Morphological traits of ectomycorrhizal fungi in symbiosis

Mycorrhiza are a mutualistic interaction between numerous fungi and approximately 90 % of land plants, making them important for diverse ecosystems. Boreal and temperate forest trees interact with ectomycorrhizal fungi (EMF) that form three characteristic morphological traits: a hyphal mantle around the fine root tips, a network of hyphae between the root cortex cells (the Hartig net) and hyphae reaching into the soil for certain distances (extraradical mycelium). In ectomycorrhizal interactions, fungi gain carbohydrates from the trees, while the trees gain access to mineral nutrients and water. Additionally, current studies underline the role of EMF in protection of trees from biotic and abiotic stressors, such as drought and herbivore attack. Still, the role of mantle, Hartig net and extraradical mycelium in these different aspects of EMF function has not been elucidated.

Here, we study the symbiotic interaction between the basidiomycete EMF Paxillus involutus and the poplar tree Paxillus x canescens in a laboratory system. We employ different P. involutus strains that differ in their ability to form a Hartig net in poplar roots. Using Petri dish mycorrhization experiments, followed by microscopic analysis of root cross sections, we examine the extent to which different strains penetrate the root cortex. These experiments are carried out in different physiological conditions regarding the nutrition status of the fungus and / or the plant partner. We aim to test the impact of Hartig net formation on nutrient exchange and protection from drought stress in potted poplar plants. Further, we perform phosphorus uptake experiments with P. involutus to understand the dependency of nutrient transfer from the substrate to the EMF and further into the plant on Hartig net formation and on environmental conditions. Our results will further the understanding of EMF functions and point out means to conquer the effects of climate change on forest ecosystems.