Cultural and Genetic Approaches to Manage Aflatoxin Contamination: Recent Insights Provide Opportunities for Improved Control

Aspergillus flavus is a morphologically complex species that can produce the group of polyketide derived carcinogenic and mutagenic secondary metabolites, aflatoxins, as well as other secondary metabolites such as cyclopiazonic acid and aflatrem. Aflatoxin causes aflatoxicosis when aflatoxins are ingested through contaminated food and feed. In addition, aflatoxin contamination is a major problem, from both an economic and health aspect, in developing countries, especially Asia and Africa, where cereals and peanuts are important food crops. Earlier measures for control of A. flavus infection and consequent aflatoxin contamination centered on creating unfavorable environments for the pathogen and destroying contaminated products. While development of atoxigenic (nonaflatoxin producing) strains of A. flavus as viable commercial biocontrol agents has marked a unique advance for control of aflatoxin contamination, particularly in Africa, new insights into the biology and sexuality of A. flavus are now providing opportunities to design improved atoxigenic strains for sustainable biological control of aflatoxin. Further, progress in the use of molecular technologies such as incorporation of antifungal genes in the host and host-induced gene silencing, is providing knowledge that could be harnessed to develop germplasm that is resistant to infection by A. flavus and aflatoxin contamination. This review summarizes the substantial progress that has been made to understand the biology of A. flavus and mitigate aflatoxin contamination with emphasis on maize. Concepts developed to date can provide a basis for future research efforts on the sustainable management of aflatoxin contamination.

RNA interference-based silencing of the alpha-amylase (amy1) gene in Aspergillus flavus decreases fungal growth and aflatoxin production in maize kernels

Expressing an RNAi construct in maize kernels that targets the gene for alpha-amylase in Aspergillus flavus resulted in suppression of alpha-amylase (amy1) gene expression and decreased fungal growth during in situ infection resulting in decreased aflatoxin production. Aspergillus flavus is a saprophytic fungus and pathogen to several important food and feed crops, including maize. Once the fungus colonizes lipid-rich seed tissues, it has the potential to produce toxic secondary metabolites, the most dangerous of which is aflatoxin. The pre-harvest control of A. flavus contamination and aflatoxin production is an area of intense research, which includes breeding strategies, biological control, and the use of genetically-modified crops. Host-induced gene silencing, whereby the host crop produces siRNA molecules targeting crucial genes in the invading fungus and targeting the gene for degradation, has shown to be promising in its ability to inhibit fungal growth and decrease aflatoxin contamination. Here, we demonstrate that maize inbred B104 expressing an RNAi construct targeting the A. flavus alpha-amylase gene amy1 effectively reduces amy1 gene expression resulting in decreased fungal colonization and aflatoxin accumulation in kernels. This work contributes to the development of a promising technology for reducing the negative economic and health impacts of A. flavus growth and aflatoxin contamination in food and feed crops.

Aspergillus flavus Secondary Metabolites: More than Just Aflatoxins

Aspergillus flavus is best known for producing the family of potent carcinogenic secondary metabolites known as aflatoxins. However, this opportunistic plant and animal pathogen also produces numerous other secondary metabolites, many of which have also been shown to be toxic. While about forty of these secondary metabolites have been identified from A. flavus cultures, analysis of the genome has predicted the existence of at least 56 secondary metabolite gene clusters. Many of these gene clusters are not expressed during growth of the fungus on standard laboratory media. This presents researchers with a major challenge of devising novel strategies to manipulate the fungus and its genome so as to activate secondary metabolite gene expression and allow identification of associated cluster metabolites. In this review, we discuss the genetic, biochemical and bioinformatic methods that are being used to identify previously uncharacterized secondary metabolite gene clusters and their associated metabolites. It is important to identify as many of these compounds as possible to determine their bioactivity with respect to fungal development, survival, virulence and especially with respect to any potential synergistic toxic effects with aflatoxin.

Advances in molecular and genomic research to safeguard food and feed supply from aflatoxin contamination

Worldwide recognition that aflatoxin contamination of agricultural commodities by the fungus Aspergillus flavus is a global problem has significantly benefitted from global collaboration for understanding the contaminating fungus, as well as for developing and implementing solutions against the contamination. The effort to address this serious food and feed safety issue has led to a detailed understanding of the taxonomy, ecology, physiology, genomics and evolution of A. flavus, as well as strategies to reduce or control pre-harvest aflatoxin contamination, including (1) biological control, using atoxigenic aspergilli, (2) proteomic and genomic analyses for identifying resistance factors in maize as potential breeding markers to enable development of resistant maize lines, and (3) enhancing host-resistance by bioengineering of susceptible crops, such as cotton, maize, peanut and tree nuts. A post-harvest measure to prevent the occurrence of aflatoxin contamination in storage is also an important component for reducing exposure of populations worldwide to aflatoxins in food and feed supplies. The effect of environmental changes on aflatoxin contamination levels has recently become an important aspect for study to anticipate future contamination levels. The ability of A. flavus to produce dozens of secondary metabolites, in addition to aflatoxins, has created a new avenue of research for understanding the role these metabolites play in the survival and biodiversity of this fungus. The understanding of A. flavus, the aflatoxin contamination problem, and control measures to prevent the contamination has become a unique example for an integrated approach to safeguard global food and feed safety.

Proteome analysis of Aspergillus flavus isolate-specific responses to oxidative stress in relationship to aflatoxin production capability

Aspergillus flavus is an opportunistic pathogen of plants such as maize and peanut under conducive conditions such as drought stress resulting in significant aflatoxin production. Drought-associated oxidative stress also exacerbates aflatoxin production by A. flavus. The objectives of this study were to use proteomics to provide insights into the pathogen responses to H₂O₂-derived oxidative stress, and to identify potential biomarkers and targets for host resistance breeding. Three isolates, AF13, NRRL3357, and K54A with high, moderate, and no aflatoxin production, were cultured in medium supplemented with varying levels of H₂O₂, and examined using an iTRAQ (Isobaric Tags for Relative and Absolute Quantification) approach. Overall, 1,173 proteins were identified and 220 were differentially expressed (DEPs). Observed DEPs encompassed metabolic pathways including antioxidants, carbohydrates, pathogenicity, and secondary metabolism. Increased lytic enzyme, secondary metabolite, and developmental pathway expression in AF13 was correlated with oxidative stress tolerance, likely assisting in plant infection and microbial competition. Elevated expression of energy and cellular component production in NRRL3357 and K54A implies a focus on oxidative damage remediation. These trends explain isolate-to-isolate variation in oxidative stress tolerance and provide insights into mechanisms relevant to host plant interactions under drought stress allowing for more targeted efforts in host resistance research.