Abstract
Root-fungal symbioses such as mycorrhizas and endophytes are key components of terrestrial ecosystems. Diverse in trophy habits (obligate, facultative or hemi-biotrophs) and symbiotic relations (from mutualism to parasitism), these associations also show great variability in their root colonization and nutritional strategies. Specialized interface structures such as arbuscules and Hartig nets are formed by certain associations while others are restricted to non-specialized intercellular or intracellular hyphae in roots. In either case, there are documented examples of active nutrient exchange, reinforcing the fact that specialized structures used to define specific mycorrhizal associations are not essential for reciprocal exchange of nutrients and plant growth promotion. In feremycorrhiza (with Austroboletus occidentalis and eucalypts), the fungal partner markedly enhances plant growth and nutrient acquisition without colonizing roots, emphasizing that a conventional focus on structural form of associations may have resulted in important functional components of rhizospheres being overlooked. In support of this viewpoint, mycobiome studies using the state-of-the-art DNA sequencing technologies have unearthed much more complexity in root-fungal relationships than those discovered using the traditional morphology-based approaches. In this review, we explore the existing literature and most recent findings surrounding structure, functioning, and ecology of root-fungal symbiosis, which highlight the fact that plant fitness can be altered by taxonomically/ecologically diverse fungal symbionts regardless of root colonization and interface specialization. Furthermore, transition from saprotrophy to biotrophy seems to be a common event that occurs in diverse fungal lineages (consisting of root endophytes, soil saprotrophs, wood decayers etc.), and which may be accompanied by development of specialized interface structures and/or mycorrhiza-like effects on plant growth and nutrition.
Highlights
Terrestrial plants moved from aquatic environments to emerged land about 400 million years ago, when the incipient soil was severely deficient in nutrients and organic matter
We examine a range of symbiotic relationships between soil-inhabiting fungi and plant roots, focusing primarily on root colonization and interface structures, to address the question of whether the focus on interface structures has limited our understanding of the wider diversity of these symbioses
Our review of the current literature suggests that the potential exchange of nutrients/metabolites and beneficial interactions in plant-fungal relationships can occur via specialized interface structures or simple non-specialized hyphae inside or outside roots
Summary
Terrestrial plants moved from aquatic environments to emerged land about 400 million years ago, when the incipient soil was severely deficient in nutrients and organic matter. The mycorrhizal structures are very similar to those of ECM and the fungal species involved are capable of forming ECM associations, so the plant species often determines the type of the association present in a given area of forest In these associations, a hyphal mantle covers the root surface but might be missing, and a Hartig net is usually restricted to epidermal and outer cortical cells due to the presence of suberin deposits and Casparian strips in the outer cortex of host roots (Münzenberger, 1991). FRE are ubiquitous in harsh environments (Crush, 1973; Postma, Olsson & Falkengren-Grerup, 2007; Newsham, Upson & Read, 2009; Orchard et al, 2016), and it would be interesting to explore their role in functionality of stressful ecosystems such as those with high altitude, soil acidity or cold temperatures
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