Mapping And Pyramiding Brown Planthopper Resistance Genes In Rice

Rice (Oryza sativa L.) is a stable food for more than half of the global population. The brown planthopper (Nilaparvata lugens Stal, BPH) has been the most devastating insect pests in rice. It has been recognized that utilization of host-plant resistance is the most desirable and economic strategy in the management of BPH. Thus, the objectives of the study were to exploit and map new BPH resistance genes, and incorporae them separately or pyramid them into some elite rice varieties, using high efficient marker-assisted (MAS) systems, which provided theoretical foundation and breeding materials for developing improved versions of hybrid rice with highly advanced BPH resistance. The main results are as follows:1. Using a BC1F2:3 population developed from ZS97 and IR02W101 (a introgression line from O. officinalis Acc.100896), a molecular genetic map containing 174 SSR markers was constructed and used for mapping QTL conferring BPH resistance. Two major QTLs, QBph4 and QBph3, were located on the short arm of chromosome 4 and long arm of chromosome 3, explaining 21.30% and 19.85% of the variation for BPH resistance, respectively, and the both of the alleles from IR02W101 increased resistance to the BPH. Screening two large BC2F2:3 populations derived form the NILs of the two QTLs for recombinants and conducting progeny test, we further fine mapped QBph4 to a 33 Kb region, and QBph3 to a 90 Kb region. Compared with the other BPH resistance genes reported, we concluded that QBph4 was 300 Kb south of Bph15, and 1.5 Mb north of Bph20, while QBph3 was 100 Kb north of Bphl4. In addition, four single-gene introgression lines (IL) and two pyramided lines (PL) carrying both or either of QBph4 and QBph3, Bph14 and Bphl5 under the background of the ZS97 were developed by MAS based backcross breeding. BPH bioassay showed that the PL-QBph4/QBph3 had the highest resistance among the lines and the order of gene effect was QBph4/QBph3> QBph4≥Bph14/Bph15≥QBph3≥Bph15≥Bph14≥ZS97 (none). The newly developed fine mapping and pyramiding system of QBph4 and QBph3 will facilitate map-based cloning and functional analysis of these genes, thus elucidating the mechanism underling BPH ressitance, and can be immediately used in hybrid improvement and production.2. Three dominant genes (Bph14, Bphl5 and Bph18) were incorporated separately and pyramided into elite restorer lines Minghui 63 and 93-11, and their hybrids using MAS. Seven genotype combinations including single, two and three genes were developed. Gene effectiveness was evaluated on the basis of seedling and adult rice resistance, honey dew weight and survival rate of BPH. BPH bioassay showed that pyramided lines or hybrids with three genes expressed highest resistance (even identical to that of the resistant check B5). The lines pyramiding two genes had lower resistance, and resistance of the single-gene lines was even lower, however, the lines with no genes had lowest resistance (indentical to that of the susceptible check TN1). All three genes had independent effects on BPH resistance and the effect varied from different genotype combinations, the order of the gene effect being 14/15/18≥ 14/15> 15/18≥ 15> 14/18 >14≥18> none. The lines pyramiding Bph14 and Bph15 expressed higher resistance than the Bphl4 or Bph15 single introgression line, indicating a strong additive effect of pyramiding these genes. However, there was partial additive effect of pyramiding Bph15 and Bph18, and no additive effect of Bph14 and Bphl8. The pyramided or single gene hybrids showed greater resistance than conventional hybrids, although the heterozygous genotypes had weaker effects than the corresponding homozygous genotypes, indicating semi-dominant of those genes. There was no significant difference on BPH resistance between improved hybrids with the same genotype combination under different genetic backgrounds, indicating the effects of these genes were affected little by backgrounds. All three genes affected BPH growth and development and had antibiotic factors, resulting in both seedling and adult resistance, furthermore,Bph15-single introgression lines or hybrids still expressed moderate resistance at adult stage even at 30 days after BPH infestating, indicating Bph15 also mediated tolerance. Field trial data demonstrated that there were no significant difference on heading date and plant height between improved versions and controls, and yields of improved versions were higher than or similar to that of the control under natural field conditions.3. In the field, pyramided hybrids with Bphl4 and Bph15 had a significantly fewer number of BPHs settling (fewer than 500 per 100 plants) than that of conventional hybrids with no genes (more than 2000 per 100 plants), indicating that pyramied hybrids could effectively decrease the population of BPH, thereby preventing BPH outbreaks, reducing the cost of labor and pesticides and increasing rice production.4. Compared with conventional hybrids, improved hybrids pyramiding Pil, Pi2, Bph14 and Bph15 (Hua 1971A/Huahui 644, Hua 1971A/Huahui 1462, etc) showed a significantly lower neck blast infection rate and higher BPH resistance, and the yields of improved hybrids were higher or similar. These improved hybrids could be of great use in breeding new varieties resistant to disease and insects.5. Based on the positional information of BPH-resistance genes previously reported, SSR and InDel markers adjacent to related genes were designed, and used to track the target genes in the segregating generation, and to test whether the markers were closed linked with genes. Thus, MAS systems of high efficiency associated with these genes were developed. Combining these MAS systems with backcross breeding, four major BPH-resistance genes, Bph10, Bphl7, Bph20 and Bph21, were incorporated into ZS97, individually and a variety of introgression and improved lines with enhanced BPH resistance were developed.