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Introduction
World cereal production increases continuously since several years, but there are subsequent losses due to contamination of produced grains by heavy metals, pesticides and mycotoxins rendering crops not suitable as food. Mycotoxins are naturally occurring secondary metabolites, produced by filamentous fungi that are toxic to humans and animals. Consumption of food containing high levels of these contaminants may cause illness. Most of these molecules are difficult to degrade; they resist high temperatures and chemicals. They are gaining worldwide attention because of the significant economic losses associated with their effects on animal and plant productivity, human health, and national and international trade.
Fusarium Head Blight (FHB) of small-grain cereals such as wheat and barley and Gibberella Ear Rot (GER) and Fusarium Ear Rot (FER) of maize are three devastating fungal diseases affecting crops worldwide. They produce the socalled field mycotoxins, zearalenone (ZEA), trichothecenes (TCTB, among which deoxynivalenol (DON) is the best known, and TCTA), and fumonisins (FB). The mycotoxins accumulate in the grains themselves, even in healthy looking ones with apparent good quality. Depending on the climatic conditions, the occurrence and level of contamination at harvest can be high.
Cereals and maize are also particularly sensitive to mycotoxin accumulation during storage. Storage mycotoxins are produced after harvest, by fungi of the genus Aspergillus and Penicillium in poor storage and ventilation conditions in silos. The main storage mycotoxins are aflatoxins and ochratoxins. Under certain climatic conditions, Aspergillus spp. and aflatoxin contamination begins in the field on maize. With the expected evolution of the climate in Europe, it is likely that field contaminations will develop significantly (Battilani et al., 2016). The aflatoxins are considered to be the most toxic of the mycotoxins, aflatoxins B1, B2, G1 and G2 being the principal aflatoxins of concern. They are produced by Aspergillus fumigatus and A. parasiticus. Ochratoxins are produced by A. ochraceus and related species, A. niger, as well as Penicillium verrucosum and certain other Penicillium species (Rodrigues, 2016).
Minimisation strategies for mycotoxins have included the use of relatively resistant varieties and good preharvest agricultural practices. Fungicides are frequently applied to manage the contamination of grains by Fusarium spp. or Aspergillus spp., but their efficacies are generally lower than 50 % (Haidukowski et al., 2005; Loos et al., 2004). In addition, the effect of fungicides is rarely statistically significant when addressing grain contamination by mycotoxins. Mycotoxin production is suspected to be part of the adaptive response of the fungus to stressful conditions such as exposure to antifungal compounds (Ponts, 2015; Martins et al., 2016), and the accumulation is modulated by complex interactions between different fungal species (Scarpino et al., 2015). This explains observations such as increasing mycotoxin levels in rice grain treated with tebuconazole in field conditions (Dors et al., 2013), or the induction of the production of DON by strobilurins during in vitro studies (D’Mello et al., 2001).
Another drawback with current fungicides is the development of resistances, including resistance to synthetic fungicides that mimic natural fungicides. Strobilurins are a recent example of synthetic fungicides which application has led to the emergence of resistant fungal strains (Walker et al., 2009). Using successively different molecules with different physiological targets in pathogenic fungi is an accepted strategy for limiting the development of resistances. Unfortunately, the catalogue of available efficient molecules is limited. Compounds that inhibit mycotoxin production without inhibiting fungal growth may be useful for mycotoxin control without incurring a rapid spread of resistant strains. Consequently, new molecules with various activities are needed in the near future. In addition, nowadays, consumers and industry ask for environment friendly solutions to tackle the problems of plant pathology and food safety due to mycotoxins. In this context, searching for natural compounds to control fungal pathogens of plants and their production of mycotoxins is a challenge shared by research institutes and actors of agriculture.
Avoiding the primary contamination of plant by minimising the inoculum is another strategy. The most important sources of inoculum are ascospores, from the sexual stage, and macroconidia, from the anamorph stage. Chlamydospores and hyphal fragments can also act as sources of inoculum (Leplat et al., 2013). They mainly originate from crop debris remaining on soil after harvest of previous diseased crop. Toxigenic Fusa rium or Aspergillus species can survive and multiply on maize stubble or small grain cereal straw and soil for several years. Crop rotation, crop residue management and biological control by treating the debris with antagonists are levers to minimize this inoculum (Sarrocco and Vannacci, 2018). Biocontrol agents acting as an antagonist, a mycoparasite and a competitor for natural substrates have been proposed (Xue et al., 2014; Palazzini et al., 2017). Microbial decomposition of crop residues is a natural process, which can be supported by adding stimulating nutrients or selected microorganisms for starving the niche of the inoculum.
Biological postharvest treatments for mycotoxin removal might represent a promising strategy to decrease mycotoxin content in contaminated grains (McCormick, 2013). Most of the microorganisms able to modify or metabolise field and storage mycotoxins are bacteria or yeasts (Martínez Tuppia et al., 2017; Wilson et al., 2017; Zhu et al., 2017; Peng et al., 2018), but filamentous fungi can also detoxify ochratoxins (Chen et al., 2018) or aflatoxins (Scarpari et al., 2014).
A mushroom forming fungus is defined as a macrofungus with a distinctive fruiting body that is large enough to be seen with the naked eye and to be picked up by hand, and can be either a Basidiomycete or an Ascomycete, aerial or underground (Chang and Miles, 1992). They generally colonize complex substrates for long times or form mycorrhizas. Among the abundant biodiversity of mushroom forming fungi, over 2,300 wild useful species have been identified, and more than 40 species are currently cultivated for producing food (Savoie et al., 2013). Most of the cultivated species are white rot fungi degrading lignocellulose organic matter. As white rot fungus, many mushroom forming fungi might have ability to compete effectively for the available nutrients in the ecological niche of crop debris on soil, and in this way colonise it and exclude the pathogen. It seems very likely that mushroom forming fungi do contain compounds with important biological properties due to the deep history of interactions between fruit bodies and potentially fungivorous animals or fungal pathogens (Largeteau and Savoie, 2010; Money, 2016). The vegetative mycelia need also antibacterial and antifungal compounds to survive in their natural environments.
We propose here a survey of potential of mushroom forming fungi as biological control agents of mycotoxigenic fungi, acting: (i) as antagonists on crop debris for reducing primary inoculum of mycotoxigenic fungi in fields, (ii) as source of active extracts with antifungal and/or antimycotoxin properties that could be used for controlling the accumulation of mycotoxin during the cultivation period and at storage of grains, (iii) as detoxifying agents in solidstate fermentation processes of contaminated cereals. The objectives are to high light the opportunities to use mushroom forming fungi to prevent and reduce mycotoxins in cereal products, and to point the remaining challenges to be faced for the development of an integrated mycotoxin management in cereals using mushroom properties.
Competition on crop debris
Infected crop debris staying on or in soils between two crops are the main inoculum source of Fusarium spp. Chlamydospores containing macroconidies allow Fu sarium species to survive unfavourable conditions and thus to overwinter in soil or on crop residues until suitable temperature and humidity facilitate a new cycle of infections. In the sexual cycle of F. graminearum, perithecia on the crop residues are abundant in spring, as warm moist weather conditions are favourable for their maturation, and they release ascospores (Shah et al., 2018). Treatment of crop debris to reduce Fusarium primary inoculum in fields is one of the biocontrol strategy that is subject of work. Microorganisms accelerating residue degradation and/or being antagonistic against F. graminearum can be used (Legrand et al., 2017). As an example, the endophytic fungus Clonos tachys rosea (formerly known as Gliocladium roseum) is a soilborne ascomycete that exhibits suppressing ability of F. graminearum perithecial production when applied on crop residues (Xue et al., 2014). Several saprophytic fungi with good ability to colonise wheat straws or maize stalks and to suppress sporulation of pathogenic Fusarium spp. had been selected for their potential as biocontrol agents. Their efficiency for reducing colonisation of stalk pieces by pathogenic Fusarium spp. were sometimes significant, but not consistent (Luongo et al., 2005). For being efficient, an antagonist has to be present in sufficient quantity at the right time and place and to use nutrients more efficiently than the pathogen in case of exploitation competition (Legrand et al., 2017).
White-rot fungi are basidiomycetes that are capable of degrading the lignin component of lignocellulose substrates in addition to cellulose and hemicelluloses and are suitable for the colonization and degradation of wood and forest litters. They play an important role in the cycling of carbon and other elements. Most of the edible mushrooms that are cultivated on agricultural by-products (straw, stalk, wood ships, etc.) are white rot fungi. We postulate here that white rot fungi inoculated on crop debris would exploit them efficiently and deprive Fusarium species of nutrients in this niche. The first challenge to be addressed is the inoculation of crop debris on soils (Legrand et al., 2017). The use of cultivated mushroom species has benefits. Due to the life cycle of mushroom forming fungi, spores have to be collected from fruiting bodies, which could be fastidious to obtain. In edible mushroom cultivation, primary mycelium can be obtained from germination of spores, from a piece of a specific fruiting body, or from several germplasm providers. For producing inoculum that is usually called the spawn, the primary mycelium is developed on sterilized cereal grain (wheat, rye, millet or sorghum) (Sanchez, 2004). The mycelium-coated grains are then mixed with various lignocellulosic wastes at rates varying from 0.8 to 8 % depending on the species and conditions, and the mycelium can rapidly colonize the substrate from a grain that is a protecting and nutritive habitat. Adding several lignocellulosic materials in the spawn improved the competitiveness of the mycelium by increasing the number of inoculation points and inducing enzymatic complexes for breaking down lignocelluloses (Mata et al., 1998). Mixing this spawn with crop debris on the soils could be a way to introduce the antagonist of Fusarium spp. Spawn making companies already exist in various countries and would rapidly be able to produce inoculum for the treatment of crop debris in fields if this treatment is adopted.
The outcome of the competition between white rot mushroom and Fusarium species will depend on the composition of the plant debris, the climatic conditions (temperature, water availability, relative concentrations of O2 and CO2) and the interaction with other microorganisms present on the debris. Changes in these factors can reverse interaction outcomes because different species display contrasting sensitivities and/ or patterns of response to these variables (Hiscox et al. 2018). Generally, a w has a greater influence on fungal growth than temperature but a w and temperature interact. Regarding water availability (a w) and temperature, there is some information available for both mycotoxinogenic fungi and white rot mushrooms
For Fusarium graminearum, maximum growth rates were obtained at the highest a w and 25 °C and no or very slight growth was observed at 5 °C, regardless of a w levels assayed (Ramirez et al., 2006). The minimum a w for growth was estimated to be 0.869 and 0.854 for F. verticillioides and F. proliferatum, respectively (Samapundo et al., 2005), but at 15 °C no growth of F. verticillioides occurred at a w lower than 0.95 (or matric water potential -7.0 MPa) (Jurado et al., 2008). Matric potential refers directly to forces required to remove water bound to the matrix (soil or plant debris). From the work of Marin et al. (1995) we extrapolate that at high a w (>0.98), growth at 15 °C represented 45 % and 55 % of growth at 25 °C for F. moniliforme and F. proli feratum respectively, since growth at 4 °C represented 16 % and 28 % of growth at 15 °C for F. moniliforme and F. proliferatum respectively. For three Aspergillus flavus strains, the minimum a w needed for mycelial growth was 0.91 at 25 and 37 °C. At 15 °C, only one isolate grew, and it was at 0.99 a w (Lahouar et al., 2016). For white rot mushrooms, linear growth rate of three species of Pleurotus (oyster mushroom) at 15 °C was measured to be 36 to 64 % that at 25°C. It was 62% for Lentinula edodes, shiitake (Zervakis et al., 2001). This highlights that mushroom forming fungi withstand better low temperatures than the mycotoxigenic fungi. Mushroom species have large geographic ranges, and adaptation of geographical populations and/or varieties of the species to different climates and habitats could be expected. As an example, Agaricus bisporus (button mushroom) is known to extend from the boreal region of Alaska to the equatorial climate of Congo and from coastal dunes to mountains of above 3000 m elevation (Largeteau et al., 2011). We postulate that it will be easy to select mushroom species and strains that are competitive for the colonisation of crop debris at temperatures below 15 °C, when mycotoxigenic fungi are inactive.
Basidiomycetes are known to be more sensitive to changes in water potential than ascomycetes. The optimum growth of various wood decay basidiomycetes, including Pleurotus ostreatus was measured between 0.999 and 0.990 a w (water potential -0.1 to -1.4 MPa) and growth ceased between -0.97 and 0.95 a w (Magan et al., 1995). For Agaricus bisporus and A. bitorquis, -3 MPa (0.985 a w) was found to stop growth, and growth decreased below -1.0 MPa (0.990 a w). In mushroom cultivation substrates based mainly of cereal straw, this favourable water potential is obtained with moisture contents in the range 60-80 % (Philippoussis and Diamantopoulou, 2011). Compared to mycotoxigenic fungi, mushroom forming fungi are less tolerant to water stress due to low water potential and would be less competitive on plant debris under dry conditions.
Finally, the strength of white rot mushrooms for colonizing the crop debris in intercrops and exclude mycotoxigenic fungi is the strong capacity to breakdown the lignocellulosic material and their better adaptation to low temperature, but their weakness is their need of higher values of water availability for expressing their potential of colonisation. The feasibility of the proposed treatment of crop debris to reduce primary inoculum of mycotoxigenic fungi in fields, have to be experimented in microcosms and in fields. The efficiency of a large diversity of mushroom species either inoculated after harvest or introduced at the following spring could be tested.
Antifungal compounds from mushroom forming fungi active against mycotoxigenic fungi in cereals
In addition to exploitation competition through resource depletion and physical exclusion, mushroom-forming fungi may act by interference competition through antibiosis, which might reinforce their ability to reduce the inoculum reservoir. In a review of the literature from 1999 to 2012 on antifungal activity of mushroom extracts and isolated compounds, the authors found that 52 species had been reported for their antifungal properties (Alves et al., 2013). More than 80% are edible mushrooms, since their extracts might be generally be recognized as safe for use. As an example, ethanolic extracts of Hydnum repandum, a mycorrhizal species commonly known as the sweet tooth or hedge-hog mushroom and broadly distributed in Europe and Asia, had strong antifungal effects on various Fusarium species (Spremo et al., 2017).
Most studies on mushrooms with antifungal activity were on crude or partially simplified extracts without identification of the compounds responsible for this property (Alves et al., 2013). Bioactive substances isolated from higher fungi belong to different chemical groups: (i) secondary metabolites (acids, terpenoids, polyphenols, sesquiterpenes, alkaloids, lactones, sterols, metal chelating agents, nucleotide analogues and vitamins) ii) proteins and glycoproteins; and (iii) high molecular mass polysaccharides (Erjavec et al., 2012). Some specific mushroom β-glucans are already used in clinical treatment for their antitumor and immunomodulatory properties, and are known as elicitors of plant defence response (Ryan, 1987), but they have no direct antifungal activity. Recently volatile compounds from edible mushrooms have been proposed as safe agents for controlling fungal diseases (Oka et al., 2015). Most investigations with mushroom metabolites have been on antagonistic effect to human pathogens for the therapeutics of human disorders, and less on the plant disease control (Chen and Huang, 2009).
Bioactive proteins from mushroom forming fungi with activity against mycotoxigenic species
A dozen of bioactive proteins and protein extracts from mushrooms have been selected for their antifungal activity on several species (Erjavec et al., 2012). In a recent overview of the biocontrol properties of Basidiomycetes, Sivanandhan et al. (2017) inventoried 15 known antifungal active compounds, half were antifungal proteins. As an example of fungal proteins that have been tested in the field of agricultural crop protection, Tamavidins are biotinbinding proteins from the oyster mushroom Pleurotus cornucopiae that can be expressed in transgenic rice and, as such, they provide resistance to the important ricepathogenic fungus Magnaporthe oryzae (Takakura et al., 2012). Homologous to genes coding for Tamavidins were found also in Galerina marginata (poisonous), Hypholoma sublateritium (edible, cultivable) (Bleuler-Martinez et al., 2012). However, humans and animals require biotin to be supplied in the diet and biotin deficiency that may result to an exposure to biotinbinding proteins can lead to developmental delays in children, or hair and skin abnormalities in affected individuals (Dakshinamurti et al., 2017).
To our knowledge, antifungal proteins from cultivable edible mushrooms have not been tested for their activity against mycotoxigenic Fusarium species. However, since several years, a group at the China Agricultural University works on mushroom proteinaceous compounds with biological activities, including antifungal activity against F. oxysporum, a soil born pathogens with a broad host range (Ng et al., 2016) (Table 1). Ganodermin from the medicinal mushroom Gano derma lucidum (Wang and Ng, 2006), Eryngin from Pleurotus eryngii (Wang and Ng, 2004), Pleurostrin from Pleurotus ostreatus (Chu et al., 2005) and Hypsin, from Hypsizigus marmoreus (Lam and Ng, 2001a), were shown to inhibit mycelial growth of F. oxysporum and other fungal plant pathogens. Hypsin and several other mushroom peptides or proteins having different molecular masses and Nterminal sequences are ribosom inactivators (Lam and Ng, 2001a): Pleuturegin from sclerotia of Pleurotus tuberingium (Wang and Ng, 2001), Lyophilin from Lyophillum shimeji (Lam and Ng, 2001b), Flammulin and Velutin, from Flammulina velutipes (Wang and Ng, 2000; Wong et al., 2010), Agrocybin from Agrocybe cylindracea (Ngai et al., 2005), Lentin from Lentinula edodes (Ngai and Ng, 2003), and Cordymin from Cordyceps militaris (Wong et al., 2011) are other mushroom proteins with antifungal properties that showed no effect on Fusarium oxysporum or Aspergillus fumigatus when tested, but they could be tested against mycotoxigenic Fusarium species and their production of mycotoxins.
Table 1: Mushroom proteins active against Fusarium oxysporum that have been extracted from fruiting bodies of mushroom forming fungi
| Name | Origin | Activity | Molecular mass kDa | IC50 or active concentration tested | References |
| Hypsin | Hypsizigus marmoreus | Ribosone-inactivation | 20 | 14.2 µM | Lam and Ng, 2001a |
| Alveolarin | Polyporus alveolaris | Ribonuclease or lectin | 28 | 120 µg/ 15 µL/disc | Wang et al., 2004 |
| Eryngin | Pleurotus eyngii | Thaumatin-like | 10 | 72 g/12 µL/disc | Wang and Ng, 2004 |
| Trichogin | Tricholoma giganteum | Ribonuclease or lectin | 27 | 16 and 80 µg/ 15 µL/ | Guo et al., 2005 |
| disc | |||||
| Ganodermin | Ganoderma lucidum | Unknown+ | 15 | 12.4 µM | Wang and Ng, 2006 |
| CMP | Cordyceps militaris | protease | 12 | >1.6 µM / disc | Park et al., 2009 |
| Unnamed | Pleurotus sajor-caju | ribonuclease | 12 | 30 µmol on disc | Ngai and Ng, 2004 |
Defensins are cysteinerich antimicrobial peptides that may also have antifungal activity. Defensins are common in vertebrates, but fungi, plants, and many invertebrates produce defensin-like peptides structurally similar to the β-defensins from vertebrates (Silva et al., 2014). Defensins are frequently reported to induce membrane permeabilization of fungal cells (Thevissen et al., 1996), or to induce deposition of chitin on the tips of growing hyphae and consequently retard hyphal growth (Lin et al., 2010). Copsin from the mushroom Coprinopsis cinerea is a famous antibacterial defensin (Essig et al., 2014). There is a potential for identifying new defensins with antifungal and anti mycotoxin properties in mushroom forming fungi.
Lectins form one major group of glycan binding proteins and can be relatively specific depending on their composition, site specific modifications and tertiary structure of the glycan. Chitin in cell wall of fungi might be the target of antifungal lectins. Several plant lectins have antifungal activities against Fusarium species (Mohsen et al., 2018). Nikitina et al. (2017) recently reviewed lectins from mycelia and fruiting bodies of basidiomycetes. The ability to synthesize lectins is widespread among higher basidiomycetes and their hemagglutinating activity is species and strain dependent (Mikiashvili et al., 2006). Carbohydrate specificity also depends on lignocellulosic growth substrate used for producing mushrooms. It is possible to regulate lectin production and obtain preparations with different carbohydrate specificities by substitution of lignocellulosic material and strains (Davitashvili, et al., 2008; 2011). The development stage of mushroom forming fungi is also to be taken into account. In P. ostreatus, the mycelial lectins were mostly specific for mannose, whereas the highest inhibition toward the fruit body lectins was obtained with N-acetyl-d-glucosamine and N-acetyl-d-galactosamine for one lectin fraction and with galactose for other ones (Davitashvili, et al., 2010). In mushrooms, lectins are factors of the interactions with other fungi. The recognition and binding of the A. bisporus fruiting body lectin to Lecanicillium fungi cola cell wall glucogalactomannan contributes to the development of the dry bubble disease on the button mushroom (Bernardo et al., 2004). Lectins of L. edo des either stabilize or activate laccases produced by the fungus (Vetchinkina et al., 2008). There is no clear report on mushroom lectins with antifungal activities against mycotoxigenic fungi, but due to their diversity, the possibility of modulation by cultivation conditions and the known activities, they are good candidates as molecules for controlling these fungi and their production of mycotoxins.
Various enzymes are produced by mushroom forming fungi to explore and use their environment, and they may contribute to an antifungal activity. Several studies reported that enzymes playing critical roles in cell wall degradation of fungi could be used for controlling fungal pathogens in vitro (Deng et al., 2018). A protease from Trichoderma harzianum inhibits spore germination of Aspergillus flavus and A. fumigatus, and is proposed as a potential means of controlling fungal diseases (Deng et al., 2018). An isolated serine protease from a Streptomyces species proved to be active against Fu sarium udum (Sing and Chhatpar, 2011). A large variety of proteases has been revealed in mushroom forming fungi and some show unique characteristics that could be exclusive to Basidiomycetes (Sabotič et al., 2007). Subtilases are a class of serine proteases ubiquitous in fungi where they play a role in nutrition acquisition or in host invasion by contributing to the digestion of the cuticles, but gene duplication and sequence diversification have likely resulted in functional diversifications of fungal subtilases beyond simple nutrient utilization (Li et al., 2017). A subgroup of subtilisin-like proteases from ascomycete and basidiomycete fungi, belonging to the pyrolysin family has been defined (Faraco et al., 2005). For instance, most proteases of the genus Pleu rotus feature the characteristics of alkaline subtilases (Eisele et al., 2011). This makes mushroom forming fungi a large, untapped pool of novel proteases with potential for controlling mycotoxigenic fungi.
Beta-1,3-glucanases are considered to be one component of a broad generalised defence mechanism of plants against pathogen attack. Beta-1,3-glucanase from seeds of Jatropha curcas exerted in vitro antifungal activity against Rhizoctonia solani Kuha and Gibbe rella zeae (anamorph Fusarium graminearum) (Wei et al., 2005). Fungi also produce beta-1,3-glucanases that have a physiological role in cell division, differentiation and nutritional role related to mycoparasitic activity. Tri choderma harzianum is an antagonist of a large variety of plant pathogens. Among different mechanisms for antagonism, it is able to produce lytic enzymes for the degradation of the fungal cell wall to assimilate further the intracellular contents of its hosts. Both β-1,3-glucanase and α-1,3-glucanase contribute to the antifungal activity (Ait-Lahsen et al., 2001; Benítez et al., 1998). These enzymes are produced in large quantities by vegetative mycelium of mushroom forming fungi that could be used as source of enzyme production or as direct antagonists of mycotoxigenic fungi.
Phenolic compounds from fruiting bodies of edible mushrooms, with potential of activity against mycotoxigenic fungi
Both flavonoids and phenolic acids are lipophilic and can diffuse through the fungal membrane to penetrate into the cell and interfere in metabolic pathways involved in cell wall and membrane structures or transmembrane transport (Ansari et al., 2013). The resistance to mycotoxin accumulation of some cultivars of cereals or maize has been correlated with their content of phenolic compounds having a role in inhibiting growth and/ or toxin production by fungi (Atanasova-Penichon et al., 2016).
Plant flavonoids have promising ability to inhibit spore development and limit mycelium hyphae elongation of plant pathogens (Mierziak et al., 2014). Flavones and flavanones were active against F. graminearum, F. culmorum, F. avenaceum with low half maximal inhibitory concentrations (IC50 <4.8 mM) (Atanasova-Penichon et al., 2016). Many reports ensure that mushroom contains flavonoids, but edible mushrooms such as A. bisporus and P. ostreatus have been shown unable to synthesize them because they do not have, in their genome, genes coding the main enzymes involved in their metabolic pathway. In addition, no significant absorption was noticed from fruiting bodies cultivated in flavonoid-enriched substrates (Gil-Ramírez et al., 2016). Phenolic acids can be divided into two major groups, derivatives of benzoic acid having C1-C6 backbone such as phydroxybenzoic or syringic acids, and derivatives of cinnamic acid having C3-C6 backbones, such as pcoumaric, caffeic, ferulic, chlorogenic acids (Ferreira et al., 2009). Ferulic acid and cinnamic acid were found to be more abundant in heads of FHB-resistant barley varieties than in those of susceptible ones, following F. graminearum inoculation (Bollina and Kushalappa, 2011). Atanasova-Penichon et al. (2016) reviewed the antioxidant secondary metabolites in cereals with potential involvement in resistance to Fusarium and mycotoxin accumulation. Their fungicidal efficiency has been characterized against different Fusarium species and the authors proposed a ranking of phenolic acids for their toxicity toward F. graminearum as follows: chlorogenic acid < p-hydroxybenzoic acid < caffeic acid < syringic acid < p-coumaric acid < ferulic acid. In fact, more lipophilic compounds display a more efficient antifungal activity on Fusarium grami nearum (Ponts et al., 2011). However, it is noteworthy that a same phenolic acid can have contrasted effects, depending on the dose used, cultivation conditions and the species. For example, Fusarium verticillioides growth was significantly reduced at 20 mM of ferulic acid, but it was stimulated at 1.0 mM (Ferrochio et al., 2013), and F. langsethiae growth was reduced at 1.0 mM but stimulated at 0.5 mM (Ferruz et al., 2016).
Phenolic acids are the main phenolic compounds in mushroom forming fungi that are proposed to be used as an easily accessible source of natural antioxidants (Nowacka et al., 2014). Recently, ethanolic extracts, known to be rich in phenolic compounds, from fruiting bodies of Hydnum repandum, and methanolic extracts of Stereum subtomentosum and Bjerkandera adusta, showed antifungal effects on F. verticillioides, F. pro liferatum and F. graminearum (Spremo et al., 2017). Phenolic acids have been measured in various species, but depending on the analytical methods used, not all the phenolic acids had been searched in all the studied mushrooms. Table 2 shows data on mushroom contents for the six phenolic acids having fungicidal activity against F. graminearum according to Atanasova-Penichon et al. (2016).
Table 2: Phenolic acids concentrations in mushroom forming fungi
| References | Mushroom species | Edibility | Phenolic acids (mg Kg-1) | |||||
| ferulic ferulic | p-coumaric acid | syringic acid | caffeic acid | p-hydroxybenzoic acid | chlorogenic acid | |||
| Barros et al., 2009 | Agaricus arvensis | edible | _ | 48.7 | _ | _ | 70.1 | _ |
| Gasecka et al., 2018 | A. arvensis | _ | 11 | 16 | nd | 13 | nd | 57 |
| Muszyńska et al., 2015 | A. biporus | edible | nd | nd | - | - | 7.0 | - |
| Kim et al., 2008 | A. bisporus | edible | nd | nd | nd | nd | nd | nd |
| Reis et al., 2013 | A. bisporus | edible | - | 2.3 | - | - | nd | - |
| Barros et al., 2009 | A. bisporus | edible | - | nd | - | - | 25.6 | - |
| Palacios et al., 2011 | A. bisporus | edible | 16 | 10.4 | 15.5 | 15.4 | 63.7 | |
| Gasecka et al., 2018 | A. bisporus (mean 7 strains) | edible | 13 | 8.7 | nd | 15.5 | 15.4 | 63.7 |
| Gasecka et al., 2018 | A. bitorquis | edible | 11 | nd | nd | 14 | nd | 56 |
| Gasecka et al., 2018 | A. campestris | edible | 18 | 11 | nd | 1 | 28 | nd |
| Barros et al., 2009 | A. romagnesii | toxic | - | nd | - | - | 32.4 | - |
| Gasecka et al., 2018 | A. silvaticus | edible | 10 | 15 | 34 | 3 | 18 | nd |
| Barros et al., 2009 | A. silvicola | edible | - | 45.7 | - | - | 238.7 | - |
| Gasecka et al., 2018 | A. subrufescens | edible | 14 | 5 | 56 | 22 | 15 | 62 |
| Kim et al., 2008 | A. subrufescens (blazei) | edible | nd | nd | 22 | nd | nd | nd |
| López-Vázquez et al., 2017 | Amanita caesarea | edible | - | - | - | - | - | 564.4 |
| Yahia et al., 2017 | A. flavocconia | non-edible | 2.1 | nd | - | nd | 145.4 | nd |
| Yahia et al., 2017 | A. pantherina | toxic | nd | nd | - | 257.6 | 90.0 | 36.9 |
| Dundar et al., 2015 | Armillaria tabescens | edible | -- | 0.6 | 62.6 | nd | - | - |
| Muszyńska et al., 2015 | Boletus badius | edible | 1.5 | 13.9 | - | - | 1.3 | - |
| Dundar et al., 2015 | B. edulis | edible | - | 5.5 | 38.9 | 6.1 | - | - |
| Yahia et al., 2017 | B. edulis | edible | nd | nd | - | - | 34.0 | nd |
| López-Vázquez et al., 2017 | B. edulis | edible | - | - | - | - | - | 1001.7 |
| Palacios et al., 2011 | B. edulis | edible | nd | 0.9 | - | 15.1 | 24.1 | 62.8 |
| Yahia et al., 2017 | B. frostii | edible | 100.0 | nd | - | nd | nd | nd |
| Yahia et al., 2017 | B. Luridus | edible | nd | nd | - | 53.0 | nd | 185.3 |
| Palacios et al., 2011 | Calocybe gambosa | edible | 14.5 | nd | - | 14.9 | 11.3 | 63.0 |
| Nowacka et al., 2014 | Calvatia excipuliformis | toxic | trace | 0.2 | - | nd | 2.19 | - |
| López-Vázquez et al., 2017 | Cantharellus cibarius | edible | - | - | - | - | - | 108.2 |
| Palacios et al., 2011 | C. cibarius | edible | 10.4 | nd | - | 16.3 | 15.7 | nd |
| Muszyńska et al., 2015 | C. cibarius | edible | nd | nd | - | - | nd | - |
| Nowacka et al., 2014 | Clitocybe cinerea | non-edible | trace | nd | - | nd | 4.5 | - |
| Nowacka et al., 2014 | Coprinus micaceus | edible | nd | 6.0 | - | nd | 25.1 | - |
| Reis et al., 2012a | Cortinarius caperatus | edible | - | nd | - | - | 25.9 | - |
| Nowacka et al., 2014 | Cratarellus cornucopiodes | edible | trace | 0.3 | - | nd | 12.7 | - |
| Palacios et al., 2011 | C. cornucopioides | edible | 14.0 | nd | - | nd | 6.3 | nd |
| Vaz et al., 2011 | Fistulina hepatica | edible | - | nd | - | - | 41.9 | - |
| Kim et al., 2008 | Flammulina velutipes | edible | 9 | nd | nd | 17 | nd | 26 |
| Kim et al., 2008 | Ganoderma lucidum | edible | nd | nd | nd | nd | nd | nd |
| Dundar et al., 2015 | Helvella leucopus | non-edible | - | 57.0 | nd | nd | - | - |
| Dundar et al., 2015 | H. queletii | non-edible | - | nd | 4.6 | nd | - | - |
| Vaz et al., 2011 | Hygrophorus agathosmus | edible | - | 8.6 | - | nd | - | |
| Vaz et al., 2011 | H. olivaceo-albus | edible | - | nd | - | - | 7.4 | - |
| Palacios et al., 2011 | H. marzuolus | edible | nd | 4.7 | - | 14.6 | 5.5 | nd |
| Kim et al., 2008 | Inonotus obliquus | edible | 22 | nd | nd | nd | 263 | nd |
| Nowacka et al., 2014 | Laccaria amnethystea | edible | nd | 1.9 | - | nd | 17.7 | - |
| Palacios et al., 2011 | Lactarius deliciosus | edible | 11.4 | nd | - | 15.5 | 21.4 | 62.7 |
| Barros et al., 2009 | L. delicious | edible | - | nd | - | - | 22.7 | - |
| Yahia et al., 2017 | L. indigo | edible | nd | 30.5 | - | nd | 11.6 | 3.2 |
| López-Vázquez et al., 2017 | L. indigo | edible | - | - | - | - | - | 222.4 |
| Vaz et al., 2011 | L. salmonicolor | edible | - | nd | - | - | 3.4 | - |
| Reis et al., 2012a | Lentinula edodes | edible | - | nd | - | - | 1.6 | - |
| Kim et al., 2008 | L. edodes | edible | nd | nd | nd | nd | nd | nd |
| Barros et al., 2009 | Lepista nuda | edible | - | 3.7 | - | - | 29.3 | - |
| Barros et al., 2009 | Lycoperdon molle | edible | - | 4.0 | - | - | 41.7 | - |
| Nowacka et al., 2014 | L. perlatum | edible | nd | 1.9 | - | nd | 3.7 | - |
| Kim et al., 2008 | Phellinus linteus | edible | nd | nd | nd | 13 | 6 | 23 |
| Nowacka et al., 2014 | Pholiota mutabilis | edible | trace | 29.1 | - | 1.4 | 24.8 | - |
| Reis et al., 2012a | Pleurotus eryngii | edible | - | 1.0 | - | - | 0.1 | - |
| Kim et al., 2008 | P. eryngii | edible | nd | nd | nd | nd | nd | nd |
| Dundar et al., 2015 | P. ostreatus | edible | - | nd | 25.4 | 2.5 | - | - |
| Reis et al., 2012a | P. ostreatus | edible | - | 0.8 | - | - | 1.6 | - |
| Palacios et al., 2011 | P. ostreatus | edible | 20.2 | 11.1 | nd | 4.7 | nd | |
| Kim et al., 2008 | P. ostreatus | edible | nd | nd | nd | nd | nd | 19 |
| Dundar et al., 2015 | Psathyrella candolleana | edible | - | 66.4 | 12.5 | 3.2 | - | - |
| Barros et al., 2009 | Ramaria botrytis | edible | - | nd | - | - | 14.0 | - |
| Yahia et al., 2017 | Russula emetica | non-edible | 5.2 | nd | - | nd | nd | nd |
| Yahia et al., 2017 | Sarcodon imbricatus | non-edible | nd | nd | - | nd | nd | 20.7 |
| Barros et al., 2009 | S. imbricatus | edible | - | nd | - | - | 33.2 | - |
| Kim et al., 2008 | Sparassis crispa | edible | nd | 37 | 5 | 18 | 34 | nd |
| Yahia et al., 2017 | Strobilomyces floccopus | non-edible | nd | nd | - | 79.9 | 80.0 | nd |
| Vaz et al., 2011 | Suillus collinitus | edible | nd | - | - | 14.1 | - | |
| Vaz et al., 2011 | S. mediterraneensis | non-edible | nd | - | - | 2.0 | - | |
| Barros et al., 2009 | Tricholoma acerbum | edible | nd | - | - | 29.7 | - | |
| Dundar et al., 2015 | T. populinum | edible | 4.2 | nd | 3.6 | - | - | |
| Vaz et al., 2011 | T. sulphureum | non-edible | nd | - | - | 2.6 | - | |
Nd= non detected, - = not searched in the analysis of extracts.
Among the three compounds with the highest antifungal effect on F. graminearum, syringic acid had not been frequently searched in mushroom forming fungi, whereas p-coumaric acid was assayed in 69 and ferulic acid in 41 mushroom samples out 76 mushrooms analysed in the works reported in Table2. Both p-coumaric acid and ferulic acid were detected in 41 % of the samples where they were searched. In table 2, the highest content of p-coumaric acid is for Psathyrella can dolleana (66.4 mg kg-1) collected in Turkey (Dündar et al., 2015). Agaricus arvensis, Lactarius indigo, Pholiota mutabilis, contain more than 25 mg p-coumaric acid kg-1. The highest contents of ferulic acid are in Boletus frostii (100 mg kg-1), an American species widely distributed, and in the cultivated oyster mushroom, Pleuro tus ostreatus (20 mg kg-1).
However, it is noteworthy that for a same species, significant differences in concentrations are reported in different articles. In Table 2, samples of A. bisporus were analysed in six different studies, and p-coumaric acid was found in only three samples (2 to 10 mg kg-1). In a study comparing different white hybrids and a brown strain of A. bisporus grown simultaneously in an experimental facility (Gasecka et al., 2018), the concentration of p-coumaric acid was higher in the brown strain than in the white ones. Ferulic acid concentration in A. bisporus was 13 and 16 mg kg-1 in the studies reported by Geseacka et al. (2018) and Palacios et al. (2011), whereas it was not detected in two other studies (Kim et al., 2008; Muszyńska et al., 2015).
Cinnamic acid has been detected at concentrations higher than 10 mg kg-1 in Agaricus subrufescens (Kim et al., 2008), A. arvensis, A. sylvicola, A. romagnesii (Barros et al., 2009), Boletus edulis (Yahia et al., 2017), Ga noderma lucidum (Heleno et al., 2013); fruiting bodies of Cantharellus cibarius and Picipes badius (formely Boletus badius) (Muszyńska et al., 2015), Hygrophorus agathosmus and Tricholoma sulphureum (Vaz et al., 2011).
Phenolic acids from mushrooms have also antifungal potentials against producers of Aflatoxins or Ochratoxins. Methanolic extracts from fruiting bodies of Cordiceps militaris containing p-hydroxybenzoic acid and cinnamic acids revealed antifungal potential against A. fumigatus and A. ochraceus (Reis et al., 2013). P-hydroxybenzoic and cinnamic acids identified in Ganoderma lucidum revealed antifungal activity against Aspergillus fu migatus, A. ochraceus, A. niger, with higher activity than the fungicides used as standards (Heleno et al., 2013). Methanolic extracts containing these compounds proved to have higher antioxidant potential than corresponding water extracts rich in polysaccharides (Heleno et al., 2012).
The present survey of the literature shows that significant levels of phenolic compounds can be found in mushroom forming fungi, but the available data varies between authors for a same species and it is difficult to select a priori mushroom species with an interesting composition in phenolic acids that could be a source of extracts as anti-Fusarium product. Apart from differences in analytical methods between laboratories expertise, the quality of the samples analysed might be affected by the development stage of the mushroom, the storing delay and conditions before analyses, the ecosystems they come from or the substrates and conditions used for their cultivation, intraspecific variations due to genetic diversity. Further screenings for obtaining extracts with interesting phenolic composition and antifungal activity should be on mushroom species with high levels of available biodiversity that can be cultivated under controlled conditions, and for which strain selection are possible. This is the case of the cultivated edible mushrooms such as several species of Agaricus or Pleurotus. Selection of very efficient strains could be completed with improvement of their capacities by breeding programs that are currently performed on these species for other traits (Savoie et al. 2013).
Other specific compounds from mushroom forming fungi with antifungal activity
Several molecules with potential as fungicides has been identified and isolated from mushroom forming fungi. There are few examples with concern on mycotoxigenic fungi. Three are given bellow.
The first fungicides in the family of Strobilurins were isolated from wood-rotting mushroom fungi, including one called Strobilurus tenacellus and chemists synthetized and improved on these natural fungicides (Vincelli, 2012). Strobilurins act to inhibit the mitochondrial respiratory chain at the level of Complex III, which results in cellular oxidative stress triggered by electrons escaping from the respiratory chain, which can be detoxified by mitochondrial superoxide dismutase (Kim et al., 2007). Strobilurin fungicides were reported to be hardly effective on several occasions against different Fusarium species few years after their introduction, but natural resistance to trifloxystrobin has been shown in populations of F. graminearum (Dubos et al., 2011). Consequently, Strobilurins are not efficient for controlling these fungi. Antimicrobial terpenoids are potentially present in many mushrooms forming fungi and cultivating mushrooms as biosources for these compounds is worth being studied (Shen et al., 2017). Rufuslactone from Lactarius rufus was found to inhibit the mycelial growth of F. graminearum (Luo et al., 2005), and chrysotrione A and B from Hygro phorus chrysodon showed moderate antifungal activity on F. verticillioides whereas they were more active for controlling another phytopathogenic fungus, Alternaria brassica (Gilardoni et al., 2007).
1-octen-3ol and 10-oxo-trans-8-decenoic acid are volatile secondary metabolites found in many mushrooms. They contribute to the “mushroom-like” aroma and possess antifungal activities against the mycotoxigenic fungus Penicillium expansum (Okull et al., 2003). This survey of potent antifungal compounds from mushroom forming fungi shows they are various, and some might be active against mycotoxigenic fungi. However, it is also worth finding natural compounds having an effect on the mycotoxin production for completing fungicide or fungistatic properties.
Anti-mycotoxin compounds from mushroom forming fungi
In this section, mushroom compounds with potential activity on the production of mycotoxins by Aspergillus spp. and other mycotoxigenic fungi have been discussed, with emphasis on phenolic compounds and antioxidants because they have been shown to inhibit the production of some mycotoxins.
Mushroom extracts inhibiting the production of aflatoxins
Published works on the control of mycotoxin production by mushroom-forming fungi are mainly for aflatoxins produced by Aspergillus parasiticus or A. flavus and lignocolous white rot fungi, Lentinula edodes, Tra metes versicolor, Pleurotus spp. The polyketide aflatoxin is looked as a secondary defence system protecting the fungus from excess reactive oxygen species (ROS) (Hong et al., 2013). Production of aflatoxin was observed to be related to the peroxidation of the fungal cellular membranes and fatty acid β-oxidation in both the peroxisome and mitochondria (Maggio-Hall et al., 2005). That explains why antioxidants have been taken into consideration, and reported for their efficacy for controlling aflatoxin production (Zhao et al., 2018).
Culture filtrates of L. edodes were shown to be able to inhibit aflatoxin production by A. parasiticus with a direct relation between aflatoxin inhibition and β-glucan content of the lyophilised filtrates (Reverberi et al., 2005). Such extracts were shown to stimulate the antioxidant system of the mycelium by the induction of a transient and modulated oxidative burst perceived by ApyapA transcription factor of A. parasiticus (Reverberi et al., 2006). Bioactive compounds secreted in the culture filtrates of T. versicolor, as well as lyophilised solid of mycelium grown on agricultural residues, inhibited the production of aflatoxin in maize kernels inoculated with A. flavus (Scarpari et al., 2014). The Afyap1 transcription factor of A. flavus was involved (Scarpari et al., 2016).
For both L. edodes and T. versicolor, significant differences in aflatoxin inhibition rate among different tested isolates were evidenced, although the same cultural conditions were used. They were attributed to differences in quality rather than the quantity of the inhibiting compounds (Zjalic et al., 2006), but improvement of the yield of β-glucan production by L. edodes strains using oxidative stress inductors leaded to higher aflatoxin inhibiting capacity (Reverberi et al., 2004). In addition to mycelium, fruiting bodies can also be a source of active compounds. A water-soluble complex of polysaccharides possessing β-(1, 4)-glucan backbones and peptides extracted at high temperature from fruiting bodies of L. edodes inhibited AFB1 production by A. flavus, with an IC50 of < 10 μg mL-1 (Li et al., 2015).
Finally, the efficacy of mushroom extracts as inhibiters of the production of aflatoxin is due, on the one hand, to the presence of compounds with intrinsic antioxidant activity and, on the other hand, to the stimulation of the antioxidant system of the toxigenic fungi.
Mushroom extracts inhibiting the production of other mycotoxins
Lentinula edodes extracts were also proposed to boost the antioxidant enzyme activity of a biocontrol agent and contribute to control patulin biosynthesis, promoting the antioxidant activity of Penicillium expansum on apples (Tolaini et al., 2010; Wang et al., 2013). They also can elicit and regulate the biosynthesis of ochratoxin A by Aspergillus ochraceus (Reverberi et al., 2008). Methanolic extracts from fruiting bodies of Agaricus boshui are also active against Penicillium verrucosum producing OTA (Reis et al., 2012b).
We did not find data on the regulation of the biosyn-thesis of Fusarium toxins by this kind of mushroom extracts. However, in addition to aflatoxin, ROS, antioxidants and Yap-like bZIP factors are also known to be involved in regulation of the biosynthesis of Fusarium mycotoxins such as trichothecenes, fumonisins and ZEN. Response to oxidative stress and fungal secondary metabolism are indeed intertwined. It seems likely that for instance, the production of DON and its acetylated derivatives is probably a part of an adaptive response to oxidative stress from the ROS produced by the host plant as an early defence mechanism in response to infection by F. graminearum (Ponts, 2015). Oxidative stress with H2O2 could be a pre-requisite for the biosynthesis of DON. Both stress responses and the regulation of DON synthesis are mediated by the transcription factor FgAP1 that activates the transcription of antioxidant enzymes (Montibus et al., 2013). A link was established between antioxidant and antiradical properties of flavonoids and their effects on tricho-thecenes production by F. culmorum and F. graminea rum (Bilska et al., 2018). Microalgae phenolic extracts rich in chlorogenic acid showed antifungal and antimy-cotoxigenic activities (Scaglioni et al., 2018), as well as maize extracts do (Atanassova-Penichon et al., 2014). Synergic effect between different compounds present in the extracts also influences their antimycotoxigenic activity (Scaglioni et al., 2018). Natural antioxidants such as ferulic acid may contribute to limit the production of fumonisins in maize kernels (Picot et al., 2013).
Phenolic compounds and antioxidant activity from mushrooms inhibiting the production of mycotoxins
Antioxidant activities appeared above as regulators of the production of various mycotoxins and searching for mushroom forming fungi with high contents in antioxidants is of interest. Mushroom extracts with antioxidant properties might contribute both to fight the up regulation of synthesis of mycotoxins by oxidative stress (Zhao et al., 2018) and to protect against their toxicity (Da Sylva et al., 2018). Actually, mushrooms are currently proposed to be used as an easily accessible source of natural antioxidants mostly due to their richness in phenolic compounds, and have attracted much interest as functional foods. The total phenol concentration is currently measured in mushroom samples for an estimation of their potential as antioxidant in functional food. It is determined most of the time in extracts by the Folin-Ciocalteu’s phenol reagent method (Savoie, 2008). Gil-Ramírez et al. (2016) who used a direct measurement of total phenolic compounds without extraction on 24 samples from 24 species belonging to 18 genera, reported values ranging from 1 to more than 14 mg gallic acid equivalent (GAE) g-1 mushroom powder, depending on the species. For the button mushroom A. bisporus, total phenolic contents ranged from 1 to 16 mg GAE g-1 when both white and brown strains, and both cultivars and wild strains were taken into account (Alvarez-Parrilla et al., 2007; Barros et al., 2009; Dubost et al., 2007; Gan et al., 2013; Gasecka et al., 2018; Gil-Ramirez et al., 2016; Palacios et al., 2011; Reis et al., 2012a; Savoie et al., 2008). Comparing these intraspecific and interspecific range shows the variability of the data and the necessity to characterize the factors responsible of variations in total phenolic contents.
The efficiency of selected phenolic acids to modulate in vitro the biosynthesis of mycotoxins has been demonstrated for various toxins. Ferulic, chlorogenic, and caffeic acids have been shown as efficient inhibitors of TCTB production by Fusarium graminearum and F. cul morum (Boutigny et al., 2009; Gauthier et al., 2016), TCTA by F. langsethiae and F. sporotrichioides (Ferruz et al., 2016), and fumonisins by F. verticillioides (Atanasova-Penichon et al., 2014; Beekrum et al., 2003).
However, there is a strain and concentration dependent effect of phenolic acids on Fusarium toxin synthesis. Ferulic acid used at concentrations that did not affected the mycelial growth rate can either limit or enhance the accumulation of TCTB by F. graminearum (Boutigny et al., 2009; Ponts et al., 2011). In regards to synthesis of fumonisins, Ferrochio et al. (2013) showed that ferulic acid activated their production at low concentrations while inhibited it at higher ones.
Richness of mushroom extracts in phenolic compounds (Table 2) is discussed above. It is noteworthy that cooccurrence of several compounds are found in various works. Mixture of phenolic acids and other active compounds, as found in natural mushroom extracts, might have different potentials than single molecules. Whereas mixtures of phenolic compounds with antioxidant activities might limit the production of mycotoxins, some studies show that there was no clear correlation between the antioxidant properties of natural and natural-like phenolic compounds tested singly and their ability to inhibit fungal growth and mycotoxinogenic capacity of Fusarium culmorum (Pani et al., 2014). Other properties of the individual compounds due to their chemical structures and mode of action might have higher effects. The potential of a mushroom extract can only partly be predicted by its known composition, and it will have to be evaluated experimentally.
Detoxification
Biotransformation of mycotoxins performed with purified enzymes isolated from bacteria, fungi and plants has recently been reviewed (Loi et al, 2017). Laccases (E.C. 1.10.3.2 benzenediol: oxygen oxidoreductase) are an interesting group of N glycosylated multicopper blue oxidases that oxidize a large number of both phenolic and nonphenolic compounds that may be involved in degradation of aflatoxins. Most of the mushroom-forming fungi are producers of laccases. Motomura et al. (2003) reported a high aflatoxin B1 degradation activity from Pleurotus ostreatus in cultures at 25 °C and pH 4.0-5.0, possibly linked to the enzymatic oxidation handled by laccases or peroxidases (Yehia, 2014). Treatments of maize seeds or milled seeds, with culture filtrates of Trametes versicolor rich in laccases activities, showed a significant degradation of aflatoxin B1 content after 7 days of the treatment (Scarpari et al., 2014). Aflatoxin B1 treated with pure laccase enzyme from T. versicolor (and recombinant laccase produced by A. niger) was significantly metabolized to degradation products with chemical properties that vastly differ from the parent molecule. Concomitant significant loss of mutagenicity, as evaluated in the Salmonella typhi murium mutagenicity assay, was observed (Alberts et al., 2009). The degrading activity of Lac2 pure enzyme from P. pulmonarius towards aflatoxin was enhanced by the addition of natural phenolic compounds as redox mediators (Loi et al., 2016).
Concerning the degradation of other mycotoxins besides the abovementioned activity towards aflatoxins, laccases are also able to degrade ZEN. This was shown with a T. versicolor preparation (Banu et al., 2013). The laccase-mediator system was used with a laccase from P. eryngii, for combined mycotoxin degradation in vitro. DON was not affected whereas AFB1, FB1, OTA, ZEN and T-2 toxin were at least partly degraded, as well as combinations of AFB1 and ZEN, but without relevant synergistic nor additive effects (Loi et al., 2018). In combination of FB1 and T-2 toxin, a strong additive effect was reported for T-2 toxin degradation whereas FB1 degradation was drastically reduced. Most of the cultivated edible mushrooms are white rot fungi, they are laccase producers and they degrade lignocellulosic substrate, which leaves a white rot residue during the degradation of lignin. Laccases extracted from mushrooms or cultivation substrates of white rot fungi mycelium have been used in the removal of many emerging contaminants that are difficult to decompose (Chang et al., 2018). Numerous laccase isoenzymes have been achieved in various species of fungi, with dissimilar physicochemical and kinetic properties. There are various laccase isoforms in a same fungal strain. These isoforms could be expressed at different phases of the fungal life cycle or under diverse cultivation conditions (Baldrian, 2006; Othman et al., 2018). It would be possible to select several ones with interesting activity for Fusarium mycotoxin degradation.
Solid-state fermentation process with edible white rot mushroom forming fungi for biological detoxification of aflatoxin B1 and other mycotoxins is worth being developed. Methods of myceliation of agricultural substrates proposed for producing functional foods with health benefits (Kelly and Langan, 2013) could also contribute to mitigate the mycotoxin risk. This was recently tested at the laboratory scale by cultivating P. eryngii on a mixture containing corn contaminated with aflatoxin B1. The mycotoxin was degraded and edible fruiting bodies free of aflatoxin B1, or its metabolite aflatoxicol, were produced (Branà et al., 2017). The resulting detoxified cultivation substrate could also be used in feed rations. Agro-based spent mushroom substrate from Pleurotus spp. and Agaricus bisporus have been known to be suitable for applications of ruminant feedstock (Mohd Hanafi et al., 2018). Otherwise, many mushroom forming fungi can be cultivated on starch rich materials such as cereal grains in solid-state fermentation, and both nutrients and antioxidant activities of the cereals are enhanced after being fermented by edible mushrooms (Han, 2003; Han et al., 2005; Zhai et al., 2015). The process could also have consequences on the final content in mycotoxins. Grains with low concentrations of mycotoxins would be cleaned up and enriched in nutrients after solid-state fermentation, and could be used as food.
Conclusion
Within this review, we were interested of the potential of mushroom-forming fungi for mitigation of mycotoxin contamination in cereal products. Many mushroom species are a food source and exhibit a rich source of bioactive compounds in preventing or treating various types of human and animal diseases. Some have been proposed as source of antifungal compounds against phytopathogenic fungi. A couple of species are subjects of works in progress for their potential as biocontrol agents of mycotoxinogenic fungi contaminating food and feed at storage. There are untapped potentials to interact with the accumulation of mycotoxins in cereals products, from the field to the final product. Extracts or purified metabolites from cultivable edible mushrooms having antifungal and/or antimycotoxin activities could be used as alternatives to fungicides in integrated crop protection strategies. Mycelium could be used as biocontrol agents being antagonists of mycotoxigenic fungi on crop debris. Mycelium could be used as degrader of mycotoxins in grains or straws and stalks and for enriching them in nutrients and antioxidant properties before their use as feed or food. Ultimately, edible mushrooms would be produced from contaminated materials that could not be detoxified.
