Advances in the Development of Host Resistance in Corn to Aflatoxin Contamination by Aspergillus flavus

MicroRNA (miRNA) profiling of maize genotypes with differential response to Aspergillus flavus implies zma-miR156-squamosa promoter binding protein (SBP) and zma-miR398/zma-miR394-F -box combinations involved in resistance mechanisms

Maize (Zea mays), a major food crop worldwide, is susceptible to infection by the saprophytic fungus Aspergillus flavus that can produce the carcinogenic metabolite aflatoxin (AF) especially under climate change induced abiotic stressors that favor mold growth. Several studies have used "-omics" approaches to identify genetic elements with potential roles in AF resistance, but there is a lack of research identifying the involvement of small RNAs such as microRNAs (miRNAs) in maize-A. flavus interaction. In this study, we compared the miRNA profiles of three maize lines (resistant TZAR102, moderately resistant MI82, and susceptible Va35) at 8 h, 3 d, and 7 d after A. flavus infection to investigate possible regulatory antifungal role of miRNAs. A total of 316 miRNAs (275 known and 41 putative novel) belonging to 115 miRNA families were identified in response to the fungal infection across all three maize lines. Eighty-two unique miRNAs were significantly differentially expressed with 39 miRNAs exhibiting temporal differential regulation irrespective of the maize genotype, which targeted 544 genes (mRNAs) involved in diverse molecular functions. The two most notable biological processes involved in plant immunity, namely cellular responses to oxidative stress (GO:00345990) and reactive oxygen species (GO:0034614) were significantly enriched in the resistant line TZAR102. Coexpression network analysis identified 34 hubs of miRNA-mRNA pairs where nine hubs had a node in the module connected to their target gene with potentially important roles in resistance/susceptible response of maize to A. flavus. The miRNA hubs in resistance modules (TZAR102 and MI82) were mostly connected to transcription factors and protein kinases. Specifically, the module of miRNA zma-miR156b-nb - squamosa promoter binding protein (SBP), zma-miR398a-3p - SKIP5, and zma-miR394a-5p - F-box protein 6 combinations in the resistance-associated modules were considered important candidates for future functional studies.

Performance of testers with contrasting provitamin A content to evaluate provitamin A maize for resistance to Aspergillus flavus infection and aflatoxin production

In sub-Saharan Africa (SSA), millions of people depend on maize as a primary staple. However, maize consumers in SSA may be exposed to malnutrition due to vitamin A deficiency (VAD) and unsafe aflatoxin levels, which can lead to serious economic and public health problems. Provitamin A (PVA) biofortified maize has been developed to alleviate VAD and may have additional benefits such as reduced aflatoxin contamination. In this study, maize inbred testers with contrasting PVA content in grain were used to identify inbred lines with desirable combining ability for breeding to enhance their level of resistance to aflatoxin. Kernels of 120 PVA hybrids generated by crossing 60 PVA inbreds with varying levels of PVA (5.4 to 51.7 µg/g) and two testers (low and high PVA, 14.4 and 25.0 µg/g, respectively) were inoculated with a highly toxigenic strain of Aspergillus flavus. Aflatoxin had a negative genetic correlation with β-carotene (r = -0.29, p < 0.0001) and PVA (r = -0.23, p < 0.0001), indicating that hybrids with high PVA content accumulated less aflatoxin than those with low to medium PVA. Both general combining ability (GCA) and specific combining ability (SCA) effects of lines and testers were significant for aflatoxin accumulation, number of spores, PVA, and other carotenoids, with additive gene actions playing a prominent role in regulating the mode of inheritance (GCA/SCA ratio >0.5). Eight inbreds had combined significant negative GCA effects for aflatoxin accumulation and spore count with significant positive GCA effects for PVA. Five testcrosses had combined significant negative SCA effects for aflatoxin with significant positive SCA effects for PVA. The high PVA tester had significant negative GCA effects for aflatoxin, lutein, β-carotene, and PVA. The study identified lines that can be used as parents to develop superior hybrids with high PVA and reduced aflatoxin accumulation. Overall, the results point out the importance of testers in maize breeding programs to develop materials that can contribute to controlling aflatoxin contamination and reducing VAD.

Identification of Early and Extra-Early Maturing Tropical Maize Inbred Lines with Multiple Disease Resistance for Enhanced Maize Production and Productivity in Sub-Saharan Africa

Maize, a staple for millions across sub-Saharan Africa (SSA), faces major biotic constraints affecting production and safety of the crop. These include northern corn leaf blight (NCLB), southern corn leaf blight (SCLB), Curvularia leaf spot (CLS), and aflatoxin contamination by Exserohilum turcicum, Bipolaris maydis, Curvularia lunata, and Aspergillus flavus, respectively. Farmers in SSA would benefit tremendously if high-yielding maize hybrids with multiple disease resistance (MDR) were developed and commercialized. In all, 49 early-maturing (EM; 90 to 95 days to physiological maturity) and 55 extra-early-maturing (EEM, 80 to 85 days to physiological maturity) inbred lines developed by the International Institute of Tropical Agriculture were identified as resistant to NCLB in field evaluations in multiple agroecologies of Nigeria in 2017 and 2018. From each maturity group, the 30 most resistant inbreds were selected for evaluation for resistance to SCLB and CLS using a detached-leaf assay. Additionally, the inbreds were screened for resistance to kernel rot and aflatoxin contamination using a kernel screening assay. In all, 7 EM and 6 EEM maize inbreds were found to be highly resistant to the three foliar pathogens while 10 inbreds were resistant to the foliar pathogens and supported significantly less (P = 0.01) aflatoxin accumulation than other inbreds. Inbreds having MDR should be tested extensively in hybrid combinations and commercialized. Large-scale use of maize hybrids with MDR would (i) increase maize production and productivity and (ii) reduce losses caused by aflatoxin contamination. Overall, planting of EM and EEM maize hybrids with MDR would contribute to food security, reduced aflatoxin exposure, and increased incomes of maize farmers in SSA.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Growth and Toxigenicity of A. flavus on Resistant and Susceptible Peanut Genotypes

Aflatoxin contamination poses serious health concerns to consumers of peanut and peanut products. This study aimed at investigating the response of peanuts to Aspergillus flavus infection and aflatoxin accumulation. Isolates of A. flavus were characterised either as aflatoxigenic or non-aflatoxigenic using multiple cultural techniques. The selected isolates were used in an in vitro seed colonisation (IVSC) experiment on two A. flavus-resistant and susceptible peanut genotypes. Disease incidence, severity, and aflatoxin accumulation were measured. Genotypes differed significantly (p < 0.001) in terms of the incidence and severity of aflatoxigenic and non-aflatoxigenic A. flavus infection with the non-aflatoxigenic isolate having significantly higher incidence and severity values. There was no accumulation of aflatoxins in peanut genotypes inoculated with non-aflatoxigenic isolate, indicating its potential as a biocontrol agent. Inoculations with the aflatoxigenic isolate resulted in the accumulation of aflatoxin B1 and G1 in all the peanut genotypes. Aflatoxin B2 was not detected in ICGV−03401 (resistant genotype), while it was present and higher in Manipinta (susceptible genotype) than L027B (resistant genotype). ICGV−03401 can resist fungal infection and aflatoxin accumulation than L027B and Manipinta. Non-aflatoxigenic isolate detected in this study could further be investigated as a biocontrol agent.

Biocontrol of Aflatoxins Using Non-Aflatoxigenic Aspergillus flavus: A Literature Review

Aflatoxins (AFs) are mycotoxins, predominantly produced by Aspergillus flavus, A. parasiticus, A. nomius, and A. pseudotamarii. AFs are carcinogenic compounds causing liver cancer in humans and animals. Physical and biological factors significantly affect AF production during the pre-and post-harvest time. Several methodologies have been developed to control AF contamination, yet; they are usually expensive and unfriendly to the environment. Consequently, interest in using biocontrol agents has increased, as they are convenient, advanced, and friendly to the environment. Using non-aflatoxigenic strains of A. flavus (AF⁻) as biocontrol agents is the most promising method to control AFs’ contamination in cereal crops. AF⁻ strains cannot produce AFs due to the absence of polyketide synthase genes or genetic mutation. AF⁻ strains competitively exclude the AF⁺ strains in the field, giving an extra advantage to the stored grains. Several microbiological, molecular, and field-based approaches have been used to select a suitable biocontrol agent. The effectiveness of biocontrol agents in controlling AF contamination could reach up to 99.3%. Optimal inoculum rate and a perfect time of application are critical factors influencing the efficacy of biocontrol agents.

Genome-wide association study leads to novel genetic insights into resistance to Aspergillus flavus in maize kernels

BACKGROUND

Fungus infection in staple grains affects the food storage and threatens food security. The Aspergillus flavus is known to infect multiple grains and produce mycotoxin Aflatoxin B1, which is mutagenic, teratogenic and causes immunosuppression in animals. However, the molecular mechanism of maize resistance to A. flavus is largely unknown.

RESULTS

Here we used corn kernels to investigate resistance genes to A. flavus using genome-wide association study (GWAS) of 313 inbred lines. We characterized the resistance levels of kernels after inoculating with A. flavus. The GWAS with 558,529 SNPs identified four associated loci involving 29 candidate genes that were linked to seed development, resistance or infection, and involved in signal pathways, seed development, germination, dormancy, epigenetic modification, and antimicrobial activity. In addition, a few candidate genes were also associated with several G-protein signaling and phytohormones that might involve in synergistic work conferring different resistance during seed development. Expression of 16 genes out of 29 during kernel development was also associated with resistance levels.

CONCLUSIONS

We characterized the resistance levels of 313 maize kernels after inoculating with A. flavus, and found four associated loci and 16 candidate maize genes. The expressed 16 genes involved in kernel structure and kernel composition most likely contribute to mature maize kernels' resistance to A. flavus, and in particular, in the development of pericarp. The linked candidate genes could be experimentally transformed to validate and manipulate fungal resistance. Thus this result adds value to maize kernels in breeding programs.

Mapping Quantitative Trait Loci Associated With Resistance to Aflatoxin Accumulation in Maize Inbred Mp719

Aflatoxins are carcinogenic and toxic compounds produced principally by fungal species Aspergillus flavus (Link: Fries) and A. parasiticus (Speare), which are common contaminants of food and feed. Aflatoxins can be found at dangerously high levels and can readily contaminate pre-harvest maize (Zea mays L.) grain. Sources of resistance to aflatoxin accumulation in maize have been identified, however, the highly quantitative nature and complex inheritance of this trait have limited the introgression of aflatoxin accumulation resistance into agronomically desirable lines. Mapping of quantitative trait loci (QTL) was performed on a bi-parental population comprised of 241 F2:3 families derived from the cross of inbred lines Mp705 (susceptible) × Mp719 (resistant). The mapping population was phenotyped in replicated field trials in three environments for resistance to aflatoxin accumulation under artificial inoculation with an A. flavus spore suspension. The genetic linkage map was constructed with 1,276 single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) molecular markers covering a total genetic distance of 1,642 cM across all ten maize chromosomes. Multiple interval mapping revealed that majority of the aflatoxin-reducing alleles and the source for the larger effect QTL identified in this study were contributed from Mp719, the resistant parent. Two QTL identified on chromosome 1 (bin 1.06-1.07) and chromosome 3 (bin 3.09) were the most stable across different environments and when combined, explained 24.6% of the total phenotypic variance across all three environments. Results from the study showed that these chromosomal regions harbor important QTL for influencing aflatoxin accumulation, which is consistent with previous reports with other different mapping populations. These stable QTL were the most promising for controlling aflatoxin accumulation in maize grain. Identifying beneficial alleles derived from Mp719 and closely linked molecular markers through QTL analysis for implementation of MAS could accelerate breeding efforts to reduce aflatoxin accumulation in maize.

Characterization of Argentinian Endemic Aspergillus flavus Isolates and Their Potential Use as Biocontrol Agents for Mycotoxins in Maize

Maize (Zea mays L.) is a highly valuable crop in Argentina, frequently contaminated with the mycotoxins produced by Aspergillus flavus. Biocontrol products formulated with atoxigenic (nontoxic) strains of this fungal species are well known as an effective method to reduce this contamination. In the present study, 83 A. flavus isolates from two maize regions of Argentina were characterized and evaluated for their ability to produce or lack of producing mycotoxins in order to select atoxigenic strains to be used as potential biocontrol agents (BCA). All of the isolates were tested for aflatoxin and cyclopiazonic acid (CPA) production in maize kernels and a liquid culture medium. Genetic diversity of the nonaflatoxigenic isolates was evaluated by analysis of vegetative compatibility groups (VCG) and confirmation of deletions in the aflatoxin biosynthesis cluster. Eight atoxigenic isolates were compared for their ability to reduce aflatoxin and CPA contamination in maize kernels in coinoculation tests. The A. flavus population was composed of 32% aflatoxin and CPA producers and 52% CPA producers, and 16% was determined as atoxigenic. All of the aflatoxin producer isolates also produced CPA. Aflatoxin and CPA production was significantly higher in maize kernels than in liquid medium. The 57 nonaflatoxigenic strains formed six VCG, with AM1 and AM5 being the dominant groups, with a frequency of 58 and 35%, respectively. In coinoculation experiments, all of the atoxigenic strains reduced aflatoxin from 54 to 83% and CPA from 60 to 97%. Members of group AM1 showed a greater aflatoxin reduction than members of AM5 (72 versus 66%) but no differences were detected in CPA production. Here, we described for the first time atoxigenic isolates of A. flavus that show promise to be used as BCA in maize crops in Argentina. This innovating biological control approach should be considered, developed further, and used by the maize industry to preserve the quality properties and food safety of maize kernels in Argentina.

Detection of Aflatoxigenic and Atoxigenic Mexican Aspergillus Strains by the Dichlorvos⁻Ammonia (DV⁻AM) Method

The dichlorvos⁻ammonia (DV⁻AM) method is a sensitive method for distinguishing aflatoxigenic fungi by detecting red (positive) colonies. In this study, the DV⁻AM method was applied for the isolation of aflatoxigenic and atoxigenic fungi from soil samples from a maize field in Mexico. In the first screening, we obtained two isolates from two soil subsamples of 20 independent samples and, in the second screening, we obtained two isolates from one subsample of these. Morphological and phylogenic analyses of the two isolates (MEX-A19-13, MEX-A19-2^nd-5) indicated that they were Aspergillus flavus located in the A. flavus clade. Chemical analyses demonstrated that one isolate could produce B-type aflatoxins, while the other produced no aflatoxins. These results demonstrate that the DV⁻AM method is useful for the isolation of both aflatoxigenic and atoxigenic Aspergilli.

Dissection of mechanisms of resistance to Aspergillus flavus and aflatoxin using tropical maize germplasm

Aspergillus flavus induced ear rots and subsequent contamination of maize (Zea mays L.) by aflatoxin is a serious food safety issue, especially in developing countries where the crop is mostly cultivated by smallholder famers for own consumption and income generation. A better understanding of the mechanisms of resistance could help breeders to develop resistant maize varieties. In this study, a set of six tropical maize inbred lines previously identified as resistant or susceptible under natural field conditions were evaluated for response to A. flavus colonisation and aflatoxin contamination. Fungal biomass was significantly higher (P<0.05) in susceptible than resistant maize inbred lines, and this was highly correlated (P=0.001) to aflatoxin levels. Maize inbred lines MRI, MR2 and MR3 had low fungal biomass and low aflatoxin levels, suggesting that resistance in these lines was mediated through restricted fungal colonisation and establishment. Among the three putatively resistant inbred lines mentioned above, MR2 had a relatively high colonisation compared to the other two lines, revealing that A. flavus could establish and colonise kernels that were injured during inoculation, but did not contain high levels of aflatoxin. This could signify the presence of host genes that interfere with the aflatoxin biosynthetic pathway.

Impact of Silver Nanoparticles on Gene Expression in Aspergillus Flavus Producer Aflatoxin B1

AIM

In this study, we evaluated the effect of silver nanoparticles (AgNPs) on the production of aflatoxin B₁ (AFB₁) through assessment the transcription activity of aflatoxin biosynthesis pathway genes in Aspergillus flavus ATCC28542.

MATERIAL AND METHODS

The mRNAs were quantitative by Real Time-polymerase chain reaction (qRT-PCR) of A. flavus grown in yeast extract sucrose (YES) medium containing AgNPs. Specific primers that are involved in the AFB₁ biosynthesis which highly specific to A. flavus, O-methyltransferase gene (omt-A), were designed and used to detect the fungus activity by quantitative PCR assay. The AFB₁ production (from A. flavus growth) which effected by AgNPs were measured in YES medium by high-pressure liquid chromatography (HPLC).

RESULTS

The AFB₁ produced by A. flavus have the highest reduction with 1.5 mg ^{-100 ml} of AgNPs were added in media those records 88.2%, 67.7% and 83.5% reduction by using AgNP HA1N, AgNP HA2N and AgNP EH, respectively. While on mycelial growth give significantly inhibitory effect. These results have been confirmed by qRT-PCR which showed that culture of A. flavus with the presence of AgNPs reduced the expression levels of omt-A gene.

CONCLUSION

Based on the results of the present study, AgNPs inhibit growth and AFB1 produced by Aspergillus flavus ATCC28542. This was confirmed through RT-PCR approach showing the effect of AgNPs on omt-A gene involved in aflatoxin biosynthesis.

Expression Profiling Coupled with In-silico Mapping Identifies Candidate Genes for Reducing Aflatoxin Accumulation in Maize

Aflatoxin, produced by Aspergillus flavus, is hazardous to health of humans and livestock. The lack of information about large effect QTL for resistance to aflatoxin accumulation is a major obstacle to employ marker-assisted selection for maize improvement. The understanding of resistance mechanisms of the host plant and the associated genes is necessary for improving resistance to A. flavus infection. A suppression subtraction hybridization (SSH) cDNA library was made using the developing kernels of Mp715 (resistant inbred) and B73 (susceptible inbred) and 480 randomly selected cDNA clones were sequenced to identify differentially expressed genes (DEGs) in response to A. flavus infection and map these clones onto the corn genome by in-silico mapping. A total of 267 unigenes were identified and majority of genes were related to metabolism, stress response, and disease resistance. Based on the reverse northern hybridization experiment, 26 DEGs were selected for semi-quantitative RT-PCR analysis in seven inbreds with variable resistance to aflatoxin accumulation at two time points after A. flavus inoculation. Most of these genes were highly expressed in resistant inbreds. Quantitative RT-PCR analysis validated upregulation of PR-4, DEAD-box RNA helicase, and leucine rich repeat family protein in resistant inbreds. Fifty-six unigenes, which were placed on linkage map through in-silico mapping, overlapped the QTL regions for resistance to aflatoxin accumulation identified in a mapping population derived from the cross between B73 and Mp715. Since majority of these mapped genes were related to disease resistance, stress response, and metabolism, these should be ideal candidates to investigate host pathogen interaction and to reduce aflatoxin accumulation in maize.

Degeneration of aflatoxin gene clusters in Aspergillus flavus from Africa and North America

Aspergillus flavus is the most common causal agent of aflatoxin contamination of food and feed. However, aflatoxin-producing potential varies widely among A. flavus genotypes with many producing no aflatoxins. Some non-aflatoxigenic genotypes are used as biocontrol agents to prevent contamination. Aflatoxin biosynthesis genes are tightly clustered in a highly conserved order. Gene deletions and presence of single nucleotide polymorphisms (SNPs) in aflatoxin biosynthesis genes are often associated with A. flavus inability to produce aflatoxins. In order to identify mechanisms of non-aflatoxigenicity in non-aflatoxigenic genotypes of value in aflatoxin biocontrol, complete cluster sequences of 35 A. flavus genotypes from Africa and North America were analyzed. Inability of some genotypes to produce aflatoxin resulted from deletion of biosynthesis genes. In other genotypes, non-aflatoxigenicity originated from SNP formation. The process of degeneration differed across the gene cluster; genes involved in early biosynthesis stages were more likely to be deleted while genes involved in later stages displayed high frequencies of SNPs. Comparative analyses of aflatoxin gene clusters provides insight into the diversity of mechanisms of non-aflatoxigenicity in A. flavus genotypes used as biological control agents. The sequences provide resources for both diagnosis of non-aflatoxigenicity and monitoring of biocontrol genotypes during biopesticide manufacture and in the environment.

Intervention trial with calcium montmorillonite clay in a south Texas population exposed to aflatoxin

South Texas currently has the highest incidence of hepatocellular carcinoma (HCC) in the United States, a disease that disproportionately affects Latino populations in the region. Aflatoxin B1 (AFB1) is a potent liver carcinogen that has been shown to be present in a variety of foods in the United States, including corn and corn products. Importantly, it is a dietary risk factor contributing to a higher incidence of HCC in populations frequently consuming AFB1-contaminated diets. In a randomised double-blind placebo controlled trial, we evaluated the effects of a 3-month administration of ACCS100 (refined calcium montmorillonite clay) on serum AFB1-lysine adduct (AFB-Lys) level and serum biochemistry in 234 healthy men and women residing in Bexar and Medina counties, Texas. Participants recruited from 2012 to 2014 received either a placebo, 1.5 g or 3 g ACCS100 each day for 3 months, and no treatment during the fourth month. Adverse event rates were similar across treatment groups and no significant differences were observed for serum biochemistry and haematology parameters. Differences in levels of AFB-Lys at 1, 3 and 4 months were compared between placebo and active treatment groups. Although serum AFB-Lys levels were decreased by month 3 for both treatment groups, the low dose was the only treatment that was significant (p = 0.0005). In conclusion, the observed effect in the low-dose treatment group suggests that the use of ACCS100 may be a viable strategy to reduce dietary AFB1 bioavailability during aflatoxin outbreaks and potentially in populations chronically exposed to this carcinogen.

Biotechnological advances for combating Aspergillus flavus and aflatoxin contamination in crops

Aflatoxins are toxic, carcinogenic, mutagenic, teratogenic and immunosuppressive byproducts of Aspergillus spp. that contaminate a wide range of crops such as maize, peanut, and cotton. Aflatoxin not only affects crop production but renders the produce unfit for consumption and harmful to human and livestock health, with stringent threshold limits of acceptability. In many crops, breeding for resistance is not a reliable option because of the limited availability of genotypes with durable resistance to Aspergillus. Understanding the fungal/crop/environment interactions involved in aflatoxin contamination is therefore essential in designing measures for its prevention and control. For a sustainable solution to aflatoxin contamination, research must be focused on identifying and improving knowledge of host-plant resistance factors to aflatoxin accumulation. Current advances in genetic transformation, proteomics, RNAi technology, and marker-assisted selection offer great potential in minimizing pre-harvest aflatoxin contamination in cultivated crop species. Moreover, developing effective phenotyping strategies for transgenic as well as precision breeding of resistance genes into commercial varieties is critical. While appropriate storage practices can generally minimize post-harvest aflatoxin contamination in crops, the use of biotechnology to interrupt the probability of pre-harvest infection and contamination has the potential to provide sustainable solution.

Genome wide association study for drought, aflatoxin resistance, and important agronomic traits of maize hybrids in the sub-tropics

The primary maize (Zea mays L.) production areas are in temperate regions throughout the world and this is where most maize breeding is focused. Important but lower yielding maize growing regions such as the sub-tropics experience unique challenges, the greatest of which are drought stress and aflatoxin contamination. Here we used a diversity panel consisting of 346 maize inbred lines originating in temperate, sub-tropical and tropical areas testcrossed to stiff-stalk line Tx714 to investigate these traits. Testcross hybrids were evaluated under irrigated and non-irrigated trials for yield, plant height, ear height, days to anthesis, days to silking and other agronomic traits. Irrigated trials were also inoculated with Aspergillus flavus and evaluated for aflatoxin content. Diverse maize testcrosses out-yielded commercial checks in most trials, which indicated the potential for genetic diversity to improve sub-tropical breeding programs. To identify genomic regions associated with yield, aflatoxin resistance and other important agronomic traits, a genome wide association analysis was performed. Using 60,000 SNPs, this study found 10 quantitative trait variants for grain yield, plant and ear height, and flowering time after stringent multiple test corrections, and after fitting different models. Three of these variants explained 5-10% of the variation in grain yield under both water conditions. Multiple identified SNPs co-localized with previously reported QTL, which narrows the possible location of causal polymorphisms. Novel significant SNPs were also identified. This study demonstrated the potential to use genome wide association studies to identify major variants of quantitative and complex traits such as yield under drought that are still segregating between elite inbred lines.

Identifying and developing maize germplasm with resistance to accumulation of aflatoxins

Efforts to identify maize germplasm with resistance to Aspergillus flavus infection and subsequent accumulation of aflatoxins were initiated by the US Department of Agriculture, Agricultural Research Service at several locations in the late 1970s and early 1980s. Research units at four locations in the south-eastern USA are currently engaged in identification and development of maize germplasm with resistance to A. flavus infection and accumulation of aflatoxins. The Corn Host Plant Resistance Research Unit, Mississippi State, MS, developed procedures for screening germplasm for resistance to A. flavus infection and accumulation of aflatoxins. Mp313E, released in 1990, was the first line released as a source of resistance to A. flavus infection. Subsequently, germplasm lines Mp420, Mp715, Mp717, Mp718, and Mp719 were released as additional sources of resistance. Quantitative trait loci associated with resistance have also been identified in four bi-parental populations. The Crop Protection and Management Research Unit and Crop Genetics and Breeding Research Unit, Tifton, GA, created a breeding population GT-MAS:gk. GT601, GT602, and GT603 were developed from GT-MAS:gk. The Food and Feed Safety Research Unit, New Orleans, LA, in collaboration with the International Institute for Tropical Agriculture used a kernel screening assay to screen germplasm and develop six germplasm lines with resistance to aflatoxins. The Plant Science Research Unit, Raleigh, NC, through the Germplasm Enhancement of Maize (GEM) Project provides to co-operators diverse germplasm that is a valuable source of resistance to A. flavus infection and accumulation of aflatoxins in maize.

Discovery and confirmation of genes/proteins associated with maize aflatoxin resistance

Maize (Zea mays L.) is one of the major crops susceptible to Aspergillus flavus infection and subsequent aflatoxin contamination. Many earlier studies indicated the roles of kernel proteins, especially constitutively expressed proteins, in maize resistance to A. flavus infection and aflatoxin production. In this review, we examined the past and current efforts in identifying maize genes and proteins from kernel, rachis, and silk tissues that may play an important role in resistance to A. flavus infection and aflatoxin contamination, as well as the efforts in determining the importance or involvement of them in maize resistance through biochemical, molecular and genetics studies. Through these studies, we gained a better understanding of host resistance mechanism: resistant lines appear to either express some stress-related and antifungal proteins at higher levels in endosperm, embryo, rachis and silk tissues before A. flavus infection or induce the expression of these proteins much faster compared to susceptible maize lines. In addition, we summarised several recent efforts in enhancing maize resistance to aflatoxin contamination using native genes from maize or heterologous and synthetic genes from other sources as well as from A. flavus. These efforts to either suppress A. flavus growth or aflatoxin production, have all shown some promising preliminary success. For example, maize plants transformed with an ?-amylase inhibitor protein from Lablab purpurea showed reduced aflatoxin levels by 56% in kernel screening assays. The antifungal potentials of transgenic maize plants expressing synthetic lytic peptides, such as cecropin-based D4E1 or tachyplesin-based AGM peptides with demonstrated anti-flavus activity (IC50 = 2.5 to 10 ?M), are yet to be assayed. Further investigation in these areas may provide a more cost-effective alternative to biocontrol in managing aflatoxin contamination in maize and other susceptible crops.

Lack of aflatoxin production by Aspergillus flavus is associated with reduced fungal growth and delayed expression of aflatoxin pathway genes

Aflatoxins, produced by Aspergillus flavus and Aspergillus parasiticus, are the most toxic fungal secondary metabolites that contaminate agricultural commodities such as peanuts, cotton and maize. Understanding the underlying mechanisms of crop resistance to fungal infection is an important step for plant breeders to develop better and improved crop varieties for safe production of human food and animal feed. Infection studies have identified a resistant (R) peanut line, GT-C20, which is able to decrease aflatoxin contamination. The mycelial growth of A. flavus NRRL3357 on the R peanut line was much lower than that on the susceptible (S) peanut line, Tifrunner. Besides reducing fungal growth, the R line compared to the S line inhibited aflatoxin production completely. Real-time RT-PCR assays of both the R and S lines infected by A. flavus showed that expression of five aflatoxin biosynthetic pathway genes, the aflR regulatory gene and the aflD, aflM, aflP and aflQ structural genes, was not reduced but was significantly delayed on the R line. The results suggested that resistance factors of the R line acted negatively on A. flavus growth and also altered fungal development. The dysfunction in development changed the timing and the pattern of aflatoxin gene expression, which in part rendered A. flavus unable to produce aflatoxins.

Development and validation of a high-performance liquid chromatography method with post-column derivatization for the detection of aflatoxins in cereals and grains

A novel, reliable and rapid high-performance liquid chromatography (HPLC) method with post-column derivatization was developed and validated. The HPLC method was used for the simultaneous determination of aflatoxin B1 (AFB1), B2 (AFB2), G1 (AFG1) and G2 (AFG2) in various cereals and grains. Samples were extracted with 80:20 (v/v) methanol:water and purified using C18 (40–63 μm) solid-phase extraction cartridges. AFs were separated using a LiChroCART–RP-18 (5 μm, 250 × 4.0 mm2) column. The mobile phase consisted of methanol:acetonitrile:buffer (17.5:17.5:65 v/v) (pH 7.4) delivered at the flow rate of 1.0 mL min−1. The fluorescence of each AF was detected at λex = 365 nm and λem = 435 nm. All four AFs were properly resolved within the total run time of 20 min. The established method was extensively validated as a final verification of the method development by the evaluation of selectivity (AFB1, AFB2, AFG1 and AFG2), linearity ( R2 ≥ 0.9994), precision (average SD ≤ 2.79), accuracy (relative mean error ≤ −5.51), robustness ( p < 0.0080), ruggedness ( p < 0.0100) and average recoveries (89.2–97.8%). The limits of quantification of AFB1, AFB2, AFG1 and AFG2 were 0.080, 0.073, 0.062 and 0.066 ng g−1, respectively. Finally, the developed method was applied for the analysis of AFs in 45 samples comprising rice ( n = 20), wheat ( n = 15) and maize ( n = 10). The results showed that 65% of rice, 20% of wheat and 80% of maize samples were found contaminated with AFs. Thus, according to the achieved results, it is suggested that the newly developed HPLC method could be effectively applied for the routine analysis of the AFs in different cereals and grains.

Susceptibility to aflatoxin contamination among maize landraces from Mexico

Maize, the critical staple food for billions of people, was domesticated in Mexico about 9,000 YBP. Today, a great array of maize landraces (MLRs) across rural Mexico is harbored in a living library that has been passed among generations since before the establishment of the modern state. MLRs have been selected over hundreds of generations by ethnic groups for adaptation to diverse environmental settings. The genetic diversity of MLRs in Mexico is an outstanding resource for development of maize cultivars with beneficial traits. Maize is frequently contaminated with aflatoxins by Aspergillus flavus, and resistance to accumulation of these potent carcinogens has been sought for over three decades. However, MLRs from Mexico have not been evaluated as potential sources of resistance. Variation in susceptibility to both A. flavus reproduction and aflatoxin contamination was evaluated on viable maize kernels in laboratory experiments that included 74 MLR accessions collected from 2006 to 2008 in the central west and northwest regions of Mexico. Resistant and susceptible MLR accessions were detected in both regions. The most resistant accessions accumulated over 99 % less aflatoxin B1 than did the commercial hybrid control Pioneer P33B50. Accessions supporting lower aflatoxin accumulation also supported reduced A. flavus sporulation. Sporulation on the MLRs was positively correlated with aflatoxin accumulation (R = 0.5336, P < 0.0001), suggesting that resistance to fungal reproduction is associated with MLR aflatoxin resistance. Results of the current study indicate that MLRs from Mexico are potentially important sources of aflatoxin resistance that may contribute to the breeding of commercially acceptable and safe maize hybrids and/or open pollinated cultivars for human and animal consumption.

Hepatocellular carcinoma: epidemiology and risk factors

Hepatocellular carcinoma is one of the major malignant tumors in the world today. The number of new cases of the tumor increases year by year, and hepatocellular carcinoma almost always runs a fulminant course and carries an especially grave prognosis. It has a low resectability rate and a high recurrence rate after surgical intervention, and responds poorly to anticancer drugs and radiotherapy. Hepatocellular carcinoma does not have a uniform geographical distribution: rather, very high incidences occur in Eastern and Southeastern Asia and in sub-Saharan Black Africans. In these regions and populations, the tumor shows a distinct shift in age distribution toward the younger ages, seen to greatest extent in sub-Saharan Black Africans. In all populations, males are more commonly affected. The most common risk factors for hepatocellular carcinoma in resource-poor populations with a high incidence of the tumor are chronic hepatitis B virus infection and dietary exposure to the fungal hepatocarcinogen aflatoxin B1. These two causative agents act either singly or synergistically. Both the viral infection and exposure to the fungus occur from early childhood, and the tumor typically presents at an early age. Chronic hepatitis C virus infection is an important cause of hepatocellular carcinoma in resource-rich countries with a low incidence of the tumor. The infection is acquired in adulthood and hepatocellular carcinoma occurs later than it does with hepatitis B virus-induced tumors. In recent years, obesity and the metabolic syndrome have increased markedly in incidence and importance as a cause of hepatocellular carcinoma in some resource-rich regions. Chronic alcohol abuse remains an important risk factor for malignant transformation of hepatocytes, frequently in association with alcohol-induced cirrhosis. Excessive iron accumulation in hereditary hemochromatosis and dietary iron overload in the Black African population and membranous obstruction of the inferior cava cause the tumor in a few countries.

Aflatoxin Resistance in Maize: What Have We Learned Lately?

Aflatoxin contamination of maize grain is a huge economic and health problem, causing death and increased disease burden in much of the developing world and income loss in the developed world. Despite the gravity of the problem, deployable solutions are still being sought. In the past 15 years, much progress has been made in creating resistant maize inbred lines; mapping of genetic factors associated with resistance; and identifying possible resistance mechanisms. This review highlights this progress, most of which has occurred since the last time a review was published on this topic. Many of the needs highlighted in the last reviews have been addressed, and several solutions, taken together, can now greatly reduce the aflatoxin problem in maize grain. Continued research will soon lead to further solutions, which promise to further reduce and even eliminate the problem completely.

Detoxification of aflatoxin in corn flour by ozone

BACKGROUND

Corn, which is one of most important agricultural products worldwide, is prone to pollution by aflatoxins (AFs) in many areas, thus seriously jeopardizing human health and threatening economic growth. This study evaluated the effects of ozone on the detoxification of AFs in corn flour (CF) and the moisture content (MC) thereof.

RESULTS

The detoxifying effects of ozone on CF became more obvious as the ozone concentration and exposure time increased. After CF was treated with 75 mg L(-1) ozone for 60 min, the contents of AFB1 , AFG1 and AFB2 decreased from 53.60, 12.08 and 2.42 µg kg(-1) to 11.38, 3.37 and 0.71 µg kg(-1) , respectively, which are lower than the maximum limits of AFB1 , AFG1 , AFB2 and total AFs (20 µg kg(-1) ) for CF regulated by the Chinese government. Ozonation significantly affected the MC of CF, and ozone at a higher concentration decreased the MC more drastically. After CF was exposed to 15, 30, 45 and 75 mg L(-1) ozone for 60 min, the MC of CF decreased from 17.4% to below 15%, fulfilling the long-period storage requirements for CF.

CONCLUSION

Ozone is potentially applicable in effectively degrading the AFs in CF and in greatly decreasing the MC of CF.

Inhibition of bacterial and filamentous fungal growth in high moisture, nonsterile corn with intermittent pumping of trans-2-hexenal vapor

Trans-2-hexenal (T2H), a plant-produced aldehyde, was intermittently pumped over a 7 d period into a small, bench top model of stored corn (nonsterile, moisture content about 23%). Naturally occurring bacteria and fungi, including added Aspergillus flavus, grew rapidly on corn not treated with T2H vapor. However, intermittently pumped T2H (30 min per 2 h or 30 min per 12 h) significantly reduced bacterial and fungal viable populations, with nearly 100% fungal viability loss observed after either (1) one day of pumping at the 30 min per 2 h rate or (2) pumping cycles of 30 min per 12 h period over the initial 48 to 72 h of incubation. Data suggest that short-term intermittent fumigation of stored corn with T2H could prevent growth of bacteria and mycotoxigenic fungi such as A. flavus.

Breeding aflatoxin-resistant maize lines using recent advances in technologies - a review

Aflatoxin contamination caused by Aspergillus flavus infection of corn is a significant and chronic threat to corn being used as food or feed. Contamination of crops at levels of 20 ng g(-1) or higher (as regulated by the USFDA) by this toxin and potent carcinogen makes the crop unsalable, resulting in a significant economic burden on the producer. This review focuses on elimination of this contamination in corn which is a major US crop and the basis of many products. Corn is also "nature's example" of a crop containing heritable resistance to aflatoxin contamination, thereby serving as a model for achieving resistance to aflatoxin contamination in other crops as well. This crop is the largest production grain crop worldwide, providing food for billions of people and livestock and critical feedstock for production of biofuels. In 2011, the economic value of the US corn crop was US$76 billion, with US growers producing an estimated 12 billion bushels, more than one-third of the world's supply. Thus, the economics and significance of corn as a food crop and the threat to food safety due to aflatoxin contamination of this major food crop have prompted the many research efforts in many parts of the world to identify resistance in corn to aflatoxin contamination. Plant breeding and varietal selection has been used as a tool to develop varieties resistance to disease. This methodology has been employed in defining a few corn lines that show resistance to A. flavus invasion; however, no commercial lines have been marketed. With the new tools of proteomics and genomics, identification of resistance mechanisms, and rapid resistance marker selection methodologies, there is an increasing possibility of finding significant resistance in corn, and in understanding the mechanism of this resistance.

Evaluation of resistance to aflatoxin contamination in kernels of maize genotypes using a GFP-expressing Aspergillus flavus strain

Resistance or susceptibility of maize inbreds to infection by Aspergillus flavus was evaluated by the kernel screening assay. A green fluorescent protein-expressing strain of A. flavus was used to measure fungal spread and aflatoxin levels in real-time following fungal infection of kernels. Among the four inbreds tested, MI82 showed the most resistance and Ga209 the least. TZAR101 was also resistant to fungal infection, whereas Va35 was susceptible to fungal infection. However, Va35 produced lower aflatoxin levels compared to the susceptible line Ga209. Fluorescence microscopy indicated that the site of entry of the fungus into the kernel was consistently through the pedicel. Entry through the pericarp was never observed in undamaged kernels. In view of these results, incorporation or overexpression of antifungal proteins should be targeted to the pedicel and basal endosperm region in developing kernels. Once the fungus has entered through the pedicel, it spreads quickly through the open spaces between the pericarp and the aleurone layer, ultimately colonising the endosperm and scutellum and, finally, the embryo. A clear correlation was established between fungal fluorescence and aflatoxin levels. This method provides a quick, reliable means of evaluating resistance to A. flavus in undamaged kernels and provides breeders with a rapid method to evaluate maize germplasm.

Current understanding on aflatoxin biosynthesis and future perspective in reducing aflatoxin contamination

Traditional molecular techniques have been used in research in discovering the genes and enzymes that are involved in aflatoxin formation and genetic regulation. We cloned most, if not all, of the aflatoxin pathway genes. A consensus gene cluster for aflatoxin biosynthesis was discovered in 2005. The factors that affect aflatoxin formation have been studied. In this report, the author summarized the current status of research progress and future possibilities that may be used for solving aflatoxin contamination.

Molecular phylogeny and phenotypic variability of clinical and environmental strains of Aspergillus flavus

Aspergillus flavus is the second most common cause of aspergillosis infection in immunocompromised patients and is responsible for the production of aflatoxins. Little is known about the population structure of A. flavus, although recent molecular and phenotypic data seem to demonstrate that different genetic lineages exist within this species. The aim of this study was to carry out a morphological, physiological, and molecular analysis of a set of clinical and environmental isolates to determine whether this variability is due to species divergence or intraspecific diversity, and to assess whether the clinical isolates form a separate group. The amdS and omtA genes were more phylogenetically informative than the other tested genes and their combined analysis inferred three main clades, with no clear distinction between clinical and environmental isolates. No important morphological and physiological differences were found between the members of the different clades, with the exception of the assimilation of d-glucosamine, which differentiates the members of the clade II from the others.

Characterization of expressed sequence tag-derived simple sequence repeat markers for Aspergillus flavus: emphasis on variability of isolates from the southern United States

Simple sequence repeat (SSR) markers were developed from Aspergillus flavus expressed sequence tag (EST) database to conduct an analysis of genetic relationships of Aspergillus isolates from numerous host species and geographical regions, but primarily from the United States. Twenty-nine primers were designed from 362 tri-nucleotide EST-SSR sequences. Eighteen polymorphic loci were used to genotype 96 Aspergillus species isolates. The number of alleles detected per locus ranged from 2 to 24 with a mean of 8.2 alleles. Haploid diversity ranged from 0.28 to 0.91. Genetic distance matrix was used to perform principal coordinates analysis (PCA) and to generate dendrograms using unweighted pair group method with arithmetic mean (UPGMA). Two principal coordinates explained more than 75 % of the total variation among the isolates. One clade was identified for A. flavus isolates (n = 87) with the other Aspergillus species (n = 7) using PCA, but five distinct clusters were present when the others taxa were excluded from the analysis. Six groups were noted when the EST-SSR data were compared using UPGMA. However, the latter PCA or UPGMA comparison resulted in no direct associations with host species, geographical region or aflatoxin production. Furthermore, there was no direct correlation to visible morphological features such as sclerotial types. The isolates from Mississippi Delta region, which contained the largest percentage of isolates, did not show any unusual clustering except for isolates K32, K55, and 199. Further studies of these three isolates are warranted to evaluate their pathogenicity, aflatoxin production potential, additional gene sequences (e.g., RPB2), and morphological comparisons.

Tissue-specific components of resistance to Aspergillus ear rot of maize

Aspergillus flavus and other Aspergillus spp. infect maize and produce aflatoxins. An important control measure is the use of resistant maize hybrids. There are several reports of maize lines that are resistant to aflatoxin accumulation but the mechanisms of resistance remain unknown. To gain a better understanding of resistance, we dissected the phenotype into 10 components: 4 pertaining to the response of silk, 4 pertaining to the response of developing kernels, and 2 pertaining to the response of mature kernels to inoculation with A. flavus. In order to challenge different tissues and to evaluate multiple components of resistance, various inoculation methods were used in experiments in vitro and under field conditions on a panel of diverse maize inbred lines over 3 years. As is typical for this trait, significant genotype-environment interactions were found for all the components of resistance studied. There was, however, significant variation in maize germplasm for susceptibility to silk and kernel colonization by A. flavus as measured in field assays. Resistance to silk colonization has not previously been reported. A significant correlation of resistance to aflatoxin accumulation with flowering time and kernel composition traits (fiber, ash, carbohydrate, and seed weight) was detected. In addition, correlation analyses with data available in the literature indicated that lines that flower later in the season tend to be more resistant. We were not able to demonstrate that components identified in vitro were associated with reduced aflatoxin accumulation in the field.

Volatile trans-2-hexenal, a soybean aldehyde, inhibits Aspergillus flavus growth and aflatoxin production in corn

Trans-2-hexenal, a volatile aldehyde, is produced by soybean (Glycine max [L.] Merr) and other plants via the lipoxygenase pathway. In vitro tests showed it significantly (P < 0.001) reduced Aspergillus flavus germinating conidial viability at 10 μM, with approximately 95% viability reduction observed at 20 μM. The viability of nongerminated conidia was not reduced. To test the effectiveness of this volatile to prevent fungal growth in stored corn, trans-2-hexenal was pumped intermittently into glass jars containing corn. Experiments were performed to determine the ability of 2 different pump cycle time-courses to prevent A. flavus growth on sterile corn (23% moisture). Intermittently (30-min pumping period) over 7 d, this volatile was pumped through 350 g of corn kernels inoculated with 1 mL of 3 × 10⁴ conidia of A. flavus. Controls consisted of (1) sterile corn, (2) corn inoculated with A. flavus with no pumped air, and (3) corn inoculated with A. flavus with intermittently pumped air. Aflatoxin B₁ (AFB₁), viability counts, and aldehyde concentration in the headspace were performed in each experiment. To determine whether an increased time period between volatile pumping would prevent A. flavus growth, a 2nd series of experiments were performed that were similar to the 1st series except that trans-2-hexenal (only) was pumped for a 30-min period every 12 h. Experiments were performed 3 times for each time course. Both experiments showed that intermittent pumping of volatile trans-2-hexenal significantly (P < 0.001) prevented A. flavus growth and aflatoxin B₁ production over a 7-d period.

PRACTICAL APPLICATION

Results from this study indicate that intermittent pumping of volatile trans-2-hexenal could be used to protect stored corn from A. flavus growth and aflatoxin contamination.

Identification of Atoxigenic Aspergillus flavus Isolates to Reduce Aflatoxin Contamination of Maize in Kenya

Aspergillus flavus has two morphotypes, the S strain and the L strain, that differ in aflatoxin-producing ability and other characteristics. Fungal communities on maize dominated by the S strain of A. flavus have repeatedly been associated with acute aflatoxin poisonings in Kenya, where management tools to reduce aflatoxin levels in maize are needed urgently. A. flavus isolates (n = 290) originating from maize produced in Kenya and belonging to the L strain morphotype were tested for aflatoxin-producing potential. A total of 96 atoxigenic isolates was identified from four provinces sampled. The 96 atoxigenic isolates were placed into 53 vegetative compatibility groups (VCGs) through complementation of nitrate non-utilizing mutants. Isolates from each of 11 VCGs were obtained from more than one maize sample, isolates from 10 of the VCGs were detected in multiple districts, and isolates of four VCGs were found in multiple provinces. Atoxigenic isolates were tested for potential to reduce aflatoxin concentrations in viable maize kernels that were co-inoculated with highly toxigenic S strain isolates. The 12 most effective isolates reduced aflatoxin levels by >80%. Reductions in aflatoxin levels caused by the most effective Kenyan isolates were comparable with those achieved with a United States isolate (NRRL-21882) used commercially for aflatoxin management. This study identified atoxigenic isolates of A. flavus with potential value for biological control within highly toxic Aspergillus communities associated with maize production in Kenya. These atoxigenic isolates have potential value in mitigating aflatoxin outbreaks in Kenya, and should be evaluated under field conditions.

Ear secondary traits related to aflatoxin accumulation in commercial maize hybrids under artificial field inoculation

Aims of the research were: (1) to evaluate and compare 24 maize hybrids for Aspergillus flavus resistance and for aflatoxin accumulation under artificial inoculation in field experiments grown during 2005 and 2006; (2) to estimate the relationship of aflatoxin concentration with ear secondary traits. Primary ears were inoculated with a fresh spore suspension (mixture of five A. flavus isolates from Northern Italy), by spraying silks, as a modification of the non-wounding silk channel inoculation technique (SCIA); controls were both non-inoculated and sterile water-inoculated ears. Ear secondary traits, such as silk channel length measured at pollination and husk coverage at maturity, were recorded for each hybrid. The severity of ear A. flavus attack was estimated using rating scales based on the percentage of kernels with visible symptoms of infection. The aflatoxin concentration in the inoculated ears resulted, during both years, higher than in the controls; this indication confirmed that the A. flavus isolates used for the inoculum procedure were successful in accumulating mycotoxin in grains. Variability was found among the hybrids under study: aflatoxin accumulation, after artificial inoculation, ranged from 0.13 to 705.25 ng/g. The data herein presented supported the implication of two ear secondary traits in determining aflatoxin accumulation. Silk channel length recorded at pollination was negatively correlated (r = -0.54; P<0.05) with aflatoxin accumulation; on the contrary, a positive correlation (r = 0.48; P<0.05) between husk coverage rating at maturity and aflatoxin concentration suggested that a looser husk coverage is associated with higher aflatoxin accumulation. The correlation between the two mentioned ear-related traits was negative (r = -0.73; P<0.05); this indicated that hybrids showing a good coverage at pollination stage, are favoured in keeping the ear tip covered until maturity, reducing the risk of aflatoxin accumulation.

Costs and efficacy of public health interventions to reduce aflatoxin-induced human disease

This study reviews available information on the economics and efficacy of aflatoxin risk-reduction interventions, and it provides an approach for analysis of the cost-effectiveness of public health interventions to reduce aflatoxin-induced human disease. Many strategies have been developed to reduce aflatoxin or its adverse effects in the body. However, a question that has been under-addressed is how likely these strategies will be adopted in the countries that need them most to improve public health. This study evaluates two aspects crucial to the adoption of new technologies and methods: the costs and the efficacy of different strategies. First, different aflatoxin risk-reduction strategies are described and categorized into pre-harvest, post-harvest, dietary, and clinical settings. Relevant data on the costs and efficacy of each strategy, in reducing either aflatoxin in food or its metabolites in the body are then compiled and discussed. In addition, we describe which crops are affected by each intervention, who is likely to pay for the control strategy, and who is likely to benefit. A framework is described for how to evaluate cost-effectiveness of strategies according to World Health Organization (WHO) standards. Finally, it is discussed which strategies are likely to be cost-effective and helpful under different conditions worldwide of regulations, local produce and soil ecology, and potential health emergencies.

Discovery and characterization of proteins associated with aflatoxin-resistance: evaluating their potential as breeding markers

Host resistance has become a viable approach to eliminating aflatoxin contamination of maize since the discovery of several maize lines with natural resistance. However, to derive commercial benefit from this resistance and develop lines that can aid growers, markers need to be identified to facilitate the transfer of resistance into commercially useful genetic backgrounds without transfer of unwanted traits. To accomplish this, research efforts have focused on the identification of kernel resistance-associated proteins (RAPs) including the employment of comparative proteomics to investigate closely-related maize lines that vary in aflatoxin accumulation. RAPs have been identified and several further characterized through physiological and biochemical investigations to determine their causal role in resistance and, therefore, their suitability as breeding markers. Three RAPs, a 14 kDa trypsin inhibitor, pathogenesis-related protein 10 and glyoxalase I are being investigated using RNAi gene silencing and plant transformation. Several resistant lines have been subjected to QTL mapping to identify loci associated with the aflatoxin-resistance phenotype. Results of proteome and characterization studies are discussed.

Variation in competitive ability among isolates of Aspergillus flavus from different vegetative compatibility groups during maize infection

ABSTRACT Aspergillus flavus, the primary causal agent of aflatoxin contamination, includes many genetically diverse vegetative compatibility groups (VCGs). Competitive ability during infection of living maize kernels was quantified for isolates from 38 VCGs. Kernels were inoculated with both a common VCG, CG136, and another VCG; after 7 days (31 degrees C), conidia were washed from kernels, and aflatoxins and DNA were extracted from kernels and conidia separately. CG136-specific single-nucleotide polymorphisms were quantified by pyrosequencing; VCGs co-inoculated with CG136 produced 46 to 85 and 51 to 84% of A. flavus DNA from kernels and conidia, respectively. Co-inoculation with atoxigenic isolates reduced aflatoxin up to 90% and, in some cases, more than predicted by competitive exclusion alone. Conidia contained up to 42 ppm aflatoxin B(1), indicating airborne conidia as potentially important sources of environmental exposure. Aflatoxin-producing potential and sporulation were negatively correlated. For some VCGs, sporulation during co-infection was greater than that predicted by kernel infection, suggesting that some VCGs increase dispersal while sacrificing competitive ability during host tissue colonization. The results indicate both life strategy and adaptive differences among A. flavus isolates and provide a basis for selection of biocontrol strains with improved competitive ability, sporulation, and aflatoxin reduction on target hosts.

Increased aflatoxin contamination of dried figs in a drought year

Dried figs (4917 samples) destined for export from Turkey to the European Union were collected between September and December during the very dry crop year of 2007 and tested for aflatoxins B(1), B(2), G(1) and G(2) by immunoaffinity column clean-up and reverse-phase high-performance liquid chromatography (RP-HPLC). While 32% of the samples contained detectable levels of total aflatoxins, 9.8% of them exceeded the European Union limits. Aflatoxin levels were in the range of 0.2-259.46 microg kg(-1) and 2.04-259.46 microg kg(-1) for all samples and samples that exceeded the limits, respectively. A substantial increase in the incidence of aflatoxins was observed in 2007 compared with previous years, most likely due to the drought stress, high temperatures and low relative humidity encountered during the period from January to September of that year. In 2007, the mean temperature was 1-2 degrees C higher, there was 300 mm less total rain, and the mean relative humidity was 10-15% lower than in 2002-06. The average concentration of individual aflatoxins present in the samples was quantified to determine whether the drought conditions promoted certain types of aflatoxins. Among the contaminated samples, aflatoxin B(1) occurred in 97% of the contaminated samples, followed by G(1) in 47%, B(2) in 24%, and G(2) in 6% of samples. Concentrations of individual aflatoxins exhibited great variability among the samples but were not significantly different from those reported in previous studies, which were conducted under conditions without drought and high temperatures.

Identifying Aflatoxin Resistance-related Proteins/Genes through Proteomics and RNAi Gene Silencing1

Aflatoxins are carcinogenic secondary metabolites produced mainly by Aspergillus flavus Link ex. Fries, and A. prarasiticus Speare during infection of susceptible crops, such as maize, cottonseed, peanuts and tree nuts. This paper will review research efforts in identifying aflatoxin resistance-related proteins/genes in maize. Similar strategies may be useful in peanut. For maize, although genotypes resistant to A. flavus infection or aflatoxin production have been identified, the incorporation of resistance into commercial lines has been slow due to the lack of selectable markers and poor understanding of host resistance mechanisms. Recently, resistance-associated proteins (RAPs) were identified through proteomic comparison of constitutive protein profiles between resistant and susceptible maize genotypes. These proteins belong to three major groups based on their peptide sequence homologies: storage proteins, stress-related proteins, and antifungal proteins. Preliminary characterization of some of these RAPs suggest that they play a direct role in host resistance, such as pathogenesis-related protein 10 (PR10), or an indirect role, such as glyoxalase I (GLX I), through enhancing the host stress tolerance. To verify whether these RAPs play a role in host resistance, RNA interference (RNAi) gene silencing technique was used to silence the expression of these genes in maize. RNAi vectors (glx I RNAi and pr10 RNAi) were constructed using Gateway technology, and then transformed into immature maize embryos using both bombardment and Agrobacterium infection. The extent of gene silencing in transgenic callus tissues ranged from 20% to over 99%. The RNAi silenced transgenic maize seeds have also been obtained from plants regenerated from Agrobacterium transformed callus lines. Kernel screen assay of the transgenic maize kernels demonstrated a significant increase in susceptibility to A. flavus colonization and aflatoxin production in some of the silenced transgenic lines compared with non-silenced control kernels, suggesting the direct involvement of these two proteins in aflatoxin resistance in maize.