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NSF 0317144  Orchid-mycorrhizae Interactions - a System for Testing Hypotheses about the Ecological Role and Distribution of Mycorrhizal Fungi in affecting Plant Distribution

Melissa McCormick & Dennis Whigham, Smithsonian Environmental Research Center, MD

Lee Taylor, University of Alaska – Fairbanks, AK

Timothy Filley, Purdue University, IN

Project Summary

This interdisciplinary, collaborative proposal presents a series of hypotheses and experiments to test fundamental components of plant-mycorrhizal interactions. We bring together a team of plant ecologists from the Smithsonian Environmental Research Center and the University of Alaska along with a molecular isotope biogeochemist from Purdue University with collective expertise in the orchid-fungal system, saprotrophic decomposition, and stable isotopes. The methods that we employ in our research permit characterization of the distribution of mycorrhizal fungi in forests to determine whether mycorrhizal fungi can regulate the distribution of plants and the ecological consequences of those interactions. Our focus is the orchid-fungus system. This proposal provides a unique opportunity to address hypotheses about the role of mycorrhizae in regulating the distribution of orchids, thus gaining insight into factors that regulate the distribution of plants in ecosystems and influence the acquisition of carbon. 

Variation in specificity underlies mycorrhizal influences on plant community structure, resilience, nutrient cycling, etc. There is almost no evidence concerning the effects of symbiont distribution on plants. Terrestrial orchids interact with a wide range of mycorrhizal fungi. Some orchids are extreme specialists, while others are generalists, hosting many different fungi. All terrestrial orchids require mycorrhizae and they can only persist when the fungus is present. 

The research that we propose will focus on two broad hypotheses. First we will test the hypothesis that the distribution of terrestrial orchids is limited by the availability of appropriate fungi. This hypothesis will be tested using field and lab experiments in which we manipulate orchid seeds, fungi, and substrates. We will develop new taxon-specific ribosomal and microsatellite primers to determine the spatial distribution of fungi. These primers will provide powerful tools for analysis of population genetics in these poorly known resupinate fungi. The second hypothesis will consider the impact of litter quality on orchid fungal dependence. In the lab, we will investigate the growth of protocorms grown on wood and leaves of varying decomposition stages. In the field and lab, we will subject orchids to water and light stress and examine the impact of each on orchid reliance on photosynthetic versus fungal carbon. 

We have isolated more than 200 orchid mycorrhizae from pelotons and have used DNA sequencing to identify them. To date, none of the fungi that we have identified can be assigned to known species, thus the orchid mycorrhizae represent fungi that are unknown or poorly known to science. Our preliminary research has demonstrated that the stable carbon isotopic signature found in orchids can vary with life cycle characteristics of the orchid species (mycotrophic or photosynthetic) or material utilized by mycorrhizae in the forest (e.g., decomposing wood, decomposing leaves, or tree roots). We have also used molecular methods to demonstrate that orchid mycorrhizae are narrowly distributed, thus potentially limiting orchid distribution. 

Our work will provide fundamental knowledge about orchid-mycorrhizal interactions and the nature of carbon acquisition by orchids under different stress (substrate quality, light, and water) regimes. Additionally, this research will substantially benefit conservation efforts. Orchids are ideal systems for testing the hypothesis that the distribution of fungi influences plant distribution, performance, and abundance because, (1) the very low number of fungal symbionts greatly simplifies testing this hypothesis, and (2) if this hypothesis is not supported in orchids, it is unlikely to be important in other, less specific, plants. 

PCR primers for analysis of orchid fungi that we have developed over the course of this grant are described here.