Endosymbiont-dependent host reproduction maintains bacterial-fungal mutualism

Intimate bacterial-fungal interaction triggers biosynthesis of archetypal polyketides in Aspergillus nidulans

Fungi produce numerous low molecular weight molecules endowed with a multitude of biological activities. However, mining the full-genome sequences of fungi indicates that their potential to produce secondary metabolites is greatly underestimated. Because most of the biosynthesis gene clusters are silent under laboratory conditions, one of the major challenges is to understand the physiological conditions under which these genes are activated. Thus, we cocultivated the important model fungus Aspergillus nidulans with a collection of 58 soil-dwelling actinomycetes. By microarray analyses of both Aspergillus secondary metabolism and full-genome arrays and Northern blot and quantitative RT-PCR analyses, we demonstrate at the molecular level that a distinct fungal-bacterial interaction leads to the specific activation of fungal secondary metabolism genes. Most surprisingly, dialysis experiments and electron microscopy indicated that an intimate physical interaction of the bacterial and fungal mycelia is required to elicit the specific response. Gene knockout experiments provided evidence that one induced gene cluster codes for the long-sought after polyketide synthase (PKS) required for the biosynthesis of the archetypal polyketide orsellinic acid, the typical lichen metabolite lecanoric acid, and the cathepsin K inhibitors F-9775A and F-9775B. A phylogenetic analysis demonstrates that orthologs of this PKS are widespread in nature in all major fungal groups, including mycobionts of lichens. These results provide evidence of specific interaction among microorganisms belonging to different domains and support the hypothesis that not only diffusible signals but intimate physical interactions contribute to the communication among microorganisms and induction of otherwise silent biosynthesis genes.

Inter-kingdom encounters: recent advances in molecular bacterium-fungus interactions

Interactions between bacteria and fungi are well known, but it is often underestimated how intimate and decisive such associations can be with respect to behaviour and survival of each participating organism. In this article we review recent advances in molecular bacterium-fungus interactions, combining the data of different model systems. Emphasis is given to the positive or negative consequences these interactions have on the microbe accommodating plants and animals. Intricate mechanisms of antagonism and tolerance have emerged, being as important for the biological control of plants against fungal diseases as for the human body against fungal infections. Bacterial growth promoters of fungal mycelium have been characterized, and these may as well assist plant-fungus mutualism as disease development in animals. Some of the toxins that have been previously associated with fungi are actually produced by endobacteria, and the mechanisms that lie behind the maintenance of such exquisite endosymbioses are fascinating. Bacteria do cause diseases in fungi, and a synergistic action between bacterial toxins and extracellular enzymes is the hallmark of such diseases. The molecular study of bacterium-fungus associations has expanded our view on microbial communication, and this promising field shows now great potentials in medicinal, agricultural and biotechnological applications.

Global distribution and evolution of a toxinogenic Burkholderia-Rhizopus symbiosis

Toxinogenic endobacteria were isolated from a collection of Rhizopus spp. representing highly diverse geographic origins and ecological niches. All endosymbionts belonged to the Burkholderia rhizoxinica complex according to matrix-assisted laser desorption ionization-time of flight biotyping and multilocus sequence typing, suggesting a common ancestor. Comparison of host and symbiont phylogenies provides insights into possible cospeciation and horizontal-transmission events.

Bacterial/Fungal interactions: from pathogens to mutualistic endosymbionts

A fundamental issue in biology is the question of how bacteria initiate and maintain pathogenic relationships with eukaryotic hosts. Despite billions of years of coexistence, far less is known about bacterial/fungal interactions than the equivalent associations formed by either of these types of microorganisms with higher eukaryotes. This review highlights recent research advances in the field of bacterial/fungal interactions, and provides examples of the various forms such interactions may assume, ranging from simple antagonism and parasitism to more intimate associations of pathogenesis and endosymbiosis. Information derived from the associations of bacteria and fungi in the context of natural and agronomic ecosystems is emphasized, including interactions observed from biological control systems, endosymbiotic relationships, diseases of cultivated mushrooms, and model systems that expand our understanding of human disease. The benefits of studying these systems at the molecular level are also emphasized.

Metabolites from symbiotic bacteria

This review describes secondary metabolites that have been shown to be synthesized by symbiotic bacteria, or for which this possibility has been discussed. It includes 365 references.

Detection and identification of bacteria intimately associated with fungi of the order Sebacinales

Because of their beneficial impact on plants, the highly diverse mycorrhizal fungi grouped in the order Sebacinales lay claim to high ecological and agricultural significance. Here, we describe for the first time associations of Sebacinoid members with bacteria. Using quantitative PCR, denaturating gradient gel electrophoresis and fluorescence in situ hybridization, we detected an intimate association between Piriformospora indica and Rhizobium radiobacter, an alpha-Proteobacterium. The stability of the association, vertical transmission of the bacteria during asexual fungal reproduction and fungal plant colonization was monitored using R. radiobacter-specific primers. Treatment of mycelium or fungal protoplasts with antibiotics highly efficient against the free bacteria failed to cure the fungus. Barley seedlings dip-inoculated with R. radiobacter showed growth promotion and systemic resistance to the powdery mildew fungus Blumeria graminis comparable to P. indica inoculation. By screening additional isolates of the Sebacina vermifera complex, three species-specific associations with bacteria from the genera Paenibacillus, Acinetobacter and Rhodococcus were found. These findings suggest that Sebacinales species regularly undergo complex interactions involving host plants and bacteria reminiscent of other ectomycorrhizal and endomycorrhizal associations.

Fungal viral mutualism moderated by ploidy

Endosymbionts and their hosts have inherently ambiguous relationships as symbionts typically depend upon their hosts for shelter, nutrition, and reproduction. Endosymbionts can acquire these needs by two alternative strategies: exploitation and cooperation. Parasites exploit hosts to advance their own reproduction at the cost of host fitness. In contrast, mutualists increase their reproductive output by increasing host fitness. Very often the distinction between parasites and mutualists is not discrete but rather contingent on the environment in which the interaction occurs, and can shift along a continuous scale from parasitism to mutualism. The cost benefit dynamics at any point along this continuum are of particular interest as they establish the likelihood of an interaction persisting or breaking down. Here we show how the interaction between the yeast Saccharomyces cerevisiae and an endosymbiotic killer virus is strongly dependent on both host ploidy and environmental pH. Additionally we elucidate the mechanisms underlying the ploidy-dependent interaction. Understanding these dynamics in the short-term is key to understanding how genetic and environmental factors impact community diversity.

Burkholderia rhizoxinica sp. nov. and Burkholderia endofungorum sp. nov., bacterial endosymbionts of the plant-pathogenic fungus Rhizopus microsporus

Several strains of the fungus Rhizopus microsporus harbour endosymbiotic bacteria for the production of the causal agent of rice seedling blight, rhizoxin, and the toxic cyclopeptide rhizonin. R. microsporus and isolated endobacteria were selected for freeze-fracture electron microscopy, which allowed visualization of bacterial cells within the fungal cytosol by their two parallel-running envelope membranes and by the fine structure of the lipopolysaccharide layer of the outer membrane. Two representatives of bacterial endosymbionts were chosen for phylogenetic analyses on the basis of full 16S rRNA gene sequences, which revealed that the novel fungal endosymbionts formed a monophyletic group within the genus Burkholderia. Inter-sequence similarities ranged from 98.94 to 100%, and sequence similarities to members of the Burkholderia pseudomallei group, the closest neighbours, were 96.74-97.38%. In addition, the bacterial strains were distinguished from their phylogenetic neighbours by their fatty acid profiles and other biochemical characteristics. The phylogenetic studies based on 16S rRNA gene sequence data, together with conclusive DNA-DNA reassociation experiments, strongly support the proposal that these strains represent two novel species within the genus Burkholderia, for which the names Burkholderia rhizoxinica sp. nov. (type strain, HKI 454T=DSM 19002T=CIP 109453T) and Burkholderia endofungorum sp. nov. (type strain, HKI 456T=DSM 19003T=CIP 109454T) are proposed.

Bacterial mycophagy: definition and diagnosis of a unique bacterial-fungal interaction

This review analyses the phenomenon of bacterial mycophagy, which we define as a set of phenotypic behaviours that enable bacteria to obtain nutrients from living fungi and thus allow the conversion of fungal into bacterial biomass. We recognize three types of bacterial strategies to derive nutrition from fungi: necrotrophy, extracellular biotrophy and endocellular biotrophy. Each is characterized by a set of uniquely sequential and differently overlapping interactions with the fungal target. We offer a detailed analysis of the nature of these interactions, as well as a comprehensive overview of methodologies for assessing and quantifying their individual contributions to the mycophagy phenotype. Furthermore, we discuss future prospects for the study and exploitation of bacterial mycophagy, including the need for appropriate tools to detect bacterial mycophagy in situ in order to be able to understand, predict and possibly manipulate the way in which mycophagous bacteria affect fungal activity, turnover, and community structure in soils and other ecosystems.

The evolution of obligate mutualism: if you can't beat 'em, join 'em

Wolbachia is best known as a facultative endosymbiotic parasite, manipulating host reproduction. However, it has also evolved as an obligate mutualist at least twice. In a recent paper, Pannebakker et al. identify a possible mechanism for such a transition from facultative parasitism to obligate mutualism in a parasitic wasp in which Wolbachia are required for producing eggs (oogenesis). Their proposed mechanism suggests that compensatory evolution in the host to counter the harmful effects of Wolbachia is the basis of this evolutionary transition.

A cryptic PKS-NRPS gene locus in the plant commensal Pseudomonas fluorescens Pf-5 codes for the biosynthesis of an antimitotic rhizoxin complex

Targeted gene inactivation and metabolic profiling revealed that the cryptic PKS-NRPS gene cluster in the genome of the plant commensal Pseudomonas fluorescens Pf-5 codes for the biosynthesis of antiproliferative and antifungal rhizoxin derivatives.