Mechanisms linking bacterial chemotaxis signaling to nitrogen fixation in beneficial plant-associated bacteria

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Bacterial inoculants can enhance crop yields and reduce reliance on expensive and environmentally harmful chemical fertilizers which are needed to meet the demand for food of the expected global population of 9.7 billion by 2050. To be useful in the field, bacterial inoculants must not only express plant growth beneficial traits but must also be able to colonize plant roots. Further, many bacteria selected in the laboratory as bio-inoculants fail to consistently produce plant growth enhancements once applied in greenhouses or in the fields . This highlights shortcomings in our understanding of the behavior and physiology of beneficial bacteria in the rhizosphere. This project aims to elucidate the molecular mechanisms by which bio-inoculant bacteria coordinate expression of plant growth beneficial traits (nitrogen fixation) with functions that are critical for competitiveness in the rhizosphere and colonization of plant roots (chemotaxis). The knowledge gained from this research will contribute toward better predictive models to guide strategies to select and/or design beneficial diazotrophs for bio-inoculation applications, to improve crop management with benefits to society. In addition to addressing challenges related to sustainable agriculture through basic research, the project will engage a diverse community of participants in developing future solutions to these challenges. The project will provide research opportunities for high school and undergraduate students, including members from underrepresented groups, with those students included as co-authors in publications, when appropriate. The principal investigator will also continue engagement of deaf-and-hard-of-hearing undergraduate students in research experiences. The project will also provide graduate students and a postdoctoral fellow with opportunities to mentor undergraduate students and to apply their research communication skills in a breadth of outreach activities.Preliminary data suggest that bacterial chemotaxis signaling proteins regulate the expression of nitrogen fixation in the beneficial plant-colonizing and bio-inoculant diazotroph, Azospirillum brasilense. The investigators aim to determine the molecular mechanisms by which bacterial chemotaxis signaling proteins coordinate the induction of nitrogen fixation in diazotroph soil bacteria used as biofertilizers worldwide, and the role of this coupling in the rhizosphere, using genetics, live cell fluorescence microscopy and biochemical approaches. They will also explore the mechanism of an apparent similar coupling between metabolism and chemotaxis signaling in the pea symbiont, Rhizobium leguminosarum bv. viciae. Objective 1 will establish the mechanism(s) of chemotaxis signaling control of nitrogen metabolism in A. brasilense and specifically test the hypothesis that chemotaxis signaling proteins interact with an energy-responsive protein(s) to affect the expression of a transcriptional regulator of nitrogen fixation, RpoN. The spatio-temporal pattern of nitrogen fixation expression in the wheat rhizosphere and how chemotaxis affects this activity will also be determined. Objective 2 will decipher how a subset of chemoreceptors adjust chemotaxis signaling to support A. brasilense nitrogen fixation in the wheat rhizosphere. The role of oxygen- and energy-sensing chemoreceptors in affecting chemotaxis sensory specificity, nitrogen fixation and root colonization will be characterized. In addition, the role of one of these chemoreceptors in maintaining chemotaxis signaling arrays structural integrity will be elucidated. Objective 3 will explore how chemotaxis signaling array composition, which is affected by nutrient starvation, regulates nitrogen-fixing nodulation in R. leguminosarum. Together, the results obtained will provide insight into how distinct molecular functions (chemotaxis and nitrogen metabolism) evolved to become integrated within cells. These findings will also produce strategies to improve the performance of diazotroph bioinoculants by elucidating general design principles for the control of traits that contribute to rhizosphere plant-growth promoting competence. The research will employ genetically tractable model systems relevant to modern agriculture to address pressing environmental issues and sustainable agricultural practices. The results of this research will contribute toward better predictive models to guide strategies to select and/or design beneficial diazotrophs for bio-inoculation applications. This award is being co-funded by the Cellular Dynamics and Function cluster in the Division of Molecular and Cellular Biosciences, along with the Plant Biotic Interactions program in the Division of Integrative Organismal Systems.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.