) were detected, while in the field test, four other QTLs (qSTV-3b, qSTV-5, qSTV-7, qSTV-11c) were mapped. At six of them (qSTV-8, qSTV-11a, qSTV-11b, qSTV-3b, qSTV-7, qSTV-11c), IR24 alleles were found to have resistance effects to RSV.

Significant differences in the ratio of disease rating index (P=0.01) were found between Asominori and six lines, CSSL5, 6, 18, 25, 36, and 63. Since all of them showed higher resistance than Asominori, their substituted chromosomal segments of IR24 may have association with RSV resistance.

Comparative analysis of RIL and CSSL shows that two chromosomal regions are in common. On chromosome 11, IR24 chromosomal segment in CSSL63 contained both qSTV-11b and qSTV-11c (Fig. 1), and these QTLs are located close to XNpb257 where Stv-bⁱ (was mapped by Hayano et al. (1998). On chromosome 3, the marker region C361-XNpb51 harboring qSTV-3b is covered by IR24 segment in CSSL18 (Fig. 1). Since QTLs for whitebacked planthopper (Yamasaki et al. 1999) and green leafhopper resistance (Wang et al. 2004) and Grh4 gene (Yasui et al. 1999) are all mapped near the marker C361, sucking insects resistance might be clustered in this region.

Although further studies are necessary to elucidate the resistance mechanism, the QTLs on chromosome 3 and 11 will be utilized for gene-pyramiding breeding program by marker-assisted selection.

Acknowledgment

Thanks to Dr. H. Yasui and A. Yoshimura (Kyushu University, Japan) for their kindly providing materials.

References

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Hayano-Saito Y., T. Tsuji, K. Fuji, K. Satio, M. Iwasaki and A. Saito, 1998. Localization of the rice stripe disease resistance gene, Stv-bⁱ, by graphical genotyping and linkage analysis with molecular markers. Theor. Appl. Genet. 96: 1044-1049.

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