SUMMARY Bacteria and fungi can form a range of physical associations that depend on various modes of molecular communication for their development and functioning. These bacterial-fungal interactions often result in changes to the pathogenicity or the nutritional influence of one or both partners toward plants or animals (including humans). They can also result in unique contributions to biogeochemical cycles and biotechnological processes. Thus, the interactions between bacteria and fungi are of central importance to numerous biological questions in agriculture, forestry, environmental science, food production, and medicine. Here we present a structured review of bacterial-fungal interactions, illustrated by examples sourced from many diverse scientific fields. We consider the general and specific properties of these interactions, providing a global perspective across this emerging multidisciplinary research area. We show that in many cases, parallels can be drawn between different scenarios in which bacterial-fungal interactions are important. Finally, we discuss how new avenues of investigation may enhance our ability to combat, manipulate, or exploit bacterial-fungal complexes for the economic and practical benefit of humanity as well as reshape our current understanding of bacterial and fungal ecology.

Pseudomonas fluorescens Pf-5, a rhizosphereinhabiting bacterium that suppresses several soilborne pathogens of plants, produces the antibiotics pyrrolnitrin, pyoluteorin, and 2,4-diacetylphloroglucinol. A gene necessary for pyrrolnitrin production by Pf-5 was identified as rpoS, which encodes the stationary-phase sigma factor as. Several pleiotropic effects of an rpoS mutation in Escherichia coli also were observed in an RpoS-mutant of Pf-5. These included sensitivities of stationary-phase cells to stresses imposed by hydrogen peroxide or high salt concentration. A plasmid containing the cloned wild-type rpoS gene restored pyrrolnitrin production and stress tolerance to the RpoS-mutant of Pf-5. The RpoS-mutant overproduced pyoluteorin and 2,4-diacetylphloroglucinol, two antibiotics that inhibit growth of the phytopathogenic fungus Pythium ultimum, and was superior to the wild type in suppression of seedling damping-off of cucumber caused by Pythium ultimum. When inoculated onto cucumber seed at high cell densities, the RpoS-mutant did not survive as well as the wild-type strain on surfaces of developing seedlings. Other stationary-phase-specific phenotypes of Pf-5, such as the production of cyanide and extracellular protease(s) were expressed by the RpoS-mutant, suggesting that a-s is only one of the sigma factors required for the transcription of genes in stationary-phase cells of P. fluorescens. These results indicate that a sigma factor encoded by rpoS influences antibiotic production, biological control activity, and survival of P. fluorescens on plant surfaces.

Summary• The mycorrhiza helper Pseudomonas fluorescens BBc6R8 promotes the presymbiotic survival and growth of the ectomycorrhizal fungus Laccaria bicolor S238N in the soil.• An in vitro fungal-bacterial confrontation bioassay mimicking the promoting effects of the bacteria on fungal growth was set up to analyse the fungal morphological and transcriptional changes induced by the helper bacteria at three successive stages of the interaction. The specificity of the P. fluorescens BBc6R8 effect was assessed in comparison with six other rhizobacterial strains possessing mycorrhiza helper or pathogen antagonistic abilities.• The helper BBc6R8 strain was the only strain to induce increases in the radial growth of the colony, hyphal apex density and branching angle. These morphological modifications were coupled with pleiotropic alterations of the fungal transcriptome, which varied throughout the interaction. Early stage-responsive genes were presumably involved in recognition processes and transcription regulation, while late stage-responsive genes encoded proteins of primary metabolism. Some of the responsive genes were partly specific to the interaction with P. fluorescens BBc6R8, whereas others were mutually regulated by different rhizobacteria.• The results highlight the fact that the helper BBc6R8 strain has a specific priming effect on growth, morphology and gene expression of its fungal associate L. bicolor S238N.

International audience• The decline of take-all disease (Gaeumannomyces graminis var. tritici), which may take place during wheat monocropping, involves plant-protecting, rootcolonizing microorganisms. So far, however, most work has focused on antagonistic fluorescent pseudomonads. Our objective was to assess the changes in rhizobacterial community composition during take-all decline of field-grown wheat. • The study was based on the development and utilization of a taxonomic 16S rRNA-based microarray of 575 probes, coupled with cloning–sequencing and quantitative PCR. Plots from one experimental field grown with wheat for 1 yr (low level of disease), 5 yr (high level of disease) or 10 yr (low level of disease, suppressiveness reached) were used. • Microarray data discriminated between the three stages. The outbreak stage (5 yr) was mainly characterized by the prevalence of Proteobacteria, notably Pseudomonas (Gammaproteobacteria), Nitrosospira (Betaproteobacteria), Rhizobacteriaceae, Sphingomonadaceae, Phyllobacteriaceae (Alphaproteobacteria), as well as Bacteroidetes and Verrucomicrobia. By contrast, suppressiveness (10 yr) correlated with the prevalence of a broader range of taxa, which belonged mainly to Acidobacteria, Planctomycetes, Nitrospira, Chloroflexi, Alphaproteobacteria (notably Azospirillum) and Firmicutes (notably Thermoanaerobacter). • In conclusion, take-all decline correlated with multiple changes in rhizobacterial community composition, far beyond the sole case of pseudomonads

Three global regulators are known to control antibiotic production by Pseudomonas fluorescens. A two-component regulatory system comprised of the sensor kinase GacS (previously called ApdA or LemA) and GacA, a member of the FixJ family of response regulators, is required for antibiotic production. A mutation inrpoS, which encodes the stationary-phase sigma factor ςS, differentially affects antibiotic production and reduces the capacity of stationary-phase cells of P. fluorescens to survive exposure to oxidative stress. ThegacA gene of P. fluorescens Pf-5 was isolated, and the influence of gacS and gacA onrpoS transcription, ςS levels, and oxidative stress response of Pf-5 was determined. We selected a gacAmutant of Pf-5 that contained a single nucleotide substitution within a predicted α-helical region, which is highly conserved among the FixJ family of response regulators. At the entrance to stationary phase, ςS content in gacS and gacAmutants of Pf-5 was less than 20% of the wild-type level. Transcription of rpoS, assessed with anrpoS-lacZ transcriptional fusion, was positively influenced by GacS and GacA, an effect that was most evident at the transition between exponential growth and stationary phase. Mutations ingacS and gacA compromised the capacity of stationary-phase cells of Pf-5 to survive exposure to oxidative stress. The results of this study provide evidence for the predominant roles of GacS and GacA in the regulatory cascade controlling stress response and antifungal metabolite production in P. fluorescens.