Grant: Discovery Grants Program - Individual

Background: Legume plants include several important food crops such as peas and lentils. Symbiotic nitrogen fixing bacteria (collectively termed rhizobia) infect the legume roots where they perform the specialized function of reducing atmospheric nitrogen to ammonia. The fixed nitrogen is assimilated by the plant to meet its nitrogen requirements. Farmers often apply a rhizobial inoculant during legume seeding to ensure consistent nitrogen fixation and enhanced yields. However, use of inoculant technologies can be inefficient because the inoculant strains are often outcompeted in the rhizosphere environment prior to infecting the legume plant, commonly referred to as the 'Competition Problem' (Triplett and Sadowsky, 1992). This challenge is further compounded by the lack of long term stability of inoculants and lower on-seed survival of inoculants resulting in decreased viable inoculant density and diminished colonization of germinating seeds. The primary objective of the research program is to improve inoculant performance through discovery- based research to unravel the complex biological traits and processes that are crucial for rhizobia to adapt to stresses encountered during both inoculant formulation and during legume host colonization. Scientific Approach:  To effectively colonize the roots of a germinating seed the inoculant strain must have the ability to outcompete less efficient nitrogen fixing rhizobial populations endemic to the soil environment. Therefore, the successful and rapid colonization of germinating seeds is a desirable trait for a rhizobia inoculant. Bacterial competition for host plant interaction can begin at the initial stages of seed germination and colonization of the spermosphere, the soil environment surrounding a germinating seed. The spermosphere is an often overlooked, but potentially important environment for the establishment of a plant-microbe symbiosis as it represents the first opportunity for soil microbes to perceive and colonize a newly developing plant root system.  Since the spermosphere environment is relatively understudied within plant microbe interactions and particularly so within legume-rhizobium systems, we will use functional genomics to identify new genetic determinants and gene regulatory networks that are important for rhizobial colonization of the spermosphere. In addition, we will investigate traits important for cell survival within the inoculant production process, such as desiccation stress that can be encountered during granular or on seed inoculant production. Using transcriptomics to study R. leguminosarum adaptation to environmentally relevant stressors will reveal novel regulatory networks within this genetically diverse rhizobial species. Collectively these experiments will provide new genetic insights into environmental microbiology and plant- microbe interactions, with the long term impact of applying rational strategies to improve rhizobial inoculants.