Molecular identification of stem growth habit and disease resistance genes in soybean
Soybean (Glycine max (L.) Merr.) is one of the most important crops in the world because of its high protein and oil content as well as its ability to fix atmospheric nitrogen with the help of symbiotic rhizobia. Improving soybean yield and enhancing its disease resistance are two challenging tasks, as it would be one of the potential solutions to meet the increasing demands for food and energy due to the large growing world population with limited resources. Taking full advantage of the availability of the soybean reference genome and the advancement of the technologies, we are at the ideal time for research on soybean genes related to specific traits of interest such as yield and disease resistance, which could eventually lead to better soybean cultivars for the world. In this thesis research, two independent studies were performed in order to identify genes regulating stem growth habit of soybean and novel genes resistant to Phytophthora Sojae. In the first project, a large mapping population consisting of 681 F 2:3 progeny rows of the mating NE3001×IA3023 was constructed. Based on the phenotypic and genotypic data, the Dt2 gene was mapped to a short genomic region which contains only 10genes. Subsequently, combined with gene sequencing analysis and phylogenetic analysis, the Dt2 candidate gene has been identified. To further extrapolation of the Dt2 candidate, different tissues at various developmental stages have been collected and analyzed to dissect the temporal and spatial expression patterns of the gene. Furthermore, the expression levels of the Dt2 candidate and Dt1 in near-isogenic lines have been monitored, revealing a model in which Dt2 would directly or indirectly repress the expression of Dt1 to initiate the early conversion of the shoot apical meristem. Introduction of the transgene successfully results in phenotypic change from indeterminate to semideterminate. In conclusion, the semideterminacy of soybean was specified by a dominant MADS-box like gene, which interacts with a flowering repressor gene, to initiate terminal flowering, and subsequently shape the stem growth habit. The research findings, coupled with the previous elucidation of Dt1, have laid the foundation for investigation of genetic diversity underlying this trait and its interaction with other traits of agronomic importance for high-yielding soybeans. In the second project, a population of 58 F2 progenies and 209 F2:3 families derived from a cross between USDA046 and the susceptible cultivar 'Williams' was used to characterize the inheritance pattern of Rps gene(s) in USDA046. Mendelian segregation ratios fitting both 3:1 and 1:2:1 were observed in the F2 and F 2:3 populations, respectively, which suggests the resistance in USDA046 is controlled by a single dominant gene (designated as Rps 11). Bulk segregation analysis (BSA) coupled with single nucleotide polymorphism (SNP) genotyping quickly identified the potential location of resistance to a genomic region around 5Mb at the beginning of the Chr.7. The entire F 2:3 mapping population was then screened with polymorphic simple sequence repeat (SSR) markers from the candidate region and a linkage map with nine SSR markers was constructed. The results showed that the Rps gene was mapped to a 0.5 cM region flanked by SSR markers SSR_07_0286 and SSR_07_0300. Co-segregating SSR markers were also identified, which will be useful in the soybean breeding program.
Ma, Purdue University.
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