| The small brown planthopper (SBPH), Laodelphax striatellus Fallén (Homoptera: Delphacide), is an economically important pest in rice (Oryza sativa L.) and distributes widely in China. It not only causes direct damage by sucking plant sap but also transmits several viral diseases such as rice stripe virus (RSV) and rice black-streaked dwarf virus (RBSDV), which often cause major yield losses. In recent years, the damage caused by SBPH feeding and the diseases transmitted by this planthopper has been increasing in China and the outbreak occurred in 2004, which caused serious rice yield reduction.Protection against SBPH pest has depended mostly on insecticides in the past years, which has led to enhanced resistance of SBPH to chemicals, natural enemy death and environmental pollution and then caused the pest resurgence. Host resistance has been recognized as one of the most economic and effective measures in controlling SBPH. The varieties, highly resistant to SBPH can control this planthopper effectively as well as RSV due to prevention of transmission of virus.Based on the trait of SBPH, a seedling screening technique suitable for SBPH has been established by way of proper modification of standard seedbox screening test. 138 rice accessions, collected from Jiangsu, Zhejiang and Yunnan Province were screened for resistance to SBPH by seedbox screening test with modification. Mechanism of resistance to SBPH in some varieties was analyzed. Then, QTLs for SBPH resistance in'DV85','Kasalath'and'Mudgo'was detected. The results were as follows:1.Out of the one hundred and thirty-eight rice accessions, twenty-five entries with different level of resistance to SBPH were detected, accounting for 18.1% of the total accessions, including 2 highly resistant, 9 resistant and 14 moderately resistant varieties. Compared with indica rice, japonica rice was more susceptible to SBPH. Antixenosis test, antibiosis test and correlation analysis were performed to elucidate resistance mechanism. The highly resistant varieties such as Rathu Heenat(iRHT), Mudgo and Kasalath and resistant IR36 expressed strong antixenosis and antibiosis against SBPH, indicating the close relationship between resistance level and these two resistance mechanisms in the four rice varieties. Antibiosis was the dominant resistance pattern in the resistant varieties Daorenqiao and Yangmaogu due to their high antibiosis but low antixenosis. Dular, ASD7 and Milyang23 had relatively strong antixenosis and antibiosis, indicating the two resistance mechanisms were significant in these three varieties. The resistant DV85 expressed relatively high level of antixenosis but low antibiosis, while Zhaiyeqing 8 and Guiyigu conferred only moderate antibiosis and antixenosis to SBPH, suggesting tolerance in these three varieties. Antibiosis and antixenosis governed the resistance to SBPH in the moderately resistant accession 9311. Antixenosis was the main resistance type in V20A. Tolerance was considered to be an important resistance mechanism in Minghui 63 and Yangjing 9538 due to their poor antibiosis and antixenosis resistance. The above accessions with strong antibiosis or antixenosis were the ideal candidate for resistance breeding.2.The indica rice'DV85'showed resistance to SBPH at the seedling stage. A mapping population consisting of 81 recombinant inbred lines (RILs), derived from a cross between a japonica cultivar Kinmaze and an indica rice DV85, was used to detect quantitative trait loci (QTLs) for the resistance to SBPH. Modified seedling screening test (MSST), along with antixenosis test and antibiosis test were applied to evaluate the resistance response of the two parents and 81 RILs to SBPH and composite interval mapping (CIM) was used for QTL analysis. When the resistance was measured by MSST method, two QTLs conferring resistance to SBPH were mapped on chromosome 11, namely Qsbph11a and Qsbph11b, with log of odds (LOD) scores 2.51 and 4.38, respectively. The two QTLs explained 16.7% and 27.8% of the phenotypic variance in this population, respectively. A total of three QTLs controlling antixenosis against SBPH were detected on chromosomes 3, 4 and 11, respectively, accounting for 37.5% of the total phenotypic variance. Two QTLs expressing antibiosis to SBPH were mapped on chromosomes 3 and 11, respectively, explaining 25.9% of the total phenotypic variance. The identified QTL located between markers XNpb202 and C1172 on chromosome 11 was detected repeatedly by three different screening methods and therefore may be important to confer the resistance to SBPH. Once confirmed in other mapping populations, these QTLs should be useful in breeding for resistance to SBPH by marker-assisted selection of different resistance genes in rice varieties.3.An indica variety,'Kasalath'is highly resistant to SBPH, which expresses strong antixenosis and antibiosis against SBPH. A mapping population of 98 BC1F9 lines (Backcross inbred lines: BILs), derived from a backcross of Nipponbare (japonica) / Kasalath (indica) // Nipponbare by the single-seed descent methods, was applied to detect quantitative trait loci (QTLs) for resistance to SBPH. In the modified seedbox screening test, three QTLs for SBPH resistance were mapped on chromosomes 3 and 11, namely Qsbph3c, Qsbph8 and Qsbph11f, with LOD scores 3.14, 2.95 and 4.12, explaining 13.8%, 12.6% and 23.5% of the phenotypic variance in this population, respectively. As indicated by the additive effect, resistance alleles at Qsbph3c, Qsbph8 and Qsbph11f derived from Kasalath, Nipponbare and Kasalath, respectively. In the antixenosis, a total of three QTLs(Qsbph3c, Qsbph8 and Qsbph11f)conferring antixenosis against SBPH were detected on chromosome 3, 8 and 11, with LOD scores 3.19, 2.58 and 3.36, respectively, accounting for 36.4% of the total phenotypic variance. In addition, two QTLs, Qsbph2 and Qsbph11g, which came from Kasalath, expressing antibiosis to SBPH were detected on chromosomes 2 and 11, with LOD scores 3.23 and 3.52, respectively. Individual QTL accounted for 13.8% and 14.7% of the phenotypic variance. Qsbph11e, Qsbph11f and Qsbph11g were located in the region between S2260 and G257 on chromosome 11, indicating the locus is significant in conferring resistance in Kasalath. The molecular markers linked to these QTLs should be useful in breeding of varieties with horizontal resistance to SBPH.4.The indica rice'Mudgo'expressed high resistance to SBPH, with strong antixenosis and antibiosis against SBPH. A genetic linkage map constructed from a F2 population, derived from a cross of'Mudgo'and'Wuyujing 3'was used for mapping QTLs associated with resistance to SBPH. The linkage map comprised of 104 SSR and 3 Indel markers and covered 1409.9 cM of the rice genome with an average marker interval of 13.2 cM. One hundred and seventy-seven F2:3 families were identified for resistance to SBPH by way of seedbox screening test with modification. A total of three QTLs such as Qsbph2b, Qsbph3d and Qsbph12a conferring resistance to SBPH were detected on chromosome 3, 8 and 11, locating in the region of RM5791～RM29, RM3199～RM5442 and I12-17～RM3331, with LOD scores 3.19, 2.58 and 3.36, respectively, accounting for 68.7 % of the total phenotypic variance observed in this population. As showed by the additive effect, resistance alleles at Qsbph2b, Qsbph3d and Qsbph12a came from Mudgo. The locus Qsbph12a, with 35.3% of variance explained, was linked tightly to the makers RM3331 and I12-17, which were useful in breeding for resistance to SBPH by rapid marker-assisted selection.
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