Collaborative Research: Dimensions: Integrating phylogenetics, ecophysiology and transcriptomics to understand the diversity of hornwort-cyanobacterium symbiosis.

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Only a few remarkable plants can form beneficial partnerships with nitrogen-fixing bacteria. These include a few species of bryophytes, ferns, cycads, and a small group of flowering plants. Because of their importance to crops, a lot of research has focused on the relationship between legumes and the bacteria that fix nitrogen for them - rhizobia. Much less is known about an even more widespread partnership between plants and cyanobacteria. Because cyanobacteria are generally less dependent on the plant host than rhizobia, they could be an excellent partner for engineering helpful nitrogen fixation into crop plants. However, our current understanding of plant-cyanobacterium partnerships is poor. The species and genetic diversity of helpful cyanobacteria is virtually unexplored. Similarly, the plant genes involved in these partnerships remain unknown. Hornworts, one of the three bryophyte lineages, are an outstanding system to study plant-cyanobacterial relationships. This study will investigate phylogenetic, functional, and genetic aspects of the hornwort-cyanobacterium partnership. In doing so, the research will explore an array of questions that will provide new insights into how these unique plants interact with their bacterial partners to fix nitrogen. One postdoctoral fellow, three graduate, ten undergraduate, and five high school students will be trained. In partnership with the Science Teachers Association of New York State, an educational video will be made. This video will showcase the various forms of plant-cyanobacterium relationships. The goals of this research are first to uncover the phylogenetic diversity of hornworts and cyanobionts. Then to elucidate the functional diversity of symbiotic nitrogen-fixation. Finally, the research will reveal the genetic components of the association between hornworts and their cyanobionts. The researchers will reconstruct the hornwort tree-of-life using a phylogenomic approach. Then, for each hornwort sample, the genetic diversity of the cyanobiont community will be characterized. Comparing the hornwort and cyanobiont diversity will shed light on host-symbiont specificity. The research will also study patterns of co-evolution, and the roles of cyanobacterial symbiosis in hornwort evolution. Next, population and microcosm studies will be used to test the interaction among hornwort microbiome diversity, environmental variables, and nitrogen-fixation rates. Finally, multiple pure hornwort and cyanobiont cultures will be isolated. Once isolated, their symbioses will be reconstituted under controlled environmental conditions in the laboratory. Time-course RNA-seq data will be generated to identify putative genes that are core for symbiosis establishment and linked to variation in symbiotic behaviors. The results of this research will enable comparisons across all three major forms of plant-microbe mutualistic interaction to fully characterize the evolutionary and genetic underpinnings of symbioses.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.