Pyramiding Resistance To Maize Rough Dwarf Disease And Southern Corn Rust By Marker-assisted Selection

Maize(Zea mays L.) is an important food and feed crop in the world. In China, maize plays an important role in agriculture and the national economy. Maize rough dwarf disease (MRDD) is a viral disease that is widely distributed in the world and causes great losses in corn production. In recent years, Southern corn rust (SCR) has become one of the major diseases of maize production. Practice has proved that disease resistance breeding is the most effective way to reduce production losses. Chinese breeders have identified and selected a number of excellent disease-resistant inbred lines such as90110which is highly resistant to MRDD and Qi319which is highly resistant to SCR after years of work. But the majority of elite self-lines and commercial hybrids in our country remain sensitive to MRDD and SCR. In general, corn growing areas are often subjected to a variety of disease infestation because of wide distribution and diversity of the disease. So selection of multi-resistant maize new varieties is the direction of future corn breeding. Making the best use of the existing disease-resistant germplasm resources, by molecular marker-assisted selection techniques, to improve the resistant to MRDD and SCR and breed disease-resistant inbred lines or hybrids is an effective way to solve the problem of diseases.Dr. Wang Fei has identified three loci which were separately located on chromosome6,7and8and denoted as Mrddl, Mrdd2and Mrdd3conferring to MRDD in90110. Li Yujie transferred the three loci from90110to the susceptible maize inbred lines L189, DH4866,502,5003, zheng58and chang7-2by multi-generation backcrossing and marker-assisted selection on the basis of the work of Wang Fei, and got the fifth backcross generation (BC5F1). The BC5F1were close to the near-isogenic lines of elite selflines, but they have a resistance to MRDD. Dr. Chen Cuixia identified a single dominant gene RppQ resistant to SCR on the short arm of chromosome10in the maize inbred line Qi319. RppQ was linked with SSR markers phi041and phi118with a genetic distance of2.45cM and3.33cM, respectively. In our previous study, we transferred the RppQ gene from Qi319to the susceptible maize inbred lines L189, DH4866,502,5003, zheng58and chang7-2by multi-generation backcrossing, and got the fourth backcross generation (BC4F1). The BC4F1have a resistance to SCR while maintaining good traits of elite selflines.In this study, we used the improved lines L189BC5F1, DH4866BC5F1,502BC5F1,5003BC5F1, zheng58BC5F1, chang7-2BC5F1,9801BC5F1containing three MRDD resistant loci as receptor and recurrent parents, and the improved lines L189BC4F1, DH4866BC4F1,502BC4F1,5003BC4F1, zheng58BC4F1, chang7-2BC4F1,9801BC4F1containing SCR resistant gene RppQ as donor parents. Finally we obtained BC2F1containing three MRDD resistant loci and RppQ by multi-generation backcrossing and marker-assisted selection.6SSR markers linked with MRDD resistant loci were used to select three MRDD resistant loci and2RppQ-linked. SSR markers was used to select RppQ in the segregating populations. In the process, only plants containing the target genes were accepted, and the background selection could considerably optimize the breeding process.During the foreground selection, BC1F1and BC2F1populations were identified for MRDD and SCR resistance in the conditions of natural occurrence. BC1F1and BC2F1, as well as90110, Qi319and elite lines L189, DH4866,502,5003, zheng58, chang7-2,9801were planted and evaluated the resistance under the same environmental conditions in the field. The result shown that elite lines L189, DH4866,502,5003, zheng58, chang7-2and9801had infected plants with different ratio, but the majority of BC1F1and BC2F1populations had a better performance than elite lines in resistance. It was concluded that the resistant genes had been pyramided into the same genome. Agronomic traits were also investigated and compared with the elite lines, and the accepted plants were semblable to the elite lines.In our work, three MRDD resistant loci and RppQ were pyramided into the same genome successfully by marker-assisted selection. Identification of resistance shown that improved lines with multi-resistance genes have high resistance to MRDD and SCR. Those improved lines can be used as intermediate of breeding process, after selfcrossing and selection, a number of excellent selflines with multi-resistance will be bred, and this is of great significance to maize multi-resistance breeding.