Role of fungal trehalose and bacterial thiamine in the improved survival and growth of the ectomycorrhizal fungus Laccaria bicolor S238N and the helper bacterium Pseudomonas fluorescens BBc6R8

Abstract

The mycorrhiza helper bacterial strain Pseudomonas fluorescens BBc6R8 enhances the establishment of Laccaria bicolor S238N ectomycorrhizae by improving the pre-symbiotic growth and survival of the fungus. Nothing is known about the effect of the ectomycorrhizal fungus on the helper bacteria or the molecules that are involved in the interaction. In this study, we have monitored the population density of the helper strain P. fluorescens BBc6R8 in soils inoculated with L. bicolor and in control soils and found that the ectomycorhizal fungus improves the survival of the helper bacteria. We investigated the identity of the fungal and bacterial metabolites involved in this reciprocal growth-promoting effect using a combination of growth measurements, chemoattractant assays, HPLC and in silico genome analyses. We showed that trehalose, a disaccharide that accumulates to high levels in the fungal hyphae, chemoattracted and promoted the growth of the helper bacteria. Meanwhile, P. fluorescens BBc6R8 produced thiamine at concentrations that enhanced the fungal growth in vitro. Altogether our data indicate that the interaction between the two microorganisms is beneficial for both species and relies, at least in part, on trophic mutualism.

Exploring functional contexts of symbiotic sustain within lichen-associated bacteria by comparative omics

“…The greenalgal strain is massively enriched within the fungal structures, while cyanobacterial Nostoc strains are acquired from the surfaces of L. pulmonaria recurrently during the life-time of the thallus to form internal organs devoted to nitrogen fixation in lichens (Hyvärinen et al, 2002, Cornejo andScheidegger, 2013). Conversely, the external presence of other bacteria in the lichen symbiosis recalls the helper bacteria of mycorrhizal symbioses, which provide multiple functions to mutually support and stabilize the root symbioses, including exchange of carbohydrates and vitamin provision (Frey-Klett et al, 2007;Deveau et al, 2010). The long-living lichen thallus is formed by tightly packed fungal hyphae, which are conglutinated by their cell walls.…”

Bacterial-Fungal Interactions: Hyphens between Agricultural, Clinical, Environmental, and Food Microbiologists

“…While diffusible molecules play a significant role in many BFIs, migration and physical contact are also important processes in the establishment of BFIs. Chemotaxis (directed movement) of bacteria toward fungi and fungally derived molecules has been demonstrated in several instances; for example, both detrimental and beneficial Pseudomonas species exhibit taxis toward fungal mycelial exudates (93,139). In the case of the chemotactic response of the biocontrol strain Pseudomonas fluorescens WCS365, Fusarium oxysporum fusaric acid has been identified as an important fungally derived chemotactic signal (95).…”

“…The essential micronutrient thiamine (vitamin B 1 ) has been implicated in the growth promotion exhibited by Bacillus sp. strain TB-1 toward the thermophilic yeast Debaryomyces vanriji (Schwanniomyces vanrijiae) (324) and in the growth-promoting effect of the mycorrhizal helper strain Pseudomonas fluorescens BBc6R8 on the ectomycorrhizal fungus L. bicolor S238N (93). Conversely, ectomycorrhizal fungi may produce organic acids or sugars, such as trehalose, that can affect the composition and growth of associated bacterial communities (116,287).…”

Profiling of differentially expressed genes in ectomycorrhizal fungus Pisolithus tinctorius responding to mycorrhiza helper Brevibacillus reuszeri MPt17

“…Auxofuran, a novel metabolite (Riedlinger et al 2006) and bacterial thiamine (Deveau et al 2010) produced by the MHB were isolated and proven to stimulate the survival and growth of the ECM fungi. More studies should be conducted to isolate effector molecules from the MHB and linke the effector molecules with related functional genes.…”