Exploring essential aspects of chemotaxis and motility in rhizosphere-dwelling bacteria

Abstract

The agricultural industry faces a constant struggle between generating profit from crop yield and the cost of production in terms of monetary loss and environmental impact. A potential solution comes from bacteria that are capable of forming a symbiotic relationship with plants to deliver nitrogen, thus alleviating the need for synthetic fertilizers that are used in excess. In addition, there are bacterial pathogens that cause disease in crops leading to reduction in yield, exacerbating the financial strain on farmers. Both groups of bacteria are capable of identifying potential hosts and biasing their movement towards them through chemotaxis and flagellar-driven motility. Key aspects for different portions of this process were examined in three agriculturally-relevant bacteria: Bradyrhizobium diazoefficiens (symbiont), Sinorhizobium meliloti (symbiont), and Agrobacterium tumefaciens (pathogen). Chapter I encompasses a literatures review of the knowledge for the chemotactic pathway and flagellar structure in the target bacteria. Additionally, it highlights the gaps in knowledge that this study set to characterize. In Chapter II we detail the investigation of the ligand binding capabilities of B. diazoefficiens chemoreceptors. A combination of bioinformatics analysis with in vitro high-throughput screening was utilized to identify chemoreceptors with periplasmic binding motifs and predict amino acid binding capabilities for the receptor Blr2932. In Chapter III we investigate the sensitivity adaptation system of S. meliloti chemoreceptors via methylation. CheR was demonstrated to crosslink to the cytosolic domain of McpX for in vitro experimental basis. We were able to demonstrate that single mutations of putative methylation sites could reduce the chemotactic ability, indicating that S. meliloti chemoreceptors likely utilizes the same residues as E. coli to adapt their signal sensitivity. Chapter IV encompasses the assembly of the A. tumefaciens strain 5A genome and characterization of its novel flagellar structure. The genome was sequenced and assembled into four complete contigs, finalizing the previously sequenced draft genome. This allowed for the characterization of individual flagellin function in motility and flagellar structure. The agricultural relevance of all three bacteria creates a growing need to characterize aspects of their functions related to plant interactions. The more knowledge we gain, the more opportunities we have to exploit them to improve formation of symbiotic relationships or to target them for reduction of disease spread.