Ecophysiology of ectomycorrhizal symbioses

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Ectomycorrhizae are ancient symbiotic relationships between fungi and plant roots. Many members of the pine, birch, willow and beech families depend heavily on these fungal partners for adequate uptake of mineral nutrients. In my lab, we look at the dual function of ectomycorrhizal fungi (EMF): as symbionts that can translocate nitrogen (N), phosphorus (P) and other mineral nutrients to their plant partners, and as soil fungi that, by excreting degradative enzymes, contribute to the breakdown of soil organic matter and the cycling of carbon (C), N, P and sulphur (S). Because EMF play a major role in plant nutrient uptake, they will be important in the migration or adaptation of trees in response to environmental change. We plan to investigate whether different EMF communities use different strategies for acquiring P depending on the types of P (organic vs inorganic) that dominant the soils in which they are found. If EMF communities are locally adapted, forest managers will need to manage both the trees and EMF if replanting programs are changed as a result of climate change. One of my students recently observed a major loss of N from pine seedlings when their EMF were supplied with nutrients after a period of starvation. This suggests that N moved from seedling to fungus, which is counter to conventional understanding of the symbiosis. We will test this hypothesis by labelling foliage, as well as hyphae, with 15N and quantify translocation of N in both directions using both mass spectrometry and sophisticated microscopic approaches. If movement of N from plant to EMF is confirmed, we will collaborate with an international expert to study behavior of N transporters at the root-fungal interface in mycorrhizal roots. We will also follow the germination and colonization of pine seedlings in the field and greenhouse to see if we can detect any short-term shift in the direction of N transport after a sudden change in soil nutrient status. This research will challenge the conventional view of N transport in ectomycorrhizal plants especially during seedling establishment, and increase understanding of N transport at the mycorrhizal interface. While it is accepted that EMF break down soil organic matter to access N, P and S, there is much debate about whether EMF meet any of their C needs from soil C, given that they have direct access to recently fixed C in roots of their plant hosts. We will test for saprotrophic behavior of EMF in the lab by growing ectomycorrhizal pine with 13C-labelled soil C. We will quantify the proportion of C acquired from soil vs roots by analyzing C isotope signatures in the phospholipids of the EMF. Through this research, and with the involvement of international collaborators, four honours, three MSc, and two PhD students will be trained in the use of sophisticated microscopic, isotopic and molecular techniques to study how EMF affect seedling establishment and nutrient cycling in Canadian forests.