Influence of arbuscular mycorrhizal mycelial exudates on soil bacterial growth and community structure

Abstract

Plant root systems colonized by arbuscular mycorrhizal (AM) fungi have previously been shown to influence soil bacterial populations; however, the direct influence of the AM extraradical mycelium itself on bacterial growth and community composition is not well understood. In this study, we investigated the effects of exudates produced by AM extraradical mycelia on the growth and development of an extracted soil bacterial community in vitro. The chemical composition of the mycelial exudates was analysed using proton nuclear magnetic resonance spectrometry. Following the addition of exudates to a bacterial community extracted from soil, bacterial growth and vitality were determined using a bacterial vitality stain and fluorescence microscopy. Changes in community composition were also analysed at various times over the course of 3 days by terminal restriction fragment length polymorphism analysis, in combination with cloning and sequencing of 16S rRNA genes. Mycelial exudates increased bacterial growth and vitality and changed bacterial community composition. Several Gammaproteobacteria, including a taxon within the Enterobacteriaceae, increased in frequency of occurrence in response to AM mycelial exudates. This study is the first attempt to identify carbohydrates from the extraradical mycelium of an AM fungus, and demonstrates the direct effects of mycelial exudates on a soil bacterial community.

Migratory Response of Soil Bacteria to Lyophyllum sp. Strain Karsten in Soil Microcosms

“…3). This clustering was primarily based on the utilization of 15 of 18 com-pounds, which have previously been postulated to be often present in fungal exudates (28), in the Biolog assay (14,21,24,25,31). The compounds utilized by B. terrae (BS001) were L-arabinose, D-arabitol, ␣-D-glucose, M-inositol, D-mannitol, D-trehalose, citric acid, D-alanine, L-aspartic acid, L-glutamic acid, L-phenylalanine, L-proline, D-serine, L-threonine, and glycerol.…”

Characterisation of microbial communities colonising the hyphal surfaces of arbuscular mycorrhizal fungi

“…As the communities that remained in solution were also different between compartments containing hyphae or glass wool, it is not unlikely that hyphal exudates have had a role in shaping these communities. A study by Toljander et al (2007), using exudates isolated from AMF hyphae, have also shown a marked influence of AMF hyphal exudates on bacterial community composition. They detected several low-molecular-mass sugars and organic acids as well as some unidentified high-molecularmass compounds in hyphal exudates.…”

“…Like plant roots, also AMF hyphae produce exudates and these exudates might explain the observed differences in soil bacterial communities. It has been found that extracted AMF hyphal exudates can stimulate bacterial growth (Filion et al, 1999) and change the bacterial community composition (Toljander et al, 2007).…”

The role of the arbuscular mycorrhiza-associated rhizobacteria in the biocontrol of soilborne phytopathogens: a review

“…Sood (2003) proved a stronger attraction of Azotobacter chrococcum and P. fluorescens by exudates collected from tomato roots colonized by G. fasciculatum than by exudates collected from non-colonized roots. Recently, a bacterial community extracted from soil was shown to be signif icantly affected after 48 h when inoculated with exudates produced by AM mycelia in comparison to a control composed of culture medium (Toljander et al, 2007). Nonetheless, in soil, reduction in exudation through defoliation of pea plants did not modify the PCR-DGGE profile of rhizosphere bacteria, whereas missing and additional bands where observed from the rhizosphere of plants precolonized with G. intraradices (Vestergård et al, 2008).…”