Alder-associated Frankia bacteria as drivers of host plant chemical phenotype and above-ground interactions

About This Grant

Red alder is a fast-growing hardwood tree that is found from California to Alaska. It plays an important role in lumber production, biofuel, erosion control, and river-shading to protect fish habitat. Like all alders, it forms a symbiosis with nitrogen-fixing bacteria that live in specialized root nodules. These bacteria convert atmospheric nitrogen, normally unavailable to plants, into a form that red alder can use, allowing it to grow in extremely poor soils and enrich these soils over time with nitrogen and organic matter. While the existence of this plant-microbe symbioses has long been recognized, it is virtually unknown which factors determine the variability, outcome and productivity of this interaction. It is also unclear whether these microbes influence their tree host in other ways, for example by producing defensive plant compounds that protect the tree from pests. This project will fill this knowledge gap and will functionally explain why some alder-bacteria partnerships are more productive and resilient than others. In addition to advancing our basic knowledge of plant-microbe relationships, the project will have broader benefits for society. It will support the economic use of alder species in forestry and restoration, and it will serve as a teaching tool to introduce K-12 students to the hidden world of plant-microbe symbioses. High school and university students will receive hands-on training in advanced lab techniques, helping them develop cutting-edge skills in biology and biotechnology. Trees and shrubs of the genus Alnus (alder) are keystone species in many temperate forests, riparian zones, and successional environments across the Northern Hemisphere. The disproportionate impact on ecosystem productivity and nitrogen cycling is largely due to their symbiosis with nitrogen-fixing Frankia bacteria. While this mutualism is well studied in terms of nitrogen provision, little is known about its broader ecological functions. Particularly, the potential bottom-up effects of Frankia on herbivores, and the top-down effects of herbivory on the Alnus-Frankia interaction, remain elusive. This project investigates the functional role of Frankia within multitrophic networks by studying two sympatric alder species in the Pacific Northwest: red alder (Alnus rubra) and Sitka alder (Alnus alnobetula subsp. sinuata). This work will assess genetic variation, host specificity, foliar chemistry, and herbivore interactions. The work will also test whether aboveground herbivory impacts belowground microbial symbiosis. The specific questions are: 1) How does host specificity, competitive ability and nitrogen-fixation efficiency among Frankia strains in sympatric red alder and Sitka alder vary across spatial and temporal scales? 2) How do different Frankia strains affect chemical leaf traits and aboveground herbivory? 3) What are the ecological costs and benefits of different Frankia strains in nature and how do plant-herbivore interactions influence the diversity of Frankia-alder associations? Answering these questions will develop a new picture of the ecological role of Frankia for alder hosts and associated ecosystems. The work combines state of the art chemical analytics and molecular analyses with laboratory and field bioassays to test our findings in nature. 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.