Roles of Fungal Volatiles from Perspective of Distinct Lifestyles in Filamentous Fungi

Impact of bacterial and fungal inoculants on the resident rhizosphere microbiome and the volatilome of tomato plants under leaf herbivory stress

Various studies have addressed the impact of microbial inoculants on the composition of the resident microbiome. How microbial inoculants impact plant metabolism and interact with the resident rhizobiota under herbivory stress remains elusive. Here, we investigated the impact of two bacterial and two fungal inoculants, inoculated as single species and as a synthetic community, on the rhizosphere microbiome and volatilome of tomato plants (Solanum lycopersicum) comparing nonstress conditions to exposed to leaf herbivory by Spodoptera exigua. Based on amplicon sequencing analysis, rhizobacterial community composition was significantly affected by all four inoculants and the magnitude of this effect was dependent on herbivory stress. Fungal community composition was altered by the microbial inoculants but independent of herbivory stress. The rhizosphere volatilome was impacted by the microbial inoculation and differences between treatments were evened under herbivory stress. Each microbial inoculant caused unique changes in the volatilome of stressed plants but also shared similar responses, in particular the enhanced production of dimethyl disulfide and benzothiazole. In conclusion, the introduction of microbial inoculants in the tomato rhizosphere caused unique as well as common changes in the rhizosphere microbiome and volatilome, but these changes were minor compared to the microbiome changes induced by herbivory stress.

Fungal volatiles have physiological properties

All fungi emit mixtures of volatile organic compounds (VOCs) during growth. The qualitative and quantitative composition of these volatile mixtures vary with the species of fungus, the age of the fungus, and the environmental parameters attending growth. In nature, fungal VOCs are found as combinations of alcohols, aldehydes, acids, ethers, esters, ketones, terpenes, thiols and their derivatives, and are responsible for the characteristic odors associated with molds, mushrooms and yeasts. One of the single most common fungal volatiles is 1-octen-3-ol also known as "mushroom alcohol" or "matsutake alcohol." Many volatiles, including 1-octen-3-ol, serve as communication agents and display biological activity as germination inhibitors, plant growth retardants or promoters, and as semiochemicals ("infochemicals") in interactions with arthropods. Volatiles are understudied and underappreciated elements of the chemical lives of fungi. This review gives a brief introduction to fungal volatiles in hopes of raising awareness of the physiological importance of these gas phase fungal metabolites to encourage mycologists and other biologists to stop "throwing away the head space."

Why Do These Yeasts Smell So Good? Volatile Organic Compounds (VOCs) Produced by Malassezia Species in the Exponential and Stationary Growth Phases

Malassezia synthesizes and releases volatile organic compounds (VOCs), small molecules that allow them to carry out interaction processes. These lipid-dependent yeasts belong to the human skin mycobiota and are related to dermatological diseases. However, knowledge about VOC production and its function is lacking. This study aimed to determine the volatile profiles of Malassezia globosa, Malassezia restricta, and Malassezia sympodialis in the exponential and stationary growth phases. The compounds were separated and characterized in each growth phase through headspace solid-phase microextraction (HS-SPME) and gas chromatography–mass spectrometry (GC–MS). We found a total of 54 compounds, 40 annotated. Most of the compounds identified belong to alcohols and polyols, fatty alcohols, alkanes, and unsaturated aliphatic hydrocarbons. Unsupervised and supervised statistical multivariate analyses demonstrated that the volatile profiles of Malassezia differed between species and growth phases, with M. globosa being the species with the highest quantity of VOCs. Some Malassezia volatiles, such as butan-1-ol, 2-methylbutan-1-ol, 3-methylbutan-1-ol, and 2-methylpropan-1-ol, associated with biological interactions were also detected. All three species show at least one unique compound, suggesting a unique metabolism. The ecological functions of the compounds detected in each species and growth phase remain to be studied. They could interact with other microorganisms or be an important clue in understanding the pathogenic role of these yeasts.

Filamentous fungi for sustainable vegan food production systems within a circular economy: Present status and future prospects

Filamentous fungi serve as potential candidates in the production of different value-added products. In the context of food, there are several advantages of using filamentous fungi for food. Among the main advantages is that the fungal biomass used food not only meets basic nutritional requirements but that it is also rich in protein, low in fat, and free of cholesterol. This speaks to the potential of filamentous fungi in the production of food that can substitute animal-derived protein sources such as meat. Moreover, life-cycle analyses and techno-economic analyses reveal that fungal proteins perform better than animal-derived proteins in terms of land use efficiency as well as global warming. The present article provides an overview of the potential of filamentous fungi as a source of food and food supplements. The commercialization potential as well as social, legal and safety issues of fungi-based food products are discussed.

Genomic Comparisons of Two Armillaria Species with Different Ecological Behaviors and Their Associated Soil Microbial Communities

Armillaria species show considerable variation in ecological roles and virulence, from mycorrhizae and saprophytes to important root pathogens of trees and horticultural crops. We studied two Armillaria species that can be found in coniferous forests of northwestern USA and southwestern Canada. Armillaria altimontana not only is considered as a weak, opportunistic pathogen of coniferous trees, but it also appears to exhibit in situ biological control against A. solidipes, formerly North American A. ostoyae, which is considered a virulent pathogen of coniferous trees. Here, we describe their genome assemblies and present a functional annotation of the predicted genes and proteins for the two Armillaria species that exhibit contrasting ecological roles. In addition, the soil microbial communities were examined in association with the two Armillaria species within a 45-year-old plantation of western white pine (Pinus monticola) in northern Idaho, USA, where A. altimontana was associated with improved tree growth and survival, while A. solidipes was associated with reduced growth and survival. The results from this study reveal a high similarity between the genomes of the beneficial/non-pathogenic A. altimontana and pathogenic A. solidipes; however, many relatively small differences in gene content were identified that could contribute to differences in ecological lifestyles and interactions with woody hosts and soil microbial communities.

Impact of Volatile Organic Compounds on the Growth of Aspergillus flavus and Related Aflatoxin B1 Production: A Review

Volatile organic compounds (VOCs) are secondary metabolites of varied chemical nature that are emitted by living beings and participate in their interactions. In addition, some VOCs called bioactive VOCs cause changes in the metabolism of other living species that share the same environment. In recent years, knowledge on VOCs emitted by Aspergillus flavus, the main species producing aflatoxin B1 (AFB1), a highly harmful mycotoxin, has increased. This review presents an overview of all VOCs identified as a result of A. flavus toxigenic (AFB1-producing) and non-toxigenic (non AFB1-producing) strains growth on different substrates, and the factors influencing their emissions. We also included all bioactive VOCs, mixes of VOCs or volatolomes of microbial species that impact A. flavus growth and/or related AFB1 production. The modes of action of VOCs impacting the fungus development are presented. Finally, the potential applications of VOCs as biocontrol agents in the context of mycotoxin control are discussed.

Endophytic Fungal Terpenoids: Natural Role and Bioactivities

Endophytic fungi are a highly diverse group of fungi that intermittently colonize all plants without causing symptoms of the disease. They sense and respond to physiological and environmental changes of their host plant and microbiome. The inter-organism interactions are largely driven by chemical networks mediated by specialized metabolites. The balance of these complex interactions leads to healthy and strong host plants. Endophytic strains have particular machinery to produce a plethora of secondary metabolites with a variety of bioactivities and unknown functions in an ecological niche. Terpenoids play a key role in endophytism and represent an important source of bioactive molecules for human health and agriculture. In this review, we describe the role of endophytic fungi in plant health, fungal terpenoids in multiple interactions, and bioactive fungal terpenoids recently reported from endophytes, mainly from plants used in traditional medicine, as well as from algae and mangroves. Additionally, we highlight endophytic fungi as producers of important chemotherapeutic terpenoids, initially discovered in plants. Despite advances in understanding endophytism, we still have much to learn in this field. The study of the role, the evolution of interactions of endophytic fungi and their terpenoids provide an opportunity for better applications in human health and agriculture.

Eight-carbon volatiles: prominent fungal and plant interaction compounds

Signaling via volatile organic compounds (VOCs) has historically been studied mostly by entomologists; however, botanists and mycologists are increasingly aware of the physiological potential of chemical communication in the gas phase. Most research to date focuses on the observed effects of VOCs on different organisms such as differential growth or metabolite production. However, with the increased interest in volatile signaling, more researchers are investigating the molecular mechanisms for these effects. Eight-carbon VOCs are among the most prevalent and best-studied fungal volatiles. Therefore, this review emphasizes examples of eight-carbon VOCs affecting plants and fungi. These compounds display different effects that include growth suppression in both plants and fungi, induction of defensive behaviors such as accumulation of mycotoxins, phytohormone signaling cascades, and the inhibition of spore and seed germination. Application of '-omics' and other next-generation sequencing techniques is poised to decipher the mechanistic basis of volatiles in plant-fungal communication.

Secondary Metabolite Production and Terpenoid Biosynthesis in Endophytic Fungi Cladosporium cladosporioides Isolated from Wild Cymbopogon martinii (Roxb.) Wats

Endophytic fungi Cladosporium cladosporioides (F1-MH810309) and Cladosporium tenuissimum (F2-MN715834) from the leaf of wild Cymbopogon martinii (MT90507) were isolated and selected based on the persistent occurrence during different seasons of the year. They were identified based on the morphological features and molecular characterization (ITS sequence), and later deposited at NCBI. Phytochemical studies on F1, F2 and host extracts showed the presence of alkaloids, flavonoids, phenols, terpenoids and tannins. The GC-MS of F1 extract (control) under the axenic condition revealed compounds like hexadecane, heptadecane,2,4-Ditert-butylphenol, E-14 hexadecenal, geraniol, geranyl acetate and cubenol similar to the host. The GC-MS of F2 extract (control) revealed metabolites that were unique. Further, both F1 and F2 were cultured in the supplementation of different concentrations (5%, 10%, 15% and 20%) of the host plant extract (an-axenic condition). The GC-MS of F1 extracts (test) exhibited good growth and showed the gradual increased production of terpenoid compounds whereas the F2 (test) did not show any growth. These compounds such as hyrdoxymenthol, nor-borneol, cedralacetate, α-cyclogeraniol, campesterol, β-cyclogeraniol, linalool oxide,2,3-boranediol, citronellyltiglate and 2,3-pinanediol were produced in a minor quantity and were known as biotransformed forms of the precursor compounds present in the host extract. In comparison, only F1 was able to produce terpenoids similar to the host species both in axenic and an-axenic conditions. Hence from the current study, the endophytic fungus F1 isolated from wild C. martinii for the first time can serve as a better resource for the bioprospection of an important terpenoid and its metabolites.

Volatile emission and biosynthesis in endophytic fungi colonizing black poplar leaves

Plant volatiles play a major role in plant-insect interactions as defense compounds or attractants for insect herbivores. Recent studies have shown that endophytic fungi are also able to produce volatiles and this raises the question of whether these fungal volatiles influence plant-insect interactions. Here, we qualitatively investigated the volatiles released from 13 endophytic fungal species isolated from leaves of mature black poplar (Populus nigra) trees. The volatile blends of these endophytes grown on agar medium consist of typical fungal compounds, including aliphatic alcohols, ketones and esters, the aromatic alcohol 2-phenylethanol and various sesquiterpenes. Some of the compounds were previously reported as constituents of the poplar volatile blend. For one endophyte, a species of Cladosporium, we isolated and characterized two sesquiterpene synthases that can produce a number of mono- and sesquiterpenes like (E)-β-ocimene and (E)-β-caryophyllene, compounds that are dominant components of the herbivore-induced volatile bouquet of black poplar trees. As several of the fungus-derived volatiles like 2-phenylethanol, 3-methyl-1-butanol and the sesquiterpene (E)-β-caryophyllene, are known to play a role in direct and indirect plant defense, the emission of volatiles from endophytic microbial species should be considered in future studies investigating tree-insect interactions.

Arcopilus aureus MaC7A as a New Source of Resveratrol: Assessment of Amino Acid Precursors, Volatiles, and Fungal Enzymes for Boosting Resveratrol Production in Batch Cultures

The chemical factors that regulate the synthesis of resveratrol (RV) in filamentous fungi are still unknown. This work reports on the RV production by Arcopilus aureus MaC7A under controlled conditions and the effect of amino acid precursors (PHE and TYR), monoterpenes (limonone, camphor, citral, thymol, menthol), and mixtures of hydrolytic enzymes (Glucanex) as elicitors for boosting fungal RV. Batch cultures with variable concentrations of PHE and TYR (50–500 mg L−1) stimulated RV production from 127.9 ± 4.6 to 221.8 ± 5.2 mg L−1 in basic cultures developed in PDB (pH 7) added with 10 g L−1 peptone at 30 °C. Maximum levels of RV and biomass were maintained during days 6–8 under these conditions, whereas a dramatic RV decrease was observed from days 10–12 without any loss of biomass. Among the tested volatiles, citral (50 mg L−1) enhanced RV production until 187.8 ± 2.2 mg L−1 in basic cultures, but better results were obtained with Glucanex (100 mg L−1; 198.3 ± 7.6 mg L−1 RV). Optimized batch cultures containing TYR (200 mg L−1), citral (50 mg L−1), thymol (50 mg L−1), and Glucanex (100 mg L−1) produced up to 237.6 ± 4.7 mg L−1 of RV. Our results suggest that low concentrations of volatiles and mixtures of isoenzymes with β-1, 3 glucanase activity increase the biosynthesis of fungal RV produced by A. aureus MaC7A in batch cultures.

High-Throughput Volatilome Fingerprint Using PTR-ToF-MS Shows Species-Specific Patterns in Mortierella and Closely Related Genera

In ecology, Volatile Organic Compounds (VOCs) have a high bioactive and signaling potential. VOCs are not only metabolic products, but are also relevant in microbial cross talk and plant interaction. Here, we report the first large-scale VOC study of 13 different species of Mortierella sensu lato (s. l.) isolated from a range of different alpine environments. Proton Transfer Reaction-Time-of-Flight Mass Spectrometry (PTR-ToF-MS) was applied for a rapid, high-throughput and non-invasive VOC fingerprinting of 72 Mortierella s. l. isolates growing under standardized conditions. Overall, we detected 139 mass peaks in the headspaces of all 13 Mortierella s. l. species studied here. Thus, Mortierella s. l. species generally produce a high number of different VOCs. Mortierella species could clearly be discriminated based on their volatilomes, even if only high-concentration mass peaks were considered. The volatilomes were partially phylogenetically conserved. There were no VOCs produced by only one species, but the relative concentrations of VOCs differed between species. From a univariate perspective, we detected mass peaks with distinctively high concentrations in single species. Here, we provide initial evidence that VOCs may provide a competitive advantage and modulate Mortierella s. l. species distribution on a global scale.