Novel root-fungus symbiosis in Ericaceae: sheathed ericoid mycorrhiza formed by a hitherto undescribed basidiomycete with affinities to Trechisporales.

Martin Vohník,Rolf Nestby,Zuzana Lhotáková,Jesse J Sadowsky,Miroslav Kolařík,Petr Kohout,Minna-Maarit Kytöviita

doi:10.1371/journal.pone.0039524

Abstract

Ericaceae (the heath family) are widely distributed calcifuges inhabiting soils with inherently poor nutrient status. Ericaceae overcome nutrient limitation through symbiosis with ericoid mycorrhizal (ErM) fungi that mobilize nutrients complexed in recalcitrant organic matter. At present, recognized ErM fungi include a narrow taxonomic range within the Ascomycota, and the Sebacinales, basal Hymenomycetes with unclamped hyphae and imperforate parenthesomes. Here we describe a novel type of basidiomycetous ErM symbiosis, termed ‘sheathed ericoid mycorrhiza’, discovered in two habitats in mid-Norway as a co-dominant mycorrhizal symbiosis in Vaccinium spp. The basidiomycete forming sheathed ErM possesses clamped hyphae with perforate parenthesomes, produces 1- to 3-layer sheaths around terminal parts of hair roots and colonizes their rhizodermis intracellularly forming hyphal coils typical for ErM symbiosis. Two basidiomycetous isolates were obtained from sheathed ErM and molecular and phylogenetic tools were used to determine their identity; they were also examined for the ability to form sheathed ErM and lignocellulolytic potential. Surprisingly, ITS rDNA of both conspecific isolates failed to amplify with the most commonly used primer pairs, including ITS1 and ITS1F + ITS4. Phylogenetic analysis of nuclear LSU, SSU and 5.8S rDNA indicates that the basidiomycete occupies a long branch residing in the proximity of Trechisporales and Hymenochaetales, but lacks a clear sequence relationship (>90% similarity) to fungi currently placed in these orders. The basidiomycete formed the characteristic sheathed ErM symbiosis and enhanced growth of Vaccinium spp. in vitro, and degraded a recalcitrant aromatic substrate that was left unaltered by common ErM ascomycetes. Our findings provide coherent evidence that this hitherto undescribed basidiomycete forms a morphologically distinct ErM symbiosis that may occur at significant levels under natural conditions, yet remain undetected when subject to amplification by ‘universal’ primers. The lignocellulolytic assay suggests the basidiomycete may confer host adaptations distinct from those provisioned by the so far investigated ascomycetous ErM fungi.

Highlights

Ericaceae comprise approx. 3000 species accommodated in 100 genera which are native to all continents except Antarctica
We report i) sheathed ericoid mycorrhizal (ErM) colonization levels in the semi-natural Vaccinium plantation and in a natural mixed population of Ericaceae in the adjacent forest, ii) morphological, anatomical, and ultrastructural characteristics of sheathed ErM, iii) the identity of the mycobiont forming sheathed ErM as determined by phylogenetic analyses of three rDNA genes, iv) results of a series of in vitro experiments evaluating the ability of the mycobiont to form sheathed ErM and its impact on the growth of ericaceous and ectomycorrhizal host plants, and v) the lignocellulolytic potential of the basidiomycetous mycobiont relative to that of commonly isolated ErM ascomycetes
Its most prominent characteristic is a dense layer of clamp-bearing hyphae over the surface of terminal parts of young hair roots (Fig. 1a–e)

Summary

Introduction

Ericaceae comprise approx. 3000 species accommodated in 100 genera which are native to all continents except Antarctica. Ericaceae display great morphological and ecological plasticity, including such different forms as Himalayan tree dominants, inconspicuous epiphytic species inhabiting tropical forests, or resilient subarctic dwarf shrubs. They include potentially invasive species, e.g., Gaultheria shallon in western coast of Canada, valued ornamentals, e.g., the genus Rhododendron, and commercially important crops, e.g., cultivated highbush blueberry, Vaccinium corymbosum. Extraradical mycelia of ErM fungi do not reach more than a few mm from the root surface into the surrounding soil [4] and, unlike ectomycorrhizae, ericoid mycorrhizae typically lack multilayered hyphal mantles, with exceptions including the superficial hyphal sheaths reported on Gaultheria procumbens by Massicotte et al [5] and the Andean clade of Ericaceae that hosts cavendishioid ectendomycorrhizae [6,7]

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