Regulatory mechanisms underlying nitrogen-induced suppression of nitrogen fixation in soybean
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Symbiotic nitrogen fixation (SNF) in soybeans is essential for converting atmospheric nitrogen into bioavailable forms through a symbiotic relationship with rhizobial bacteria. This dissertation investigates the regulatory mechanisms behind nitrogen-induced suppression of SNF in soybean, focusing on the molecular and physiological pathways involved in nodulation and nitrogen fixation. By employing RNA sequencing, small RNA analysis, gene expression studies, and the acetylene reduction assay to measure nitrogen fixation activity, I examined both plant (Glycine max) and bacterial (Bradyrhizobium diazoefficiens USDA110) transcriptomes in nodules under high nitrogen conditions and compared them to nodules grown under low nitrogen conditions, which are more typical of field cultivation. The results reveal nitrogen’s strong regulatory effect on nodules and leaves, particularly under high nitrogen conditions, where high-yielding soybeans are frequently supplemented with nitrogen fertilizer. This practice suppresses SNF, making it an inefficient approach for sustainable agriculture. I also developed a novel, non-destructive method to assess belowground nitrogen fixation activity using aboveground leaf RNA markers, offering a practical tool for evaluating nitrogen fixation status without disrupting plant growth. Furthermore, I identified differential gene expression patterns and nitrogen-responsive biomarkers, providing new insights into nitrogen metabolism in both the host plant and its rhizobial symbiont. These findings offer strategies to enhance nitrogen use efficiency and reduce dependence on synthetic fertilizers, contributing to more sustainable agricultural practices. Collectively, this research deepens our understanding of SNF regulation and its potential to optimize soybean cultivation in both high- and low-nitrogen environments.
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Ph. D.