Mosaic genome of endobacteria in arbuscular mycorrhizal fungi: Transkingdom gene transfer in an ancient mycoplasma-fungus association

Interspecific cross-talk: The catalyst driving microbial biosynthesis of secondary metabolites

Microorganisms co-exist and co-evolve in nature, forming intricate ecological communities. The interspecies cross-talk within these communities creates and sustains their great biosynthetic potential, making them an important source of natural medicines and high-value-added chemicals. However, conventional investigations into microbial metabolites are typically carried out in pure cultures, resulting in the absence of specific activating factors and consequently causing a substantial number of biosynthetic gene clusters to remain silent. This, in turn, hampers the in-depth exploration of microbial biosynthetic potential and frequently presents researchers with the challenge of rediscovering compounds. In response to this challenge, the coculture strategy has emerged to explore microbial biosynthetic capabilities and has shed light on the study of cross-talk mechanisms. These elucidated mechanisms will contribute to a better understanding of complex biosynthetic regulations and offer valuable insights to guide the mining of secondary metabolites. This review summarizes the research advances in microbial cross-talk mechanisms, with a particular focus on the mechanisms that activate the biosynthesis of secondary metabolites. Additionally, the instructive value of these mechanisms for developing strategies to activate biosynthetic pathways is discussed. Moreover, challenges and recommendations for conducting in-depth studies on the cross-talk mechanisms are presented.

Signatures of Mollicutes-related endobacteria in publicly available Mucoromycota genomes

Mucoromycota fungi and their Mollicutes-related endobacteria (MRE) are an ideal system for studying bacterial-fungal interactions and evolution due to the long-term and intimate nature of their interactions. However, methods for detecting MRE face specific challenges due to the poor representation of MRE in sequencing databases coupled with the high sequence divergence of their genomes, making traditional similarity searches unreliable. This has precluded estimations on the diversity of MRE associated with Mucoromycota. To determine the prevalence of previously undetected MRE in fungal genome sequences, we scanned 389 Mucoromycota genome assemblies available from the National Center for Biotechnology Information for the presence of MRE sequences using publicly available tools to map contigs from fungal assemblies to publicly available MRE genomes. We demonstrate a higher diversity of MRE genomes than previously described in Mucoromycota and a lack of cophylogeny between MRE and the majority of their fungal hosts. This supports the late invasion hypothesis regarding MRE acquisition across most of the examined fungal families. In contrast with other Mucoromycota lineages, MRE from the Gigasporaceae displayed some degree of cophylogeny with their hosts, which may indicate that horizontal transmission is restricted between members of this family or that transmission is strictly vertical. These results underscore the need for a refined process to capture sequencing data from potential fungal endosymbionts to discern their evolution and transmission. Screens of fungal genomes for MRE can help improve the quality of fungal genome assemblies while identifying new MRE lineages to further test hypotheses on their origin and evolution.IMPORTANCEMollicutes-related endobacteria (MRE) are obligate intracellular bacteria found within Mucoromycota fungi. Despite their frequent detection, MRE roles in host functioning are still unknown. Comparative genomic investigations can improve our understanding of the impact of MRE on their fungal hosts by identifying similarities and differences in MRE genome evolution. However, MRE genomes have only been assembled from a small fraction of Mucoromycota hosts. Here, we demonstrate that MRE can be present yet undetected in publicly available Mucoromycota genome assemblies. We use these newfound sequences to assess the broader diversity of MRE and their phylogenetic relationships with respect to their hosts. We demonstrate that publicly available tools can be used to extract novel MRE sequences from assembled fungal genomes leading to insights on MRE evolution. This work contributes to a greater understanding of the fungal microbiome, which is crucial to improving knowledge on the dynamics and impacts of fungi in microbial ecosystems.

Cross-kingdom nutrient exchange in the plant-arbuscular mycorrhizal fungus-bacterium continuum

The association between plants and arbuscular mycorrhizal fungi (AMF) affects plant performance and ecosystem functioning. Recent studies have identified AMF-associated bacteria as cooperative partners that participate in AMF-plant symbiosis: specific endobacteria live inside AMF, and hyphospheric bacteria colonize the soil that surrounds the extraradical hyphae. In this Review, we describe the concept of a plant-AMF-bacterium continuum, summarize current advances and provide perspectives on soil microbiology. First, we review the top-down carbon flow and the bottom-up mineral flow (especially phosphorus and nitrogen) in this continuum, as well as how AMF-bacteria interactions influence the biogeochemical cycling of nutrients (for example, carbon, phosphorus and nitrogen). Second, we discuss how AMF interact with hyphospheric bacteria or endobacteria to regulate nutrient exchange between plants and AMF, and the possible molecular mechanisms that underpin this continuum. Finally, we explore future prospects for studies on the hyphosphere to facilitate the utilization of AMF and hyphospheric bacteria in sustainable agriculture.

Assembly and comparative analyses of the Geosiphon pyriformis metagenome

Geosiphon pyriformis, a representative of the fungal sub-phylum Glomeromycotina, is unique in its endosymbiosis with cyanobacteria within a fungal cell. This symbiotic relationship occurs in bladders containing nuclei of G. pyriformis, Mollicutes-like bacterial endosymbionts (MRE), and photosynthetically active and dividing cells of Nostoc punctiforme. Recent genome analyses have shed light on the biology of G. pyriformis, but the genome content and biology of its endosymbionts remain unexplored. To fill this gap, we gathered and examined metagenomic data from the bladders of G. pyriformis, where N. punctiforme and MRE are located. This ensures that our analyses are focused on the organs directly involved in the symbiosis. By comparing this data with the genetic information of related cyanobacteria and MREs from other species of Arbuscular Mycorrhizal Fungi, we aimed to reveal the genetic content of these organisms and understand how they interact at a genetic level to establish a symbiotic relationship. Our analyses uncovered significant gene expansions in the Nostoc endosymbiont, particularly in mobile elements and genes potentially involved in xenobiotic degradation. We also confirmed that the MRE of Glomeromycotina are monophyletic and possess a highly streamlined genome. These genomes show dramatic differences in both structure and content, including the presence of enzymes involved in environmental sensing and stress response.

Genome analysis and hyphal movement characterization of the hitchhiker endohyphal Enterobacter sp. from Rhizoctonia solani

Bacterial-fungal interactions are pervasive in the rhizosphere. While an increasing number of endohyphal bacteria have been identified, little is known about their ecology and impact on the associated fungal hosts and the surrounding environment. In this study, we characterized the genome of an Enterobacter sp. Crenshaw (En-Cren), which was isolated from the generalist fungal pathogen Rhizoctonia solani, and examined the genetic potential of the bacterium with regard to the phenotypic traits associated with the fungus. Overall, the En-Cren genome size was typical for members of the genus and was capable of free-living growth. The genome was 4.6 MB in size, and no plasmids were detected. Several prophage regions and genomic islands were identified that harbor unique genes in comparison with phylogenetically closely related Enterobacter spp. Type VI secretion system and cyanate assimilation genes were identified from the bacterium, while some common heavy metal resistance genes were absent. En-Cren contains the key genes for indole-3-acetic acid (IAA) and phenylacetic acid (PAA) biosynthesis, and produces IAA and PAA in vitro, which may impact the ecology or pathogenicity of the fungal pathogen in vivo. En-Cren was observed to move along hyphae of R. solani and on other basidiomycetes and ascomycetes in culture. The bacterial flagellum is essential for hyphal movement, while other pathways and genes may also be involved.IMPORTANCEThe genome characterization and comparative genomics analysis of Enterobacter sp. Crenshaw provided the foundation and resources for a better understanding of the ecology and evolution of this endohyphal bacteria in the rhizosphere. The ability to produce indole-3-acetic acid and phenylacetic acid may provide new angles to study the impact of phytohormones during the plant-pathogen interactions. The hitchhiking behavior of the bacterium on a diverse group of fungi, while inhibiting the growth of some others, revealed new areas of bacterial-fungal signaling and interaction, which have yet to be explored.

Biofilms communities in the soil: characteristic and interactions using mathematical model

There are many different kinds of microorganisms in the soil, and many of them are biofilms because they can make supracellular compounds. Surface-associated microorganisms in a biofilm are encased in a hydrated extracellular polymeric substance that aids in adherence and survival. Numerous different kinds of microorganisms call the soil home. Strong interactions with and among species are made possible by biofilms; this, in turn, might increase the effectiveness with which organic compounds and poisons in soil are degraded. This encouraged us to take a close look at soil biofilm ecosystems, which we do in this paper. In this research, we will look at how soil biofilms arise and how that affects the composition of microbial communities and their function in the soil. Recent years have seen an uptick in interest in questions about biofilm structure and the social interactions of various bacteria. Many concepts elucidating the underlying mathematics of biofilm growth are also presented. Since biofilms are so widespread, this breakthrough in soil biofilm inquiry might help scientists understand soil microbiomes better. Mathematical models further extrapolate the relationships between microbial communities and gives a more precise information as to what is happening in a biofilm. Biofilms can help plants cope with a variety of environmental challenges. Soil quality, plant nourishment, plant protection, bioremediation, and climate change are all influenced by the interplay of biofilm communities. Thus, biofilms play an important role in the development of environmentally friendly and sustainable agriculture.

Hyphosphere microorganisms facilitate hyphal spreading and root colonization of plant symbiotic fungus in ammonium-enriched soil

Anthropogenic nitrogen inputs lead to a high ammonium (NH4+)/nitrate (NO3-) ratio in the soil, which restricts hyphal spreading of soil fungi. Access of symbiotic fungi to roots is a prerequisite for plant-fungal interactions. Hyphosphere bacteria protect fungi from environmental stress, yet the impact of hyphosphere bacteria on adaptation of host fungi to NH4+-enriched conditions remains unclear. By developing soil microcosm assays, we report that a plant-symbiotic fungus, Phomopsis liquidambaris, harbors specific hyphosphere bacteria that facilitate hyphal spreading and assist in the root colonization in NH4+-enriched soil. Genetic manipulation, 16S rRNA gene analysis and coinoculation assays revealed that the genus Enterobacter was enriched in the hyphosphere of NH4+-sensitive wild-type compared to NH4+-preferring nitrite reductase-deficient strain. The representative Enterobacter sp. SZ2-promoted hyphal spreading is only evident in nonsterilized soil. We further identified an increased abundance and diversity of ammonia-oxidizing archaea (AOA) and a synchronously decreased NH4+:NO3- ratio following SZ2 inoculation. Microbial supplementation and inhibitor assays showed that AOA-mediated reduction in NH4+:NO3- ratio is responsible for SZ2-enhanced fungal adaptation to NH4+-enriched conditions. The Ph. liquidambaris-Enterobacter-AOA triple interaction promoted rice growth in NH4+-enriched soil. Our study reveals the essential role of hyphosphere microorganism-based hyphal spreading in plant-fungal symbiosis establishment within nitrogen-affected agroecosystems.

Comparative genomics of Mollicutes-related endobacteria supports a late invasion into Mucoromycota fungi

Diverse members of early-diverging Mucoromycota, including mycorrhizal taxa and soil-associated Mortierellaceae, are known to harbor Mollicutes-related endobacteria (MRE). It has been hypothesized that MRE were acquired by a common ancestor and transmitted vertically. Alternatively, MRE endosymbionts could have invaded after the divergence of Mucoromycota lineages and subsequently spread to new hosts horizontally. To better understand the evolutionary history of MRE symbionts, we generated and analyzed four complete MRE genomes from two Mortierellaceae genera: Linnemannia (MRE-L) and Benniella (MRE-B). These genomes include the smallest known of fungal endosymbionts and showed signals of a tight relationship with hosts including a reduced functional capacity and genes transferred from fungal hosts to MRE. Phylogenetic reconstruction including nine MRE from mycorrhizal fungi revealed that MRE-B genomes are more closely related to MRE from Glomeromycotina than MRE-L from the same host family. We posit that reductions in genome size, GC content, pseudogene content, and repeat content in MRE-L may reflect a longer-term relationship with their fungal hosts. These data indicate Linnemannia and Benniella MRE were likely acquired independently after their fungal hosts diverged from a common ancestor. This work expands upon foundational knowledge on minimal genomes and provides insights into the evolution of bacterial endosymbionts.

The endohyphal microbiome: current progress and challenges for scaling down integrative multi-omic microbiome research

As microbiome research has progressed, it has become clear that most, if not all, eukaryotic organisms are hosts to microbiomes composed of prokaryotes, other eukaryotes, and viruses. Fungi have only recently been considered holobionts with their own microbiomes, as filamentous fungi have been found to harbor bacteria (including cyanobacteria), mycoviruses, other fungi, and whole algal cells within their hyphae. Constituents of this complex endohyphal microbiome have been interrogated using multi-omic approaches. However, a lack of tools, techniques, and standardization for integrative multi-omics for small-scale microbiomes (e.g., intracellular microbiomes) has limited progress towards investigating and understanding the total diversity of the endohyphal microbiome and its functional impacts on fungal hosts. Understanding microbiome impacts on fungal hosts will advance explorations of how "microbiomes within microbiomes" affect broader microbial community dynamics and ecological functions. Progress to date as well as ongoing challenges of performing integrative multi-omics on the endohyphal microbiome is discussed herein. Addressing the challenges associated with the sample extraction, sample preparation, multi-omic data generation, and multi-omic data analysis and integration will help advance current knowledge of the endohyphal microbiome and provide a road map for shrinking microbiome investigations to smaller scales. Video Abstract.

Sequencing the Genomes of the First Terrestrial Fungal Lineages: What Have We Learned?

The first genome sequenced of a eukaryotic organism was for Saccharomyces cerevisiae, as reported in 1996, but it was more than 10 years before any of the zygomycete fungi, which are the early-diverging terrestrial fungi currently placed in the phyla Mucoromycota and Zoopagomycota, were sequenced. The genome for Rhizopus delemar was completed in 2008; currently, more than 1000 zygomycete genomes have been sequenced. Genomic data from these early-diverging terrestrial fungi revealed deep phylogenetic separation of the two major clades-primarily plant-associated saprotrophic and mycorrhizal Mucoromycota versus the primarily mycoparasitic or animal-associated parasites and commensals in the Zoopagomycota. Genomic studies provide many valuable insights into how these fungi evolved in response to the challenges of living on land, including adaptations to sensing light and gravity, development of hyphal growth, and co-existence with the first terrestrial plants. Genome sequence data have facilitated studies of genome architecture, including a history of genome duplications and horizontal gene transfer events, distribution and organization of mating type loci, rDNA genes and transposable elements, methylation processes, and genes useful for various industrial applications. Pathogenicity genes and specialized secondary metabolites have also been detected in soil saprobes and pathogenic fungi. Novel endosymbiotic bacteria and viruses have been discovered during several zygomycete genome projects. Overall, genomic information has helped to resolve a plethora of research questions, from the placement of zygomycetes on the evolutionary tree of life and in natural ecosystems, to the applied biotechnological and medical questions.

Emergence of the fungal immune system

Investigation of fungal biology has been frequently motivated by the fact that many fungal species are important plant and animal pathogens. Such efforts have contributed significantly toward our understanding of fungal pathogenic lifestyles (virulence factors and strategies) and the interplay with host immune systems. In parallel, work on fungal allorecognition systems leading to the characterization of fungal regulated cell death determinants and pathways, has been instrumental for the emergent concept of fungal immunity. The uncovered evolutionary trans-kingdom parallels between fungal regulated cell death pathways and innate immune systems incite us to reflect further on the concept of a fungal immune system. Here, I briefly review key findings that have shaped the fungal immunity paradigm, providing a perspective on what I consider its most glaring knowledge gaps. Undertaking to fill such gaps would establish firmly the fungal immune system inside the broader field of comparative immunology.

On Holobionts, Holospecies, and Holoniches: the Role of Microbial Symbioses in Ecology and Evolution

My goal in writing this is to increase awareness of the roles played by microbial symbionts in eukaryote ecology and evolution. Most eukaryotes host one or more species of symbiotic microorganisms, including prokaryotes and fungi. Many of these have profound impacts on the biology of their hosts. For example, microbial symbionts may expand the niches of their hosts, cause rapid adaptation of the host to the environment and re-adaptation to novel conditions via symbiont swapping, facilitate speciation, and fundamentally alter our concept of the species. In some cases, microbial symbionts and multicellular eukaryote hosts have a mutual dependency, which has obvious conservation implications. Hopefully, this contribution will stimulate a reevaluation of important ecological and evolutionary concepts including niche, adaptation, the species, speciation, and conservation of multicellular eukaryotes.

Investigating Endobacteria that Thrive Within Mucoromycota

Metagenomics approaches have revealed the importance of Mucoromycota in the evolution and functioning of plant microbiomes. Comprised of three subphyla (Glomeromycotina, Mortierellomycotina, and Mucoromycotina), this early diverging lineage of fungi encompasses species of mycorrhizal fungi, root endophytes, plant pathogens, and many decomposers of plant debris. Interestingly, several taxa of Mucoromycota share a common feature, that is, the presence of endobacteria within their mycelia and spores. The study of these endosymbiotic bacteria is still a challenging task. However, given recent improvements in the sensitivity of culture-free approaches, a deeper understanding of such microbial interactions is now possible and fuels an emerging research field. In this chapter, we report how Mucoromycota, in particular Mortierellomycotina, and their endobacteria can be investigated using a combination of diverse cellular biology, microscopy, and molecular techniques.

Old genes in new places: A taxon-rich analysis of interdomain lateral gene transfer events

Vertical inheritance is foundational to Darwinian evolution, but fails to explain major innovations such as the rapid spread of antibiotic resistance among bacteria and the origin of photosynthesis in eukaryotes. While lateral gene transfer (LGT) is recognized as an evolutionary force in prokaryotes, the role of LGT in eukaryotic evolution is less clear. With the exception of the transfer of genes from organelles to the nucleus, a process termed endosymbiotic gene transfer (EGT), the extent of interdomain transfer from prokaryotes to eukaryotes is highly debated. A common critique of studies of interdomain LGT is the reliance on the topology of single-gene trees that attempt to estimate more than one billion years of evolution. We take a more conservative approach by identifying cases in which a single clade of eukaryotes is found in an otherwise prokaryotic gene tree (i.e. exclusive presence). Starting with a taxon-rich dataset of over 13,600 gene families and passing data through several rounds of curation, we identify and categorize the function of 306 interdomain LGT events into diverse eukaryotes, including 189 putative EGTs, 52 LGTs into Opisthokonta (i.e. animals, fungi and their microbial relatives), and 42 LGTs nearly exclusive to anaerobic eukaryotes. To assess differential gene loss as an explanation for exclusive presence, we compare branch lengths within each LGT tree to a set of vertically-inherited genes subsampled to mimic gene loss (i.e. with the same taxonomic sampling) and consistently find shorter relative distance between eukaryotes and prokaryotes in LGT trees, a pattern inconsistent with gene loss. Our methods provide a framework for future studies of interdomain LGT and move the field closer to an understanding of how best to model the evolutionary history of eukaryotes.

Detection and Identification of Novel Intracellular Bacteria Hosted in Strains CBS 648.67 and CFCC 80795 of Biocontrol Fungi Metarhizium

"Endosymbiosis" is a cohesive form of a symbiotic association. Endobacteria exist in many fungi and play important roles in fungal host biology. Metarhizium spp. are important entomopathogenic fungi for insect pest control. In the present study, we performed comprehensive ana-lyses of strains of Metarhizium bibionidarum and M. anisopliae using PCR, phylogenetics, and fluorescent electron microscopy to identify endobacteria within hyphae and conidia. The results of the phylogenetic ana-lysis based on 16S rRNA gene sequences indicated that these endobacteria were the most closely related to Pelomonas puraquae and affiliated with Betaproteobacteria. Ultrastructural observations indicated that endobacteria were coccoid and less than 500‍ ‍nm in diameter. The basic characteristics of endobacteria in M. bibionidarum and M. anisopliae were elucidated, and biological questions were raised regarding their biological functions in the Metarhizium hosts.

Third Generation Genome Sequencing Reveals That Endobacteria in Nematophagous Fungi Esteya vermicola Contain Multiple Genes Encoding for Nematicidal Proteins

Esteya vermicola is the first recorded endoparasitic nematophagous fungus with high infectivity capacity, attacking the pinewood nematode Bursaphelenchus xylophilus which causes pine wilt disease. Endosymbionts are found in the cytoplasm of E. vermicola from various geographical areas. We sequenced the genome of endobacteria residing in E. vermicola to discover possible biological functions of these widespread endobacteria. Multilocus phylogenetic analyses showed that the endobacteria form a previously unidentified lineage sister to Phyllobacterium myrsinacearum species. The number of genes in the endobacterium was 4542, with 87.8% of the proteins having a known function. It contained a high proportion of repetitive sequences, as well as more Acyl-CoA synthetase genes and genes encoding the electron transport chain, compared with compared with plant-associated P. zundukense Tri 48 and P. myrsinacearum DSM 5893. Thus, this symbiotic bacterium is likely to be more efficient in regulating gene expression and energy release. Furthermore, the endobacteria in nematophagous fungi Esteya vermicola contained multiple nematicidal subtilase/subtilisin encoding genes, so it is likely that endobacteria cooperate with the host to kill nematodes.

Fungi as mediators linking organisms and ecosystems

Fungi form a major and diverse component of most ecosystems on Earth. They are both micro and macroorganisms with high and varying functional diversity as well as great variation in dispersal modes. With our growing knowledge of microbial biogeography, it has become increasingly clear that fungal assembly patterns and processes differ from other microorganisms such as bacteria, but also from macroorganisms such as plants. The success of fungi as organisms and their influence on the environment lies in their ability to span multiple dimensions of time, space, and biological interactions, that is not rivalled by other organism groups. There is also growing evidence that fungi mediate links between different organisms and ecosystems, with the potential to affect the macroecology and evolution of those organisms. This suggests that fungal interactions are an ecological driving force, interconnecting different levels of biological and ecological organisation of their hosts, competitors, and antagonists with the environment and ecosystem functioning. Here we review these emerging lines of evidence by focusing on the dynamics of fungal interactions with other organism groups across various ecosystems. We conclude that the mediating role of fungi through their complex and dynamic ecological interactions underlie their importance and ubiquity across Earth's ecosystems.

Transcriptional activity and epigenetic regulation of transposable elements in the symbiotic fungus Rhizophagus irregularis

Arbuscular mycorrhizal (AM) fungi form mutualistic relationships with most land plant species. AM fungi have long been considered as ancient asexuals. Long-term clonal evolution would be remarkable for a eukaryotic lineage and suggests the importance of alternative mechanisms to promote genetic variability facilitating adaptation. Here, we assessed the potential of transposable elements for generating such genomic diversity. The dynamic expression of TEs during Rhizophagus irregularis spore development suggests ongoing TE activity. We find Mutator-like elements located near genes belonging to highly expanded gene families. Whole-genome epigenomic profiling of R. irregularis provides direct evidence of DNA methylation and small RNA production occurring at TE loci. Our results support a model in which TE activity shapes the genome, while DNA methylation and small RNA-mediated silencing keep their overproliferation in check. We propose that a well-controlled TE activity directly contributes to genome evolution in AM fungi.

A potential microbiological approach to the evaluation of earthquake-induced soil liquefaction

Earthquakes occur thousands of times every day around the world. They are naturally destructive seismic events and often result in soil liquefaction. Soil microbiota plays a vital role in soil environments and may serve as an effective indicator to assess soil liquefaction after earthquakes. This study aimed to detect the microbial community abundance and composition in soil samples of different depths. Soil samples were collected in Southern Taiwan immediately after the 2010 earthquake. Their physical characteristics were determined, and their microbial communities were analyzed through 16S amplicon sequencing. The results revealed that Nitrospirae phylum dominated in the liquefied layer. In particular, the genus HB118, dominant in the liquefied layer, was not detected at other soil depths or in the expelled liquefied soil. This finding not only provides valuable insights into changes in microbial community composition at different soil depths after earthquakes but also suggests a useful indicator for monitoring liquefied soil.

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This study characterized the microbial composition of liquefied soil after an earthquake

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Most abundant phylum Nitrospirae found in liquefied soil if 3 most abundant phyla removed

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HB118 spp is correlated with liquefied soil

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We set up the alternative monitoring methods of soil liquefaction after seismic events

The genome of Geosiphon pyriformis reveals ancestral traits linked to the emergence of the arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal fungi (AMF) (subphylum Glomeromycotina)¹ are among the most prominent symbionts and form the Arbuscular Mycorrhizal symbiosis (AMS) with over 70% of known land plants.^2,3 AMS allows plants to efficiently acquire poorly soluble soil nutrients⁴ and AMF to receive photosynthetically fixed carbohydrates. This plant-fungus symbiosis dates back more than 400 million years⁵ and is thought to be one of the key innovations that allowed the colonization of lands by plants.⁶ Genomic and genetic analyses of diverse plant species started to reveal the molecular mechanisms that allowed the evolution of this symbiosis on the host side, but how and when AMS abilities emerged in AMF remain elusive. Comparative phylogenomics could be used to understand the evolution of AMS.^7,8 However, the availability of genome data covering basal AMF phylogenetic nodes (Archaeosporales, Paraglomerales) is presently based on fragmentary protein coding datasets.⁹Geosiphon pyriformis (Archaeosporales) is the only fungus known to produce endosymbiosis with nitrogen-fixing cyanobacteria (Nostoc punctiforme) presumably representing the ancestral AMF state.^10-12 Unlike other AMF, it forms long fungal cells ("bladders") that enclose cyanobacteria. Once in the bladder, the cyanobacteria are photosynthetically active and fix nitrogen, receiving inorganic nutrients and water from the fungus. Arguably, G. pyriformis represents an ideal candidate to investigate the origin of AMS and the emergence of a unique endosymbiosis. Here, we aimed to advance knowledge in these questions by sequencing the genome of G. pyriformis, using a re-discovered isolate.

SeSaMe: Metagenome Sequence Classification of Arbuscular Mycorrhizal Fungi-associated Microorganisms

Arbuscular mycorrhizal fungi (AMF) are plant root symbionts that play key roles in plant growth and soil fertility. They are obligate biotrophic fungi that form coenocytic multinucleated hyphae and spores. Numerous studies have shown that diverse microorganisms live on the surface of and inside their mycelia, resulting in a metagenome when whole-genome sequencing (WGS) data are obtained from sequencing AMF cultivated in vivo. The metagenome contains not only the AMF sequences, but also those from associated microorganisms. In this study, we introduce a novel bioinformatics program, Spore-associated Symbiotic Microbes (SeSaMe), designed for taxonomic classification of short sequences obtained by next-generation DNA sequencing. A genus-specific usage bias database was created based on amino acid usage and codon usage of a three consecutive codon DNA 9-mer encoding an amino acid trimer in a protein secondary structure. The program distinguishes between coding sequence (CDS) and non-CDS, and classifies a query sequence into a genus group out of 54 genera used as reference. The mean percentages of correct predictions of the CDS and the non-CDS test sets at the genus level were 71% and 50% for bacteria, 68% and 73% for fungi (excluding AMF), and 49% and 72% for AMF (Rhizophagus irregularis), respectively. SeSaMe provides not only a means for estimating taxonomic diversity and abundance but also the gene reservoir of the reference taxonomic groups associated with AMF. Therefore, it enables users to study the symbiotic roles of associated microorganisms. It can also be applicable to other microorganisms as well as soil metagenomes. SeSaMe is freely available at www.fungalsesame.org.

Upregulation of a marine fungal biosynthetic gene cluster by an endobacterial symbiont

Fungal-bacterial associations are present in nature, playing important roles in ecological, evolutionary and medicinal processes. Here we report a fungus-bacterial symbiont from marine sediment. The bacterium lives inside the fungal mycelium yet is robust enough to survive independent of its host; the independently grown bacterium can infect the fungal host in vitro and continue to grow progenitively. The bacterial symbiont modulates the fungal host to biosynthesize a polyketide antimicrobial, spiromarmycin. Spiromarmycin appears to endow upon the symbiont pair a protective/defensive means of warding off competitor organisms, be they prokaryotic or eukaryotic microorganisms. Genomic analyses revealed the spiromarmycin biosynthetic machinery to be encoded, not by the bacterium, but rather the fungal host. This unique fungal-bacterial symbiotic relationship and the molecule/s resulting from it dramatically expand our knowledge of marine microbial diversity and shed important insights into endosymbionts and fungal-bacterial relationships.

Shao et al. show that a bacterial symbiont drives its fungal host to biosynthesize a polyketide antimicrobial, spiromarmycin, fending off their competitors. They find that the spiromarmycin biosynthetic machinery is encoded by the fungal host. This study provides insights into the evolution of marine microbial diversity.

Photosynthetic Traits and Nitrogen Uptake in Crops: Which Is the Role of Arbuscular Mycorrhizal Fungi?

Arbuscular mycorrhizal (AM) fungi are root symbionts that provide mineral nutrients to the host plant in exchange for carbon compounds. AM fungi positively affect several aspects of plant life, improving nutrition and leading to a better growth, stress tolerance, and disease resistance and they interact with most crop plants such as cereals, horticultural species, and fruit trees. For this reason, they receive expanding attention for the potential use in sustainable and climate-smart agriculture context. Although several positive effects have been reported on photosynthetic traits in host plants, showing improved performances under abiotic stresses such as drought, salinity and extreme temperature, the involved mechanisms are still to be fully discovered. In this review, some controversy aspects related to AM symbiosis and photosynthesis performances will be discussed, with a specific focus on nitrogen acquisition-mediated by AM fungi.

Microbial biofilms in nature: unlocking their potential for agricultural applications

Soil environments are dynamic and the plant rhizosphere harbours a phenomenal diversity of micro-organisms which exchange signals and beneficial nutrients. Bipartite beneficial or symbiotic interactions with host roots, such as mycorrhizae and various bacteria, are relatively well characterized. In addition, a tripartite interaction also exists between plant roots, arbuscular mycorrhizal fungi (AMF) and associated bacteria. Bacterial biofilms exist as a sheet of bacterial cells in association with AMF structures, embedded within a self-produced exopolysaccharide matrix. Such biofilms may play important functional roles within these tripartite interactions. However, the details about such interactions in the rhizosphere and their relevant functional relationships have not been elucidated. This review explores the current understanding of naturally occurring microbial biofilms, and their interaction with biotic surfaces, especially AMF. The possible roles played by bacterial biofilms and the potential for their application for a more productive and sustainable agriculture is discussed in this review.

Symbiotic bacteria of plant-associated fungi: friends or foes?

Many bacteria form symbiotic associations with plant-associated fungi. The effects of these symbionts on host fitness usually depend on symbiont or host genotypes and environmental conditions. However, bacterial endosymbionts, that is those living within fungal cells, may positively regulate host performance as their survival is often heavily dependent on host fitness. Contrary to this, bacteria that establish ectosymbiotic associations with fungi, that is those located on the hyphal surface or in close vicinity to fungal mycelia, may not have an apparent net effect on fungal performance due to the low level of fitness dependency on their host. Our analysis supports the hypothesis that endosymbiotic bacteria of fungi are beneficial symbionts, and that effects of ectosymbiotic bacteria on fungal performance depends on the bacterial type involved in the interaction (e.g. helper versus pathogen of fungi). Ecological scenarios, where the presence of beneficial bacterial endosymbionts of fungi could be compromised, are also discussed.

Beneficial soil-borne bacteria and fungi: a promising way to improve plant nitrogen acquisition

Nitrogen (N) is an essential element for plant productivity, thus, it is abundantly applied to the soil in the form of organic or chemical fertilizers that have negative impacts on the environment. Exploiting the potential of beneficial microbes and identifying crop genotypes that can capitalize on symbiotic associations may be possible ways to significantly reduce the use of N fertilizers. The best-known example of symbiotic association that can reduce the use of N fertilizers is the N2-fixing rhizobial bacteria and legumes. Bacterial taxa other than rhizobial species can develop associative symbiotic interactions with plants and also fix N. These include bacteria of the genera Azospirillum, Azotobacter, and Bacillus, some of which are commercialized as bio-inoculants. Arbuscular mycorrhizal fungi are other microorganisms that can develop symbiotic associations with most terrestrial plants, favoring access to nutrients in a larger soil volume through their extraradical mycelium. Using combinations of different beneficial microbial species is a promising strategy to boost plant N acquisition and foster a synergistic beneficial effect between symbiotic microorganisms. Complex biological mechanisms including molecular, metabolic, and physiological processes dictate the establishment and efficiency of such multipartite symbiotic associations. In this review, we present an overview of the current knowledge and future prospects regarding plant N nutrition improvement through the use of beneficial bacteria and fungi associated with plants, individually or in combination.

Phylum-level diversity of the microbiome of the extremophilic basidiomycete fungus Pisolithus arhizus (Scop.) Rauschert: An island of biodiversity in a thermal soil desert

We used high‐throughput DNA sequencing methods combined with bio‐geochemical profiles to characterize the internal environment and community structure of the microbiome of the basidiomycete fungus Pisolithus arhizus (Scop.) Rauschert from soils within a geothermal feature of Yellowstone National Park. Pisolithus arhizus is unique in that it forms closed fruiting bodies that sequester visible sulfur within. Fourier transform infrared spectroscopy (FTIR) analysis demonstrates that the P. arhizus fruiting body also concentrates copper, manganese, nickel, and zinc and contains pure granular silica. Gas chromatography‐mass spectrometry (GC‐MS) analysis indicates an environment rich in hydrocarbons. Oxygen probe analysis reveals that zones of up to 4× atmospheric oxygen exist within nanometers of zones of near anoxia. Analysis of microbial community structure using high‐throughput DNA sequencing methods shows that the fruiting body supports a microbiome that reflects the physiochemical environment of the fruiting body. Diversity and richness measures indicate a microbiome that is significantly richer and more diverse than that of the soils in which P. arhizus grows. Further, P. arhizus sporocarps are enriched significantly in Proteobacteria (primarily Burkholderia) Gemmatimonadetes, Bacteroidetes, Verrucomicrobia, Nitrospirae, Elusimicrobia, and Latescibacteria (WS3) while soils are enriched in Actinobacteria (primarily Mycobacterium), Dormibacteraeota (AD3), and Eremiobacteraeota (WPS‐2). Finally, pairwise % similarity comparisons indicate that P. arhizus harbors two lineages that may represent new groups in the candidate phylum radiation (CPR). Together, these results demonstrate that P. arhizus provides a novel environment for microbiome studies and provides for interesting hypotheses regarding the evolution, origins, and functions of symbioses and novel microbes.

Palmitoylation of Metazoan Carotenoid Oxygenases

Abundant in nature, carotenoids are a class of fat-soluble pigments with a polyene tetraterpenoid structure. They possess antioxidant properties and their consumption leads to certain health benefits in humans. Carotenoid cleavage oxygenases (CCOs) are a superfamily of enzymes which oxidatively cleave carotenoids and they are present in all kingdoms of life. Complexity of CCO evolution is high. For example, in this study we serendipitously found a new family of eukaryotic CCOs, the apocarotenoid oxygenase-like (ACOL) family. This family has several members in animal genomes and lacks the animal-specific amino acid motif PDPCK. This motif is likely to be associated with palmitoylation of some animal CCOs. We recently demonstrated that two mammalian members of the carotenoid oxygenase family retinal pigment epithelial-specific 65 kDa protein (RPE65) and beta-carotene oxygenase 2 (BCO2) are palmitoylated proteins. Here we used the acyl-resin-assisted capture (acyl-RAC) method to demonstrate protein palmitoylation and immunochemistry to localize mouse BCO2 (mBCO2) in COS7 cell line in the absence and presence of its substrate β-carotene. We demonstrate that mBCO2 palmitoylation depends on the evolutionarily conserved motif PDPCK and that metazoan family members lacking the motif (Lancelet beta-carotene oxygenase-like protein (BCOL) and Acropora ACOL) are not palmitoylated. Additionally, we observed that the palmitoylation status of mBCO2 and its membrane association depend on the presence of its substrate β-carotene. Based on our results we conclude that most metazoan carotenoid oxygenases retain the evolutionarily conserved palmitoylation PDPCK motif to target proteins to internal membranes depending on substrate status. Exceptions are in the secreted BCOL subfamily and the strictly cytosolic ancient ACOL subfamily of carotenoid oxygenases.

A Tripartite Interaction among the Basidiomycete Rhodotorula mucilaginosa, N2-Fixing Endobacteria, and Rice Improves Plant Nitrogen Nutrition

Nitrogen (N) limits crop yield, and improvement of N nutrition remains a key goal for crop research; one approach to improve N nutrition is identifying plant-interacting, N2-fixing microbes. Rhodotorula mucilaginosa JGTA-S1 is a basidiomycetous yeast endophyte of narrowleaf cattail (Typha angustifolia). JGTA-S1 could not convert nitrate or nitrite to ammonium but harbors diazotrophic (N2-fixing) endobacteria (Pseudomonas stutzeri) that allow JGTA-S1 to fix N2 and grow in a N-free environment; moreover, P. stutzeri dinitrogen reductase was transcribed in JGTA-S1 even under adequate N. Endobacteria-deficient JGTA-S1 had reduced fitness, which was restored by reintroducing P. stutzeri JGTA-S1 colonizes rice (Oryza sativa), significantly improving its growth, N content, and relative N-use efficiency. Endofungal P. stutzeri plays a significant role in increasing the biomass and ammonium content of rice treated with JGTA-S1; also, JGTA-S1 has better N2-fixing ability than free-living P. stutzeri and provides fixed N to the plant. Genes involved in N metabolism, N transporters, and NODULE INCEPTION-like transcription factors were upregulated in rice roots within 24 h of JGTA-S1 treatment. In association with rice, JGTA-S1 has a filamentous phase and P. stutzeri only penetrated filamentous JGTA-S1. Together, these results demonstrate an interkingdom interaction that improves rice N nutrition.

At the nexus of three kingdoms: the genome of the mycorrhizal fungus Gigaspora margarita provides insights into plant, endobacterial and fungal interactions

As members of the plant microbiota, arbuscular mycorrhizal fungi (AMF, Glomeromycotina) symbiotically colonize plant roots. AMF also possess their own microbiota, hosting some uncultivable endobacteria. Ongoing research has revealed the genetics underlying plant responses to colonization by AMF, but the fungal side of the relationship remains in the dark. Here, we sequenced the genome of Gigaspora margarita, a member of the Gigasporaceae in an early diverging group of the Glomeromycotina. In contrast to other AMF, G. margarita may host distinct endobacterial populations and possesses the largest fungal genome so far annotated (773.104 Mbp), with more than 64% transposable elements. Other unique traits of the G. margarita genome include the expansion of genes for inorganic phosphate metabolism, the presence of genes for production of secondary metabolites and a considerable number of potential horizontal gene transfer events. The sequencing of G. margarita genome reveals the importance of its immune system, shedding light on the evolutionary pathways that allowed early diverging fungi to interact with both plants and bacteria.

Molecular host mimicry and manipulation in bacterial symbionts

It is common among intracellular bacterial pathogens to use eukaryotic-like proteins that mimic and manipulate host cellular processes to promote colonization and intracellular survival. Eukaryotic-like proteins are bacterial proteins with domains that are rare in bacteria, and known to function in the context of a eukaryotic cell. Such proteins can originate through horizontal gene transfer from eukaryotes or, in the case of simple repeat proteins, through convergent evolution. Recent studies of microbiomes associated with several eukaryotic hosts suggest that similar molecular strategies are deployed by cooperative bacteria that interact closely with eukaryotic cells. Some mimics, like ankyrin repeats, leucine rich repeats and tetratricopeptide repeats are shared across diverse symbiotic systems ranging from amoebae to plants, and may have originated early, or evolved independently in multiple systems. Others, like plant-mimicking domains in members of the plant microbiome are likely to be more recent innovations resulting from horizontal gene transfer from the host, or from microbial eukaryotes occupying the same host. Host protein mimics have only been described in a limited set of symbiotic systems, but are likely to be more widespread. Systematic searches for eukaryote-like proteins in symbiont genomes could lead to the discovery of novel mechanisms underlying host-symbiont interactions.

A unique life-strategy of an endophytic yeast Rhodotorula mucilaginosa JGTA-S1-a comparative genomics viewpoint

Endophytic yeasts of genus Rhodotorula are gaining importance for their ability to improve plant growth. The nature of their interaction with plants, however, remains unknown. Rhodotorula mucilaginosa JGTA-S1 was isolated as an endophyte of Typha angustifolia and promoted growth in the host. To investigate the life-strategy of the yeast from a genomics perspective, we used Illumina and Oxford Nanopore reads to generate a high-quality annotated draft assembly of JGTA-S1 and compared its genome to three other Rhodotorula yeasts and the close relative Rhodosporidium toruloides. JGTA-S1 is a haploid yeast possessing several genes potentially facilitating its endophytic lifestyle such as those responsible for solubilizing phosphate and producing phytohormones. An intact mating-locus in JGTA-S1 raised the possibility of a yet unknown sexual reproductive cycle in Rhodotorula yeasts. Additionally, JGTA-S1 had functional anti-freezing genes and was also unique in lacking a functional nitrate-assimilation pathway—a feature that is associated with obligate biotrophs. Nitrogen-fixing endobacteria were found within JGTA-S1 that may circumvent this defective N-metabolism. JGTA-S1 genome data coupled with experimental evidence give us an insight into the nature of its beneficial interaction with plants.

Microbe-driven chemical ecology: past, present and future

In recent years, research in the field of Microbial Ecology has revealed the tremendous diversity and complexity of microbial communities across different ecosystems. Microbes play a major role in ecosystem functioning and contribute to the health and fitness of higher organisms. Scientists are now facing many technological and methodological challenges in analyzing these complex natural microbial communities. The advances in analytical and omics techniques have shown that microbial communities are largely shaped by chemical interaction networks mediated by specialized (water-soluble and volatile) metabolites. However, studies concerning microbial chemical interactions need to consider biotic and abiotic factors on multidimensional levels, which require the development of new tools and approaches mimicking natural microbial habitats. In this review, we describe environmental factors affecting the production and transport of specialized metabolites. We evaluate their ecological functions and discuss approaches to address future challenges in microbial chemical ecology (MCE). We aim to emphasize that future developments in the field of MCE will need to include holistic studies involving organisms at all levels and to consider mechanisms underlying the interactions between viruses, micro-, and macro-organisms in their natural environments.

Mechanisms and Impact of Symbiotic Phosphate Acquisition

Phosphorous is important for life but often limiting for plants. The symbiotic pathway of phosphate uptake via arbuscular mycorrhizal fungi (AMF) is evolutionarily ancient and today occurs in natural and agricultural ecosystems alike. Plants capable of this symbiosis can obtain up to all of the phosphate from symbiotic fungi, and this offers potential means to develop crops less dependent on unsustainable P fertilizers. Here, we review the mechanisms and insights gleaned from the fine-tuned signal exchanges that orchestrate the intimate mutualistic symbiosis between plants and AMF. As the currency of trade, nutrients have signaling functions beyond being the nutritional goal of mutualism. We propose that such signaling roles and metabolic reprogramming may represent commitments for a mutualistic symbiosis that act across the stages of symbiosis development.

Effect of Long-Term Farming Practices on Agricultural Soil Microbiome Members Represented by Metagenomically Assembled Genomes (MAGs) and Their Predicted Plant-Beneficial Genes

To follow the hypothesis that agricultural management practices affect structure and function of the soil microbiome regarding soil health and plant-beneficial traits, high-throughput (HT) metagenome analyses were performed on Chernozem soil samples from a long-term field experiment designated LTE-1 carried out at Bernburg-Strenzfeld (Saxony-Anhalt, Germany). Metagenomic DNA was extracted from soil samples representing the following treatments: (i) plough tillage with standard nitrogen fertilization and use of fungicides and growth regulators, (ii) plough tillage with reduced nitrogen fertilization (50%), (iii) cultivator tillage with standard nitrogen fertilization and use of fungicides and growth regulators, and (iv) cultivator tillage with reduced nitrogen fertilization (50%). Bulk soil (BS), as well as root-affected soil (RS), were considered for all treatments in replicates. HT-sequencing of metagenomic DNA yielded approx. 100 Giga bases (Gb) of sequence information. Taxonomic profiling of soil communities revealed the presence of 70 phyla, whereby Proteobacteria, Actinobacteria, Bacteroidetes, Planctomycetes, Acidobacteria, Thaumarchaeota, Firmicutes, Verrucomicrobia and Chloroflexi feature abundances of more than 1%. Functional microbiome profiling uncovered, i.a., numerous potential plant-beneficial, plant-growth-promoting and biocontrol traits predicted to be involved in nutrient provision, phytohormone synthesis, antagonism against pathogens and signal molecule synthesis relevant in microbe-plant interaction. Neither taxonomic nor functional microbiome profiling based on single-read analyses revealed pronounced differences regarding the farming practices applied. Soil metagenome sequences were assembled and taxonomically binned. The ten most reliable and abundant Metagenomically Assembled Genomes (MAGs) were taxonomically classified and metabolically reconstructed. Importance of the phylum Thaumarchaeota for the analyzed microbiome is corroborated by the fact that the four corresponding MAGs were predicted to oxidize ammonia (nitrification), thus contributing to the cycling of nitrogen, and in addition are most probably able to fix carbon dioxide. Moreover, Thaumarchaeota and several bacterial MAGs also possess genes with predicted functions in plant-growth-promotion. Abundances of certain MAGs (species resolution level) responded to the tillage practice, whereas the factors compartment (BS vs. RS) and nitrogen fertilization only marginally shaped MAG abundance profiles. Hence, soil management regimes promoting plant-beneficial microbiome members are very likely advantageous for the respective agrosystem, its health and carbon sequestration and accordingly may enhance plant productivity. Since Chernozem soils are highly fertile, corresponding microbiome data represent a valuable reference resource for agronomy in general.

Phylogenomics of Endogonaceae and evolution of mycorrhizas within Mucoromycota

Endogonales (Mucoromycotina), composed of Endogonaceae and Densosporaceae, is the only known non-Dikarya order with ectomycorrhizal members. They also form mycorrhizal-like association with some nonspermatophyte plants. It has been recently proposed that Endogonales were among the earliest mycorrhizal partners with land plants. It remains unknown whether Endogonales possess genomes with mycorrhizal-lifestyle signatures and whether Endogonales originated around the same time as land plants did. We sampled sporocarp tissue from four Endogonaceae collections and performed shotgun genome sequencing. After binning the metagenome data, we assembled and annotated the Endogonaceae genomes. We performed comparative analysis on plant-cell-wall-degrading enzymes (PCWDEs) and small secreted proteins (SSPs). We inferred phylogenetic placement of Endogonaceae and estimated the ages of Endogonaceae and Endogonales with expanded taxon sampling. Endogonaceae have large genomes with high repeat content, low diversity of PCWDEs, but without elevated SSP/secretome ratios. Dating analysis estimated that Endogonaceae originated in the Permian-Triassic boundary and Endogonales originated in the mid-late Silurian. Mycoplasma-related endobacterium sequences were identified in three Endogonaceae genomes. Endogonaceae genomes possess typical signatures of mycorrhizal lifestyle. The early origin of Endogonales suggests that the mycorrhizal association between Endogonales and plants might have played an important role during the colonization of land by plants.

Genome and evolution of the arbuscular mycorrhizal fungus Diversispora epigaea (formerly Glomus versiforme) and its bacterial endosymbionts

Arbuscular mycorrhizal (AM) fungi form endosymbioses with most plants, and they themselves are hosts for Mollicutes/Mycoplasma-related endobacteria (MRE). Despite their significance, genomic information for AM fungi and their MRE are relatively sparse, which hinders our understanding of their biology and evolution. We assembled the genomes of the AM fungus Diversispora epigaea (formerly Glomus versiforme) and its MRE and performed comparative genomics and evolutionary analyses. The D. epigaea genome showed a pattern of substantial gene duplication and differential evolution of gene families, including glycosyltransferase family 25, whose activities are exclusively lipopolysaccharide biosynthesis. Genes acquired by horizontal transfer from bacteria possibly function in defense against foreign DNA or viruses. The MRE population was diverse, with multiple genomes displaying characteristics of differential evolution and encoding many MRE-specific genes as well as genes of AM fungal origin. Gene family expansion in D. epigaea may enhance adaptation to both external and internal environments, such as expansion of kinases for signal transduction upon external stimuli and expansion of nucleoside salvage pathway genes potentially for competition with MRE, whose genomes lack purine and pyrimidine biosynthetic pathways. Collectively, this metagenome provides high-quality references and begins to reveal the diversity within AM fungi and their MRE.

Bacterial-fungal interactions: ecology, mechanisms and challenges

Fungi and bacteria are found living together in a wide variety of environments. Their interactions are significant drivers of many ecosystem functions and are important for the health of plants and animals. A large number of fungal and bacterial families engage in complex interactions that lead to critical behavioural shifts of the microorganisms ranging from mutualism to antagonism. The importance of bacterial-fungal interactions (BFI) in environmental science, medicine and biotechnology has led to the emergence of a dynamic and multidisciplinary research field that combines highly diverse approaches including molecular biology, genomics, geochemistry, chemical and microbial ecology, biophysics and ecological modelling. In this review, we discuss recent advances that underscore the roles of BFI across relevant habitats and ecosystems. A particular focus is placed on the understanding of BFI within complex microbial communities and in regard of the metaorganism concept. We also discuss recent discoveries that clarify the (molecular) mechanisms involved in bacterial-fungal relationships, and the contribution of new technologies to decipher generic principles of BFI in terms of physical associations and molecular dialogues. Finally, we discuss future directions for research in order to stimulate synergy within the BFI research area and to resolve outstanding questions.

Conserved Proteins of the RNA Interference System in the Arbuscular Mycorrhizal Fungus Rhizoglomus irregulare Provide New Insight into the Evolutionary History of Glomeromycota

Horizontal gene transfer (HGT) is an important mechanism in the evolution of many living organisms particularly in Prokaryotes where genes are frequently dispersed between taxa. Although, HGT has been reported in Eukaryotes, its accumulative effect and its frequency has been questioned. Arbuscular mycorrhizal fungi (AMF) are an early diverged fungal lineage belonging to phylum Glomeromycota, whose phylogenetic position is still under debate. The history of AMF and land plant symbiosis dates back to at least 460 Ma. However, Glomeromycota are estimated to have emerged much earlier than land plants. In this study, we surveyed genomic and transcriptomic data of the model arbuscular mycorrhizal fungus Rhizoglomus irregulare (synonym Rhizophagus irregularis) and its relatives to search for evidence of HGT that occurred during AMF evolution. Surprisingly, we found a signature of putative HGT of class I ribonuclease III protein-coding genes that occurred from autotrophic cyanobacteria genomes to R. irregulare. At least one of two HGTs was conserved among AMF species with high levels of sequence similarity. Previously, an example of intimate symbiosis between AM fungus and cyanobacteria was reported in the literature. Ribonuclease III family enzymes are important in small RNA regulation in Fungi together with two additional core proteins (Argonaute/piwi and RdRP). The eukaryotic RNA interference system found in AMF was conserved and showed homology with high sequence similarity in Mucoromycotina, a group of fungi closely related to Glomeromycota. Prior to this analysis, class I ribonuclease III has not been identified in any eukaryotes. Our results indicate that a unique acquisition of class I ribonuclease III in AMF is due to a HGT event that occurred from cyanobacteria to Glomeromycota, at the latest before the divergence of the two Glomeromycota orders Diversisporales and Glomerales.

Biology of Fungi and Their Bacterial Endosymbionts

Heritable symbioses, in which endosymbiotic bacteria (EB) are transmitted vertically between host generations, are an important source of evolutionary novelties. A primary example of such symbioses is the eukaryotic cell with its EB-derived organelles. Recent discoveries suggest that endosymbiosis-related innovations can be also found in associations formed by early divergent fungi in the phylum Mucoromycota with heritable EB from two classes, Betaproteobacteria and Mollicutes. These symbioses exemplify novel types of host-symbiont interactions. Studies of these partnerships fuel theoretical models describing mechanisms that stabilize heritable symbioses, control the rate of molecular evolution, and enable the establishment of mutualisms. Lastly, by altering host phenotypes and metabolism, these associations represent an important instrument for probing the basic biology of the Mucoromycota hosts, which remain one of the least explored filamentous fungi.

Horizontal Gene Transfer From Bacteria and Plants to the Arbuscular Mycorrhizal Fungus Rhizophagus irregularis

Arbuscular mycorrhizal fungi (AMF) belong to Glomeromycotina, and are mutualistic symbionts of many land plants. Associated bacteria accompany AMF during their lifecycle to establish a robust tripartite association consisting of fungi, plants and bacteria. Physical association among this trinity provides possibilities for the exchange of genetic materials. However, very few horizontal gene transfer (HGT) from bacteria or plants to AMF has been reported yet. In this study, we complement existing algorithms by developing a new pipeline, Blast2hgt, to efficiently screen for putative horizontally derived genes from a whole genome. Genome analyses of the glomeromycete Rhizophagus irregularis identified 19 fungal genes that had been transferred between fungi and bacteria/plants, of which seven were obtained from bacteria. Another 18 R. irregularis genes were found to be recently acquired from either plants or bacteria. In the R. irregularis genome, gene duplication has contributed to the expansion of three foreign genes. Importantly, more than half of the R. irregularis foreign genes were expressed in various transcriptomic experiments, suggesting that these genes are functional in R. irregularis. Functional annotation and available evidence showed that these acquired genes may participate in diverse but fundamental biological processes such as regulation of gene expression, mitosis and signal transduction. Our study suggests that horizontal gene influx through endosymbiosis is a source of new functions for R. irregularis, and HGT might have played a role in the evolution and symbiotic adaptation of this arbuscular mycorrhizal fungus.

Bacterial Endosymbionts: Master Modulators of Fungal Phenotypes

The ecological modes of fungi are shaped not only by their intrinsic features and the environment in which they occur, but also by their interactions with diverse microbes. Here we explore the ecological and genomic features of diverse bacterial endosymbionts-endohyphal bacteria-that together are emerging as major determinants of fungal phenotypes and plant-fungi interactions. We first provide a historical perspective on the study of endohyphal bacteria. We then propose a functional classification of three main groups, providing an overview of their genomic, phylogenetic, and ecological traits. Last, we explore frontiers in the study of endohyphal bacteria, with special attention to those facultative and horizontally transmitted bacteria that associate with some of the most diverse lineages of fungi. Overall, our aim is to synthesize the rich literature from nearly 50 years of studies on endohyphal bacteria as a means to highlight potential applications and new research directions.

The genome and microbiome of a dikaryotic fungus (Inocybe terrigena, Inocybaceae) revealed by metagenomics

Recent advances in molecular methods have increased our understanding of various fungal symbioses. However, little is known about genomic and microbiome features of most uncultured symbiotic fungal clades. Here, we analysed the genome and microbiome of Inocybaceae (Agaricales, Basidiomycota), a largely uncultured ectomycorrhizal clade known to form symbiotic associations with a wide variety of plant species. We used metagenomic sequencing and assembly of dikaryotic fruiting-body tissues from Inocybe terrigena (Fr.) Kuyper, to classify fungal and bacterial genomic sequences, and obtained a nearly complete fungal genome containing 93% of core eukaryotic genes. Comparative genomics reveals that I. terrigena is more similar to ectomycorrhizal and brown rot fungi than to white rot fungi. The reduction in lignin degradation capacity has been independent from and significantly faster than in closely related ectomycorrhizal clades supporting that ectomycorrhizal symbiosis evolved independently in Inocybe. The microbiome of I. terrigena fruiting-bodies includes bacteria with known symbiotic functions in other fungal and non-fungal host environments, suggesting potential symbiotic functions of these bacteria in fungal tissues regardless of habitat conditions. Our study demonstrates the usefulness of direct metagenomics analysis of fruiting-body tissues for characterizing fungal genomes and microbiome.

Mycoplasma-related endobacteria within Mortierellomycotina fungi: diversity, distribution and functional insights into their lifestyle

Bacterial interactions with animals and plants have been examined for over a century; by contrast, the study of bacterial-fungal interactions has received less attention. Bacteria interact with fungi in diverse ways, and endobacteria that reside inside fungal cells represent the most intimate interaction. The most significant bacterial endosymbionts that have been studied are associated with Mucoromycota and include two main groups: Burkholderia-related and Mycoplasma-related endobacteria (MRE). Examples of Burkholderia-related endobacteria have been reported in the three Mucoromycota subphyla. By contrast, MRE have only been identified in Glomeromycotina and Mucoromycotina. This study aims to understand whether MRE dwell in Mortierellomycotina and, if so, to determine their impact on the fungal host. We carried out a large-scale screening of 394 Mortierellomycotina strains and employed a combination of microscopy, molecular phylogeny, next-generation sequencing and qPCR. We detected MRE in 12 strains. These endosymbionts represent novel bacterial phylotypes and show evidence of recombination. Their presence in Mortierellomycotina demonstrates that MRE occur within fungi across Mucoromycota and they may have lived in their common ancestor. We cured the fungus of its endosymbionts with antibiotics and observed improved biomass production in isogenic lines lacking MRE, demonstrating that these endobacteria impose some fitness costs to their fungal host. Here we provided the first functional insights into the lifestyle of MRE. Our findings indicate that MRE may be antagonistic to their fungal hosts, and adapted to a non-lethal parasitic lifestyle in the mycelium of Mucoromycota. However, context-dependent adaptive benefits to their host at minimal cost cannot not be excluded. Finally, we conclude that Mortierellomycotina represent attractive model organisms for exploring interactions between MRE and fungi.

A phylum-level phylogenetic classification of zygomycete fungi based on genome-scale data

Zygomycete fungi were classified as a single phylum, Zygomycota, based on sexual reproduction by zygospores, frequent asexual reproduction by sporangia, absence of multicellular sporocarps, and production of coenocytic hyphae, all with some exceptions. Molecular phylogenies based on one or a few genes did not support the monophyly of the phylum, however, and the phylum was subsequently abandoned. Here we present phylogenetic analyses of a genome-scale data set for 46 taxa, including 25 zygomycetes and 192 proteins, and we demonstrate that zygomycetes comprise two major clades that form a paraphyletic grade. A formal phylogenetic classification is proposed herein and includes two phyla, six subphyla, four classes and 16 orders. On the basis of these results, the phyla Mucoromycota and Zoopagomycota are circumscribed. Zoopagomycota comprises Entomophtoromycotina, Kickxellomycotina and Zoopagomycotina; it constitutes the earliest diverging lineage of zygomycetes and contains species that are primarily parasites and pathogens of small animals (e.g. amoeba, insects, etc.) and other fungi, i.e. mycoparasites. Mucoromycota comprises Glomeromycotina, Mortierellomycotina, and Mucoromycotina and is sister to Dikarya. It is the more derived clade of zygomycetes and mainly consists of mycorrhizal fungi, root endophytes, and decomposers of plant material. Evolution of trophic modes, morphology, and analysis of genome-scale data are discussed.