Mycorrhizal fungi are a fundamental component of nearly all terrestrial ecosystems, creating the critical link between above- and below-ground systems by establishing an intimate association with the roots of >90% of land plants. This relationship provides plant partners with improved nutrient acquisition from the soil, stress tolerance, and pathogen protection and, in most mycorrhizal associations, allows the fungal partner to obtain the photosynthetically fixed carbon sources they need to survive and reproduce. Mycorrhizal fungi impact the function and biodiversity of entire ecosystems, connect unrelated individual plant species, and function as a sink for atmospheric carbon dioxide. Because of the many benefits they provide to plants, mycorrhizal fungi are now widely used in organic agriculture, plant nurseries, and restoration of mine tailings to improve the growth of economically important species. The factors that drive fungi to establish and maintain mycorrhizal associations are critically important to understanding their function in governing biodiversity, nutrient cycling, and plant nutrition but are still very poorly understood. While the economic and ecological value of plant- mycorrhizal interactions has been identified and understood as important from human and ecosystem perspective, interactions between fungi and endobacteria are now at the frontier of understanding plant-mycorrhizal interactions.
Endobacteria in mycorrhizal fungi: A recent breakthrough in understanding what allows fungi to establish mycorrhizal associations with plants has come from the discovery of bacteria living within fungal cells (endobacteria), which are now proposed as drivers of plant-fungus symbioses. There is increasing evidence that bacteria living within fungal cells are widespread among mycorrhizal fungi and that their dynamics may be crucial to development and function of mycorrhizal associations. It was recently revealed that some endobacterial genes are critical to the establishment of mycorrhizal associations and that endobacteria alter the metabolic profile and growth of host fungi, such that without endobacteria fungi respond inappropriately to plant exudates.
Despite these advances, the overall picture of how endobacteria and fungi interact in the mycorrhizal association is still limited and fragmentary. Some types of mycorrhizal fungi, such as orchid mycorrhizal fungi, are only facultatively associated with plants, allowing the potential to generate a much wider array of fungi free of endobacteria. Such a system would facilitate a wide range of experiments elucidating the roles endobacteria play in affecting how fungi function in ecosystems and in interactions with their plant hosts.
Orchid mycorrhizae, an ideal study system: Approximately 10% of all plant species are orchids and >50% of North American orchids are threatened or endangered. Orchids predominantly form mycorrhizal associations with saprotrophic fungi that are common, widespread, and easy to culture, but poorly studied. The Smithsonian Environmental Research Center (SERC) maintains the largest known culture collection of orchid mycorrhizal fungi in the world (>450 fungal isolates from >40 native orchids).
We are working to identify the endobacteria that live within orchid mycorrhizal fungi and to determine how they affect the fungus’ ability to support orchid growth. We have identified the endobacteria within several orchid mycorrhizal fungi and are working to determine how they affect orchid germination (which requires a functional mycorrhizal association) and whether different endobacteria affect mycorrhizal function differently.