Antifungal Activity in Transgenic Peanut (Arachis hypogaea L.) Conferred by a Nonheme Chloroperoxidase Gene

A Critical Review of Aflatoxin Contamination of Peanuts in Malawi and Zambia: The Past, Present, and Future

Peanut (Arachis hypogaea L.) is an important crop in Malawi and Zambia. The crop is valued for soil improvement in cereal-based cropping systems, for improving the livelihoods of farming households who consume it and also sell it for cash, and for earning foreign exchange when exported. Research and development efforts have resulted in an increase in both peanut production area and productivity. However, a key challenge that still needs to be solved in these countries is how to produce peanuts with acceptable levels of aflatoxin contamination. Data continues to show that aflatoxin continues to be a problem in both formal and informal trade. As a result, unlike 30 years ago, most of the peanut trade has now shifted to domestic and regional markets that do not restrict the sale of aflatoxin-contaminated peanuts. Impacts of aflatoxin contamination on health and also on the full cost burden of control are not well documented. Technologies are available for mitigating against aflatoxin contamination. The advantages, disadvantages, and gaps associated with these technologies are discussed. Considerable money and effort continues to be invested in Malawi and Zambia into mitigating aflatoxin contamination, but evidence of long-term success is limited. Based on past and current initiatives, the prospects of eliminating aflatoxin in the near future at the household level and in trade are not promising.

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.

Progress in genetic engineering of peanut (Arachis hypogaea L.)--a review

Peanut (Arachis hypogaea L.) is a major species of the family, Leguminosae, and economically important not only for vegetable oil but as a source of proteins, minerals and vitamins. It is widely grown in the semi-arid tropics and plays a role in the world agricultural economy. Peanut production and productivity is constrained by several biotic (insect pests and diseases) and abiotic (drought, salinity, water logging and temperature aberrations) stresses, as a result of which crop experiences serious economic losses. Genetic engineering techniques such as Agrobacterium tumefaciens and DNA-bombardment-mediated transformation are used as powerful tools to complement conventional breeding and expedite peanut improvement by the introduction of agronomically useful traits in high-yield background. Resistance to several fungal, virus and insect pest have been achieved through variety of approaches ranging from gene coding for cell wall component, pathogenesis-related proteins, oxalate oxidase, bacterial chloroperoxidase, coat proteins, RNA interference, crystal proteins etc. To develop transgenic plants withstanding major abiotic stresses, genes coding transcription factors for drought and salinity, cytokinin biosynthesis, nucleic acid processing, ion antiporter and human antiapoptotic have been used. Moreover, peanut has also been used in vaccine production for the control of several animal diseases. In addition to above, this study also presents a comprehensive account on the influence of some important factors on peanut genetic engineering. Future research thrusts not only suggest the use of different approaches for higher expression of transgene(s) but also provide a way forward for the improvement of crops.

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.

Expression of chloroperoxidase from Pseudomonas pyrrocinia in tobacco plastids for fungal resistance

The chloroperoxidase (cpo) gene from Pseudomonas pyrrocinia was transformed into the plastid genome (plastome) of Nicotiana tabacum var. Petit Havana and transplastomic lines were compared with a nuclear transformant for the same gene. Southern analysis confirmed integration in the plastome and western blotting confirmed the presence of the chloroperoxidase protein (CPO) in higher abundance in transplastomic plants than in cpo nuclear transformants. Northern analysis of primary plastome transformants for cpo showed 15-fold higher transcript abundance than in the nuclear transformant, yet this extent of enhancement was not observed in western blot, enzyme or bioassay, indicating a bottleneck at the post-transcriptional level. Representative plants from the two transplastomic lines showed resistance to fungal pathogens in vitro (Aspergillus flavus, Fusarium verticillioides, and Verticillium dahliae) and in planta (Alternaria alternata).

Increased resistance to late leaf spot disease in transgenic peanut using a combination of PR genes

Peanut (Arachis hypogaea L.) is the sixth most important oil seed crop in the world. Yield loss due to Cercospora leaf spot (early and late leaf spots) is a serious problem in cultivating this crop. Non-availability of resistant genes within crossable germplasms of peanut necessitates the use of a genetic engineering strategy to develop genetic resistance against various biotic stresses. The pathogenesis-related (PR) proteins are a group of plant proteins that are toxic to invading fungal pathogens, but are present in trace amounts in plants. The PR proteins, PR-5 and defensins, are potent antifungal proteins. A double gene construct with SniOLP (Solanum nigrum osmotin-like protein) and Rs-AFP2 (Raphanus sativus antifungal protein-2) genes under separate constitutive 35S promoters was used to transform peanut plants. Transgenic peanut plants expressing the SniOLP and Rs-AFP2 genes showed enhanced disease resistance to late leaf spot based on a reduction in number and size of lesions on leaves and delay in the onset of Phaeoisariopsis personata leaf spot disease. PCR, RT-PCR, and Southern hybridization analyses confirmed stable integration and expression of these genes in peanut transgenics. The results demonstrate the potential of SniOLP and Rs-AFP2 genes in developing late leaf spot disease resistance in transgenic peanut.

Developing resistance to aflatoxin in maize and cottonseed

At this time, no "magic bullet" for solving the aflatoxin contamination problem in maize and cottonseed has been identified, so several strategies must be utilized simultaneously to ensure a healthy crop, free of aflatoxins. The most widely explored strategy for the control of aflatoxin contamination is the development of preharvest host resistance. This is because A. flavus infects and produces aflatoxins in susceptible crops prior to harvest. In maize production, the host resistance strategy has gained prominence because of advances in the identification of natural resistance traits. However, native resistance in maize to aflatoxin contamination is polygenic and complex and, therefore, markers need to be identified to facilitate the transfer of resistance traits into agronomically viable genetic backgrounds while limiting the transfer of undesirable traits. Unlike maize, there are no known cotton varieties that demonstrate enhanced resistance to A. flavus infection and aflatoxin contamination. For this reason, transgenic approaches are being undertaken in cotton that utilize genes encoding antifungal/anti-aflatoxin factors from maize and other sources to counter fungal infection and toxin production. This review will present information on preharvest control strategies that utilize both breeding and native resistance identification approaches in maize as well as transgenic approaches in cotton.