Subcellular nutrient element localization and enrichment in ecto- and arbuscular mycorrhizas of field-grown beech and ash trees indicate functional differences.

Jasmin Seven,Andrea Polle,Erika Kothe

doi:10.1371/journal.pone.0114672

Abstract

Mycorrhizas are the chief organ for plant mineral nutrient acquisition. In temperate, mixed forests, ash roots (Fraxinus excelsior) are colonized by arbuscular mycorrhizal fungi (AM) and beech roots (Fagus sylvatica) by ectomycorrhizal fungi (EcM). Knowledge on the functions of different mycorrhizal species that coexist in the same environment is scarce. The concentrations of nutrient elements in plant and fungal cells can inform on nutrient accessibility and interspecific differences of mycorrhizal life forms. Here, we hypothesized that mycorrhizal fungal species exhibit interspecific differences in mineral nutrient concentrations and that the differences correlate with the mineral nutrient concentrations of their associated root cells. Abundant mycorrhizal fungal species of mature beech and ash trees in a long-term undisturbed forest ecosystem were the EcM Lactarius subdulcis, Clavulina cristata and Cenococcum geophilum and the AM Glomus sp. Mineral nutrient subcellular localization and quantities of the mycorrhizas were analysed after non-aqueous sample preparation by electron dispersive X-ray transmission electron microscopy. Cenococcum geophilum contained the highest sulphur, Clavulina cristata the highest calcium levels, and Glomus, in which cations and P were generally high, exhibited the highest potassium levels. Lactarius subdulcis-associated root cells contained the highest phosphorus levels. The root cell concentrations of K, Mg and P were unrelated to those of the associated fungal structures, whereas S and Ca showed significant correlations between fungal and plant concentrations of those elements. Our results support profound interspecific differences for mineral nutrient acquisition among mycorrhizas formed by different fungal taxa. The lack of correlation between some plant and fungal nutrient element concentrations may reflect different retention of mineral nutrients in the fungal part of the symbiosis. High mineral concentrations, especially of potassium, in Glomus sp. suggest that the well-known influence of tree species on chemical soil properties may be related to their mycorrhizal associates.

Highlights

Trees can be considered as ecosystem engineers because they influence soil properties and belowground community structure by species-specific effects, which in turn may have feedback effects on tree functions and vegetation composition [1,2,3,4,5,6]
In the present study we investigated the distribution of nutrient elements between fungal and plant structures in ectomycorrhizal fungi (EcM) and arbuscular mycorrhizal fungi (AM)-colonized root tips of mature beech and ash trees that grow in a long-term undisturbed, deciduous forest (National Park Hainich, Germany)
P was more enriched in cells towards the inner side of the mantle and some intensely stained P-containing granules were observed in the cells (Fig. 2C). In some of these granules, which could be detected by their dark staining in the Scanning transmission electron microscopy (STEM) image (Fig. 2A), S and K were present (Fig. 2D,E), whereas Ca was not detected in them (Fig. 2F)

Summary

Introduction

Trees can be considered as ecosystem engineers because they influence soil properties and belowground community structure by species-specific effects, which in turn may have feedback effects on tree functions and vegetation composition [1,2,3,4,5,6]. Beech litter has a high C-to-N ratio .50, a high lignin content and a slow decomposition rate, whereas ash produces high quality litter with a low C-toN ratio ,30, a low lignin content and a fast decomposition rate [10, 11]. In roots of these tree species nutrient element concentrations differ, with higher concentrations of phosphorus (P), sulphur (S), potassium (K) and magnesium (Mg) and lower concentrations of calcium (Ca) in ash than in beech [12]. Mycorrhizal fungi have profound effects on plant nutrient acquisition, very little information is available on the role of AM for tree nutrition in the temperate region

Methods

Results

Discussion

Conclusion