Ubiquitous eight-carbon volatiles of fungi are infochemicals for a specialist fungivore

Cross-talk between immunity and behavior: insights from entomopathogenic fungi and their insect hosts

Insects are one of the most successful animals in nature, and entomopathogenic fungi play a significant role in the natural epizootic control of insect populations in many ecosystems. The interaction between insects and entomopathogenic fungi has continuously coevolved over hundreds of millions of years. Many components of the insect innate immune responses against fungal infection are conserved across phyla. Additionally, behavioral responses, which include avoidance, grooming, and/or modulation of body temperature, have been recognized as important mechanisms for opposing fungal pathogens. In an effort to investigate possible cross-talk and mediating mechanisms between these fundamental biological processes, recent studies have integrated and/or explored immune and behavioral responses. Current information indicates that during discrete stages of fungal infection, several insect behavioral and immune responses are altered simultaneously, suggesting important connections between the two systems. This review synthesizes recent advances in our understanding of the physiological and molecular aspects influencing cross-talk between behavioral and innate immune antifungal reactions, including chemical perception and olfactory pathways.

Analysis of Streptomyces Volatilomes Using Global Molecular Networking Reveals the Presence of Metabolites with Diverse Biological Activities

Streptomyces species produce a wide variety of specialized metabolites, some of which are used for communication or competition for resources in their natural environments. In addition, many natural products used in medicine and industry are derived from Streptomyces, and there has been interest in their capacity to produce volatile organic compounds (VOCs) for different industrial and agricultural applications. Recently, a machine-learning workflow called MSHub/GNPS was developed, which enables auto-deconvolution of gas chromatography-mass spectrometry (GC-MS) data, molecular networking, and library search capabilities, but it has not been applied to Streptomyces volatilomes. In this study, 131 Streptomyces isolates from the island of Newfoundland were phylogenetically typed, and 37 were selected based on their phylogeny and growth characteristics for VOC analysis using both a user-guided (conventional) and an MSHub/GNPS-based approach. More VOCs were annotated by MSHub/GNPS than by the conventional method. The number of unknown VOCs detected by the two methods was higher than those annotated, suggesting that many novel compounds remain to be identified. The molecular network generated by GNPS can be used to guide the annotation of such unknown VOCs in future studies. However, the number of overlapping VOCs annotated by the two methods is relatively small, suggesting that a combination of analysis methods might be required for robust volatilome analysis. More than half of the VOCs annotated with high confidence by the two approaches are plant-associated, many with reported bioactivities such as insect behavior modulation. Details regarding the properties and reported functions of such VOCs are described. IMPORTANCE This study represents the first detailed analysis of Streptomyces volatilomes using MSHub/GNPS, which in combination with a routinely used conventional method led to many annotations. More VOCs could be annotated using MSHub/GNPS as compared to the conventional method, many of which have known antimicrobial, anticancer, and insect behavior-modulating activities. The identification of numerous plant-associated VOCs by both approaches in the current study suggests that their production could be a more widespread phenomenon by members of the genus, highlighting opportunities for their large-scale production using Streptomyces. Plant-associated VOCs with antimicrobial activities, such as 1-octen-3-ol, octanol, and phenylethyl alcohol, have potential applications as fumigants. Furthermore, many of the annotated VOCs are reported to influence insect behavior, alluding to a possible explanation for their production based on the functions of other recently described Streptomyces VOCs in dispersal and nutrient acquisition.

Plant growth-promoting and non-promoting rhizobacteria from avocado trees differentially emit volatiles that influence growth of Arabidopsis thaliana

Microbial volatile organic compounds (mVOCs) play important roles in inter- and intra-kingdom interactions, and they are also important as signal molecules in physiological processes acting either as plant growth-promoting or negatively modulating plant development. We investigated the effects of mVOCs emitted by PGPR vs non-PGPR from avocado trees (Persea americana) on growth of Arabidopsis thaliana seedlings. Chemical diversity of mVOCs was determined by SPME-GC-MS; selected compounds were screened in dose-response experiments in A. thaliana transgenic lines. We found that plant growth parameters were affected depending on inoculum concentration. Twenty-six compounds were identified in PGPR and non-PGPR with eight of them not previously reported. The VOCs signatures were differential between those groups. 4-methyl-2-pentanone, 1-nonanol, 2-phenyl-2-propanol and ethyl isovalerate modified primary root architecture influencing the expression of auxin- and JA-responsive genes, and cell division. Lateral root formation was regulated by 4-methyl-2-pentanone, 3-methyl-1-butanol, 1-nonanol and ethyl isovalerate suggesting a participation via JA signalling. Our study revealed the differential emission of volatiles by PGPR vs non-PGPR from avocado trees and provides a general view about the mechanisms by which those volatiles influence plant growth and development. Rhizobacteria strains and mVOCs here reported are promising for improvement the growth and productivity of avocado crop.

Effects of Drying Process on the Volatile and Non-Volatile Flavor Compounds of Lentinula edodes

In this study, fresh Lentinula edodes was dehydrated using freeze-drying (FD), hot-air drying (HAD), and natural drying (ND), and the volatile and non-volatile flavor compounds were analyzed. The drying process changed the contents of eight-carbon compounds and resulted in a weaker “mushroom flavor” for dried L. edodes. HAD mushrooms had higher levels of cyclic sulfur compounds (56.55 μg/g) and showed a stronger typical shiitake mushroom aroma than those of fresh (7.24 μg/g), ND (0.04 μg/g), and FD mushrooms (3.90 μg/g). The levels of 5′-nucleotide increased, whereas the levels of organic acids and free amino acids decreased after the drying process. The dried L. edodes treated with FD had the lowest levels of total free amino acids (29.13 mg/g). However, it had the highest levels of umami taste amino acids (3.97 mg/g), bitter taste amino acids (6.28 mg/g) and equivalent umami concentration (EUC) value (29.88 g monosodium glutamate (MSG) per 100 g). The results indicated that FD was an effective drying method to produce umami flavor in dried mushrooms. Meanwhile, HAD can be used to produce a typical shiitake mushroom aroma. Our results provide a theoretical basis to manufacture L. edodes products with a desirable flavor for daily cuisine or in a processed form.

Biotransformation of [U-13C]linoleic acid suggests two independent ketonic- and aldehydic cycles within C8-oxylipin biosynthesis in Cyclocybe aegerita (V. Brig.) Vizzini

Although the typical aroma contributing compounds in fungi of the phylum Basidiomycota are known for decades, their biosynthetic pathways are still unclear. Amongst these volatiles, C8-compounds are probably the most important ones as they function, in addition to their specific perception of fungal odour, as oxylipins. Previous studies focused on C8-oxylipin production either in fruiting bodies or mycelia. However, comparisons of the C8-oxylipin biosynthesis at different developmental stages are scarce, and the biosynthesis in basidiospores was completely neglected. In this study, we addressed this gap and were able to show that the biosynthesis of C8-oxylipins differs strongly between different developmental stages. The comparison of mycelium, primordia, young fruiting bodies, mature fruiting bodies, post sporulation fruiting bodies and basidiospores revealed that the occurance of the two main C8-oxylipins octan-3-one and oct-1-en-3-ol distinguished in different stages. Whereas oct-1-en-3-ol levels peaked in the mycelium and decreased with ongoing maturation, octan-3-one levels increased during maturation. Furthermore, oct-2-en-1-ol, octan-1-ol, oct-2-enal, octan-3-ol, oct-1-en-3-one and octanal contributed to the C8-oxylipins but with drastically lower levels. Biotransformations with [U-13C]linoleic acid revealed that early developmental stages produced various [U-13C]oxylipins, whereas maturated developmental stages like post sporulation fruiting bodies and basidiospores produced predominantly [U-13C]octan-3-one. Based on the distribution of certain C8-oxylipins and biotransformations with putative precursors at different developmental stages, two distinct biosynthetic cycles were deduced with oct-2-enal (aldehydic-cycle) and oct-1-en-3-one (ketonic-cycle) as precursors.

Transcriptome of different fruiting stages in the cultivated mushroom Cyclocybe aegerita suggests a complex regulation of fruiting and reveals enzymes putatively involved in fungal oxylipin biosynthesis

BACKGROUND

Cyclocybe aegerita (syn. Agrocybe aegerita) is a commercially cultivated mushroom. Its archetypal agaric morphology and its ability to undergo its whole life cycle under laboratory conditions makes this fungus a well-suited model for studying fruiting body (basidiome, basidiocarp) development. To elucidate the so far barely understood biosynthesis of fungal volatiles, alterations in the transcriptome during different developmental stages of C. aegerita were analyzed and combined with changes in the volatile profile during its different fruiting stages.

RESULTS

A transcriptomic study at seven points in time during fruiting body development of C. aegerita with seven mycelial and five fruiting body stages was conducted. Differential gene expression was observed for genes involved in fungal fruiting body formation showing interesting transcriptional patterns and correlations of these fruiting-related genes with the developmental stages. Combining transcriptome and volatilome data, enzymes putatively involved in the biosynthesis of C8 oxylipins in C. aegerita including lipoxygenases (LOXs), dioxygenases (DOXs), hydroperoxide lyases (HPLs), alcohol dehydrogenases (ADHs) and ene-reductases could be identified. Furthermore, we were able to localize the mycelium as the main source for sesquiterpenes predominant during sporulation in the headspace of C. aegerita cultures. In contrast, changes in the C8 profile detected in late stages of development are probably due to the activity of enzymes located in the fruiting bodies.

CONCLUSIONS

In this study, the combination of volatilome and transcriptome data of C. aegerita revealed interesting candidates both for functional genetics-based analysis of fruiting-related genes and for prospective enzyme characterization studies to further elucidate the so far barely understood biosynthesis of fungal C8 oxylipins.

Volatilomes of Cyclocybe aegerita during different stages of monokaryotic and dikaryotic fruiting

Volatile organic compounds (VOC) are characteristic for different fungal species. However, little is known about VOC changes during development and their biological role. Therefore, we established a laboratory cultivation system in modified crystallizing dishes for analyzing VOC during fruiting body development of the dikaryotic strainCyclocybe aegeritaAAE-3 as well as four monokaryotic offspring siblings exhibiting different fruiting phenotypes. From these, VOC were extracted directly from the headspace (HS) and analyzed by means of gas chromatography-mass spectrometry (GC-MS). For all tested strains, alcohols and ketones, including oct-1-en-3-ol, 2-methylbutan-1-ol and cyclopentanone, were the dominant substances in the HS of early developmental stages. In the dikaryon, the composition of the VOC altered with ongoing fruiting body development and, even more drastically, during sporulation. At the latter stage, sesquiterpenes, especially Δ6-protoilludene, α-cubebene and δ-cadinene, were the dominant substances. After sporulation, the amount of sesquiterpenes decreased, while additional VOC, mainly octan-3-one, appeared. In the HS of the monokaryons, less VOC were present of which all were detectable in the HS of the dikaryonC. aegeritaAAE-3. The results of the present study show that the volatilome ofC. aegeritachanges considerably depending on the developmental stage of the fruiting body.

Fungal volatile organic compounds show promise as potent molluscicides

BACKGROUND

Slugs and snails constitute major crop pests. Withdrawal of metaldehyde has prompted a search for more environmentally friendly yet fast acting molluscicides. This study investigated the response of representative molluscs to conidia and volatile organic compounds (VOCs) of the insect pathogenic fungus Metarhizium brunneum Petch.

RESULTS

Conidia of M. brunneum had antifeedant/repellent properties with repellency being dependent upon the fungal strain and conidia concentration. Three commonly produced fungal VOCs, 1-octene, 3-octanone and 1-octen-3-ol, were repellent at low doses (1-5 μL) but could kill slugs and snails on contact or fumigation. At the highest dose tested (10 μL), 100% mortality was achieved for Cornu aspersum Muller (garden snail) and Derocerus reticulatum Muller (grey field slug) within 1 h post-treatment with the first deaths being recorded in <11 min. Aqueous formulations (20% v/v) of the most potent VOCs, 3-octanone and 1-octen-3-ol, could be sprayed onto plants to kill or drive the pest of the crop with no phytotoxic effects.

CONCLUSION

The sensitivity of terrestrial molluscs to 3-octanone and 1-octen-3-ol and the ephemeral nature of these compounds makes these excellent candidates for development as mollusc repellents or molluscicides. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Analysis of volatile compounds of Lentinula edodes grown in different culture substrate formulations

Volatile compounds of Lentinula edodes grown in different culture substrate (CS) formulations were analyzed to reveal (i) the relationship between volatile compound production and CS formulations, (ii) the contribution of volatile compounds to L. edodes flavor, (iii) the activities of LOX and γ-GGT enzymes, (iv) γ-GGT gene expression, and (v) the correlation between enzyme activity and volatile compound production. Our results showed that 82 kinds of volatile compounds were analyzed; 25 volatile compounds were considered key flavor components, and sulfur containing compounds, eight-carbon compounds, and aldehyde compounds also had great contributions to mushroom flavor. Bagasse could be used as a partial substitute for sawdust as a carbon source. LOX and γ-GGT activities showed a weak correlation with the volatile end products. The results indicated that the mechanisms by which CS formulations influence volatile compounds production were complex.

Aroma-volatile profile of black morel (Morchella importuna) grown in Israel

BACKGROUND

A headspace solid-phase microextraction method with gas chromatography-mass spectrometry was used to profile the aroma volatiles of mature fruiting bodies of Morchella importuna grown in Israel.

RESULTS

We tentatively identified 40 aroma compounds and seven unknown volatiles. The M. importuna aroma profile consisted of 14 aldehydes, six alcohols, 10 methyl esters, four heterocyclic/sulfur compounds, 10 carbohydrates and three other compounds (i.e. one acid, one ketone and one butyl ester). The most abundant volatiles were carbohydrates, with a total relative peak area of 29.3%, followed by alcohols (27.7%), aldehydes (21.6%), methyl esters (10.8%), heterocyclic/sulfur compounds (3.1%) and other compounds (5.8%). The 8-carbon (C8) compounds imparting typical mushroom-like aroma were very abundant in M. importuna, accounting for 27.9% of the total peak area and including, amongst others, 1-octen-3-ol (80% of total C8), octanal and 2-octenal (Z- and E-).

CONCLUSION

The aroma volatile profile of morels has much in common with that of other mushrooms, with a few unique characteristics. To our knowledge, this is the first detailed report of the aroma profile of M. importuna. © 2017 Society of Chemical Industry.

Fungal volatiles - a survey from edible mushrooms to moulds

Covering: up to January 2017This review gives a comprehensive overview of the production of fungal volatiles, including the history of the discovery of the first compounds and their distribution in the various investigated strains, species and genera, as unravelled by modern analytical methods. Biosynthetic aspects and the accumulated knowledge about the bioactivity and biological functions of fungal volatiles are also covered. A total number of 325 compounds is presented in this review, with 247 cited references.

Fungal allelochemicals in insect pest management

Interactions between insects and fungi are widespread, and important mediators of these interactions are fungal chemicals that can therefore be considered as allelochemicals. Numerous studies suggest that fungal chemicals can affect insects in many different ways. Here, we apply the terminology established by insect-plant ecologists for categorizing the effect of fungal allelochemicals on insects and for evaluating the application potential of these chemicals in insect pest management. Our literature survey shows that fungal volatile and non-volatile chemicals have an enormous potential to influence insect behavior and fitness. Many of them still remain to be discovered, but some recent examples of repellents and toxins could open up new ways for developing safe insect control strategies. However, we also identified shortcomings in our understanding of the chemical ecology of insect-fungus interactions and the way they have been investigated. In particular, the mode-of-action of fungal allelochemicals has often not been appropriately designated or examined, and the way in which induction by insects affects fungal chemical diversity is poorly understood. This review should raise awareness that in-depth ecological studies of insect-fungus interactions can reveal novel allelochemicals of particular benefit for the development of innovative insect pest management strategies.

Chemical diversity of microbial volatiles and their potential for plant growth and productivity

Microbial volatile organic compounds (MVOCs) are produced by a wide array of microorganisms ranging from bacteria to fungi. A growing body of evidence indicates that MVOCs are ecofriendly and can be exploited as a cost-effective sustainable strategy for use in agricultural practice as agents that enhance plant growth, productivity, and disease resistance. As naturally occurring chemicals, MVOCs have potential as possible alternatives to harmful pesticides, fungicides, and bactericides as well as genetic modification. Recent studies performed under open field conditions demonstrate that efficiently adopting MVOCs may contribute to sustainable crop protection and production. We review here the chemical diversity of MVOCs by describing microbial-plants and microbial-microbial interactions. Furthermore, we discuss MVOCs role in inducing phenotypic plant responses and their potential physiological effects on crops. Finally, we analyze potential and actual limitations for MVOC use and deployment in field conditions as a sustainable strategy for improving productivity and reducing pesticide use.