Naturally occurring phenols: a detoxification strategy for fumonisin B1

Volatilome of the maize phytopathogenic fungus Fusarium verticillioides: potential applications in diagnosis and biocontrol

BACKGROUND

Fusarium verticillioides is a maize fungal phytopathogen and a producer of volatile organic compounds (VOCs) and fumonisin B1 (FB1). Our aim was to study the volatilome, conidial production, ergosterol and FB1 biosynthesis in maize cultures over a 30-day incubation period (5, 10, 15, 20, 25, 30 days post inoculation [DPI]). The effect of pure VOCs on the same parameters was then evaluated to study their potential role as biocontrol agents.

RESULTS

In total, 91 VOCs were detected, with volatile profiles being more similar between 5 and 10 DPI compared with 15, 20, 25 and 30 DPI. Ergosterol content increased steadily with incubation time, and three growth stages were identified: a lag phase (0 to 15 DPI), an exponential phase (15 to 20 DPI) and a stationary phase (20 to 30 DPI). The maximum concentration of FB1 was detected at 25 (0.030 μg FB1/μg ergosterol) and 30 DPI (0.037 μg FB1/μg ergosterol), whereas conidial production showed a maximum value at 15 DPI (4.3 ± 0.2 × 105 conidia/μg ergosterol). Regarding pure VOCs, minimal inhibitory concentration values ranged from 0.3 mm for 4-hexen-3-one to 7.4 mm for 2-undecanone. Pure VOCs reduced radial growth, conidial production and ergosterol and FB1 biosynthesis.

CONCLUSIONS

The marked resemblance between VOC profiles at 5 and 10 DPI suggests that they could act as early indicators of fungal contamination, particularly 4-ethylguaiacol, 4-ethyl-2-methoxyanisole, heptanol and heptyl acetate. On the other hand, their role as inhibitors of fungal growth and FB1 biosynthesis prove their great potential as safer alternatives to control phytopathogenic fungi. © 2024 Society of Chemical Industry.

A secondary function of trehalose-6-phosphate synthase is required for resistance to oxidative and desiccation stress in Fusarium verticillioides

The disaccharide trehalose has long been recognized for its role as a stress solute, but in recent years some of the protective effects previously ascribed to trehalose have been suggested to arise from a function of the trehalose biosynthesis enzyme trehalose-6-phosphate (T6P) synthase that is distinct from its catalytic activity. In this study, we use the maize pathogenic fungus Fusarium verticillioides as a model to explore the relative contributions of trehalose itself and a putative secondary function of T6P synthase in protection against stress as well as to understand why, as shown in a previous study, deletion of the TPS1 gene coding for T6P synthase reduces pathogenicity against maize. We report that a TPS1-deletion mutant of F. verticillioides is compromised in its ability to withstand exposure to oxidative stress meant to simulate the oxidative burst phase of maize defense and experiences more ROS-induced lipid damage than the wild-type strain. Eliminating T6P synthase expression also reduces resistance to desiccation, but not resistance to phenolic acids. Expression of catalytically-inactive T6P synthase in the TPS1-deletion mutant leads to a partial rescue of the oxidative and desiccation stress-sensitive phenotypes, suggesting the importance of a T6P synthase function that is independent of its role in trehalose synthesis.

Plant Metabolites Affect Fusarium proliferatum Metabolism and In Vitro Fumonisin Biosynthesis

Fusarium proliferatum is a common hemi-biotrophic pathogen that infect a wide range of host plants, often leading to substantial crop loss and yield reduction. F. proliferatum synthesizes various mycotoxins, and fumonisins B are the most prevalent. They act as virulence factors and specific effectors that elicit host resistance. The effects of selected plant metabolites on the metabolism of the F. proliferatum strain were analyzed in this study. Quercetin-3-glucoside (Q-3-Glc) and kaempferol-3-rutinoside (K-3-Rut) induced the pathogen's growth, while DIMBOA, isorhamnetin-3-O-rutinoside (Iso-3-Rut), ferulic acid (FA), protodioscin, and neochlorogenic acid (NClA) inhibited fungal growth. The expression of seven F. proliferatum genes related to primary metabolism and four FUM genes was measured using RT-qPCR upon plant metabolite addition to liquid cultures. The expression of CPR6 and SSC1 genes was induced 24 h after the addition of chlorogenic acid (ClA), while DIMBOA and protodioscin reduced their expression. The transcription of FUM1 on the third day of the experiment was increased by all metabolites except for Q-3-Glc when compared to the control culture. The expression of FUM6 was induced by protodioscin, K-3-Rut, and ClA, while FA and DIMBOA inhibited its expression. FUM19 was induced by all metabolites except FA. The highest concentration of fumonisin B₁ (FB₁) in control culture was 6.21 µg/mL. Protodioscin did not affect the FB content, while DIMBOA delayed their synthesis/secretion. Flavonoids and phenolic acids displayed similar effects. The results suggest that sole metabolites can have lower impacts on pathogen metabolism and mycotoxin synthesis than when combined with other compounds present in plant extracts. These synergistic effects require additional studies to reveal the mechanisms behind them.

Phenolic compounds as natural microbial toxin detoxifying agents

Despite the abundance of promising studies, developments, and improvements about the elimination of microbial toxins from food matrices, they are still considered as one of the major food safety problems due to the lack of their complete avoidance even today. Every year, many crops and foodstuffs have to be discarded due to unconstrained contamination and/or production of microbial toxins. Furthermore, the difficulty for the detection of toxin presence and determination of its level in foods may lead to acute or chronic health problems in many individuals. On the other hand, phenolic compounds might be considered as microbial toxin detoxification agents because of their inhibition effect on the toxin synthesis of microorganisms or exhibiting protective effects against varying damaging mechanisms caused by toxins. In this study, the effect of phenolic compounds on the synthesis of bacterial toxins and mycotoxins is comprehensively reviewed. The potential curing effect of phenolic compounds against toxin-induced damages has also been discussed. Consequently, phenolic compounds are indicated as promising, and considerable natural preservatives against toxin damages and their detoxification potentials are pronounced.

In Search of Resistance Against Fusarium Ear Rot: Ferulic Acid Contents in Maize Pericarp Are Associated With Antifungal Activity and Inhibition of Fumonisin Production

Fusarium verticillioides is a fungal pathogen of maize that causes seedling blight, stem rot, and Fusarium ear rot. Fungal infestation of maize kernels and ears affects grain quality from the ensuing mycotoxin buildup. Among the mycotoxins produced by F. verticillioides, fumonisins accumulate to high levels in Fusarium-infected maize kernels, fumonisin B1 (FB1) being the most abundant in naturally infected maize. Achieving resistance to Fusarium ear rot has been challenging, as various environmental factors facilitate fungal infection. Among the maize grain components that contribute to resistance to F. verticillioides infection, the pericarp is the first barrier faced by the fungus and thus plays a key role. Phenolic acids are major constituents of maize pericarp, of which ferulic acid (FA) is the predominant molecular species. In this work, we explored the relationship between FA levels, fungal infection, and FB1 production in 51 maize genotypes and whether the antioxidant activity of FA might play a role. We confirmed that FA is a major component of the seed pericarp, whose levels as bound FA varied between 4.5 and 26.3 mg/g across maize genotypes. We selected two pools of five maize varieties, with contrasting FA contents: low FA (LFA; 6.14 ± 0.40 mg/g) and high FA (HFA; 15.49 ± 1.31 mg/g). In vitro, HFA extracts inhibited fungal growth with effects comparable to FA concentrations in the 0.25-0.50 mM range. We also established a kernel assay to study F. verticillioides colonization and FB1 production in the LFA and HFA genotypes. Fungal colonization was significantly lower in HFA genotypes relative to LFA genotypes, based on ergosterol levels. Moreover, FB1 production was also inhibited in the HFA genotypes. Importantly, the antioxidant activity of maize pericarp extracts was associated with FA contents, with HFA extracts exhibiting a greater antioxidant activity than LFA extracts. Overall, our results highlight the role of FA and its antioxidant activity on resistance to Fusarium ear rot and provide the basis of a phenotypic trait that can be deployed for breeding selection.

Naturally occurring prenylated stilbenoids: food sources, biosynthesis, applications and health benefits

Prenylated stilbenoids are a unique class of natural phenolic compounds consisting of C6-C2-C6 skeleton with prenyl substitution. They are potential nutraceuticals and dietary supplements presented in some edible plants. Prenylated stilbenoids demonstrate promising health benefits, including antioxidant, anti-cancer, anti-inflammatory, anti-microbial activities. This review reports the structure, bioactivity and potential application of prenylated stilbeniods in food industry. Edible sources of these compounds are compiled and summarized. Structure-activity relationship of prenylated stilbenoids are also highlighted. The biosynthesis strategies of prenylated stilbenoids are reviewed. The findings of these compounds as food preservative, nutraceuticals and food additive are discussed. This paper combines the up-to-date information and gives a full image of prenylated stilbenoids.

Bioprospecting Phenols as Inhibitors of Trichothecene-Producing Fusarium: Sustainable Approaches to the Management of Wheat Pathogens

Fusarium spp. are ubiquitous fungi able to cause Fusarium head blight and Fusarium foot and root rot on wheat. Among relevant pathogenic species, Fusarium graminearum and Fusarium culmorum cause significant yield and quality loss and result in contamination of the grain with mycotoxins, mainly type B trichothecenes, which are a major health concern for humans and animals. Phenolic compounds of natural origin are being increasingly explored as fungicides on those pathogens. This review summarizes recent research activities related to the antifungal and anti-mycotoxigenic activity of natural phenolic compounds against Fusarium, including studies into the mechanisms of action of major exogenous phenolic inhibitors, their structure-activity interaction, and the combined effect of these compounds with other natural products or with conventional fungicides in mycotoxin modulation. The role of high-throughput analysis tools to decipher key signaling molecules able to modulate the production of mycotoxins and the development of sustainable formulations enhancing potential inhibitors' efficacy are also discussed.

Peanut Seed Coat Acts as a Physical and Biochemical Barrier against Aspergillus flavus Infection

Aflatoxin contamination is a global menace that adversely affects food crops and human health. Peanut seed coat is the outer layer protecting the cotyledon both at pre- and post-harvest stages from biotic and abiotic stresses. The aim of the present study is to investigate the role of seed coat against A. flavus infection. In-vitro seed colonization (IVSC) with and without seed coat showed that the seed coat acts as a physical barrier, and the developmental series of peanut seed coat showed the formation of a robust multilayered protective seed coat. Radial growth bioassay revealed that both insoluble and soluble seed coat extracts from 55-437 line (resistant) showed higher A. flavus inhibition compared to TMV-2 line (susceptible). Further analysis of seed coat biochemicals showed that hydroxycinnamic and hydroxybenzoic acid derivatives are the predominant phenolic compounds, and addition of these compounds to the media inhibited A. flavus growth. Gene expression analysis showed that genes involved in lignin monomer, proanthocyanidin, and flavonoid biosynthesis are highly abundant in 55-437 compared to TMV-2 seed coats. Overall, the present study showed that the seed coat acts as a physical and biochemical barrier against A. flavus infection and its potential use in mitigating the aflatoxin contamination.

Current Perspectives of Biocontrol Agents for Management of Fusarium verticillioides and Its Fumonisin in Cereals-A Review

Fusarium verticillioides is the most predominant fungal phytopathogen of cereals and it is posing great concern from a global perspective. The fungus is mainly associated with maize, rice, sorghum, wheat, sugarcane, banana, and asparagus and causes cob, stalk, ear, root, crown, top, and foot rot. F. verticillioides produces fumonisins as the major secondary metabolite along with trace levels of beauvericin, fusaric acid, fusarin C, gibberiliformin, and moniliformin. Being a potential carcinogen, fumonisins continue to receive major attention as they are common contaminants in cereals and its processed food products. The importance of elimination of F. verticillioides growth and its associated fumonisin from cereals cannot be overemphasized considering the significant health hazards associated with its consumption. Physical and chemical approaches have been shown to reduce fumonisin B1 concentrations among feeds and food products but have proved to be ineffective during the production process. Hence, biological control methods using microorganisms, plant extracts, antioxidants, essential oils, phenolic compounds, and other advanced technologies such as growing disease-resistant crops by applying genetic engineering, have become an effective alternative for managing F. verticillioides and its toxin. The different methods, challenges, and concerns regarding the biocontrol of F. verticillioides and production of fumonisin B1 have been addressed in the present review.

Antifungal and mycotoxin detoxification ability of essential oils: A review

With increased popular awareness of food safety and environmental protection, plant essential oil has attracted interest due to the absence of residue, its high efficiency, antioxidant, immune regulation, antibacterial, insecticidal, and other advantages. Their application in degradation and elimination of mycotoxin toxicity has attracted increasing attention. This paper reviews the structure, antibacterial activity, antibacterial mechanism, and toxic effects of essential oils. The inhibitory effects of various essential oils on different mycotoxins were studied. The research progress on the inhibitory effects of plant essential oils on fungi and mycotoxins in recent years was summarized to provide reference for the application of plant essential oils.

The Sorghum Grain Mold Disease Complex: Pathogens, Host Responses, and the Bioactive Metabolites at Play

Grain mold is a major concern in sorghum [Sorghum bicolor (L.) Moench] production systems, threatening grain quality, safety, and nutritional value as both human food and livestock feed. The crop's nutritional value, environmental resilience, and economic promise poise sorghum for increased acreage, especially in light of the growing pressures of climate change on global food systems. In order to fully take advantage of this potential, sorghum improvement efforts and production systems must be proactive in managing the sorghum grain mold disease complex, which not only jeopardizes agricultural productivity and profitability, but is also the culprit of harmful mycotoxins that warrant substantial public health concern. The robust scholarly literature from the 1980s to the early 2000s yielded valuable insights and key comprehensive reviews of the grain mold disease complex. Nevertheless, there remains a substantial gap in understanding the complex multi-organismal dynamics that underpin the plant-pathogen interactions involved - a gap that must be filled in order to deliver improved germplasm that is not only capable of withstanding the pressures of climate change, but also wields robust resistance to disease and mycotoxin accumulation. The present review seeks to provide an updated perspective of the sorghum grain mold disease complex, bolstered by recent advances in the understanding of the genetic and the biochemical interactions among the fungal pathogens, their corresponding mycotoxins, and the sorghum host. Critical components of the sorghum grain mold disease complex are summarized in narrative format to consolidate a collection of important concepts: (1) the current state of sorghum grain mold in research and production systems; (2) overview of the individual pathogens that contribute to the grain mold complex; (3) the mycotoxin-producing potential of these pathogens on sorghum and other substrates; and (4) a systems biology approach to the understanding of host responses.

Effect of Naturally Occurring Compounds on Fumonisin Production and fum Gene Expression in Fusarium verticillioides

Fusarium verticillioides, one of the most common pathogens in maize, is responsible for yield losses and reduced kernel quality due to contamination by fumonisins (FBs). Two F. verticillioides isolates that differed in their ability to produce FBs were treated with a selection of eight natural phenolic compounds with the aim of identifying those that were able to decrease toxin production at concentrations that had a limited effect on fungal growth. Among the tested compounds, ellagic acid and isoeugenol, which turned out to be the most effective molecules against fungal growth, were assayed at lower concentrations, while the first retained its ability to inhibit toxin production in vitro, the latter improved both the fungal growth and FB accumulation. The effect of the most effective phenolic compounds on FB accumulation was also tested on maize kernels to highlight the importance of appropriate dosages in order to avoid conditions that are able to promote mycotoxin biosynthesis. An expression analysis of genes involved in FB production allowed more detailed insights into the mechanisms underlying the inhibition of FBs by phenolic compounds. The expression of the fum gene was generally down-regulated by the treatments; however, some treatments in the low-producing F. verticillioides strain up-regulated fum gene expression without improving FB production. This study showed that although different phenolic compounds are effective for FB reduction, they can modulate biosynthesis at the transcription level in opposite manners depending on strain. In conclusion, on the basis of in vitro and in vivo screening, two out of the eight tested phenols (ellagic acid and carvacrol) appear to be promising alternative molecules for the control of FB occurrence in maize.

Sphinganine-Analog Mycotoxins (SAMs): Chemical Structures, Bioactivities, and Genetic Controls

Sphinganine-analog mycotoxins (SAMs) including fumonisins and A. alternata f. sp. Lycopersici (AAL) toxins are a group of related mycotoxins produced by plant pathogenic fungi in the Fusarium genus and in Alternaria alternata f. sp. Lycopersici, respectively. SAMs have shown diverse cytotoxicity and phytotoxicity, causing adverse impacts on plants, animals, and humans, and are a destructive force to crop production worldwide. This review summarizes the structural diversity of SAMs and encapsulates the relationships between their structures and biological activities. The toxicity of SAMs on plants and animals is mainly attributed to their inhibitory activity against the ceramide biosynthesis enzyme, influencing the sphingolipid metabolism and causing programmed cell death. We also reviewed the detoxification methods against SAMs and how plants develop resistance to SAMs. Genetic and evolutionary analyses revealed that the FUM (fumonisins biosynthetic) gene cluster was responsible for fumonisin biosynthesis in Fusarium spp. Sequence comparisons among species within the genus Fusarium suggested that mutations and multiple horizontal gene transfers involving the FUM gene cluster were responsible for the interspecific difference in fumonisin synthesis. We finish by describing methods for monitoring and quantifying SAMs in food and agricultural products.

The association of maize characteristics with resistance to Fusarium verticillioides and fumonisin accumulation in commercial maize cultivars

Fusarium verticillioides is the primary fungus that causes Fusarium ear rot (FER) of maize. Infection results in reduced grain yield and quality due to moulding and the contamination of grain with toxic compounds namely fumonisins. Resistance to fungal infection and fumonisin accumulation in maize and maize grain is governed at different levels. In this study, the structural, physico-chemical and genetic basis of resistance to F. verticillioides was investigated in two, replicated field trials at Potchefstroom and Vaalharts in South Africa. Phenotypic data (silk length, husk coverage, pericarp thickness hundred-kernel mass and kernel hardness), physico-chemical data (kernel pH, moisture content, total nitrogen and carbon as well as phenolic acid content) and the expression of pathogenesis-related-5 gene (PR5) and peroxidase gene expression was evaluated in 15 commercial cultivars under artificially inoculated and natural infection conditions. The data were correlated to FER severity, fumonisin accumulation and fungal DNA (referred to as infection indicators). Disease development and fumonisin contamination in Vaalharts was significantly more than in Potchefstroom. There were no significant correlations (r=≥0.60) between phenotypic characteristics and infection indicators. Kernel pH was the most important trait associated with disease development and was negatively correlated (between r=-0.58 and r=-0.75) to all infection indicators. PR5 gene expression had significant positive correlations (r=0.69 and r=0.72) with the fungal and fumonisin levels, respectively. This study presents of the first data demonstrating the use of gene expression in identifying FER/fumonisin-resistant plant material and could aid breeders and growers in selecting resistant material more effectively.

Methanolic Extracts from Cultivated Mushrooms Affect the Production of Fumonisins B and Fusaric Acid by Fusarium verticillioides

The maize pathogen Fusarium verticillioides and their mycotoxins cause damage to plants, animals, and human health. This work aimed to evaluate the effect of crude extracts (CEs) from Agaricus subrufescens, Lentinula edodes, and Pleurotus ostreatus fruiting bodies on in vitro production of biomass and mycotoxins by two strains of F. verticillioides. Stipes and pilei were separated before extraction for A. subrufescens and L. edodes. Comparative metabolomics and dereplication of phenolic compounds were used to analyze all CEs. Mushroom CEs did not significantly inhibit the production of mycelial biomass at concentrations of 2 mg mL⁻1. CEs from A. subrufescens (stipes and pilei) and L. edodes pilei inhibited the production of fumonisins B1 + B2 + B3 by 54% to 80%, whereas CE from P. ostreatus had no effect. In contrast, CE from L. edodes stipes dramatically increased the concentration of fumonisins in culture media. Fusaric acid concentration was decreased in cultures by all CEs except L. edodes stipes. Differences in phenolic composition of the extracts may explain the different effects of the CE treatments on the production of mycotoxins. The opposing activities of stipes and pilei from L. edodes offer an opportunity to search for active compounds to control the mycotoxin production by F. verticillioides.

Rapid elimination of foodborne and environmental fungal contaminants by benzo analogs

BACKGROUND

Contamination of food or the environment by fungi, especially those resistant to conventional fungicides or drugs, represents a hazard to human health. The objective of this study is to identify safe, natural antifungal agents that can remove fungal pathogens or contaminants rapidly from food and / or environmental sources.

RESULTS

Fifteen antifungal compounds (nine benzo derivatives as candidates; six conventional fungicides as references) were investigated. Three benzo analogs, namely octyl gallate (OG), trans-cinnamaldehyde (CA), and 2-hydroxy-5-methoxybenzaldehyde (2H5M), at 1 g L^-1 (3.54 mmol), 1 mL L^-1 (7.21 mmol), 1 mL L^-1 (5.39 mmol), respectively, achieved ≥99.9% fungal death after 0.5, 2.5 or 24 h of treatments, respectively, in in vitro phosphate-buffered saline (PBS) bioassay. However, when OG, CA, and 2H5M were examined in commercial food matrices, organic apple, or grape juices, only CA maintained a similar level of antifungal activity, compared with a PBS bioassay. trans-Cinnamaldehyde showed higher antifungal activity at pH 3.5, equivalent to that of commercial fruit juices, than at pH 5.6. In soil sample tests, the application of 1 mL L^-1 (7.21 mmol) CA to conventional maize / tomato soil samples (pH 6.8) for 2.5 h resulted in ≥99.9% fungal death, indicating CA could also eliminate fungal contaminants in soil. While the conventional fungicide thiabendazole exerted antifungal activity comparable to CA, thiabendazole enhanced the production of carcinogenic aflatoxins by Aspergillus flavus, an undesirable side effect.

CONCLUSION

trans-Cinnamaldehyde could be developed as a potent antifungal agent in food processing or soil sanitation by reducing the time / cost necessary for fungal removal. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Fumonisins and their analogues in contaminated corn and its processed foods - a review

One of the food security problems faced worldwide is the occurrence of mycotoxins in grains and their foods. Fumonisins (FBs) are mycotoxins which are prevalent in corn (Zea mays L.) and its based foods. Their intake and exposure have been epidemiologically and inconclusively associated with oesophageal cancer and neural tube defects in humans, and other harmful health effects in animals. The toxic effects of FBs can be acute or chronic and these metabolites bioaccumulate mainly in liver and kidney tissues. Among FBs, fumonisin B₁ (FB₁) is the most relevant moiety although the 'hidden' forms produced after food thermal processes are becoming relevant. Corn is the grain most susceptible to Fusarium and FBs contamination and the mould growth is affected both by abiotic and biotic factors during grain maturation and storage. Mould counts are mainly affected by the grain water activity, the environmental temperature during grain maturation and insect damage. The abiotic factors affected by climatic change patterns have increased their incidence in other regions of the world. Among FBs, the hidden forms are the most difficult to detect and quantify. Single or combined physical, chemical and biological methods are emerging to significantly reduce FBs in processed foods and therefore diminish their toxicological effects.

Application of Fungicides and Microalgal Phenolic Extracts for the Direct Control of Fumonisin Contamination in Maize

Fungicides and, for the first time, microalgal phenolic extracts (MPE) from Spirulina sp. and Nannochloropsis sp. were applied on maize culture media under field conditions to evaluate their ability to minimize Fusarium species development and fumonisin production. An in vitro assay against F. verticillioides was carried out using maize grains as the culture medium. An open-field experiment was carried out in Northwest Italy under natural infection conditions. The compared treatments were factorial combinations of two insecticide treatments (an untreated control and pyrethroid, used against European Corn Borer), four antifungal treatments (an untreated control, MPE from Spirulina sp., MPE from Nannochloropsis sp., and a synthetic fungicide), and two timings of the application of the antifungal compounds (at maize flowering and at the milk stage). The MPE compounds were capable of inhibiting fumonisin production in vitro more efficiently than tebuconazole. Insecticide application reduced the infection by Fusarium species and subsequent fumonisin contamination. However, fumonisins in maize fields were not significantly controlled by either fungicide or MPE application.

Inhibition of aflatoxin B1 production by verbascoside and other olive polyphenols

In this study, the effects of pure olive phenolic compounds and olive mill wastewater (OMWW) (after membrane filtration treatments) on Aspergillus flavus growth and aflatoxin B1 (AFB1) production, were investigated. Five OMWWs coming from Greek (Lianolia, Koroneiki and Asprolia) and Italian (Cellina di Nardò and Coratina) olive oil cultivars, opportunely filtered using a membrane system, were added at two concentrations (5 and 15%) to growth medium, in order to evaluate their effect on A. flavus growth and AFB1 production. The OMWW fractions treatment, after 6 days of incubation, did not inhibit the fungal growth rate, but at 15% concentration significantly reduced the AFB1 production (ranging from 88 to 100%). A similar approach was used for caffeic acid, hydroxytyrosol, tyrosol and verbascoside, the major pure phenolic compounds identified in OMWW fractions. They were evaluated at increasing doses (10, 50 and 100 µg/ml) following both AFB1 production and fungal growth. At the highest concentration (100 µg/ml) all pure compounds showed a reduction of about 99% of AFB1 production without any influence on fungal growth. This is the first time in which OMWWs and their main phenolics were used in the treatments against AFB1 production. The results obtained could provide possible new strategies for preventing AFB1 food contamination using olive polyphenols and OMWW fractions with anti-aflatoxigenic effect, and permitting to harness in a sustainable way an olive oil by-product.

Correlation of Deoxynivalenol Accumulation in Fusarium-Infected Winter and Spring Wheat Cultivars with Secondary Metabolites at Different Growth Stages

Fusarium infection in wheat causes Fusarium head blight, resulting in yield losses and contamination of grains with trichothecenes. Some plant secondary metabolites inhibit accumulation of trichothecenes. Eighteen Fusarium infected wheat cultivars were harvested at five time points and analyzed for the trichothecene deoxynivalenol (DON) and 38 wheat secondary metabolites (benzoxazinoids, phenolic acids, carotenoids, and flavonoids). Multivariate analysis showed that harvest time strongly impacted the content of secondary metabolites, more distinctly for winter wheat than spring wheat. The benzoxazinoid 2-β-glucopyranoside-2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA-glc), α-tocopherol, and the flavonoids homoorientin and orientin were identified as potential inhibitors of DON accumulation. Several phenolic acids, lutein and β-carotene also affected DON accumulation, but the effect varied for the two wheat types. The results could form a basis for choosing wheat cultivars using metabolite profiling as a marker for selecting wheat cultivars with improved resistance against Fusarium head blight and accumulation of trichothecene toxins in wheat heads.

Biologically Based Methods for Control of Fumonisin-Producing Fusarium Species and Reduction of the Fumonisins

Infection by the fumonisin-producing Fusarium spp. and subsequent fumonisin contamination of maize adversely affect international trade and economy with deleterious effects on human and animal health. In developed countries high standards of the major food suppliers and retailers are upheld and regulatory controls deter the importation and local marketing of fumonisin-contaminated food products. In developing countries regulatory measures are either lacking or poorly enforced, due to food insecurity, resulting in an increased mycotoxin exposure. The lack and poor accessibility of effective and environmentally safe control methods have led to an increased interest in practical and biological alternatives to reduce fumonisin intake. These include the application of natural resources, including plants, microbial cultures, genetic material thereof, or clay minerals pre- and post-harvest. Pre-harvest approaches include breeding for resistant maize cultivars, introduction of biocontrol microorganisms, application of phenolic plant extracts, and expression of antifungal proteins and fumonisin degrading enzymes in transgenic maize cultivars. Post-harvest approaches include the removal of fumonisins by natural clay adsorbents and enzymatic degradation of fumonisins through decarboxylation and deamination by recombinant carboxylesterase and aminotransferase enzymes. Although, the knowledge base on biological control methods has expanded, only a limited number of authorized decontamination products and methods are commercially available. As many studies detailed the use of natural compounds in vitro, concepts in reducing fumonisin contamination should be developed further for application in planta and in the field pre-harvest, post-harvest, and during storage and food-processing. In developed countries an integrated approach, involving good agricultural management practices, hazard analysis and critical control point (HACCP) production, and storage management, together with selected biologically based treatments, mild chemical and physical treatments could reduce fumonisin contamination effectively. In rural subsistence farming communities, simple, practical, and culturally acceptable hand-sorting, maize kernel washing, and dehulling intervention methods proved to be effective as a last line of defense for reducing fumonisin exposure. Biologically based methods for control of fumonisin-producing Fusarium spp. and decontamination of the fumonisins could have potential commercial application, while simple and practical intervention strategies could also impact positively on food safety and security, especially in rural populations reliant on maize as a dietary staple.

Antioxidant Secondary Metabolites in Cereals: Potential Involvement in Resistance to Fusarium and Mycotoxin Accumulation

Gibberella and Fusarium Ear Rot and Fusarium Head Blight are major diseases affecting European cereals. These diseases are mainly caused by fungi of the Fusarium genus, primarily Fusarium graminearum and Fusarium verticillioides. These Fusarium species pose a serious threat to food safety because of their ability to produce a wide range of mycotoxins, including type B trichothecenes and fumonisins. Many factors such as environmental, agronomic or genetic ones may contribute to high levels of accumulation of mycotoxins in the grain and there is an urgent need to implement efficient and sustainable management strategies to reduce mycotoxin contamination. Actually, fungicides are not fully efficient to control the mycotoxin risk. In addition, because of harmful effects on human health and environment, their use should be seriously restricted in the near future. To durably solve the problem of mycotoxin accumulation, the breeding of tolerant genotypes is one of the most promising strategies for cereals. A deeper understanding of the molecular mechanisms of plant resistance to both Fusarium and mycotoxin contamination will shed light on plant-pathogen interactions and provide relevant information for improving breeding programs. Resistance to Fusarium depends on the plant ability in preventing initial infection and containing the development of the toxigenic fungi while resistance to mycotoxin contamination is also related to the capacity of plant tissues in reducing mycotoxin accumulation. This capacity can result from two mechanisms: metabolic transformation of the toxin into less toxic compounds and inhibition of toxin biosynthesis. This last mechanism involves host metabolites able to interfere with mycotoxin biosynthesis. This review aims at gathering the latest scientific advances that support the contribution of grain antioxidant secondary metabolites to the mechanisms of plant resistance to Fusarium and mycotoxin accumulation.

Assessing pigmented pericarp of maize kernels as possible source of resistance to fusarium ear rot, Fusarium spp. infection and fumonisin accumulation

One of the purposes of maize genetic improvement is the research of genotypes resistant to fusarium ear rot (FER) and fumonisin accumulation. Flavonoids in the pericarp of the kernels are considered particularly able to reduce the fumonisin accumulation (FUM). The aim of this field study was to assess the effect of flavonoids, associated with anti-insect protection and Fusarium verticillioides inoculation, on FER symptoms and fumonisin contamination in maize kernels. Two isogenic hybrids, one having pigmentation in the pericarp (P1-rr) and the other without it (P1-wr), were compared. P1-rr showed lower values of FER symptoms and FUM contamination than P1-wr only if the anti-insect protection and the F. verticillioides inoculations were applied in combination. Fusarium spp. kernel infection was not influenced by the presence of flavonoids in the pericarp. Artificial F. verticillioides inoculation was more effective than anti-insect protection in enhancing the inhibition activity of flavonoids toward FUM contamination. The interactions between FUM contamination levels and FER ratings were better modeled in the pigmented hybrid than in the unpigmented one. The variable role that the pigment played in kernel defense against FER and FUM indicates that flavonoids alone may not be completely effective in the resistance of fumonisin contamination in maize.

Maize-Pathogen Interactions: An Ongoing Combat from a Proteomics Perspective

Maize (Zea mays L.) is a host to numerous pathogenic species that impose serious diseases to its ear and foliage, negatively affecting the yield and the quality of the maize crop. A considerable amount of research has been carried out to elucidate mechanisms of maize-pathogen interactions with a major goal to identify defense-associated proteins. In this review, we summarize interactions of maize with its agriculturally important pathogens that were assessed at the proteome level. Employing differential analyses, such as the comparison of pathogen-resistant and susceptible maize varieties, as well as changes in maize proteomes after pathogen challenge, numerous proteins were identified as possible candidates in maize resistance. We describe findings of various research groups that used mainly mass spectrometry-based, high through-put proteomic tools to investigate maize interactions with fungal pathogens Aspergillus flavus, Fusarium spp., and Curvularia lunata, and viral agents Rice Black-streaked Dwarf Virus and Sugarcane Mosaic Virus.

Augmenting the Activity of Monoterpenoid Phenols against Fungal Pathogens Using 2-Hydroxy-4-methoxybenzaldehyde that Target Cell Wall Integrity

Disruption of cell wall integrity system should be an effective strategy for control of fungal pathogens. To augment the cell wall disruption efficacy of monoterpenoid phenols (carvacrol, thymol), antimycotic potency of benzaldehyde derivatives that can serve as chemosensitizing agents were evaluated against strains of Saccharomyces cerevisiae wild type (WT), slt2Δ and bck1Δ (mutants of the mitogen-activated protein kinase (MAPK) and MAPK kinase kinase, respectively, in the cell wall integrity pathway). Among fourteen compounds investigated, slt2Δ and bck1Δ showed higher susceptibility to nine benzaldehydes, compared to WT. Differential antimycotic activity of screened compounds indicated "structure-activity relationship" for targeting the cell wall integrity, where 2-hydroxy-4-methoxybenzaldehyde (2H4M) exhibited the highest antimycotic potency. The efficacy of 2H4M as an effective chemosensitizer to monoterpenoid phenols (viz., 2H4M + carvacrol or thymol) was assessed in yeasts or filamentous fungi (Aspergillus, Penicillium) according to European Committee on Antimicrobial Susceptibility Testing or Clinical Laboratory Standards Institute M38-A protocols, respectively. Synergistic chemosensitization greatly lowers minimum inhibitory or fungicidal concentrations of the co-administered compounds. 2H4M also overcame the tolerance of two MAPK mutants (sakAΔ, mpkCΔ) of Aspergillus fumigatus to fludioxonil (phenylpyrrole fungicide). Collectively, 2H4M possesses chemosensitizing capability to magnify the efficacy of monoterpenoid phenols, which improves target-based (viz., cell wall disruption) antifungal intervention.

Metabolomics to Decipher the Chemical Defense of Cereals against Fusarium graminearum and Deoxynivalenol Accumulation

Fusarium graminearum is the causal agent of Fusarium head blight (FHB) and Gibberella ear rot (GER), two devastating diseases of wheat, barley, and maize. Furthermore, F. graminearum species can produce type B trichothecene mycotoxins that accumulate in grains. Use of FHB and GER resistant cultivars is one of the most promising strategies to reduce damage induced by F. graminearum. Combined with genetic approaches, metabolomic ones can provide powerful opportunities for plant breeding through the identification of resistant biomarker metabolites which have the advantage of integrating the genetic background and the influence of the environment. In the past decade, several metabolomics attempts have been made to decipher the chemical defense that cereals employ to counteract F. graminearum. By covering the major classes of metabolites that have been highlighted and addressing their potential role, this review demonstrates the complex and integrated network of events that cereals can orchestrate to resist to F. graminearum.

Secondary metabolites in fungus-plant interactions

Fungi and plants are rich sources of thousands of secondary metabolites. The genetically coded possibilities for secondary metabolite production, the stimuli of the production, and the special phytotoxins basically determine the microscopic fungi-host plant interactions and the pathogenic lifestyle of fungi. The review introduces plant secondary metabolites usually with antifungal effect as well as the importance of signaling molecules in induced systemic resistance and systemic acquired resistance processes. The review also concerns the mimicking of plant effector molecules like auxins, gibberellins and abscisic acid by fungal secondary metabolites that modulate plant growth or even can subvert the plant defense responses such as programmed cell death to gain nutrients for fungal growth and colonization. It also looks through the special secondary metabolite production and host selective toxins of some significant fungal pathogens and the plant response in form of phytoalexin production. New results coming from genome and transcriptional analyses in context of selected fungal pathogens and their hosts are also discussed.

Endogenous cross-talk of fungal metabolites

Non-ribosomal peptide (NRP) synthesis in fungi requires a ready supply of proteogenic and non-proteogenic amino acids which are subsequently incorporated into the nascent NRP via a thiotemplate mechanism catalyzed by NRP synthetases. Substrate amino acids can be modified prior to or during incorporation into the NRP, or following incorporation into an early stage amino acid-containing biosynthetic intermediate. These post-incorporation modifications involve a range of additional enzymatic activities including but not exclusively, monooxygenases, methyltransferases, epimerases, oxidoreductases, and glutathione S-transferases which are essential to effect biosynthesis of the final NRP. Likewise, polyketide biosynthesis is directly by polyketide synthase megaenzymes and cluster-encoded ancillary decorating enzymes. Additionally, a suite of additional primary metabolites, for example: coenzyme A (CoA), acetyl CoA, S-adenosylmethionine, glutathione (GSH), NADPH, malonyl CoA, and molecular oxygen, amongst others are required for NRP and polyketide synthesis (PKS). Clearly these processes must involve exquisite orchestration to facilitate the simultaneous biosynthesis of different types of NRPs, polyketides, and related metabolites requiring identical or similar biosynthetic precursors or co-factors. Moreover, the near identical structures of many natural products within a given family (e.g., ergot alkaloids), along with localization to similar regions within fungi (e.g., conidia) suggests that cross-talk may exist, in terms of biosynthesis and functionality. Finally, we speculate if certain biosynthetic steps involved in NRP and PKS play a role in cellular protection or environmental adaptation, and wonder if these enzymatic reactions are of equivalent importance to the actual biosynthesis of the final metabolite.

Inhibition of ochratoxin A production in Aspergillus carbonarius by hydroxycinnamic acids from grapes

Hydroxycinnamic acids (HCAs), phenolic components of wine, are known to have antimicrobial properties. Aspergillus carbonarius is one of the most important ochratoxin A (OTA) producing fungi in wine. Strategies for the control and prevention of A. carbonarius contamination are important for the maintenance of wine safety. This study sought to determine the potential of HCAs, such as caffeic, p-coumaric and ferulic acids, as antifungal natural compounds for the control of A. carbonarius growth and OTA production. The HCAs were tested at the increasing concentrations of 0.30, 0.65 and 1.10 mg/ml in minimal medium (MM) and grape juice. Germination of conidia was not affected in neither of the two media in presence of HCAs. At all the concentrations tested, OTA biosynthesis in MM was reduced and the dose effect was more evident for p-coumaric and ferulic acids; in grape juice the reduction trend was confirmed, and ferulic acid showed the highest inhibitory effect. Moreover, the expression level of genes encoding a polyketide synthase (AcOTApks) and a nonribosomal peptide synthetase (AcOTAnrps) involved in OTA biosynthesis, was evaluated by real-time PCR in A. carbonarius grown in presence of 0.65 mg/ml of HCAs. From gene expression analysis only the AcOTApks gene showed a marked reduction of transcription level in presence of p-coumaric and ferulic acids. On the contrary, caffeic acid seemed to not influence the expression levels of the genes analysed in this study, suggesting a different mechanism of action on the regulation of OTA biosynthesis.

Bioguided isolation, characterization, and biotransformation by Fusarium verticillioides of maize kernel compounds that inhibit fumonisin production

Fusarium verticillioides infects maize ears, causing ear rot disease and contamination of grain with fumonisin mycotoxins. This contamination can be reduced by the presence of bioactive compounds in kernels that are able to inhibit fumonisin biosynthesis. To identify such compounds, we used kernels from a maize genotype with moderate susceptibility to F. verticillioides, harvested at the milk-dough stage (i.e., when fumonisin production initiates in planta), and applied a bioguided fractionation approach. Chlorogenic acid was the most abundant compound in the purified active fraction and its contribution to fumonisin inhibitory activity was up to 70%. Moreover, using a set of maize genotypes with different levels of susceptibility, chlorogenic acid was shown to be significantly higher in immature kernels of the moderately susceptible group. Altogether, our data indicate that chlorogenic acid may considerably contribute to either maize resistance to Fusarium ear rot, fumonisin accumulation, or both. We further investigated the mechanisms involved in the inhibition of fumonisin production by chlorogenic acid and one of its hydrolyzed products, caffeic acid, by following their metabolic fate in supplemented F. verticillioides broths. Our data indicate that F. verticillioides was able to biotransform these phenolic compounds and that the resulting products can contribute to their inhibitory activity.

Effect of aqueous extracts of Mentha arvensis (mint) and Piper betle (betel) on growth and citrinin production from toxigenic Penicillium citrinum

Due to growing concern of consumers about chemical residues in food products, the demand for safe and natural food is increasing greatly. The use of natural additives such as spices and herbal oil as seasoning agents for their antimicrobial activity has been extensively investigated. This paper discusses the efficacy of the aqueous extract of mint (Mentha arvensis) and betel (Piper betle) on the mycelial growth and citrinin production of Penicillium citrinum. The present investigation revealed that mint extract inhibited citrinin production up to 73 % without inhibiting the mycelium growth. The citrinin production decreased with increase in the concentration of mint extract as observed from the data obtained from High pressure liquid chromatography. The samples also showed reduced cytotoxicity on HeLa cells. On the other hand betel extract resulted in stimulatory effect on citrinin production and mycelial growth. The study showed that mint extract has the potential to be used safely for restraining citrinin contamination.

Use of benzo analogs to enhance antimycotic activity of kresoxim methyl for control of aflatoxigenic fungal pathogens

The aim of this study was to examine two benzo analogs, octylgallate (OG) and veratraldehyde (VT), as antifungal agents against strains of Aspergillus parasiticus and A.flavus (toxigenic or atoxigenic). Both toxigenic and atoxigenic strains used were capable of producing kojic acid, another cellular secondary product. A. fumigatus was used as a genetic model for this study. When applied independently, OG exhibits considerably higher antifungal activity compared to VT. The minimum inhibitory concentrations (MICs) of OG were 0.3–0.5 mM, while that of VT were 3.0–5.0 mM in agar plate-bioassays. OG or VT in concert with the fungicide kresoxim methyl (Kre-Me; strobilurin) greatly enhanced sensitivity of Aspergillus strains to Kre-Me. The combination with OG also overcame the tolerance of A. fumigatus mitogen-activated protein kinase (MAPK) mutants to Kre-Me. The degree of compound interaction resulting from chemosensitization of the fungi by OG was determined using checkerboard bioassays, where synergistic activity greatly lowered MICs or minimum fungicidal concentrations. However, the control chemosensitizer benzohydroxamic acid, an alternative oxidase inhibitor conventionally applied in concert with strobilurin, did not achieve synergism. The level of antifungal or chemosensitizing activity was also “compound—strain” specific, indicating differential susceptibility of tested strains to OG or VT, and/or heat stress. Besides targeting the antioxidant system, OG also negatively affected the cell wall-integrity pathway, as determined by the inhibition of Saccharomyces cerevisiae cell wall-integrity MAPK pathway mutants. We concluded that certain benzo analogs effectively inhibit fungal growth. They possess chemosensitizing capability to increase efficacy of Kre-Me and thus, could reduce effective dosages of strobilurins and alleviate negative side effects associated with current antifungal practices. OG also exhibits moderate antiaflatoxigenic activity.

Evaluation of ability of ferulic acid to control growth and fumonisin production of Fusarium verticillioides and Fusarium proliferatum on maize based media

The aim of this study was to evaluate the efficacy of ferulic acid (1, 10, 20 and 25 mM) at different water activity (aw) values (0.99, 0.98, 0.96 and 0.93) at 25 °C on growth and fumonisin production by Fusarium verticillioides and Fusarium proliferatum on maize based media. For both Fusarium species, the lag phase significantly decreased (p ≤ 0.001), and the growth rates increased (p ≤ 0.001) at the lowest ferulic acid concentration used (1mM), regardless of the aw. However, high doses of ferulic acid (10 to 25 mM) significantly reduced (p ≤ 0.001) the growth rate of both Fusarium species, regardless of the a(w). In general, growth rate inhibition increased as ferulic acid doses increased and as media aw decreased. Fumonisin production profiles of both Fusarium species showed that low ferulic acid concentrations (1-10mM) significantly increased (p ≤ 0.001) toxin production, regardless of the aw. High doses of ferulic acid (20-25 mM) reduced fumonisin production, in comparison with the controls, by both Fusarium species but they were not statistically significant in most cases. The results show that the use of ferulic acid as a post-harvest strategy to reduce mycotoxin accumulation on maize needs to be discussed.

Equisetum arvense hydro-alcoholic extract: phenolic composition and antifungal and antimycotoxigenic effect against Aspergillus flavus and Fusarium verticillioides in stored maize

BACKGROUND

Maize is a very important cereal for human and animal diet, but it can be contaminated by moulds and their mycotoxins. On the other hand, natural plant products with antimicrobial properties could possibly used to control mycotoxigenic fungi in foods and feeds. In this study, Equisetum arvense extract was tested for the efficacy on Aspergillus section Flavi and Fusarium section Liseola growth. Natural contaminated maize was used in this study and extract was added under different water activities (a(w)) - 0.90 and 0.95 - for Aspergillus section Flavi and Fusarium section Liseola, respectively. Moulds were inoculated in maize and incubated during 30 days.

RESULTS

We confirm that E. arvense extract may be effective for the inhibition of Aspergillus section Flavi in maize with high levels of this mould. Moreover, this extract showed a good inhibition of growth on Fusarium section Liseola levels. Aflatoxin and fumonisin production was not affected by the extract.

CONCLUSIONS

E. arvense extract could be an alternative to synthetic fungicides to control maize mycobiota level in moist grain.

Root exudates from grafted-root watermelon showed a certain contribution in inhibiting Fusarium oxysporum f. sp. niveum

Grafting watermelon onto bottle gourd rootstock is commonly used method to generate resistance to Fusarium oxysporum f. sp. niveum (FON), but knowledge of the effect of the root exudates of grafted watermelon on this soil-borne pathogen in rhizosphere remains limited. To investigate the root exudate profiles of the own-root bottle gourd, grafted-root watermelon and own-root watermelon, recirculating hydroponic culture system was developed to continuously trap these root exudates. Both conidial germination and growth of FON were significantly decreased in the presence of root exudates from the grafted-root watermelon compared with the own-root watermelon. HPLC analysis revealed that the composition of the root exudates released by the grafted-root watermelon differed not only from the own-root watermelon but also from the bottle gourd rootstock plants. We identified salicylic acid in all 3 root exudates, chlorogenic acid and caffeic acid in root exudates from own-root bottle gourd and grafted-root watermelon but not own-root watermelon, and abundant cinnamic acid only in own-root watermelon root exudates. The chlorogenic and caffeic acid were candidates for potentiating the enhanced resistance of the grafted watermelon to FON, therefore we tested the effects of the two compounds on the conidial germination and growth of FON. Both phenolic acids inhibited FON conidial germination and growth in a dose-dependent manner, and FON was much more susceptible to chlorogenic acid than to caffeic acid. In conclusion, the key factor in attaining the resistance to Fusarium wilt is grafting on the non-host root stock, however, the root exudates profile also showed some contribution in inhibiting FON. These results will help to better clarify the disease resistance mechanisms of grafted-root watermelon based on plant-microbe communication and will guide the improvement of strategies against Fusarium-mediated wilt of watermelon plants.

Maize kernel antioxidants and their potential involvement in Fusarium ear rot resistance

The potential involvement of antioxidants (α-tocopherol, lutein, zeaxanthin, β-carotene, and ferulic acid) in the resistance of maize varieties to Fusarium ear rot was the focus of this study. These antioxidants were present in all maize kernel stages, indicating that the fumonisin-producing fungi (mainly Fusarium verticillioides and Fusarium proliferatum ) are likely to face them during ear colonization. The effect of these compounds on fumonisin biosynthesis was studied in F. verticillioides liquid cultures. In carotenoid-treated cultures, no inhibitory effect of fumonisin accumulation was observed while a potent inhibitory activity was obtained for sublethal doses of α-tocopherol (0.1 mM) and ferulic acid (1 mM). Using a set of genotypes with moderate to high susceptibility to Fusarium ear rot, ferulic acid was significantly lower in immature kernels of the very susceptible group. Such a relation was nonexistent for tocopherols and carotenoids. Also, ferulic acid in immature kernels ranged from 3 to 8.5 mg/g, i.e., at levels consistent with the in vitro inhibitory concentration. Overall, our data support the fact that ferulic acid may contribute to resistance to Fusarium ear rot and/or fumonisin accumulation.

Chlorogenic acid and maize ear rot resistance: a dynamic study investigating Fusarium graminearum development, deoxynivalenol production, and phenolic acid accumulation

Fusarium graminearum is the causal agent of Gibberella ear rot and produces trichothecene mycotoxins. Basic questions remain unanswered regarding the kernel stages associated with trichothecene biosynthesis and the kernel metabolites potentially involved in the regulation of trichothecene production in planta. In a two-year field study, F. graminearum growth, trichothecene accumulation, and phenolic acid composition were monitored in developing maize kernels of a susceptible and a moderately resistant variety using quantitative polymerase chain reaction and liquid chromatography coupled with photodiode array or mass spectrometry detection. Infection started as early as the blister stage and proceeded slowly until the dough stage. Then, a peak of trichothecene accumulation occurred and infection progressed exponentially until the final harvest time. Both F. graminearum growth and trichothecene production were drastically reduced in the moderately resistant variety. We found that chlorogenic acid is more abundant in the moderately resistant variety, with levels spiking in the earliest kernel stages induced by Fusarium infection. This is the first report that precisely describes the kernel stage associated with the initiation of trichothecene production and provides in planta evidence that chlorogenic acid may play a role in maize resistance to Gibberella ear rot and trichothecene accumulation.

Role of hydroxycinnamic acids in the infection of maize silks by Fusarium graminearum Schwabe

In the current study, the hydroxycinnamic acids in silks of diverse maize inbred lines differing in Fusarium resistance were determined at several times after inoculation with Fusarium graminearum or sterile water as control. The main objective was to determine the possible relationship between the hydroxycinnamic acid changes in silks and ear rot resistance. Several changes in the cell-wall-bound hydroxycinnamic acid concentrations were observed after inoculation with F. graminearum, although these changes were not directly correlated with genotypic resistance to this fungus. Ester-bound ferulic acid decreased, probably due to degradation of hemicellulose by hydrolytic enzymes produced by Fusarium spp., while p-coumaric acid and diferulates showed slight increases that, in conjunction, did not result in delayed F. graminearum progression through the silks. It is important to note that the decrease of ferulic acid in the F. graminearum treatment was faster in susceptible than in resistant genotypes, suggesting a differential hemicellulose degradation in silk tissues. Therefore, the ability of the maize genotypes to slow down that process through hemicellulose structural features or xylanase inhibitors needs to be addressed in future studies.

Cinnamic-derived acids significantly affect Fusarium graminearum growth and in vitro synthesis of type B trichothecenes

The impact of five phenolic acids (ferulic, coumaric, caffeic, syringic, and p-hydroxybenzoic acids) on fungal growth and type B trichothecene production by four strains of Fusarium graminearum was investigated. All five phenolic acids inhibited growth but the degree of inhibition varied between strains. Our results suggested that the more lipophilic phenolic acids are, the higher is the effect they have on growth. Toxin accumulation in phenolic acid-supplemented liquid glucose, yeast extract, and peptone cultures was enhanced in the presence of ferulic and coumaric acids but was reduced in the presence of p-hydroxybenzoic acid. This modulation was shown to correlate with a regulation of TRI5 transcription. In this study, addition of phenolic acids with greater antioxidant properties resulted in a higher toxin accumulation, indicating that the modulation of toxin accumulation may be linked to the antioxidant properties of the phenolic acids. These data suggest that, in planta, different compositions in phenolic acids of kernels from various cultivars may reflect different degrees of sensitivity to "mycotoxinogenesis."

Regulation of trichothecene biosynthesis in Fusarium: recent advances and new insights

Trichothecenes are toxic secondary metabolites produced by filamentous fungi mainly belonging to the Fusarium genus. Production of these mycotoxins occurs during infection of crops and is a threat to human and animal health. Although the pathway for biosynthesis of trichothecenes is well established, the regulation of the Tri genes implicated in the pathway remains poorly understood. Most of the Tri genes are gathered in a cluster which contains two transcriptional regulators controlling the expression of the other Tri genes. The regulation of secondary metabolites biosynthesis in most fungal genera has been recently shown to be controlled by various regulatory systems in response to external environment. The control of the "Tri cluster" by non-cluster regulators in Fusarium was not clearly demonstrated until recently. This review covers the recent advances concerning the regulation of trichothecene biosynthesis in Fusarium and highlights the potential implication of various general regulatory circuits. Further studies on the role of these regulatory systems in the control of trichothecene biosynthesis might be useful in designing new strategies to reduce mycotoxin accumulation.

In vitro inhibitory effect of tetrahydrocurcuminoids on Fusarium proliferatum growth and fumonisin B₁ biosynthesis

Many plant pathogens produce toxic metabolites when growing on food and feed. Some antioxidative components seem to prevent fungal growth and mycotoxin formation. Recently, we synthesized a new class of powerful antioxidative compounds, i.e. tetrahydrocurcuminoids, and its structure/antioxidant activity relationships have been established. The South West of France produces large amounts of corn, which can be infected by Fusarium species, particularly F. proliferatum. In this context, the efficiency of tetrahydrocurcuminoids, which can be obtained from natural curcuminoids, was investigated to control in vitro the growth of F. proliferatum and the production of its associated mycotoxin, fumonisin B₁. The relation between structure and antifungal activity was studied. Tetrahydrocurcumin (THC1), with two guaiacyl phenolic subunits, showed the highest inhibitory activity (measured as radial growth on agar medium) against the F. proliferatum development (67% inhibition at a concentration of 13.6 µmol ml⁻¹). The efficiencies of THC2 (36% at a concentration of 11.5 µmol ml⁻¹), which contains syringyl phenolic units, and THC3 (30% at a concentration of 13.6 µmol ml⁻¹), which does not have any substituent on the aromatic rings, were relatively close. These results indicate that the simultaneous presence of guaiacyl phenols and the enolic function of the β-diketone moiety play an important role in the inhibition mechanisms. The importance of this combination was confirmed using n-propylguaiacol and acetylacetone as molecular models. Under the same conditions, ferulic acid and eugenol, other natural phenolic antioxidants, were less efficient in inhibiting fungal growth. THC1 also reduced fumonisin B₁ production in liquid medium by approximately 35, 50 and 75% at concentrations of 0.8, 1.3, and 1.9 µmol ml⁻¹, respectively. These very low inhibitory concentrations show that tetrahydrocurcuminoids could be one of the most promising biobased molecules for the control of mycotoxinogen fungal strains.

Chemosensitization of aflatoxigenic fungi to antimycin A and strobilurin using salicylaldehyde, a volatile natural compound targeting cellular antioxidation system

Various species of fungi in the genus Aspergillus are the most common causative agents of invasive aspergillosis and/or producers of hepato-carcinogenic mycotoxins. Salicylaldehyde (SA), a volatile natural compound, exhibited potent antifungal and anti-mycotoxigenic activities to A. flavus and A. parasiticus. By exposure to the volatilized SA, the growth of A. parasiticus was inhibited up to 10-75% at 9.5 mM ≤ SA ≤ 16.0 mM, while complete growth inhibition was achieved at 19.0 mM ≤ SA. Similar trends were also observed with A. flavus. The aflatoxin production, i.e., aflatoxin B(1) and B(2) (AFB(1), AFB(2)) for A. flavus and AFB(1), AFB(2), AFG(1), and AFG(2) for A. parasiticus, in the SA-treated (9.5 mM) fungi was reduced by ~13-45% compared with the untreated control. Using gene deletion mutants of the model yeast Saccharomyces cerevisiae, we identified the fungal antioxidation system as the molecular target of SA, where sod1Δ [cytosolic superoxide dismutase (SOD)], sod2Δ (mitochondrial SOD), and glr1Δ (glutathione reductase) mutants showed increased sensitivity to this compound. Also sensitive was the gene deletion mutant, vph2Δ, for the vacuolar ATPase assembly protein, suggesting vacuolar detoxification plays an important role for fungal tolerance to SA. In chemosensitization experiments, co-application of SA with either antimycin A or strobilurin (inhibitors of mitochondrial respiration) resulted in complete growth inhibition of Aspergillus at much lower dose treatment of either agent, alone. Therefore, SA can enhance antifungal activity of commercial antifungal agents required to achieve effective control. SA is a potent antifungal and anti-aflatoxigenic volatile that may have some practical application as a fumigant.