Digging Deeper: In Search of the Mechanisms of Carbon and Nitrogen Exchange in Ectomycorrhizal Symbioses

Abstract

Symbiosis with ectomycorrhizal (ECM) fungi is an advantageous partnership for trees in nutrient-limited environments. Ectomycorrhizal fungi colonize the roots of their hosts and improve their access to nutrients, usually nitrogen (N) and, in exchange, trees deliver a significant portion of their photosynthetic carbon (C) to the fungi. This nutrient exchange affects key soil processes and nutrient cycling, as well as plant health, and is therefore central to forest ecosystem functioning. Due to their ecological importance, there is a need to more accurately understand ECM fungal mediated C and N movement within forest ecosystems such that we can better model and predict their role in soil processes both now and under future climate scenarios. There are a number of hurdles that we must overcome, however, before this is achievable such as understanding how the evolutionary history of ECM fungi and their inter- and intra- species variability affect their function. Further, there is currently no generally accepted universal mechanism that appears to govern the flux of nutrients between fungal and plant partners. Here, we consider the current state of knowledge on N acquisition and transport by ECM fungi and how C and N exchange may be related or affected by environmental conditions such as N availability. We emphasize the role that modern genomic analysis, molecular biology techniques and more comprehensive and standardized experimental designs may have in bringing cohesion to the numerous ecological studies in this area and assist us in better understanding this important symbiosis. These approaches will help to build unified models of nutrient exchange and develop diagnostic tools to study these fungi at various scales and environments.

Keywords: carbon cycle; carbon sequestration; ecosystem modelling; mutualism; mycorrhizae; nutrition; symbiosis.

Figure 1

Physiology and phylogeny of ECM fungi (A) Eucalyptus grandis roots colonized by Pisolithus microcarpus and (B) Pinus radiata colonized roots showing the typical morphology of mycorrhizal root tips. (C) Cross section of a mycorrhizal root tip, showing the fungal mantle (M) surrounding the root and ingrowth of fungal hyphae between the root epidermal cells (RC). Fungal hyphae (green) is stained with WGA-FITC. (D) Phylogenetic diversity of the major ECM containing orders of fungi with selected model ECM species (adapted from Kohler et al., 2015 and Nagy et al., 2016).

Figure 2

Major ecosystem parameters affected by or affecting ECM fungi.