Document Type

Dissertation - Open Access

Award Date

2024

Degree Name

Doctor of Philosophy (PhD)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Senthil Subramanian

Abstract

Biological Nitrogen Fixation (BNF) has surged as a natural strategy to provide available nitrogen to crops by taking advantage of diazotrophs, microorganisms that can reduce atmospheric nitrogen to ammonia. Soybean, a major crop, greatly benefits from BNF by forming endosymbiotic relationships with rhizobacteria from the genus Bradyrhizobium, which culminates in the formation of specialized organs called nodules, where nitrogen fixation occurs. This is known as Symbiotic Nitrogen Fixation (SNF). Inoculation of soybean fields with high nitrogen fixing Bradyrhizobium strains to stimulate SNF is being used as a strategy to increase crop productivity and nitrogen content. However, in North America, SNF cannot entirely meet the soybean nitrogen demand partially due to rhizobial competition for nodule occupancy, where the high-efficient inoculant strain is outcompeted by the indigenous soil population exhibiting poor nitrogen fixing capacity. Despite its agricultural importance, the mechanisms that mediate Bradyrhizobium competition for nodulation in soybean are not well understood. Therefore, the goal of this study was to identify the mechanisms that Bradyrhizobium strains employ for competition during symbiosis with soybean. First, we determined that graphene nanoparticles could be safely utilized as a root or seed coating agent; This would enable potential use of such coatings for preferential strain binding in soybean- Bradyrhizobium symbiosis. Next, we characterized the symbiotic properties of nine Bradyrhizobium strains to gain mechanistic insights on their competitiveness. We then classified them into high, intermediate, and low nitrogen-fixing capacity groups and generated whole genome sequence information that could partially explain their phenotypes during symbiosis. We later evaluated the competition of two strains with opposite nitrogen-fixing capacities (Fix+ USDA 110 vs Fix-USDA 126) using two approaches - (1) dual inoculation of molecular tagged bacterial strains onto a single root system and (2) a split root system wherein the separate root halves are inoculated with different strains. Our results indicated that competition outcomes measured as nodule occupancy patters could be detected in both approaches as early as 10 dpi. We sought to distinguish the combined effect of soybean systemic sanctioning and Bradyrhizobium colonization mechanisms (dual inoculation) from soybean sanctioning mechanisms (split root) on nodule occupancy patterns. Consequently, we designed and tested a dual RNAseq experiment to simultaneously evaluate the Bradyrhizobium and soybean gene expression responses in competitive split root systems. Preliminary results showed the preferential activation of distinct bacterial and soybean pathways in response to USDA 110 and USDA 126 inoculation. Overall, our study suggests that beyond their intrinsic nitrogen fixing capacity, root colonization and endosymbiont lifestyle adaptation are essential traits for Bradyrhizobium competitiveness and soybean selective nodulation.

Publisher

South Dakota State University

Download

Share

COinS
 

Rights Statement

In Copyright