Author ORCID Identifier

Date Available


Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation


Agriculture, Food and Environment


Plant and Soil Sciences

First Advisor

Dr. Hongyan Zhu


Legumes form a mutualistic partnership with nitrogen-fixing bacteria to meet their nitrogen requirements. This collaboration leads to the development of root nodules, offering an ideal environment for the bacteria to convert atmospheric nitrogen into ammonia. A salient aspect of this symbiotic relationship is host range specificity, such that nodulation capacity and nitrogen fixation efficiency differ significantly between different host-bacterial combinations. Understanding the genetic basis of this specificity could pave the way for strategies to overcome the host range barrier. Sinorhizobium meliloti and Sinorhizobium medicae are two closely related species that display differential symbiotic behaviors with legume plants of the Medicago genus. Some Medicago species selectively interact with one over the other. In this study, we identified a receptor-like kinase in Medicago, named Nodulation Specificity 2 (NS2). This kinase distinguishes between the two bacterial species, acting as a genetic barrier against infection by most S. medicae strains. Intriguingly, NS2, located on chromosome 8, is part of an extensive kinase gene cluster. Similar kinase gene clusters are present in other legumes and even in nonlegumes. Analyzing various Medicago accessions revealed that most lack allelic variants of NS2, indicating potential evolutionary pressures to limit its distribution. Activation of NS2-mediated nodulation restriction requires the bacterial rns2 gene product. This gene encodes a glycine-rich octapeptide repeat protein, having distinct variants that differentiate S. medicae from S. meliloti. Our research delves deep into the molecular intricacies steering the legume-rhizobial symbiosis. The pivotal roles of NS2 and Rns2 in defining host range specificity enrich our understanding of symbiotic relationships' coevolution. This knowledge offers promising implications for agricultural practices, suggesting potential avenues to optimize symbiotic relationships to sustain crop production.

Digital Object Identifier (DOI)

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