Document Type

Dissertation - Open Access

Award Date

2024

Degree Name

Doctor of Philosophy (PhD)

Department / School

Biology and Microbiology

First Advisor

Volker Brözel

Abstract

With increasing demand for soybean, maximizing plant yield is important yiel is dependent on nitrogen availability, but nitrogen fertilizers can meet the plant's needs but have severe environmental impacts. An economically and environmentally sustainable approach is to enhance nitrogen availability through Biological Nitrogen Fixation by symbiotic rhizobia, Bradyrhizobium, an efficient nitrogen fixer for soybean. This symbiosis begins with attachment of rhizobia to legume roots. Root and rhizobia surface properties play a significant role in this attachment. Another major factor affecting root colonization is bacterial competition ability. There is a lack of knowledge on the role of bacterial surface properties, competition mechanisms, and root properties in nodulation. I aimed to gain a better understanding of the ecophysiology of Bradyrhizobium in response to rhizosphere nutrient environment specifically root exudates and exploring its competition ability. I investigated whether factors like motility, hydrophobicity, surface sugar moieties and attachment of Bradyrhizobium to the root are influenced by growth in a soil nutrient environment. Four Bradyrhizobium strains USDA 26, 110, 126 and 3384 were cultured in medium with arabinose and soil-extracted soluble organic matter (SESOM), a soybean field soil representative. Phenotypic and attachment related properties offered significantly between SESOM, and arabinose. SESOM-grown populations had reduced motility, simpler surface sugar profiles, increased cell surface hydrophobicity (CSH) and enhanced binding to soybean roots. These findings suggest that growth in a soil environment affects surface and attachment related properties of rhizobia. Therefore, I further evaluated surface and attachment related properties of Bradyrhizobium diazoefficiens USDA 110 under the influence of 11 abundantly present soybean root exudate chemical compounds (RECCs). USDA 110 exhibited significant surface phenotypic plasticity when exposed to different RECCs. Specific RECCs modulated motility and root attachment, with serine enhancing CSH and attachment, gluconate promoting EPS production and biofilm formation but reducing attachment, and raffinose increasing motility and chemotaxis. CSH significantly influenced root attachment. My findings suggest that attachment phenotype is shaped by individual RECCs at different steps of attachment process, challenging conclusions drawn from standard carbon sources. Further, I studied the competition ability of USDA 110 as in soil it is crucial to compete for survival and colonization. I discovered unique occurrence of sibling rivalry and selfgrowth suppression in USDA 110. Swimming colonies of USDA 110 develop intercolony zones (ICZ) between them. This is due to growth suppression of cells in the ICZ, as cells had reduced metabolic activity and motility. Growth inhibition also occurs in other Bradyrhizobium species and soil bacteria. I confirmed that suppression is due to a diffusible compound whose exact nature remains elusive. The compound has short lifespan and is sensitive to UV and heat but, is not a protein, strong acid, or base. This counterintuitive self-suppression suggests an adaptive strategy resource conservation and phenomenon of antagonism against other soil bacteria and Bradyrhizobium.

Library of Congress Subject Headings

Soybean -- Roots.
Soybean -- Yields.
Rhizobium.
Nitrogen -- Fixation.
Ecophysiology.

Publisher

South Dakota State University

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Rights Statement

In Copyright