Rice stripe disease is one of the most serious viral disease in East Asia, such as China, Japanese and Korea. The rice stripe virus (RSV) is transmitted by small brown planthooper (SPBH), Laodelphax striatellus Fall. It was indicated that most japonica varieties are susceptible, while indica and upland varieties are highly resistant.

A mapping population of 98 BC₁F₉ lines (Backcross inbred lines: BIL), derived from a backcross of Nipponbare (japonica)/Kasalath (indica)//Nipponbare by the single-seed descent method, was used for characterizing QTL resistant to rice stripe virus. About 35 seedlings of each BIL and parent were used to evaluate the reactions to virus by inoculating small brown planthopper containing rice stripe virus. The rate of viruliferous SBPH was around 40% by dot-immunobinding assay. We inoculated rice stripe virus to the population by individual inoculation and field test.

By individual inoculation test, twenty-five to thirty seedlings of each BIL were planted within 9cm × 9cm space in a plastic tray filled with nutrient soil. When the rice plants reached 1.5-2.1 leaf stage, a plastic cylinder 9 cm in diameter covered with gauze was put on each RIL. Inoculation was practiced by releasing 2^nd to 3^rd instar nymphs of vector insects into the plastic cylinder at the rate of five insects per plant. During the inoculation, the insects were scattered twice one day. After inoculation, the insects were removed and the inoculated plants were taken normal care of. The symptom class of BIL population and parents were evaluated according to Washio. O.’s criteria. (Washio. O. et.al 1968).

The field test was done in 2004 at Jianyan, Jiangsu province, P.R.C., where rice stripe disease was epidemic. The lines were planted in May, which was the most susceptible period to the natural infection. Each test line was represented by a row of 35 plants. The RSV incidence was symptomatically evaluated at about 60 days after planting.

QTL of RSV-resistance was mapped with MAPMAKER/QTL1.1b. By individual inoculation test, a total of two QTL controlling RSV resistance were detected on chromosomes 7 and 11, respectively. Individual QTL accounted for 8.96 and 31.79% of the phenotypic variance. The Kasalath alleles increased the resistance to RSV (Table 1, Fig. 1, Fig. 3). By field test, one QTL only accounting for resistance to RSV was detected at the same location as above on chromosome 11 (Table 2, Fig. 2, Fig. 4). The results indicated that this QTL on chromo-

some 11 was important to control the resistance to RSV.

Hayano-Satio et al. (1998) detected Stv-bⁱ,an incompletely dominant gene resistant to RSV, coming from Modan, located between XNpb220 and XNpb257 (XNpb254) on chromosome11. Maeda H. et al. (1999) had detected one QTL controlling RSV resistance located around XNpb257 in the middle of XNpb202 and C1172. According to the markers’ locations and resources, we found that the QTL on chromosome 11 in our study was located in the same location as what Maeda H. and Hayano-Satio had detected. It was indicated that the interval between C1172 and XNpb202 on chromosome 11 was closely associated with genes resistant to RSV and a very conserved interval during evolution. More studies about the interval will help us know the evolution of plant disease resistance.

References

Hayano-Saito, Y., T. Tsuji and K. Fuji, 1998. Localization of the rice stripe disease resistance gene, Stv-bⁱ, by graphical genotyping and linkage analysis with molecular markers. Theor. Appl. Genet. 96(8): 1044-49.

Maeda, H., H. Nemoto, T. Yagi and Y. Fukuta, 1999. QTL analysis for rice stripe disease resistance using recombinant inbred lines (RILs) derived from crossing between Milyang and Akihikari. In: China Association of Agricultural Science Societies, China National Rice Research Institute, China National Hybrid Rice Research and Development Center, China Foundation Society for Agricultural Science and Education (eds). Prospects of rice genetics and breeding for the 21st century-Paper collection of international rice genetics and breeding symposium. Beijing: China Agricultural Science Technology Press. 53~57.

Washio, O., A. Ezuka, K. Toriyama and Y. Sakurai, 1968. Testing method for genetic of and breeding for resistance to rice stripe disease. Bull. Chugoku Natl. Agr. Exp. Stn. A16: 39~197.