Elucidating genetic and molecular mechanisms underlying symbiotic specificity in the Medicago-Sinorhizobium mutualism

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The availability of nitrogen in the soil is a major limiting factor for agricultural production. Although the use of industrial fertilizers has contributed substantially to feeding the world, the production of nitrogen fertilizers requires fossil fuels and excessive fertilizers cause serious environmental pollution. In contrast, plants of the legume family can make their own nitrogen fertilizer by forming symbiosis with nitrogen-fixing soil bacteria, called rhizobia. This symbiosis culminates in the formation of specialized root organs, called nodules, within which the bacteria convert atmospheric nitrogen into ammonia for use by the plant. One important property of the legume-rhizobial symbiosis is its high level of specificity. Such specificity can take place at various stages of the interaction, ranging from initial bacterial infection and nodulation (nodulation specificity) to late nodule development associated with nitrogen fixation efficiency (nitrogen fixation specificity). This project aims to elucidate genetic and molecular mechanisms that regulate symbiotic specificity in the legume-rhizobia interaction. Such knowledge will facilitate development of novel strategies to enhance the benefits of symbiotic nitrogen fixation for sustainable agriculture. This project will contribute to the training of students at graduate, undergraduate and high school levels. The research group will particularly encourage the participation of members of under-represented undergraduates to join the project.Despite recent advances in our understanding of the signaling pathways leading to root nodule development, the molecular mechanisms underlying natural variation in nodulation capacity and nitrogen fixation efficiency are still not well understood. In the Medicago truncatula- Sinorhizobium meliloti interaction, the Zhu laboratory recently cloned two host genes, NFS1 and NFS2 that regulate nitrogen fixation specificity concerning S. meliloti Rm41 and an NS1 locus containing two receptor-like kinases that restricts infection and nodulation by Rm41. The polymorphic NFS genes encode nodule-specific cysteine-rich (NCR) peptides. In contrast to the predominant notion of NCR peptides as effectors of endosymbionts' differentiation to nitrogen-fixing bacteroids, this finding demonstrated that specific NCRs negatively regulate symbiotic persistence. NFS1 and NFS2 provoke bacterial cell death and early nodule senescence in an allele-specific and rhizobial strain-specific manner, and their function is dependent on the host's genetic background. This project addresses how NCR peptides, on one hand, can induce terminal bacteroid differentiation needed for nitrogen fixation, and on the other hand, some NCR variants result in lysis of bacteroids. Specifically, the project will determine whether the pro- and anti-symbiotic peptides interact with the same bacterial targets, how amino-acids of the peptides affect their function, and how the functions of these peptides are affected by the genetic background. The specific objectives of the research include: (1) characterization of the NFS1 and NFS2 genes and their roles in negative regulation of symbiotic development with Rm41; (2) cloning and characterization of a suppressor of NFS1; and (3) characterization of the NS1 locus that restricts nodulation by Rm41.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.