Volatile organic compounds produced by Saccharomyces cerevisiae inhibit the in vitro development of Guignardia citricarpa, the causal agent of citrus black spot

Development of an Application Method for Volatile Compounds Derived from Mushroom Fungi Beds as Plant Growth-Promoting Biostimulants

Volatile compounds (VCs) from fungi can promote plant growth, but their application methods are limited. Edible mushroom fungi beds (FBs) provide a readily available alternative source of fungal VCs, although their biostimulatory functions remain unvalidated. In this study, a novel, non-contact exposure method for applying VCs emitted from FBs to rice seedlings was developed. This marks the first evaluation of mushroom FBs as a direct source of bioactive VCs for plant growth promotion. Volatiles from two different edible mushroom FBs promoted shoot growth and increased biomass for rice seedlings. VCs from shiitake FBs significantly increased biomass by 67.4% while VCs from enokitake FBs by 39.5% compared to the control. The biomass-increasing effects were influenced by the quantity of shiitake FBs applied, with significant increases at 15 g, 30 g and 60 g applications. The VCs effects remained significant even when the FBs were covered with two types of gas-permeable polymer film. Chemical analysis of VCs from FBs identified several organic compounds and subsequent bioassays using synthetic VCs determined key bioactive VCs contributing to biomass increase at specific concentrations. This study presents a utilization method of waste mushroom FBs as sustainable, scalable, and cost-effective agricultural biostimulants.

Isolation and identification of antifungal, antibacterial and nematocide agents from marine bacillus gottheilii MSB1

Pathogenic fungi employ numerous strategies to colonize plants, infect them, reduce crop yield and quality, and cause significant losses in agricultural production. The increasing use of chemical pesticides has led to various ecological and environmental issues, including the emergence of resistant weeds, soil compaction, and water pollution, all negatively impacting agricultural sustainability. Additionally, the extensive development of synthetic fungicides has adverse effects on animal and human health, prompting the exploration of alternative approaches and green strategies for phytopathogen control. Microorganisms living in sponges represent a promising source of novel bioactive secondary metabolites, potentially useful in developing new nematicidal and antimicrobial agents. This study focuses on extracting bioactive compounds from endosymbiotic bacteria associated with the marine sponge Hyrtios erect sp. (collected from NIOF Station, Hurghada, Red Sea, Egypt) using various organic solvents. Bacillus sp. was isolated and identified through 16 S rRNA gene sequencing. The biocidal activity of Bacillus gotheilii MSB1 extracts was screened against plant pathogenic bacteria, fungi, and nematodes. The n-butanol extract showed significant potential as a biological fungicide against Alternaria alternata and Fusarium oxysporum. Both n-hexane and ethyl acetate extracts exhibited negative impacts against the plant pathogenic bacteria Erwinia carotovora and Ralstonia solanacearum, whereas the n-butanol extract had a positive effect. Regarding nematicidal activity, ethyl acetate and n-butanol extracts demonstrated in-vitro activity against the root-knot nematode Meloidogyne incognita, which causes serious vegetable crop diseases, but the n-hexane extract showed no positive effects. The findings suggest that bioactive compounds from endosymbiotic bacteria associated with marine sponges, particularly B. gotheilii MSB1, hold significant potential as alternative biological control agents against plant pathogens. The n-butanol extract, in particular, displayed promising biocidal activities against various plant pathogenic fungi, bacteria, and nematodes. These results support further exploration and development of such bioactive compounds as sustainable, environmentally friendly alternatives to synthetic pesticides and fungicides in agricultural practices.

Characterization and biocontrol potential of Wickerhamomyces subpelliculosus yeasts isolated from dates: Volatile compounds-mediated antifungal activity against mycotoxigenic Penicillium strains

Seven yeast strains were isolated from Tunisian dates. The strains were identified by sequence analysis of the D1/D2 domain of the nuclear large subunit (LSU) rRNA gene. Based on this all strains in the study were almost identical with that of the type strain of Wickerhamomyces subpelliculosus (CBS 5767) indicating that they belong to this species. All strains were characterized physiologically and biochemically. All strains grew in the presence of 50 % sucrose, 10 % sodium chloride and at 42 °C. The potential of these yeasts as biocontrol agent against mycotoxigenic Penicillium species inhabiting date, was evaluated. All yeast strains inhibited the growth of P. citrinum P10 and P. chrysogenum C17 previously isolated from dates, with inhibition percentages ranging between 43.6 % and 70.3 % on dual culture plate assays. Moreover, the volatile compounds (VCs) produced by these yeasts inhibited the mycelial growth rate and sporulation of both fungus strains, up to 76.5 and 100 %, respectively, on inverted culture plate assay. The VCs of W. subpelliculosus strains Y4 and Y24, which exhibit strong inhibitory activity against toxigenic Penicillium, were determined by head-space solid-phase microextraction (HS-SPME) combined with gas chromatography coupled with mass spectrometry (GC-MS) analysis. Results revealed significant levels of alcohols (27.36 % for Y4 and 23.35 % for Y24) and esters (66.19 % for Y4 and 75.82 % for Y24). Their significant bioactivity, along with the lack of reported adverse effects on consumer health or the environment, makes them a sustainable and effective alternative to synthetic fungicides for the biocontrol of mycotoxigenic Penicillium affecting stored dates.

Transcriptomic analysis of Ralstonia solanacearum in response to antibacterial volatiles of Bacillus velezensis FZB42

Volatile organic compounds (VOCs), produced by a variety of microbial species and used as biological agents, have been demonstrated to play a significant role in controlling phytopathogens. In continuation of our previous studies, we aim to elucidate the underlying mechanisms and pathways involved in interactions between pathogens and microbial VOCs. In the current study, we tested how VOCs produced by Bacillus velezensis FZB42 affect the growth of Ralstonia solanacearum TBBS1 in vitro.Query The result showed that the colony growth of R. solanacearum was reduced with an inhibition rate of 0.83 ± 0.043 as compared to the control 1.7 ± 0.076, respectively. The number of viable cells of R. solanacearum was significantly decreased to 7.68 CFU/mL as compared to the control (9.02 CFU/mL). In addition, transcriptomic analysis of R. solanacearum in response to VOCs produced by FZB42 was performed to better understand the effect of VOCs on R. solanacearum. The transcriptional response of R. solanacearum to FZB42-VOCs was determined using an Illumina RNA-seq approach. The results revealed significant changes in the expression of 2094 R. solanacearum genes, including 593 upregulated and 1501 downregulated genes. To validate the RNA-seq results, the expression of 10 genes was quantified using RT-qPCR. Furthermore, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases were used to functionally annotate differentially expressed genes. Significant changes were observed in genes directly or indirectly related to virulence, including those related to bacterial invasion, motility, chemotaxis, and secretion systems. Overall, RNA-seq profiling provides new insights into the possible fundamental molecular mechanisms that are responsible for the reduction in growth and virulence of R. solanacearum upon application of FZB42-VOC.

Co-cultivation of Mortierellaceae with Pseudomonas helmanticensis affects both their growth and volatilome

Volatile organic compounds (VOCs) might mediate microbial interactions, especially in spatially structured environments, such as soil. However, the variety and specificity of VOC production are poorly understood. Here, we studied 25 Mortierellaceae strains belonging to the genera Linnemannia and Entomortierella in both pure and co-culture with Pseudomonas helmanticensis under laboratory conditions. We analysed both the fungal growth depending on co-cultivation and the cultures' volatilomes applying proton-transfer-reaction time-of-flight and gas chromatography-mass spectrometry (PTR-ToF-MS and GC-MS). In a strain-specific manner, we found the fungi's radial growth rate and colony morphology affected by the presence of P. helmanticensis. The fungus seemed to generally reduce the bacterial growth. The volatilomes of the fungal and bacterial pure and co-cultures were diverse. While the fungi frequently consumed VOCs, P. helmanticensis produced a higher diversity and amount of VOCs than any fungal strain. Our results support that both the pure and co-culture volatilomes are taxonomically conserved. Taken together, our data supports the relevance of VOCs in Mortierellaceae-P. helmanticensis interaction. We also discuss individual VOCs that appear relevant in the interaction.

Volatile compounds for biotechnological applications produced during competitive interactions between yeasts and fungi

Fungi, yeasts and bacteria produce volatile compounds during their metabolism. In this study, the volatile compounds produced by yeast strains (Saccharomyces cerevisiae and Rhodotorula mucilaginosa) and fungal strains (Aspergillus carbonarius and Aspergillus ochraceus) during competitive interactions were investigated by solid-phase microextraction coupled with gas chromatography-mass spectrometry. Fifty-six volatile compounds were identified representing alcohols, aldehydes, esters, ketones, aromatic compounds, acids, furans, phenols, and nitrogen compounds, being the largest amount in the class of esters and alcohols. Eight compounds were identified only in interactive culture conditions such as 2-amino-1-propanol, isopropylamine, dimethylamine, pentyl propanoate, ethyl-2-aminopropanoate, acetone, oxalic acid, and β-elemene and five of these were produced in cocultures including A. carbonarius. These will be developed for future biotechnological applications such as in the pharmaceutical and biological industry to produce drugs. Antimicrobial and antifungal activities; Solvent and herbicide; flavoring ingredient; solvent, plastic synthesis, nail polish remover and thinner, pesticide and herbicide; important in the complexation of minerals in the soil; and plant-environment interactions, defending predators, pathogens, and competitors.

The power of the smallest: The inhibitory activity of microbial volatile organic compounds against phytopathogens

Plant diseases caused by phytopathogens result in huge economic losses in agriculture. In addition, the use of chemical products to control such diseases causes many problems to the environment and to human health. However, some bacteria and fungi have a mutualistic relationship with plants in nature, mainly exchanging nutrients and protection. Thus, exploring those beneficial microorganisms has been an interesting and promising alternative for mitigating the use of agrochemicals and, consequently, achieving a more sustainable agriculture. Microorganisms are able to produce and excrete several metabolites, but volatile organic compounds (VOCs) have huge biotechnology potential. Microbial VOCs are small molecules from different chemical classes, such as alkenes, alcohols, ketones, organic acids, terpenes, benzenoids and pyrazines. Interestingly, volatilomes are species-specific and also change according to microbial growth conditions. The interaction of VOCs with other organisms, such as plants, insects, and other bacteria and fungi, can cause a wide range of effects. In this review, we show that a large variety of plant pathogens are inhibited by microbial VOCs with a focus on the in vitro and in vivo inhibition of phytopathogens of greater scientific and economic importance in agriculture, such as Ralstonia solanacearum, Botrytis cinerea, Xanthomonas and Fusarium species. In this scenario, some genera of VOC-producing microorganisms stand out as antagonists, including Bacillus, Pseudomonas, Serratia and Streptomyces. We also highlight the known molecular and physiological mechanisms by which VOCs inhibit the growth of phytopathogens. Microbial VOCs can provoke many changes in these microorganisms, such as vacuolization, fungal hyphal rupture, loss of intracellular components, regulation of metabolism and pathogenicity genes, plus the expression of proteins important in the host response. Furthermore, we demonstrate that there are aspects to investigate by discussing questions that are still not very clear in this research area, especially those that are essential for the future use of such beneficial microorganisms as biocontrol products in field crops. Therefore, we bring to light the great biotechnological potential of VOCs to help make agriculture more sustainable.

Microbial Volatile Organic Compounds: An Alternative for Chemical Fertilizers in Sustainable Agriculture Development

Microorganisms are exceptional at producing several volatile substances called microbial volatile organic compounds (mVOCs). The mVOCs allow the microorganism to communicate with other organisms via both inter and intracellular signaling pathways. Recent investigation has revealed that mVOCs are chemically very diverse and play vital roles in plant interactions and microbial communication. The mVOCs can also modify the plant's physiological and hormonal pathways to augment plant growth and production. Moreover, mVOCs have been affirmed for effective alleviation of stresses, and also act as an elicitor of plant immunity. Thus, mVOCs act as an effective alternative to various chemical fertilizers and pesticides. The present review summarizes the recent findings about mVOCs and their roles in inter and intra-kingdoms interactions. Prospects for improving soil fertility, food safety, and security are affirmed for mVOCs application for sustainable agriculture.

Volatile Organic Compound (VOC) Profiles of Different Trichoderma Species and Their Potential Application

Fungi emit a broad spectrum of volatile organic compounds (VOCs), sometimes producing species-specific volatile profiles. Volatilomes have received over the last decade increasing attention in ecological, environmental and agricultural studies due to their potential to be used in the biocontrol of plant pathogens and pests and as plant growth-promoting factors. In the present study, we characterised and compared the volatilomes from four different Trichoderma species: T. asperellum B6; T. atroviride P1; T. afroharzianum T22; and T. longibrachiatum MK1. VOCs were collected from each strain grown both on PDA and in soil and analysed using proton transfer reaction quadrupole interface time-of-flight mass spectrometry (PTR-Qi-TOF-MS). Analysis of the detected volatiles highlighted a clear separation of the volatilomes of all the four species grown on PDA whereas the volatilomes of the soil-grown fungi could be only partially separated. Moreover, a limited number of species-specific peaks were found and putatively identified. In particular, each of the four Trichoderma species over-emitted somevolatiles involved in resistance induction, promotion of plant seed germination and seedling development and antimicrobial activity, as 2-pentyl-furan, 6PP, acetophenone and p-cymene by T. asperellum B6, T. atroviride P1, T. afroharzianum T22 and T. longibrachiatum MK1, respectively. Their potential role in interspecific interactions from the perspective of biological control is briefly discussed.

Biological Extraction, HPLC Quantification and Medical Applications of Astaxanthin Extracted from Crawfish “Procambarus clarkii” Exoskeleton By-Product

Simple Summary

The study aims to provide an eco-friendly method for the extraction of the natural pigment called “astaxanthin” from crawfish powder. Bacterial and fungal strains that are beneficial to humans were used as an alternative method of extraction instead of using chemicals. Astaxanthin concentration was determined using an analytical tool referred to as “high-performance liquid chromatography” (HPLC). The results were promising; however, future studies can provide more effective methods for obtaining better results. Several tests were done to evaluate the biological activity of extracted astaxanthin such as antifungal, anti-inflammatory, antioxidant, and anticancer. Further purification of the extracted astaxanthin from crawfish exoskeleton needed to be done to assure that the results of the tests obtained were only due to the action of astaxanthin.

Abstract

The main challenge of astaxanthin extraction is to provide an eco-friendly method of extraction instead of chemical methods that harm human health. This study provided an eco-friendly method for astaxanthin extraction using two bacterial and fungal probiotics (Bifidobacterium lactis, Lactobacillus lactis, Candida utilis, and Saccharomyces cerevisiae, respectively) and determined the astaxanthin concentration by high-performance liquid chromatography (HPLC) analysis. The results showed that the highest concentration was obtained by S. cerevisiae (45.69 µg/g). Several biological tests were done on the exoskeleton containing astaxanthin of crawfish. Antifungal activity was effective against C. utilis (inhibition zone is 12.3 ± 0.5 mm). The scavenging percentage of 2,2-diphenyl-1-picrylhydrazyl (DPPH scavenging percentage) was 72.1% at 1000 µg/mL concentration of exoskeleton containing astaxanthin. The Hemolysis inhibition percentage was 65% at the same concentration used previously. Furthermore, the IC50 value of human liver cancer cell line (HepG2), human hepatocellular carcinoma (HCT), and breast cancer cell line MCF-7 were 24 µg/mL, 11 µg/mL, and 9.5 µg/mL, respectively. The least cell viability percentage was 19% (using breast cancer cell line (MCF-7)) at 100 µg/mL of exoskeleton containing astaxanthin. Thus, using microorganisms can be an alternative and promising way of astaxanthin extraction. Furthermore, purification of extracted astaxanthin is essential for medical applications.

Biological Control of Fusarium culmorum, Fusarium graminearum and Fusarium poae by Antagonistic Yeasts

The genus Fusarium is considered to be one of the most pathogenic, phytotoxic and toxin-producing group of microorganisms in the world. Plants infected by these fungi are characterized by a reduced consumer and commercial value, mainly due to the contamination of crops with mycotoxins. Therefore, effective methods of reducing fungi of the genus Fusarium must be implemented already in the field before harvesting, especially with alternative methods to pesticides such as biocontrol. In this study we identified yeasts that inhibit the growth of the pathogenic fungi Fusarium culmorum, F. graminearum and F. poae. Tested yeasts came from different culture collections, or were obtained from organic and conventional cereals. The greater number of yeast isolates from organic cereals showed antagonistic activity against fungi of the genus Fusarium compared to isolates from the conventional cultivation system. Cryptococcus carnescens (E22) isolated from organic wheat was the only isolate that limited the mycelial growth of all three tested fungi and was the best antagonist against F. poae. Selected yeasts showed various mechanisms of action against fungi, including competition for nutrients and space, production of volatile metabolites, reduction of spore germination, production of siderophores or production of extracellular lytic enzymes: chitinase and β-1,3-glucanase. Of all the investigated mechanisms of yeast antagonism against Fusarium, competition for nutrients and the ability to inhibit spore germination prevailed.

Mapping Aspergillus niger Metabolite Biomarkers for In Situ and Early Evaluation of Table Grapes Contamination

The Aspergillus niger exometabolome was recently investigated using advanced gas chromatography in tandem with multivariate analysis, which allowed a metabolite biomarker pattern to be proposed. Microbial metabolomics patterns have gained enormous relevance, mainly due to the amount of information made available, which may be useful in countless processes. One of the great challenges in microbial metabolomics is related to applications in more complex systems of metabolomics information obtained from studies carried out in culture media, as complications may occur due to the dynamic nature of biological systems. Thus, the main objective of this research was to evaluate the applicability of the A. niger metabololite biomarkers pattern for in situ and early evaluation of table grapes contamination, used as study model. A. niger is a ubiquitous fungus responsible for food contamination, being reported as one of the main agents of the black mold disease, a serious post-harvest pathology of table grapes. This work included analysis from 1 day of growth time of pure A. niger cultures, A. niger cultures obtained from previously contaminated grapes, and finally, an in situ solid-phase microextraction (SPME) approach directly on previously contaminated table grapes. Supervised multivariate analysis was performed which revealed that after 1 day of inoculation it was possible to detect A. niger biomarkers, which can be extremely useful in making this type of method possible for the rapid detection of food contamination. The results obtained confirm the potential applicability of the pattern of A. niger biomarkers for early detection of the fungi (after 1 day of contamination), and may be further explored for access food susceptibility to fungi contamination, based on direct analysis of the food item.

Elucidating the interaction of carbon, nitrogen, and temperature on the biosynthesis of Aureobasidium pullulans antifungal volatiles

The combined biochemical impact of carbon, nitrogen and temperature on the biosynthesis of the antifungal volatile organic compounds (VOCs): ethanol, 2-methyl-1-propanol, 3-methyl-1-butanol and 2-phenylethanol produced by Aureobasidium pullulans A1 and A3 was investigated using a Box-Behnken experimental design and response surface methodology (RSM). Normalized peak areas derived from solid phase micro extraction-gas chromatography-mass spectrometry (SPME-GC-MS) analysis, indicated that initial carbon content had a significant influence on the biosynthesis of ethanol and alcohols with greater than three carbon atoms. This result suggests a dominant activity of the A. pullulans anabolic pathway to biosynthesize three higher alcohols via de novo biosynthesis of amino acids from sugar metabolism. Low concentrations of carbon (3-13 g l^-1 ) with nitrogen as both ammonium and amino acids in the growth medium resulted in a higher number of significant linear and quadratic relationships. Nitrogen availability and growth temperature had significant negative linear and quadratic correlations with VOCs biosynthesis in most instances. Isolate-dependant metabolic response was evident for all abiotic parameters tested on alcohol production. The findings of this study offer new perspectives to improve the production of key antifungal compounds by antagonists in biological control systems.

Debaryomyces hansenii Strains Isolated From Danish Cheese Brines Act as Biocontrol Agents to Inhibit Germination and Growth of Contaminating Molds

The antagonistic activities of native Debaryomyces hansenii strains isolated from Danish cheese brines were evaluated against contaminating molds in the dairy industry. Determination of chromosome polymorphism by use of pulsed-field gel electrophoresis (PFGE) revealed a huge genetic heterogeneity among the D. hansenii strains, which was reflected in intra-species variation at the phenotypic level. 11 D. hansenii strains were tested for their ability to inhibit germination and growth of contaminating molds, frequently occurring at Danish dairies, i.e., Cladosporium inversicolor, Cladosporium sinuosum, Fusarium avenaceum, Mucor racemosus, and Penicillium roqueforti. Especially the germination of C. inversicolor and P. roqueforti was significantly inhibited by cell-free supernatants of all D. hansenii strains. The underlying factors behind the inhibitory effects of the D. hansenii cell-free supernatants were investigated. Based on dynamic headspace sampling followed by gas chromatography-mass spectrometry (DHS-GC-MS), 71 volatile compounds (VOCs) produced by the D. hansenii strains were identified, including 6 acids, 22 alcohols, 15 aldehydes, 3 benzene derivatives, 8 esters, 3 heterocyclic compounds, 12 ketones, and 2 phenols. Among the 71 identified VOCs, inhibition of germination of C. inversicolor correlated strongly with three VOCs, i.e., 3-methylbutanoic acid, 2-pentanone as well as acetic acid. For P. roqueforti, two VOCs correlated with inhibition of germination, i.e., acetone and 2-phenylethanol, of which the latter also correlated strongly with inhibition of mycelium growth. Low half-maximal inhibitory concentrations (IC50) were especially observed for 3-methylbutanoic acid, i.e., 6.32-9.53 × 10-5 and 2.00-2.67 × 10-4 mol/L for C. inversicolor and P. roqueforti, respectively. For 2-phenylethanol, a well-known quorum sensing molecule, the IC50 was 1.99-7.49 × 10-3 and 1.73-3.45 × 10-3 mol/L for C. inversicolor and P. roqueforti, respectively. For acetic acid, the IC50 was 1.35-2.47 × 10-3 and 1.19-2.80 × 10-3 mol/L for C. inversicolor and P. roqueforti, respectively. Finally, relative weak inhibition was observed for 2-pentanone and acetone. The current study shows that native strains of D. hansenii isolated from Danish brines have antagonistic effects against specific contaminating molds and points to the development of D. hansenii strains as bioprotective cultures, targeting cheese brines and cheese surfaces.

Volatile Organic Compounds (VOCs) Produced by Gluconobacter cerinus and Hanseniaspora osmophila Displaying Control Effect against Table Grape-Rot Pathogens

Table grapes (Vitis vinifera) are affected by botrytis bunch rot and summer bunch rot, the latter a complex disease caused by Botrytis cinerea, Aspergillus spp., Penicillium expansum and Rhizopus stolonifer. To search for biocontrol alternatives, a new bioproduct composed of Gluconobacter cerinus and Hanseniaspora osmophila, a consortium called PUCV-VBL, was developed for the control of fungal rots in table grapes. Since this consortium presents new biocontrol species, the effect of their VOCs (volatile organic compounds) was evaluated under in vitro and in vivo conditions. The VOCs produced by the PUCV-VBL consortium showed the highest mycelial inhibition against Botrytis cinerea (86%). Furthermore, H. osmophila was able to inhibit sporulation of A. tubingensis and P. expansum. VOCs' effect in vivo was evaluated using berries from Red Globe, Thompson Seedless and Crimson Seedless grapes cultivars, demonstrating a mycelial inhibition by VOCs greater than 70% for all evaluated fungal species. The VOC identification of the PUCV-VBL consortium was analyzed by solid-phase microextraction coupled to gas chromatography-mass spectrometry (SPME-GCMS). A total 26 compounds were identified, including 1-butanol 3-methyl, propanoic acid ethyl ester, ethyl acetate, phenylethyl alcohol, isobutyl acetate and hexanoic acid ethyl ester. Our results show that VOCs are an important mode of action of the PUCV-VBL biological consortium.

Microbial-based Biocontrol Solutions for Fruits and Vegetables: Recent Insight, Patents, and Innovative Trends

BACKGROUND

Fruits and vegetables are susceptible to colonisation by undesired microflora, which, in pre- and post-harvest conditions, negatively impact the quality of these products, leading to a reduction of yield, shelf-life, and marketability. In the few last years, the use of microbial Biological Control Agents (BCAs) has assumed international relevance in order to control harmful microorganisms, as a promising alternative to chemical interventions.

OBJECTIVE

The purpose of this review is to discuss the microbial-based solutions applicable for the biocontrol of the main microbial spoilers, phytopathogens, and human food-borne pathogens affecting fruits and vegetables during their production and storage.

RESULTS

A comprehensive overview of the scientific literature investigating the effectiveness of BCA-based products available on the market is provided, as well as of the most recent patents protecting biotechnological applications in this field. Innovative trends are discussed, with a particular focus on the integration of BCAs to minimise spoilage phenomena and microbiological risks adopting combined approaches.

CONCLUSION

This study underlines the growing interest about biocontrol strategies to counteract the growth of spoilage and/or pathogenic microorganisms indicating that in the next years a considerable increase of commercial products and patents will be developed worldwide to exploit innovative biotechnological solutions in the sector.

A Comparison of Collection Methods for Microbial Volatiles

Recently, there has been an increase in the number of reports that highlight the role of microbes and their volatile metabolites in interactions with plants and insects, including interactions which may benefit agricultural production. Accurate and reproducible volatile collection is crucial to investigations of chemical-mediated communication between organisms. Accordingly, accurate detection of volatiles emitted from microbe-inoculated media is a research priority. Though numerous classes of volatile organic compounds are emitted from plants, insects, and microbes, emissions from microbes typically contain polar compounds of high volatility. Therefore, commonly used plant or insect volatile collection techniques may not provide an accurate representation of microbe-specific volatile profiles. Here, we present and compare the volatile data derived via three solventless collection techniques: direct headspace injection, solid-phase microextraction (SPME), and active sampling with a sorptive matrix blend specifically designed to prevent collection filter breakthrough of VOCs (solid-phase extraction, SPE). These methods were applied to a synthetic floral nectar media containing a nectar-inhabiting yeast, Metschnikowia reukaufii, and sunflower (Helianthus annus) pollen. The yeast contributed alcohols, ketones, and esters, and the pollen provided terpenoids. Direct headspace injections were not effective, and the resultant chromatography was poor despite the use of on-column cryofocusing. SPME and SPE detected a similar number of volatiles, but with varying relative abundances. SPE collected a greater abundance of microbial volatiles than SPME, a difference driven by high ethanol capture in SPE. Both SPE and SPME are appropriate for analysis of microbial volatiles, though the sorbent type and amount, and other collection parameters should be further evaluated for each studied system.

Effect of a Debaryomyces hansenii and Lactobacillus buchneri Starter Culture on Aspergillus westerdijkiae Ochratoxin A Production and Growth during the Manufacture of Short Seasoned Dry-Cured Ham

Recently, specific dry-cured hams have started to be produced in San Daniele and Parma areas. The ingredients are similar to protected denomination of origin (PDO) produced in San Daniele or Parma areas, and include pork leg, coming from pigs bred in the Italian peninsula, salt and spices. However, these specific new products cannot be marked as a PDO, either San Daniele or Parma dry cured ham, because they are seasoned for 6 months, and the mark PDO is given only to products seasoned over 13 months. Consequently, these products are called short-seasoned dry-cured ham (SSDCH) and are not branded PDO. During their seasoning period, particularly from the first drying until the end of the seasoning period, many molds, including Eurotium spp. and Penicillium spp., can grow on the surface and work together with other molds and tissue enzymes to produce a unique aroma. Both of these strains typically predominate over other molds. However, molds producing ochratoxins, such as Aspergillus ochraceus and Penicillium nordicum, can simultaneously grow and produce ochratoxin A (OTA). Consequently, these dry-cured hams may represent a potential health risk for consumers. Recently, Aspergillus westerdijkiae has been isolated from SSDCHs, which could represent a potential problem for consumers. Therefore, the aim of this study was to inhibit A. westerdijkiae using Debaryomyces hansenii or Lactobacillus buchneri or a mix of both microorganisms. Six D. hansenii and six L. buchneri strains were tested in vitro for their ability to inhibit A. westerdijkiae. The strains D. hansenii (DIAL)1 and L. buchneri (Lb)4 demonstrated the highest inhibitory activity and were selected for in situ tests. The strains were inoculated or co-inoculated on fresh pork legs for SSDCH production with OTA-producing A. westerdijkiae prior to the first drying and seasoning. At the end of seasoning (six months), OTA was not detected in the SSDCH treated with both microorganisms and their combination. Because both strains did not adversely affect the SSDCH odor or flavor, the combination of these strains are proposed for use as starters to inhibit OTA-producing A. westerdijkiae.

Bioactivity of volatile organic compounds by Aureobasidium species against gray mold of tomato and table grape

Aureobasidium strains isolated from diverse unconventional environments belonging to the species A. pullulans, A. melanogenum, and A. subglaciale were evaluated for Volatile Organic Compounds (VOCs) production as a part of their modes of action against Botrytis cinerea of tomato and table grape. By in vitro assay, VOCs generated by the antagonists belonging to the species A. subglaciale showed the highest inhibition percentage of the pathogen mycelial growth (65.4%). In vivo tests were conducted with tomatoes and grapes artificially inoculated with B. cinerea conidial suspension, and exposed to VOCs emitted by the most efficient antagonists of each species (AP1, AM10, AS14) showing that VOCs of AP1 (A. pullulans) reduced the incidence by 67%, partially confirmed by the in vitro results. Conversely, on table grape, VOCs produced by all the strains did not control the fungal incidence but were only reducing the infection severity (< 44.4% by A. pullulans; < 30.5% by A. melanogenum, and A. subglaciale). Solid-phase microextraction (SPME) and subsequent gas chromatography coupled to mass spectrometry identified ethanol, 3-methyl-1-butanol, 2-methyl-1-propanol as the most produced VOCs. However, there were differences in the amounts of produced VOCs as well as in their repertoire. The EC50 values of VOCs for reduction of mycelial growth of B. cinerea uncovered 3-methyl-1-butanol as the most effective compound. The study demonstrated that the production and the efficacy of VOCs by Aureobasidium could be directly related to the specific species and pathosystem and uncovers new possibilities for searching more efficient VOCs producing strains in unconventional habitats other than plants.

Biological control of mites by xerophile Eurotium species isolated from the surface of dry cured ham and dry beef cecina

Some meat dry products, including dry cured ham and dry beef cecina, are cured in cellars at moderately cold temperature allowing the growth of a lawn of fungi on their surface. During the curing process, frequently these products became contaminated with fungivore mites of the Acaridae family that feed on fungal mycelium and spores.

AIMS

The aim of this article is to study the possible biological control of mites by fungi that form part of the normal microbiota of these meat products.

METHODS AND RESULTS

Some yellow/orange pigmented fungi growing on the ham surface decreased the proliferation of mites; therefore, we isolated from ham and cecina xerophilic yellow/orange coloured fungal strains that were identified as members of the genus Eurotium (recently reclassified as Aspergillus section Aspergillus). Using molecular genetic tools, we have identified 158 strains as Eurotium rubrum (Aspergillus ruber), Eurotium repens (Aspergillus pseudoglaucus) and Eurotium chevalieri (Aspergillus chevalieri). Two strains, E. rubrum C47 and E. rubrum C49, showed strong miticidal activity. The toxic compound(s) are associated with the formation of cleistothecia. In synchronized mite development experiments, we observed that all stages of the mite lifecycle were inhibited by the E. rubrum C47 strain. In addition, we searched for miticidal activity in 13 culture collection Eurotium strains isolated from different habitats, and found that only one, Eurotium cristatum NRRL 4222 (Aspergillus cristatus) has a strong miticidal activity.

CONCLUSIONS

These fungal strains have proliferated on the surface of ham and cecina for decades, and possibly have acquired miticidal activity as a resistance mechanism against fungivores.

SIGNIFICANCE AND IMPACT OF THE STUDY

Biological control of infecting mites by favouring growth of E. rubrun C47, in place of the normal mixed population of Aspergillus and Penicillium, is an attractive approach to control mite infestations.

Antifungal Effects of Volatiles Produced by Bacillus subtilis Against Alternaria solani in Potato

Antifungal activities of plant-beneficial Bacillus have been widely studied in recent years. Numerous studies have studied the antifungal mechanisms of soluble non-volatile bioactive compounds such as lipopeptides and proteins produced by Bacillus against soil-borne diseases. However, the antagonistic mechanisms of volatile organic compounds (VOCs) from Bacillus against airborne phytopathogens are still largely unknown, and the function of Alternaria solani pathogenic genes has not been well identified. Here, we first isolated a Bacillus strain with strong antifungal activity and finally identified it as B. subtilis ZD01. Then, the antagonistic mechanisms of VOCs produced by strain ZD01, against A. solani, an airborne fungal pathogen that can cause early blight diseases of potato, were studied. We showed that VOCs produced by strain ZD01 can reduce the colony size and mycelial penetration and can cause serious morphological changes of A. solani. Scanning electron microscope (SEM) observation showed that VOCs released by ZD01 could cause more flaccid and gapped hyphae of A. solani. Also, we found that VOCs produced by ZD01 can inhibit the conidia germination and reduce the lesion areas and number of A. solani in vivo significantly. Meanwhile, based on gas chromatography/mass spectrometry (GC/MS) analysis, 29 volatile compounds produced by strain ZD01 were identified. Out of 29 identified VOCs, 9 VOCs showed complete growth inhibition activities against A. solani. Moreover, we identified two virulence-associated genes (slt2 and sod) in A. solani. slt2 is a key gene that regulates the mycelial growth, penetration, sporulation, and virulence in vivo in A. solani. In addition, sod plays a significant role in the SOD synthetic pathway in A. solani. Results from qRT-PCR showed that the transcriptional expression of these two genes was down-regulated after being treated by VOCs produced by ZD01. These results are useful for a better understanding of the biocontrol mechanism of Bacillus and offer a potential method for potato early blight disease control.

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

Volatile compounds (VOCs) are not only media for communication within a species but also effective tools for sender to manipulate behavior and physiology of receiver species. Although the influence of VOCs on the interactions among organisms is evident, types of VOCs and specific mechanisms through which VOCs work during such interactions are only beginning to become clear. Here, we review the fungal volatile compounds (FVOCs) and their impacts on different recipient organisms from perspective of distinct lifestyles of the filamentous fungi. Particularly, we discuss the possibility that different lifestyles are intimately associated with an ability to produce a repertoire of FVOCs in fungi. The FVOCs discussed here have been identified and analyzed as relevant signals under a range of experimental settings. However, mechanistic insight into how specific interactions are mediated by such FVOCs at the molecular levels, amidst complex community of microbes and plants, requires further testing. Experimental designs and advanced technologies that attempt to address this question will facilitate our understanding and applications of FVOCs to agriculture and ecosystem management.

Biocontrol ability and volatile organic compounds production as a putative mode of action of yeast strains isolated from organic grapes and rye grains

The inhibiting activity of three yeast strains belonging to Pichia kudriavzevii, Pichia occidentalis, and Meyerozyma quilliermondii/Meyerozyma caribbica genera against common plant pathogens representing Mucor spp., Penicillium chrysogenum, Penicillium expansum, Aspergillus flavus, Fusarium cereals, Fusarium poae, as well as Botrytis cinerea genera was investigated. The yeast strains tested had a positive impact on growth inhibition of all target plant pathogens. The degree of inhibition was more than 50% and varied depending on both the yeast antagonist and the mold. Ethyl esters of medium-chain fatty acids, phenylethyl alcohol, and its acetate ester prevailed among the analyzed volatile organic compounds (VOCs) emitted by yeasts in the presence of the target plant pathogens. Due to the method used, assuming no contact between the antagonist and the pathogen, the antagonistic activity of the yeast strains studied resulted mainly from the production of biologically active VOCs. Moreover, the antagonistic activity was not only restricted to a single plant pathogen but effective towards molds of different genera, making the yeast strains studied very useful for potential application in biological control.

Aureobasidium pullulans volatilome identified by a novel, quantitative approach employing SPME-GC-MS, suppressed Botrytis cinerea and Alternaria alternata in vitro

Volatile organic compounds (VOCs) produced by Aureobasidium pullulans were investigated for antagonistic actions against Alternaria alternata and Botrytis cinerea. Conidia germination and colony growth of these two phytopathogens were suppressed by A. pullulans VOCs. A novel experimental setup was devised to directly extract VOCs using solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) from antagonist-pathogen culture headspace. The proposed system is a robust method to quantify microbial VOCs using an internal standard. Multivariate curve resolution-alternating least squares deconvolution of SPME-GC-MS spectra identified fourteen A. pullulans VOCs. 3-Methyl-1-hexanol, acetone, 2-heptanone, ethyl butyrate, 3-methylbutyl acetate and 2-methylpropyl acetate were newly identified in A. pullulans headspace. Partial least squares discriminant analysis models with variable importance in projection and selectivity ratio identified four VOCs (ethanol, 2-methyl-1-propanol, 3-methyl-1-butanol and 2-phenylethanol), with high explanatory power for discrimination between A. pullulans and pathogen. The antifungal activity and synergistic interactions of the four VOCs were evaluated using a Box-Behnken design with response surface modelling. Ethanol and 2-phenylethanol are the key inhibitory A. pullulans VOCs against both B. cinerea and A. alternata. Our findings introduce a novel, robust, quantitative approach for microbial VOCs analyses and give insights into the potential use of A. pullulans VOCs to control B. cinerea and A. alternata.

Scent of a Killer: Microbial Volatilome and Its Role in the Biological Control of Plant Pathogens

The use of synthetic fungicides represents the most common strategy to control plant pathogens. Excessive and/or long-term distribution of chemicals is responsible for increased levels of environmental pollution, as well as adverse health consequence to humans and animals. These issues are deeply influencing public perception, as reflected by the increasing demand for safer and eco-friendly agricultural commodities and their by-products. A steadily increasing number of research efforts is now devoted to explore the use of safer and innovative approaches to control plant pathogens. The use of microorganisms as biological control agents (BCAs) represents one of the most durable and promising strategies. Among the panoply of microbial mechanisms exerted by BCAs, the production of volatile organic compounds (VOCs) represents an intriguing issue, mostly exploitable in circumstances where a direct contact between the pathogen and its antagonist is not practicable. VOCs are potentially produced by all living microorganisms, and may be active in the biocontrol of phytopathogenic oomycetes, fungi, and bacteria by means of antimicrobial activity and/or other cross-talk interactions. Their biological effects, the reduced residuals in the environment and on agricultural commodities, and the ease of application in different agricultural systems make the use of VOCs a promising and sustainable approach to replace synthetic fungicides in the control of plant pathogens. In this review, we focus on VOCs produced by bacteria and fungi and on their role in the cross-talk existing between the plant pathogens and their host. Biologic systemic effect of the microbial volatile blends on both pathogen and host plant cells is also briefly reviewed.

Aureobasidium pullulans volatile organic compounds as alternative postharvest method to control brown rot of stone fruits

Volatile compounds produced by L1 and L8 strains were assayed against mycelia and conidia growth of Monilinia laxa, M. fructicola, M. polystroma, and M. fructigena of stone fruits. Results showed that volatile metabolites inhibited significantly pathogens growth, in particular M. fructigena mycelium growth (70% by L1 and 50% by L8) and M. fructicola conidia germination (85% by L1 and 70% by L8) compared to the control. Moreover, the antagonistic activity was enhanced by the addition of asparagine (120 mg L^-1) in the culture media composition. Synthetic pure compounds were tested in vitro on pathogens mycelial and conidia growth and their EC₅₀ values were estimated, confirming 2-phenethyl as the most active compound. For this reason 2-phenethyl and VOCs of both yeast strains were assayed in vivo on cherry, peach, and apricot fruits. Regarding peach fruit, both treatments, yeasts and pure compounds, displayed the best inhibiting action against all the pathogens especially against M. laxa (100% by L1, 84% by L8 and 2-phenethyl). ATR/IR spectroscopy analysis showed how VOCs produced by both strains increase the fruit waxes complexity reducing the pathogens attack so playing an essential role in the antagonistic activity of both yeast strains and on fruit structural composition.

Biocontrol yeasts: mechanisms and applications

Yeasts occur in all environments and have been described as potent antagonists of various plant pathogens. Due to their antagonistic ability, undemanding cultivation requirements, and limited biosafety concerns, many of these unicellular fungi have been considered for biocontrol applications. Here, we review the fundamental research on the mechanisms (e.g., competition, enzyme secretion, toxin production, volatiles, mycoparasitism, induction of resistance) by which biocontrol yeasts exert their activity as plant protection agents. In a second part, we focus on five yeast species (Candida oleophila, Aureobasidium pullulans, Metschnikowia fructicola, Cryptococcus albidus, Saccharomyces cerevisiae) that are or have been registered for the application as biocontrol products. These examples demonstrate the potential of yeasts for commercial biocontrol usage, but this review also highlights the scarcity of fundamental studies on yeast biocontrol mechanisms and of registered yeast-based biocontrol products. Yeast biocontrol mechanisms thus represent a largely unexplored field of research and plentiful opportunities for the development of commercial, yeast-based applications for plant protection exist.

In vitro interactions between Trichoderma harzianum and pathogenic fungi damaging horse-chestnut (Aesculus hippocastanum) leaves and fruits

Interactions between 3 pathogenic fungi damaging horse-chestnut (Aesculus hippocastanum) leaves and fruits – Phyllosticta sphaeropsoidea, Phomopsis carposchiza, and Diaporthe padi – and the antagonistic fungus Trichoderma harzianum were studied to determine their mutual influence in vitro. Antibiosis of colonies developing on 5 nutrient media was tested. The 3 studied T. harzianum isolates differed in their antagonistic potential. although T. harzianum isolates significantly inhibited the growth of Phomopsis carposchiza, the mycelium growth of some of the re-isolates on fresh medium indicates an inadequate antagonistic effect of T. harzianum on this species. The tested Trichoderma isolates showed stronger antagonism towards the other pathogens, reflected in overgrowing of Phyllosticta sphaeropso-idea and Diaporthe padi and reducing their growth. Granulation of the cytoplasm and lysis of hyphae of the fungal pathogens were the most frequently observed effects of the interaction.

The Role of Volatile Organic Compounds and Rhizosphere Competence in Mode of Action of the Non-pathogenic Fusarium oxysporum FO12 Toward Verticillium Wilt

Verticillium wilts caused by Verticillium spp. are among the most challenging plant diseases to control and affect numerous hosts worldwide. Due to the lack of effective, conventional control methods, integrated control strategies provide a promising approach to manage these diseases. The non-pathogenic Fusarium oxysporum strain FO12 was reported in previous studies to be an effective biocontrol agent against Verticillium dahliae, however, its mode of action remains to be elucidated. In this study, complementary in vitro and in vivo experiments were conducted in order to explore the implications of inhibitory substances and rhizosphere competence in antagonistic effects of FO12 against V. dahliae and V. longisporum. Volatile organic compounds and soluble substances produced by FO12, which caused significant inhibition of mycelial growth and microsclerotia viability in the two tested Verticillium species, were identified by means of gas and liquid chromatography-mass spectrometry. We showed that the antagonistic effect of F. oxysporum FO12 is partially due to the production of bioactive compounds such as 3-methyl-1-butanol and 2-methyl-1-butanol, among others. Several metabolic pathways of FO12 were altered upon contact with V. dahliae ELV22 volatiles. The reduced production of alpha, alpha-trehalose, a metabolite used in starch and sucrose metabolism, suggests that the biocontrol agent activates its stress response in the presence of the phytopathogen. Microscopic analysis using sGFP-tagged FO12 on oil seed rape as a model plant suggests that the biocontrol strain is an efficient root colonizer, which could compete with V. dahliae in the same ecological niche. The findings obtained in this study provide new insights into the mode of action of this potential biocontrol agent, which are relevant for controlling Verticillium wilt through an ecologically friendly approach.

Plant Volatiles Reduce the Viability of the Root-Knot Nematode Meloidogyne incognita Either Directly or When Retained in Water

Volatile organic compounds (VOC) produced by green residues for the management of plant-parasitic nematodes are poorly studied for oilseed plants and some Brassica spp. To investigate the activity of VOC in vitro and as biofumigants, dry and aqueous macerates of broccoli (Brassica oleracea var. italica) shoots and sunflower (Helianthus annuus) seed were used against the root-knot nematode Meloidogyne incognita. VOC produced by sunflower seed caused higher mortality of M. incognita second-stage juveniles (J₂) than VOC produced by broccoli shoots but both plant species were equally effective in decreasing the infectivity and reproduction of this nematode. The number of galls and eggs produced by the nematode in tomato roots was reduced by 89 and 95%, respectively, on average, at the highest concentrations of broccoli and sunflower seed macerates tested as biofumigants. When nematodes were placed in water exposed to broccoli VOC, J₂ immobility increased and the number of galls and eggs produced by the nematode in tomato roots decreased 80 and 96%, respectively. Water exposed to sunflower seed VOC had no effect on the viability of the nematode. Gas chromatography was used to identify five and six chemical groups in broccoli and in sunflower seed macerates, respectively, but only alcohols, sulfurated VOC, and terpenes were detected in the water exposed to these plant macerates. Sulfurated VOC from the water exposed to broccoli macerates were found to be involved in its activity against M. incognita. The purified VOC dimethyl disulfide (DMDS) and 3-pentanol were tested directly against J₂ and showed a lethal concentration of 176 and 918 µg/ml (ppm), respectively, whereas dimethyl sulfide had no effect against M. incognita. Furthermore, DMDS and 3-pentanol retained in water killed J₂ and reduced gall formation and the number of eggs of M. incognita on tomato roots. Both these plant species produced toxic VOC to M. incognita, whereas only VOC retained in water exposed to broccoli had activity against M. incognita.

Characterization and Synergistic Effect of Antifungal Volatile Organic Compounds Emitted by the Geotrichum candidum PF005, an Endophytic Fungus from the Eggplant

Plant-associated endophytes are recognized as sources of novel bioactive molecules having diverse applications. In this study, an endophytic yeast-like fungal strain was isolated from the fruit of eggplant (Solanum melongena) and identified as Geotrichum candidum through phenotypic and genotypic characterizations. This endophytic G. candidum isolate PF005 was found to emit fruity scented volatiles. The compositional profiling of volatile organic compounds (VOCs) revealed the presence of 3-methyl-1-butanol, ethyl 3-methylbutanoate, 2-phenylethanol, isopentyl acetate, naphthalene, and isobutyl acetate in significant proportion when analyzed on a time-course basis. The VOCs from G. candidum exhibited significant mycelial growth inhibition (54%) of phytopathogen Rhizoctonia solani, besides having mild antifungal activity against a few other fungi. The source of carbon as a nutrient was found to be an important factor for the enhanced biosynthesis of antifungal VOCs. The antifungal activity against phytopathogen R. solani was improved up to 91% by feeding the G. candidum with selective precursors of alcohol and ester volatiles. Furthermore, the antifungal activity of VOCs was enhanced synergistically up to 92% upon the exogenous addition of naphthalene (1.0 mg/plate). This is the first report of G. candidum as an endophyte emitting antifungal VOCs, wherein 2-penylethanol, isopentyl acetate, and naphthalene were identified as important contributors to its antifungal activity. Possible utilization of G. candidum PF005 as a mycofumigant has been discussed based upon its antifungal activity and the qualified presumption of safety status.

Volatile organic compounds from Wickerhamomyces anomalus, Metschnikowia pulcherrima and Saccharomyces cerevisiae inhibit growth of decay causing fungi and control postharvest diseases of strawberries

The effectiveness of Wickerhamomyces anomalus, Metschnikowia pulcherrima and Saccharomyces cerevisiae as biocontrol agents on postharvest decay of strawberry (Fragaria x ananassa, cv. 'Alba') fruit, and their inhibitory activities on some decay-causing fungi were evaluated. Volatile organic compounds from these yeasts decreased mycelial growth of Botrytis cinerea by 69%, and by less for Monilinia fructicola, Alternaria alternata, Aspergillus carbonarius, Penicillium digitatum, Cladosporium spp., and Colletotrichum spp. Strawberry fruit exposed to 6-day-old liquid cultures of W. anomalus, M. pulcherrima and S. cerevisiae for 48h showed 89%, 40%, and 32% reductions, respectively, in gray mold McKinney Index. Vapours of ethyl acetate, the main volatile organic compound of these yeasts, completely inhibited B. cinerea growth at 8.97mg/cm³, and suppressed gray mold on strawberry fruit at 0.718mg/cm³. The biocontrol activities of these yeasts can be ascribed to ethyl acetate, which can be used for control of postharvest gray mold of strawberry fruit.

Mining the Volatilomes of Plant-Associated Microbiota for New Biocontrol Solutions

Microbial lifeforms associated with land plants represent a rich source for crop growth- and health-promoting microorganisms and biocontrol agents. Volatile organic compounds (VOCs) produced by the plant microbiota have been demonstrated to elicit plant defenses and inhibit the growth and development of numerous plant pathogens. Therefore, these molecules are prospective alternatives to synthetic pesticides and the determination of their bioactivities against plant threats could contribute to the development of control strategies for sustainable agriculture. In our previous study we investigated the inhibitory impact of volatiles emitted by Pseudomonas species isolated from a potato field against the late blight-causing agent Phytophthora infestans. Besides the well-documented emission of hydrogen cyanide, other Pseudomonas VOCs impeded P. infestans mycelial growth and sporangia germination. Current advances in the field support the emerging concept that the microbial volatilome contains unexploited, eco-friendly chemical resources that could help select for efficient biocontrol strategies and lead to a greener chemical disease management in the field.

Mycofumigation through production of the volatile DNA-methylating agent N-methyl-N-nitrosoisobutyramide by fungi in the genus Muscodor

Antagonistic microorganisms produce antimicrobials to inhibit the growth of competitors. Although water-soluble antimicrobials are limited to proximal interactions via aqueous diffusion, volatile antimicrobials are able to act at a distance and diffuse through heterogeneous environments. Here, we identify the mechanism of action of Muscodor albus, an endophytic fungus known for its volatile antimicrobial activity toward a wide range of human and plant pathogens and its potential use in mycofumigation. Proposed uses of the Muscodor species include protecting crops, produce, and building materials from undesired fungal or bacterial growth. By analyzing a collection of Muscodor isolates with varying toxicity, we demonstrate that the volatile mycotoxin, N-methyl-N-nitrosoisobutyramide, is the dominant factor in Muscodor toxicity and acts primarily through DNA methylation. Additionally, Muscodor isolates exhibit higher resistance to DNA methylation compared with other fungi. This work contributes to the evaluation of Muscodor isolates as potential mycofumigants, provides insight into chemical strategies that organisms use to manipulate their environment, and provokes questions regarding the mechanisms of resistance used to tolerate constitutive, long-term exposure to DNA methylation.

Performance evaluation of volatile organic compounds by antagonistic yeasts immobilized on hydrogel spheres against gray, green and blue postharvest decays

Wickerhamomyces anomalus, Metschnikowia pulcherrima, Aureobasidium pullulans and Saccharomyces cerevisiae yeasts were tested for their ability to survive and synthesize antifungal volatile organic compounds (VOCs) both in vitro and in vivo conditions when immobilized on commercial hydrogel spheres. The results showed a good survival of all yeasts on hydrogel spheres up to 10 days of incubation. Moreover, VOCs produced in vitro by tested yeasts inhibited Botrytis cinerea, Penicillium digitatum and P. italicum radial growth and conidial germination, with the highest antagonistic activity reported for W. anomalus and A. pullulans strains. Experimental in vivo trials performed on strawberry and mandarin fruits proved the ability of VOCs to reduce significantly postharvest decays on artificially wounded tissues. Comprehensively, the best efficacy was detected for W. anomalus, which totally inhibited gray mold decay on strawberry fruits and significantly reduced green mold infections on mandarin fruits. On the other hand, blue mold decay on mandarin fruits was more effectively managed by A. pullulans VOCs. Accordingly, hydrogel spheres used as a support for VOC-generating yeasts could open a new way for the employment of this polymeric material as a bio-emitter in postharvest packaging.

Biocontrol ability and putative mode of action of yeasts against Geotrichum citri-aurantii in citrus fruit

Sour rot is a major postharvest disease of citrus fruit and is caused by the fungal pathogen Geotrichum citri-aurantii. A lack of chemicals certified for the control of this disease has led to the consideration of alternative methods and strategies, such as the use of yeasts as biocontrol agents. The purpose of the present study was to test the ability of yeasts isolated from leaves, flowers, fruit, and soil, and six Saccharomyces cerevisiae isolates to control citrus sour rot, to assess the mechanisms of action of the yeast isolates that were demonstrated to be effective for biocontrol, and to identify the most effective yeast isolates for the biocontrol of G. citri-aurantii. In in vivo assays, three yeast isolates (ACBL-23, ACBL-44, and ACBL-77) showed a potential for controlling sour rot in citrus fruits, both preventatively and curatively. Most of the eight yeast isolates that were assessed for a mechanism of action did not produce antifungal compounds in an amount sufficient to inhibit the growth of the pathogen. Additionally, nutrient competition among the yeast strains was not found to be a biocontrol strategy. Instead, killer activity and hydrolytic enzyme production were identified as the major mechanisms involved in the biocontrol activity of the yeasts. Isolates ACBL-23, ACBL-44, and ACBL-77, which controlled sour rot most effectively, were identified as Rhodotorula minuta, Candida azyma, and Aureobasidium pullulans, respectively. To our knowledge, this is the first report of the potential of C. azyma as a biological control agent against a postharvest pathogen and its ability to produce a killer toxin.

Proteomic response of the phytopathogen Phyllosticta citricarpa to antimicrobial volatile organic compounds from Saccharomyces cerevisiae

Volatile organic compounds (VOCs) released by Saccharomyces cerevisiae inhibit plant pathogens, including the filamentous fungus Phyllosticta citricarpa, causal agent of citrus black spot. VOCs mediate relevant interactions between organisms in nature, and antimicrobial VOCs are promising, environmentally safer fumigants to control phytopathogens. As the mechanisms by which VOCs inhibit microorganisms are not well characterized, we evaluated the proteomic response in P. citricarpa after exposure for 12h to a reconstituted mixture of VOCs (alcohols and esters) originally identified in S. cerevisiae. Total protein was extracted and separated by 2D-PAGE, and differentially expressed proteins were identified by LC-MS/MS. About 600 proteins were detected, of which 29 were downregulated and 11 were upregulated. These proteins are involved in metabolism, genetic information processing, cellular processes, and transport. Enzymes related to energy-generating pathways, particularly glycolysis and the tricarboxylic acid cycle, were the most strongly affected. Thus, the data indicate that antimicrobial VOCs interfere with essential metabolic pathways in P. citricarpa to prevent fungal growth.

In vivo determination of the volatile metabolites of saprotroph fungi by comprehensive two-dimensional gas chromatography

In this work, we discuss the use of multiway principal component analysis combined with comprehensive two-dimensional gas chromatography to study the volatile metabolites of the saprophytic fungus Memnoniella sp. isolated in vivo by headspace solid-phase microextraction. This fungus has been identified as having the ability to induce plant resistance against pathogens, possibly through its volatile metabolites. Adequate culture media were inoculated, and its headspace was then sampled with a solid-phase microextraction fiber and chromatographed every 24 h over seven days. The raw chromatogram processing using multiway principal component analysis allowed the determination of the inoculation period, during which the concentration of volatile metabolites was maximized, as well as the discrimination of the appropriate peaks from the complex culture media background. Several volatile metabolites not previously described in the literature on biocontrol fungi were observed, as well as sesquiterpenes and aliphatic alcohols. These results stress that, due to the complexity of multidimensional chromatographic data, multivariate tools might be mandatory even for apparently trivial tasks, such as the determination of the temporal profile of metabolite production and extinction. However, when compared with conventional gas chromatography, the complex data processing yields a considerable improvement in the information obtained from the samples.