Uncovering limitations and opportunities in nitrogen fixation in soybean using genetic mapping and transgenic approaches
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Soybean [Glycine max (L.) Merr.] is one of the most valuable crops world-wide and is the most cultivated legume. Symbiotic N fixation (SNF) evolved as it confers a competitive advantage to soybean in environments where N is limited and is important in modern agriculture as it benefits farmers and the environment by eliminating the requirement of synthetic N fertilizers to achieve high yields. Development of legume nodules and active SNF are complex processes which involve numerous genes. As SNF is correlated with plant biomass and yield, understanding of the genetic architecture of SNF and SNF-related traits may be of use to soybean breeders. Using genome-wide association and bi-parental mapping approaches in both semi-controlled and field conditions, novel genomic regions controlling SNF traits including nodule number, nodule size, number area, total N accumulation, percent N from the atmosphere, and plant biomass were identified in soybean. The quantitative trait loci (QTLs) identified here are valuable data for soybean breeders as they can be leveraged for marker assisted selection of varieties with improved SNF or used for genomic prediction and accelerate breeding progress. In addition to the immediate practical use of QTL data, the genomic regions marked by the QTL can be further investigated for causative genes and the physiological implications of their allelic variation. Investigations of SNF through genetic mapping is an important area of plant biology as it provides opportunities for improvement of SNF in soybean, which has not yet been fully exploited. Besides broad reaching genetic mapping methods, more focused approaches prove useful in testing hypotheses of specific routes to improve SNF. Here, we present results from transgenic soybean expressing the Phaseolus vulgaris ureide permease 1 (PvUPS1) gene, a ureide transporter expressed in vascular endodermis cells of the nodule, leaf, stem, roots, and seed coat. Previous evidence from controlled environments suggests that increasing UPS1 expression may lead to improved SNF, biomass accumulation, and seed yield. Our results from field trials showed no increase in SNF or yield, but revealed an increase in seed protein concentration of up to 6 percent compared to wild type control plants. Causes of discrepancies between controlled and field studies were not determined; however, modification of UPS1 expression was shown to be a possible avenue for improvement of soybean seed protein concentration. Through genetic mapping and transgenic approaches, the work presented here investigated genotypic variation in SNF traits and uncovered routes for improvement of SNF.
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Ph. D.