Pseudomonas fluorescens BBc6R8 type III secretion mutants no longer promote ectomycorrhizal symbiosis

Abstract

The Mycorrhiza Helper Bacterium (MHB) Pseudomonas fluorescens BBc6R8 promotes the ectomycorrhizal symbiosis between Douglas fir roots and Laccaria bicolor. In this study, we identified a non-flagellar type III secretion system (T3SS) in the draft genome of BBc6R8 similar to that described in the biocontrol strain P. fluorescens SBW25. We examined whether this T3SS plays a role in the BBc6R8 mycorrhizal helper effect by creating a deletion in the rscRST genes encoding the central channel of the injectisome. The in vitro effect of BBc6R8 T3SS mutants on the radial growth rate of L. bicolor was unchanged compared with the parental strain. In contrast, T3SS mutants were unable to promote mycorrhization, suggesting that type III secretion plays an important role in the mycorrhizal helper effect of P. fluorescens BBc6R8 independent of the promotion of hyphal growth that BBc6R8 exhibits in vitro.

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

“…Surprisingly, evidence for the translocation of T3SS effectors into the fungal cytoplasm does not exist; however, two recent studies employing T3SS mutants suggested that these systems may play a role in BFIs (87,206). Cusano et al (87) demonstrated that a disruption of the T3SS of Pseudomonas fluorescens BBc6R8 abolished its mycorrhizal helper effect, although whether the plant of the fungus was the T3SS target is unknown. Lackner et al (206) found that the disruption of a T3SS gene cluster in the endobacterium Burkholderia rhizoxinica resulted in a reduced intrahyphal survival of the bacterium and a failure to elicit the sporulation of the Rhizopus microsporus host.…”

“…The heterologous expression of T3SS effector proteins from a range of plant-and animal-pathogenic bacteria in S. cerevisiae has been used successfully to identify their potential cellular functions (368). Surprisingly, evidence for the translocation of T3SS effectors into the fungal cytoplasm does not exist; however, two recent studies employing T3SS mutants suggested that these systems may play a role in BFIs (87,206). Cusano et al (87) demonstrated that a disruption of the T3SS of Pseudomonas fluorescens BBc6R8 abolished its mycorrhizal helper effect, although whether the plant of the fungus was the T3SS target is unknown.…”

Distinct Ectomycorrhizospheres Share Similar Bacterial Communities as Revealed by Pyrosequencing-Based Analysis of 16S rRNA Genes

“…Apart from its physical and chemical characteristics, which differ from those of the rhizosphere (26, 28), the ectomycorrhizosphere is characterized by diverse fungal and bacterial communities that inhabit the same environment. Consequently, the functioning of the ectomycorrhizal symbiosis is influenced by each partner of the ectomycorrhizal (ECM) complex (6,8, 9).Cultivation-dependent and -independent studies have demonstrated the structuring effect of the mycorrhizal fungi on the soil bacterial communities. They revealed that the bacterial communities colonizing the ectomycorrhizal roots differed from those of uncolonized roots and that the ectomycorrhizal species differentially impacted the structure of ectomycorrhizosphere bacterial communities (2,11,12,22,35,37).…”

“…Apart from its physical and chemical characteristics, which differ from those of the rhizosphere (26,28), the ectomycorrhizosphere is characterized by diverse fungal and bacterial communities that inhabit the same environment. Consequently, the functioning of the ectomycorrhizal symbiosis is influenced by each partner of the ectomycorrhizal (ECM) complex (6,8,9).…”

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

“…Many studies have focused on the physiological aspects of mycorrhizal interactions between plant root systems and fungi. The mechanisms of MHB stimulating mycorrhiza formation, however, still remain unclear (Deveau et al 2007;Cusano et al 2011). Especially, the molecular mechanisms of the interaction between MHB and mycorrhiza fungi are poorly understood.…”