Fungal Competitors Research Articles

Transcriptomics reveals the regulation of the immune system of the mushroom-forming fungus Schizophyllum commune during interaction with four competitors

Mushroom-forming fungi frequently encounter competitors during their lifecycle, but their defense mechanisms remain largely unexplored. We studied the response of the mushroom-forming fungus Schizophyllum commune during interaction with the fungal competitors Trichoderma harzianum, Trichoderma aggressivum and Purpureocillium lilacinum and the bacterial competitor Serratia quinivorans. Transcriptomics revealed 632 up-regulated genes in the direct interaction zone, which were enriched in small secreted proteins and transporters. A set of 26 genes were up-regulated during all interactions, indicating a core transcriptomic defense response. In the non-interacting edge of the mycelium of S. commune, there were 154 up-regulated genes, suggesting that there is a systemic response due to a signal that reaches unaffected areas. The GATA zinc finger transcription factor gene gat1 was up-regulated during interaction and a Δgat1 strain displayed increased colonization by T. harzianum. Previously linked to mushroom development, this transcription factor apparently has a dual role. Moreover, 138 genes were up-regulated during both interaction and mushroom development, indicating priming of the defense response during development to prepare the fruiting body for future interactions. Overall, we unveiled a defensive response of S. commune during interaction with fungal and bacterial competitors and identified a regulator of this response.

Isolation and Identification of Competitive Fungi on Medium for Black Wood Ear Mushroom in Korea and In Vitro Selection of Potential Biocontrol Agents

Black wood ear mushroom (<i>Auricularia auricula-judae</i>) is one of the most economically important mushrooms in China, Japan, and Korea. The cultivation of wood ear mushrooms on artificial substrates is more efficient in terms of time and cost compared with their natural growth on trees. However, if the substrate cultivation is infected by fast-growing fungi, the relatively slow-growing ear mushroom will be outcompeted, leading to economic losses. In this study, we investigated the competitive fungal isolates from substrates infected with fast-growing fungi for the cultivation of ear mushrooms in Jangheung and Sunchon, Korea. We collected 54 isolates and identified them by sequencing their internal transcribed spacer region with morphological identification. Among the isolates, the dominant isolates were <i>Trichoderma</i> spp. (92.6%), <i>Penicillium</i> spp. (5.6%), and <i>Talaromyces</i> sp. (1.8%). To find an appropriate eco-friendly biocontrol agent, we used five <i>Streptomyces</i> spp. and Benomyl, as controls against <i>Trichoderma</i> spp. and <i>Penicillium</i> spp. Among the six <i>Streptomyces</i> spp., <i>Streptomyces</i> sp. JC203-3 effectively controlled the fungi <i>Trichoderma</i> spp. and <i>Penicillium</i> spp., which pose a significant problem for the substrates of black wood ear mushrooms. This result indicated that this <i>Streptomyces</i> sp. JC203-3 can be used as biocontrol agents to protect against <i>Trichoderma</i> and <i>Penicillium</i> spp.

Morpho-Molecular Identification and First Report of Trichoderma Aggressivum is Causing Green Rot on White Button Mushroom in Iraq

The aim of this study was to morphological isolation and identification of fungus associated with the fruits and culture media of the white mushroom Agaricus bisporus. The results of isolation and identification from the sampling areas included Baghdad, Babylon, and Diwaniyah. Besides, Sulaymaniyah governorates showed the association of seven species of fungi associated the fruiting body and culture media of the white mushroom A. bisporus represented by the two pathogens Cladobotryum spp. and Trichoderma spp. As well as, the competitive fungi of A. bisporus culture medium, such as Aspergillus spp. and Penicillium spp. and Phoma sp. and Rhizopus spp., and Ulocladium sp. The most common pathogenic fungus was Cladobotryum spp. with a frequency of 56.2%, followed by Trichoderma spp., with a frequency of 54.1%, which appeared in samples from two sampling areas. Conversely, species T. aggressivum recorded a frequency of 42.5%, which is the first record in Iraq on the fruits and media of white fungus. The fungus Aspergillus spp. showed the existence percentage reached 65.5%, which is the highest among the competing fungi, followed by Penicillium spp., with an existence percentage of 26.6%. Then, Rhizopus spp., Phoma sp., and Ulocladium sp. with a frequency of 16.6%, 10.4%, and 10.4%, respectively. The results of the pathogenicity of the fungus isolate Trichoderma spp. indicated that all the isolates tested for the pathogenicity of the fruiting bodies of the white mushroom A. bisporus led to the infection of the fruiting bodies and their transformation into brown color and damage. The isolates of the fungus T. aggressivum (isolate 1 and isolate 2) recorded a disease severity of 60.0% and 60.0%, respectively. While, at a temperature of 4 °C, the isolates of the fungus T. aggressivum (1 and 2) recorded a disease severity amounted to 40.0% and 50.0%, respectively, compared to the control treatment without fungus, where the disease severity amounted to 0.0%. The results of the molecular identification of the DNA of the T. aggressivum, which showed the highest pathogenicity of A. bisporus fruiting bodies, using the specialized primer ITSI/ITS4, as it gave bands with a molecular weight of 600 bp. These findings were compared with the GenBank with a high similarity for the fungus T. aggressivum and it has been deposited in the Gen Bank with the accession number (OQ109172).

Antifungal Properties of Nanosilver Clay Composites Against Fungal Pathogens of Agaricus bisporus

AbstractThis study aimed to determine the possible control of selected pathogenic and competitive fungi of A. bisporus by three nanosilver clay composites and establish the effective concentrations of the composites for inhibiting pathogens. Nanosilver (AgNPs) clay (zeolite, montmorillonite, and palygorskite) composites were synthesized by a microwave‐assisted surface functionalization technique, and various techniques characterized the products. Zeolite and montmorillonite composites showed uniformly distributed spherical AgNPs with an average size of 3.33 nm and 2.85 nm, respectively, whereas palygorskite presented agglomerated and unevenly distributed AgNPs. The influence of the various composites on 9 fungi, including strains of T. aggressivum f. aggressivum, L. fungicola, C. dendroides, and Mycogone sp., was determined in vitro at different concentrations. At 10 mg/mL, AgNP‐zeolite and AgNP‐montmorillonite inhibited 8 out of 9 pathogens, while AgNP‐palygorskite only inhibited 1 pathogen. The nanosilver clay composites tested against A. bisporus revealed no adverse effects on mycelial growth at any concentrations tested. This study confirms that AgNP‐zeolite and AgNP‐montmorillonite composites have effective antifungal properties and can be used as alternative fungicides against mushroom pathogens without affecting A. bisporus growth. However, further investigation is required to unravel the mechanism of selective antifungal activity observed in this study.

Mycobiota of compost, air, fruiting bodies of Pleurotus ostreatus (EM-1) and the mycotoxigenic potential of some resident Aspergillus species

The commercial and cottage industry entrepreneurs in Ghana have cultivated edible mushrooms such as Pleurotus ostreatus with appreciable success. However, the presence of fungal competitors and potential pathogens in the substrate obstructs yield. Selected farms in Greater Accra (6) and Central Region (1) were sampled for baseline data on the resident mycobiota of the compost (Triplochiton scleroxylon ‘wawa’ sawdust), fruiting body and aeromycoflora of the cropping rooms using conventional serial dilution technique and the Plate Exposure method respectively. Twenty (20) fungal species belonging to eleven (11) genera Aspergillus, Cladosporium, Didymella, Fusarium, Mycelia, Penicillium, Rhizopus, Trichoderma, Verticillium and Saccharomyces were encountered in the compost. The total fungal populations in compost varied significantly and with the highest recorded by E90 Mushroom Farm (4.0 to 5.24 log10CFU/g) whereas 4E Farm (2.28 to 2.93 log10CFU/g) obtained the lowest population. Eighteen (18) fungal species from ten (10) genera were isolated from the fruiting body of P. ostreatus from the farms. The most predominant species were Aspergillus species followed by Cladosporium, Penicillium, Trichoderma, Rhizopus, Saccharomyces and Talaromyces. The total mycoflora population resident in the fruiting body of the mushroom was lowest for Immaculate Gold Enterprise (2.59 to 3.22 log10CFU/g) whereas PMC Mushroom Farm (4.71 to 5.34 log10CFU/g) ranked highest. Aeromycoflora analysis showed nineteen (19) different fungal species isolated from the seven farms. Findings from the present data indicate fungi isolates from cropping rooms were also isolated from fruiting bodies and the compost. Potential toxin-producing Aspergillus species (A. flavus, A. parasiticus, A. niger, A. alutaceus, A. fumigatus) were present in the compost, the fruiting body of P. ostreatus and aeromycoflora. Keywords: Mycobiota, Trichoderma harzianum, fruiting body, Pleurotus ostreatus, aeromycoflora of cropping rooms, toxin-producing Aspergillus species.

Heat stress can change the competitive outcome between fungi: insights from a modelling approach

Under a changing climate, soil fungal communities will increasingly be subject to periods of heat stress. These periods can affect the performance of individual fungi and their competition for space and resources. Competition between fungi is strongly controlled by the exudation of inhibitory compounds, resulting in different competitive outcomes that range from overgrowth of the inferior competitor to a deadlock, where the competing fungi inhibit each other. As heat stress can alter the competitive outcome between fungi, the community composition can also change strongly. So far, a general understanding of the mechanisms that drive the competitive outcome between fungi under heat stress is still missing. However, this understanding is essential to assess important community functions, such as decomposition or mediation of plant nutrition, which strongly depend on the fungal community composition.Here, we used a partial differential equation (PDE) model simulating two fungal competitors in a two‐dimensional space, to mechanistically explain the observed change of fungal competition under heat stress. The model describes mycelial growth, the production and secretion of antifungal compounds and the synthesis of heat shock proteins of interacting colonies. We found a heat stress‐induced lag phase favouring the accumulation of antifungal compounds and the build‐up of inhibitor fields. This led to a qualitative change of the competitive outcome, reducing the occurrence of overgrowth by two thirds. The changes in competitive outcome favoured slower growing species, which benefit more strongly from the additional time during a stress‐induced lag to build up a defence or block territory that would otherwise be quickly claimed by faster competitors.Our work is an important step towards understanding how environmental changes may lead to qualitative changes in competitive outcomes. Our results show the importance of explicitly including species interactions into studies of climate change effects.

Articles of Significant Interest in This Issue

Baiome et al. (e00155-22) investigated the composition and antifungal properties of volatile organic compounds (VOCs) produced by the worm symbiont Xenorhabdus indica strain AB. Methyl anthranilate in the VOC mix was particularly effective against fungal phytopathogens. The secretion of these antifungal compounds allows the bacteria to kill fungal competitors and can be harnessed in biocontrol applications.

Validation and Ecological Niche Investigation of a New Fungal Intraspecific Competitor as a Biocontrol Agent for the Sustainable Containment of Aflatoxins on Maize Fields.

Crop yield and plant products quality are directly or indirectly affected by climate alterations. Adverse climatic conditions often promote the occurrence of different abiotic stresses, which can reduce or enhance the susceptibility to pests or pathogens. Aflatoxin producing fungi, in particular, whose diffusion and deleterious consequences on cereals commodities have been demonstrated to highly depend on the temperature and humidity conditions that threaten increasingly larger areas. Biological methods using intraspecific competitors to prevent fungal development and/or toxin production at the pre-harvest level are particularly promising, even if their efficacy could be affected by the ecological interaction within the resident microbial population. A previously characterized Aspergillus flavus atoxigenic strain was applied in two maize fields to validate its effectiveness as a biocontrol agent against aflatoxin contamination. At one month post-application, at the harvest stage, its persistence within the A. flavus population colonizing the maize kernels in the treated area was assessed, and its efficacy was compared in vitro with a representation of the isolated atoxigenic population. Results proved that our fungal competitor contained the aflatoxin level on maize grains as successfully as a traditional chemical strategy, even if representing less than 30% of the atoxigenic strains re-isolated, and achieved the best performance (in terms of bio-competitive potential) concerning endogenous atoxigenic isolates.

Delving into defence: identifying the Pseudomonas protegens Pf-5 gene suite involved in defence against secreted products of fungal, oomycete and bacterial rhizosphere competitors.

Competitive behaviours of plant growth promoting rhizobacteria (PGPR) are integral to their ability to colonize and persist on plant roots and outcompete phytopathogenic fungi, oomycetes and bacteria. PGPR engage in a range of antagonistic behaviours that have been studied in detail, such as the production and secretion of compounds inhibitory to other microbes. In contrast, their defensive activities that enable them to tolerate exposure to inhibitory compounds produced by their neighbours are less well understood. In this study, the genes involved in the Pseudomonas protegens Pf-5 response to metabolites from eight diverse rhizosphere competitor organisms, Fusarium oxysporum, Rhizoctonia solani, Gaeumannomyces graminis var. tritici, Pythium spinosum, Bacillus subtilis QST713, Pseudomonas sp. Q2-87, Streptomyces griseus and Streptomyces bikiniensis subspecies bikiniensi, were examined. Proximity induced excreted metabolite responses were confirmed for Pf-5 with all partner organisms through HPLC before culturing a dense Pf-5 transposon mutant library adjacent to each of these microbes. This was followed by transposon-directed insertion site sequencing (TraDIS), which identified genes that influence Pf-5 fitness during these competitive interactions. A set of 148 genes was identified that were associated with increased fitness during competition, including cell surface modification, electron transport, nucleotide metabolism, as well as regulatory genes. In addition, 51 genes were identified for which loss of function resulted in fitness gains during competition. These included genes involved in flagella biosynthesis and cell division. Considerable overlap was observed in the set of genes observed to provide a fitness benefit during competition with all eight test organisms, indicating commonalities in the competitive response to phylogenetically diverse micro-organisms and providing new insight into competitive processes likely to take place in the rhizosphere.

Circular alternatives to peat in growing media: A microbiome perspective

Peat use in horticulture is associated with a large ecological footprint. Peat is the predominant growing media in Europe. Modern cropping systems rely heavily on dynamic interactions of the crop with the microorganisms in the growing media and yet, in the search for sustainable peat-alternatives, the microbiome of the growing media has often been ignored. In mushroom cultivation, peat is a prime determinant of productivity, in the form of a casing soil which supplies beneficial microbes. In this study we describe the microbial composition, interactions, and activity of four circular substrates used to proportionally replace peat in mushroom growing media. We also evaluate various physico-chemical characteristics of the peat-alternatives. We characterize the impact of sanitary pre-treatments such as steaming and acidification on the microbiome as well as the agronomical performance of the peat-reduced growing media. We found that grass fibres from agricultural residue streams, peat-moss farmed in degraded peatlands, and spent casing soil recycled from previous cultivation cycles can be used to successfully replace peat in mushroom growing media. Peat moss and spent casing were expectedly similar to peat in physical, chemical, and microbiological properties. However, the grass fibres had unique characteristics, such as high organic matter content, low water holding capacity and a diverse and competitive microbiome. Pre-treatment of the substrates by acidification and steaming significantly affected the microbiome, and reduced the presence of pests, pathogens and competitive fungi in the peat-reduced media. Strong trade-offs existed between the productivity and disease pressure in the circular cropping system, which are also governed by the microbial composition of the growing media. Knowledge on the accessibility, sustainability, and economic viability of these peat-alternatives will further determine the transition away from peat use and towards sustainable growing media.

A Comprehensive Phylogenetic and Bioinformatics Survey of Lectins in the Fungal Kingdom.

Fungal lectins are a large family of carbohydrate-binding proteins with no enzymatic activity. They play fundamental biological roles in the interactions of fungi with their environment and are found in many different species across the fungal kingdom. In particular, their contribution to defense against feeders has been emphasized, and when secreted, lectins may be involved in the recognition of bacteria, fungal competitors and specific host plants. Carbohydrate specificities and quaternary structures vary widely, but evidence for an evolutionary relationship within the different classes of fungal lectins is supported by a high degree of amino acid sequence identity. The UniLectin3D database contains 194 fungal lectin 3D structures, of which 129 are characterized with a carbohydrate ligand. Using the UniLectin3D lectin classification system, 109 lectin sequence motifs were defined to screen 1223 species deposited in the genomic portal MycoCosm of the Joint Genome Institute. The resulting 33,485 putative lectin sequences are organized in MycoLec, a publicly available and searchable database. These results shed light on the evolution of the lectin gene families in fungi.

Coordinated control of the type IV pili and c-di-GMP-dependent antifungal antibiotic production in Lysobacter by the response regulator PilR.

In the soil gammaproteobacterium Lysobacter enzymogenes, a natural fungal predator, the response regulator PilR controls type IV pili (T4P)‐mediated twitching motility as well as synthesis of the heat‐stable antifungal factor (HSAF). Earlier we showed that PilR acts via the second messenger, c‐di‐GMP; however, the mechanism remained unknown. Here, we describe how PilR, c‐di‐GMP signalling, and HSAF synthesis are connected. We screened genes for putative diguanylate cyclases (c‐di‐GMP synthases) and found that PilR binds to the promoter region of lchD and down‐regulates its transcription. The DNA‐binding affinity of PilR, and therefore its repressor function, are enhanced by phosphorylation by its cognate histidine kinase, PilS. The lchD gene product is a diguanylate cyclase, and the decrease in LchD levels shifts the ratio of c‐di‐GMP‐bound and c‐di‐GMP‐free transcription factor Clp, a key activator of the HSAF biosynthesis operon expression. Furthermore, Clp directly interacts with LchD and enhances its diguanylate cyclase activity. Therefore, the PilS–PilR two‐component system activates T4P‐motility while simultaneously decreasing c‐di‐GMP levels and promoting HSAF production via the highly specific LchD–c‐di‐GMP–Clp pathway. Coordinated increase in motility and secretion of the “long‐distance” antifungal weapon HSAF is expected to ensure safer grazing of L. enzymogenes on soil or plant surfaces, unimpeded by fungal competitors, or to facilitate bacterial preying on killed fungal cells. This study uncovered the mechanism of coregulated pili‐based motility and production of an antifungal antibiotic in L. enzymogenes, showcased the expanded range of functions of the PilS–PilR system, and highlighted exquisite specificity in c‐di‐GMP‐mediated circuits.

EFFECT OF CASING MATERIAL MANAGEMENT ON YIELD AND YIELD ATTRIBUTES OF MILKY WHITE MUSHROOM (CALOCYBE INDICA)

General practice in Bangladesh is, after harvest producers leave the non-effective fruiting body of milky white mushroom in the spawn packet as it is. But every fruiting body primordia take some nutrient from the substrate, and the dried & rotten primordia may encourage other competitive fungi or harmful microorganism to grow on the upper surface of the substrate which may affect the fruiting body formation and yield in the subsequent flushes. Therefore, the present study was under taken to know the effect of casing material management technique on yield and yield attributes of milky white mushroom. Five different casing material management technique were practiced in this experiment, such as T1= removal of dried non effective fruiting bodies after each harvest; T2= removal of dried non effective fruiting bodies and filling the casing hole with fresh casing material after each harvest; T3= scraping the upper surface of the substrate after each harvest; T4= scraping the upper surface of the substrate and adding 10% fresh casing material after each harvest; and T5= no disturbance of the casing material (control). Number of effective fruiting body (NEFB), number of flushes, days to total harvest and size of fruiting body were significantly affected by casing material management technique but economic yield and biological efficiency among the treatments were insignificant. Considering all the parameters removal of dried non effective fruiting bodies and filling the casing hole with fresh casing material after each harvest (T2) was the best technique for casing material management.

An agent-based model of the foraging ascomycete hypothesis

Abstract Most trees host hundreds of species of fungi asymptomatically in their internal tissues, known collectively as fungal endophytes. The Foraging Ascomycete (FA) hypothesis proposed that some fungal endophytes inhabit the internal leaf tissue of forest trees in order to enhance dispersal to substrates on the forest floor, by using leaves as vectors and as refugia during periods of environmental stress. This dispersal strategy has recently been termed viaphytism and is now thought to apply to fungal groups beyond ascomycetes. Viaphytism implies that many fungi may be in continuous and cyclical flux between life stages as endophytes in the forest canopy and as wood-decomposing fungi on the forest floor. This cycle may represent a very common and previously-ignored process in the ecology of forests, with implications for forest health. The ecological consequences of the viaphyte lifestyle are complex, so we constructed an agent-based model to explore it. Our model is intended to serve as both an explicit conceptual explanation of viaphytism, and as an exploration of the conditions in which a strategy of endophytism accompanied by leaf dispersal may be advantageous for fungi. In a scenario of a viaphytic fungal species on a model forest landscape, without fungal competitors, viaphytism is predicted to be a plausible alternative to dispersal to substrates by spores alone, allowing the fungus to persist reliably in the landscape. In a scenario that allows competition from aggressively dispersed non-viaphytic fungi, the model predicts some competitive benefits to fungal dispersal via leaves. However, these benefits are conditional, requiring sufficient retention through time of endophyte infections by host trees, and sufficient host trees in the landscape. In the model, loss of these fungal populations can result from increased local disturbances of forest canopy, and deforestation.

A trait-based understanding of wood decomposition by fungi

As the primary decomposers of organic material in terrestrial ecosystems, fungi are critical agents of the global carbon cycle. Yet our ability to link fungal community composition to ecosystem functioning is constrained by a limited understanding of the factors accounting for different wood decomposition rates among fungi. Here we examine which traits best explain fungal decomposition ability by combining detailed trait-based assays on 34 saprotrophic fungi from across North America in the laboratory with a 5-y field study comprising 1,582 fungi isolated from 74 decomposing logs. Fungal growth rate (hyphal extension rate) was the strongest single predictor of fungal-mediated wood decomposition rate under laboratory conditions, and accounted for up to 27% of the in situ variation in decomposition in the field. At the individual level, decomposition rate was negatively correlated with moisture niche width (an indicator of drought stress tolerance) and with the production of nutrient-mineralizing extracellular enzymes. Together, these results suggest that decomposition rates strongly align with a dominance-tolerance life-history trade-off that was previously identified in these isolates, forming a spectrum from slow-growing, stress-tolerant fungi that are poor decomposers to fast-growing, highly competitive fungi with fast decomposition rates. Our study illustrates how an understanding of fungal trait variation could improve our predictive ability of the early and midstages of wood decay, to which our findings are most applicable. By mapping our results onto the biogeographic distribution of the dominance-tolerance trade-off across North America, we approximate broad-scale patterns in intrinsic fungal-mediated wood decomposition rates.

Co-culture of the bacterium Pseudomonas aeruginosa with the fungus Fusarium tricinctum induces bacterial antifungal and quorum sensing signaling molecules

Co-cultivation of viable cells of the human pathogenic bacterium Pseudomonas aeruginosa with the endophytic fungus Fusarium tricinctum induced silent bacterial biosynthetic gene clusters. Visual and chemical inspection of the co-cultures by comparison with the axenic cultures showed a clear reaction of P. aeruginosa towards its fungal competitor which was reflected by the deep greenish coloration of the co-cultures. Inspection of the HPLC chromatograms of the co-cultures revealed the induction of the pseudomonal quorum sensing molecule 2-heptyl-4-hydroxy-quinolone (HHQ) along with the antifungal phenazine alkaloids, phenazine-1-carboxylic acid (PCA) and phenazine-1-carboxamide (PCN), which are well-known metabolites of P. aeruginosa. The latter compounds were not detected in axenic bacterial cultures. In contrast to P. aeruginosa, no metabolic responses were detected for F. tricinctum which is in sharp contrast to previous co-culture experiments with this fungus using other bacteria.

Why Does Phlebiopsis gigantea not Always Inhibit Root and Butt Rot in Conifers?

This review aims to identify possible causes of differing effectiveness of artificial biological control of Heterobasidion root rot by the saprotrophic fungus Phlebiopsis gigantea. We describe published information in terms of pathogen–competitor relationships and the impact of environmental and genetic factors. We also revisit data from original research performed in recent years at the Forest Research Institute in Poland. We hypothesized that, in many cases, competition in roots and stumps of coniferous trees between the necrotrophic Heterobasidion spp. and the introduced saprotroph, Phlebiopsis gigantea, is affected by growth characteristics and enzymatic activity of the fungi, the characteristics of the wood, and environmental conditions. We concluded that both wood traits and fungal enzymatic activity during wood decay in roots and stumps, and the richness of the fungal biota, may limit biological control of root rot. In addition, we identify the need for research on new formulations and isolates of the fungal competitor, Phlebiopsis gigantea, as well as on approaches for accurately identifying the infectious threat from pathogens.

Serendipity in the wrestle between Trichoderma and Metarhizium

The fungal species Trichoderma is frequently found in soil antagonizing plant-pathogenic fungi as well as parasitizing plant-pathogenic nematodes. Metarhizium species are insect-pathogenic fungi that are used throughout the world to control agricultural insect pests. Here, we determine whether the antagonism (A) of Trichoderma atroviride to Metarhizium robertsii during growth and spore formation can impact the stress biology of M. robertsii conidia. Cultures of M. robertsii were either produced without exposure to T. atroviride (control) or in the presence of T. atroviride. M. robertsii was grown in dual culture with T. atroviride on potato dextrose agar (PDA) using the following treatments: 1) Trichoderma inoculated at the same time with Metarhizium (A0); 2) Trichoderma inoculated two days after the inoculation of Metarhizium (A2); 3) Trichoderma inoculated four days after Metarhizium (A4); 4) Trichoderma inoculated 6 d after Metarhizium (A6); 5) M. robertsii grown alone on PDA medium (control); and 6) M. robertsii grown alone on minimal medium (Czapek–Dox medium without sucrose) (MM). Germination of M. robertsii conidia from all six treatments was then assessed under osmotic, oxidative, UV-B, and thermal stress. M. robertsii conidia produced on MM were the most tolerant to all stress conditions. For all stress conditions, conidia from treatments A0 and A2 were not viable. For osmotic stress, conidia produced in treatment A4 were the most tolerant, followed by conidia from treatment A6, which were both more tolerant than the control. For oxidative stress, conidia produced in both A4 and A6 treatments were similarly tolerant and more tolerant than conidia produced in the control. For thermal stress, conidia produced in treatments A4, A6, and control (PDA) were similarly heat-tolerant. For UV-B stress, conidia produced in treatments A4 and A6 were equally tolerant and more tolerant than conidia produced in the control. The germination speed of conidia produced in all treatments, A0, A2, A4, and A6 was also tested. Conidia produced on MM germinated faster than the other treatments. Conidia produced in the A4 treatment were the second fastest, followed by conidia produced in treatment A6. Both A4 and A6 conidia germinated faster than conidia produced in the control treatment. Conidia produced in the treatments A0 and A2 did not germinate in 24 h. In summary, moderate levels of biotic stress from a fungal competitor or low-nutrient conditions can enhance the stress tolerance of M. robertsii conidia.

A New Front in Microbial Warfare-Delivery of Antifungal Effectors by the Type VI Secretion System.

Microbes typically exist in mixed communities and display complex synergistic and antagonistic interactions. The Type VI secretion system (T6SS) is widespread in Gram-negative bacteria and represents a contractile nano-machine that can fire effector proteins directly into neighbouring cells. The primary role assigned to the T6SS is to function as a potent weapon during inter-bacterial competition, delivering antibacterial effectors into rival bacterial cells. However, it has recently emerged that the T6SS can also be used as a powerful weapon against fungal competitors, and the first fungal-specific T6SS effector proteins, Tfe1 and Tfe2, have been identified. These effectors act via distinct mechanisms against a variety of fungal species to cause cell death. Tfe1 intoxication triggers plasma membrane depolarisation, whilst Tfe2 disrupts nutrient uptake and induces autophagy. Based on the frequent coexistence of bacteria and fungi in microbial communities, we propose that T6SS-dependent antifungal activity is likely to be widespread and elicited by a suite of antifungal effectors. Supporting this hypothesis, homologues of Tfe1 and Tfe2 are found in other bacterial species, and a number of T6SS-elaborating species have been demonstrated to interact with fungi. Thus, we envisage that antifungal T6SS will shape many polymicrobial communities, including the human microbiota and disease-causing infections.

Patterns of Hericium coralloides growth with competitive fungi.

Patterns of <i>Hericium coralloides</i> growth with competitive fungi.