Aflatoxin B1--its effects on an in vitro plant system

A perspective on varied fungal virulence factors causing infection in host plants

Pathogenic fungi and their spores are ubiquitously present and invade the tissues of higher living plants causing pathogenesis and inevitably death or retarded growth. A group of fungi kills its hosts and consume the dead tissues (necrotrophs), while others feed on living tissue (biotrophs) or combination of two (hemibiotrophs). A number of virulent factors is used by fungal pathogens to inhabit new hosts and cause illness. Fungal pathogens develop specialized structures for complete invasion into plant organs to regulate pathogenic growth. Virulence factors like effectors, mycotoxins, cell wall degrading enzymes and organic acids have varied roles depending on the infection strategy and assist the pathogens to possess control on living tissues of the plants. Infection strategies employed by fungi generally masks the plant defense mechanism, however necrotrophs are best known to harm plant tissues with their poisonous secretion. Interestingly, the effector chemicals released by Biotrophs reduce plant cell growth and regulate plant metabolism in their advantage causing no direct death. All these virulence tools cause huge loss to the agricultural product of pre- harvest crops and post-harvest yields causing low output leading to huge economic losses. This review focusses on comprehensive study of range of virulence factors of the pathogenic fungi responsible for their invasion inside the healthy tissues of plants. The compiled information would influence researchers to design antidote against all virulence factors of fungi relevant to their area of research which could pave way for protection against plant pathogenesis.

Unveiling the spatial distribution of aflatoxin B1 and plant defense metabolites in maize using AP-SMALDI mass spectrometry imaging

In order to cope with the presence of unfavorable compounds, plants can biotransform xenobiotics, translocate both parent compounds and metabolites, and perform compartmentation and segregation at the cellular or tissue level. Such a scenario also applies to mycotoxins, fungal secondary metabolites with a pre-eminent role in plant infection. In this work, we aimed to describe the effect of the interplay between Zea mays (maize) and aflatoxin B1 (AFB1) at the tissue and organ level. To address this challenge, we used atmospheric pressure scanning microprobe matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-SMALDI MSI) to investigate the biotransformation, localization and subsequent effects of AFB1 on primary and secondary metabolism of healthy maize plants, both in situ and from a metabolomics standpoint. High spatial resolution (5 µm) provided fine localization of AFB1, which was located within the root intercellular spaces, and co-localized with its phase-I metabolite aflatoxin M2. We provided a parallel visualization of maize metabolic changes, induced in different organs and tissues by an accumulation of AFB1. According to our untargeted metabolomics investigation, anthocyanin biosynthesis and chlorophyll metabolism in roots are most affected. The biosynthesis of these metabolites appears to be inhibited by AFB1 accumulation. On the other hand, metabolites found in above-ground organs suggest that the presence of AFB1 may also activate the biochemical response in the absence of an actual fungal infection; indeed, several plant secondary metabolites known for their antimicrobial or antioxidant activities were localized in the outer tissues, such as phenylpropanoids, benzoxazinoids, phytohormones and lipids.

The Aspergilli and Their Mycotoxins: Metabolic Interactions With Plants and the Soil Biota

Species of the highly diverse fungal genus Aspergillus are well-known agricultural pests, and, most importantly, producers of various mycotoxins threatening food safety worldwide. Mycotoxins are studied predominantly from the perspectives of human and livestock health. Meanwhile, their roles are far less known in nature. However, to understand the factors behind mycotoxin production, the roles of the toxins of Aspergilli must be understood from a complex ecological perspective, taking mold-plant, mold-microbe, and mold-animal interactions into account. The Aspergilli may switch between saprophytic and pathogenic lifestyles, and the production of secondary metabolites, such as mycotoxins, may vary according to these fungal ways of life. Recent studies highlighted the complex ecological network of soil microbiotas determining the niches that Aspergilli can fill in. Interactions with the soil microbiota and soil macro-organisms determine the role of secondary metabolite production to a great extent. While, upon infection of plants, metabolic communication including fungal secondary metabolites like aflatoxins, gliotoxin, patulin, cyclopiazonic acid, and ochratoxin, influences the fate of both the invader and the host. In this review, the role of mycotoxin producing Aspergillus species and their interactions in the ecosystem are discussed. We intend to highlight the complexity of the roles of the main toxic secondary metabolites as well as their fate in natural environments and agriculture, a field that still has important knowledge gaps.

Transcriptome analysis reveals downregulation of virulence-associated genes expression in a low virulence Verticillium dahliae strain

Verticillium dahliae causes wilt diseases and early senescence in numerous plants, including agricultural crops such as cotton. In this study, we studied two closely related V. dahliae strains, and found that V991w showed significantly reduced virulence on cotton than V991b. Comprehensive transcriptome analysis revealed various differentially expressed genes between the two strains, with more genes repressed in V991w. The downregulated genes in V991w were involved in production of hydrophobins, melanin, predicted aflatoxin, and membrane proteins, most of which are related to pathogenesis and multidrug resistance. Consistently, melanin production in V991w in vitro was compromised. We next obtained genomic variations between the two strains, demonstrating that transcription factor genes containing fungi specific transcription factor domain and fungal Zn2-Cys6 binuclear cluster domain were enriched in V991w, which might be related to pathogenicity-related genes downregulation. Thus, this study supports a model in which some virulence factors involved in V. dahliae pathogenicity were pre-expressed during in vitro growth before host interaction.

R, Pestka J, Williams D, Glenn A. The current state of mycotoxin biomarker development in humans and animals and the potential for application to plant systems

Filamentous fungi that contaminate livestock feeds and human food supply often produce toxigenic secondary metabolites known as mycotoxins. Among the hundreds of known mycotoxins, aflatoxins, deoxynivalenol, fumonisins, ochratoxin A and zearalenone are considered the most commercially important. Intense research on these mycotoxins, especially aflatoxin, has resulted in the development of 'biomarkers' used to link exposure to disease risk. In the case of aflatoxin this effort has led to the discovery of both exposure and mechanism-based biomarkers, which have proven essential for understanding aflatoxin's potential for causing disease in humans, including subtle effects on growth and immune response. Fumonisin biomarkers have also been used extensively in farm and laboratory animals to study the fumonisin-induced disruption of cellular and systemic physiology which leads to disease. This review summarises the status of mycotoxin biomarker development in humans and animals for the commercially important mycotoxins. Since the fungi responsible for the production of these mycotoxins are often endophytes that infect and colonise living plant tissues, accumulation of mycotoxins in the plant tissues may at times be associated with development of plant disease symptoms. The presence of mycotoxins, even in the absence of disease symptoms, may still have subtle biological effects on the physiology of plants. This review examines the question of whether or not the knowledge gained from mechanistic studies and development of biomarkers in animal and human systems is transferable to the study of mycotoxin effects on plant systems. Thus far, fumonisin has proven amenable to development of mechanism-based biomarkers to study maize seedling disease caused by the fumonisin producer, Fusarium verticillioides. Expanding our knowledge of mechanisms of toxicity and the overt and subtle effects on animal, human, and plant systems through the identification and validation of biomarkers will further our ability to monitor and limit the damage and economic impact of mycotoxins.

Biodiversity ofAspergillussectionFlaviin the United States: A review

Fungi belonging to Aspergillus section Flavi are of great economic importance in the United States due to their ability to produce toxic and carcinogenic aflatoxins in agricultural commodities. Development of control strategies against A. flavus and A. parasiticus, the major aflatoxin-producing species, is dependent upon a basic understanding of their diversity in agricultural ecosystems. This review summarizes our current knowledge of species and population diversity in the United States in relation to morphology, mycotoxin production and genetic characters. The high genetic diversity in populations of aflatoxigenic fungi is a reflection of their versatile habits in nature, which include saprotrophic colonization of plant debris in soil and parasitism of seeds and grain. Genetic variation within populations may originate from a cryptic sexual state. The advent of intensive monoculture agriculture not only increases population size but also may introduce positive selective pressure for aflatoxin production due to its link with pathogenicity in crops. Important goals in population research are to determine how section Flavi diversity in agricultural ecosystems is changing and to measure the direction of this evolution.

Phytohormone levels in germinating seeds of Zea mays L. exposed to selenium and aflatoxines

Seeds of Zea mays L. were exposed to aflatoxine B1 (AFB1), aflatoxine G1 (AFG1) and selenium (Se) alone and in combination and allowed to germinate. Phytohormone levels of GA-like substances (GAs), trans-Zeatin (t-Z) and Indole-3-acetic acid (IAA) were determined by High Performance Liquid Chromatography (HPLC) when the roots of the germinating seeds reach 1.5-3.0 cm in length. The levels of endogenous hormones decreased in seeds treated with AFB1 and AFG1 compared to control; however an increase was noted in seeds exposed to AFG1 and Se together. AFB1 and Se treatment caused reduced hormone levels in most of the treatments. When plants were exposed to Se alone, the highest levels of GAs, t-Z and IAA were observed in the application of 800 ppm Se. The highest levels of GAs, t-Z and IAA were observed when seeds were treated with 0.2 ppm AFG1 + 8 ppm Se, 0.2 ppm AFG1 + 8 ppm Se and 0.2 ppm AFG1 + 0.08 ppm Se, respectively, whereas the lowest levels of the hormones were observed in 0.2 ppm AFB1 + 8 ppm Se, 0.2 ppm AFB1 + 0.08 ppm Se and 0.1 ppm AFB1, respectively. In conclusion, the levels of phytohormones were reduced by the treatment of AFB1 and AFG1 alone. However Se removed the negative effect of AFB1 on phytohormones, but not AFB1.

Aflatoxin biosynthesis gene clusters and flanking regions

AIMS

To compare the biosynthetic gene cluster sequences of the main aflatoxin (AF)-producing Aspergillus species.

METHODS AND RESULTS

Sequencing was on fosmid clones selected by homology to Aspergillus parasiticus sequence. Alignments revealed that gene order is conserved among AF gene clusters of Aspergillus nomius, A. parasiticus, two sclerotial morphotypes of Aspergillus flavus, and an unnamed Aspergillus sp. Phylogenetic relationships were established using the maximum likelihood method implemented in PAUP. Based on the Eurotiomycete/Sordariomycete divergence time, the A. flavus-type cluster has been maintained for at least 25 million years. Such conservation of the genes and gene order reflects strong selective constraints on rearrangement. Phylogenetic comparison of individual genes in the cluster indicated that ver-1, which has homology to a melanin biosynthesis gene, experienced selective forces distinct from the other pathway genes. Sequences upstream of the polyketide synthase-encoding gene vary among the species, but a four-gene sugar utilization cluster at the distal end is conserved, indicating a functional relationship between the two adjacent clusters.

CONCLUSIONS

The high conservation of cluster components needed for AF production suggests there is an adaptive value for AFs in character-shaping niches important to those taxa.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first comparison of the complete nucleotide sequences of gene clusters harbouring the AF biosynthesis genes of the main AF-producing species. Such a comparison will aid in understanding how AF biosynthesis is regulated in experimental and natural environments.

Sequence breakpoints in the aflatoxin biosynthesis gene cluster and flanking regions in nonaflatoxigenic Aspergillus flavus isolates

Aspergillus flavus populations are genetically diverse. Isolates that produce either, neither, or both aflatoxins and cyclopiazonic acid (CPA) are present in the field. We investigated defects in the aflatoxin gene cluster in 38 nonaflatoxigenic A. flavus isolates collected from southern United States. PCR assays using aflatoxin-gene-specific primers grouped these isolates into eight (A-H) deletion patterns. Patterns C, E, G, and H, which contain 40 kb deletions, were examined for their sequence breakpoints. Pattern C has one breakpoint in the cypA 3' untranslated region (UTR) and another in the verA coding region. Pattern E has a breakpoint in the amdA coding region and another in the ver1 5'UTR. Pattern G contains a deletion identical to the one found in pattern C and has another deletion that extends from the cypA coding region to one end of the chromosome as suggested by the presence of telomeric sequence repeats, CCCTAATGTTGA. Pattern H has a deletion of the entire aflatoxin gene cluster from the hexA coding region in the sugar utilization gene cluster to the telomeric region. Thus, deletions in the aflatoxin gene cluster among A. flavus isolates are not rare, and the patterns appear to be diverse. Genetic drift may be a driving force that is responsible for the loss of the entire aflatoxin gene cluster in nonaflatoxigenic A. flavus isolates when aflatoxins have lost their adaptive value in nature.

Molecular genetic analysis and regulation of aflatoxin biosynthesis

Aflatoxins, produced by some Aspergillus species, are toxic and extremely carcinogenic furanocoumarins. Recent investigations of the molecular mechanism of AFB biosynthesis showed that the genes required for biosynthesis are in a 70 kb gene cluster. They encode a DNA-binding protein functioning in aflatoxin pathway gene regulation, and other enzymes such as cytochrome p450-type monooxygenases, dehydrogenases, methyltransferases, and polyketide and fatty acid synthases. Information gained from these studies has led to a better understanding of aflatoxin biosynthesis by these fungi. The characterization of genes involved in aflatoxin formation affords the opportunity to examine the mechanism of molecular regulation of the aflatoxin biosynthetic pathway, particularly during the interaction between aflatoxin-producing fungi and plants.