Cloning and functional expression of the gene encoding an inhibitor against Aspergillus flavus alpha-amylase, a novel seed lectin from Lablab purpureus (Dolichos lablab)

Natural inhibitors: A sustainable way to combat aflatoxins

Among a few hundred mycotoxins, aflatoxins had always posed a major threat to the world. Apart from A. flavus, A. parasiticus, and A. nomius of Aspergillus genus, which are most toxin-producing strains, several fungal bodies including Fusarium, Penicillium, and Alternaria that can biosynthesis aflatoxins. Basically, there are four different types of aflatoxins (Aflatoxin B1 (AFB1), Aflatoxin B2 (AFB2), Aflatoxin G1 (AFG1), Aflatoxin G2 (AFG2)) are produced as secondary metabolites. There are certainly other types of aflatoxins found but they are the by-products of these toxins. The fungal agents generally infect the food crops during harvesting, storing, and/or transporting; making a heavy post-harvest as well as economic loss in both developed and developing countries. And while ingesting the crop products, these toxins get into the dietary system causing aflatoxicosis, liver cirrhosis, etc. Therefore, it is imperative to search for certain ways to control the spread of infections and/or production of these toxins which may also not harm the crop harvest. In this review, we are going to discuss some sustainable methods that can effectively control the spread of infection and inhibit the biosynthesis of aflatoxins.

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.

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.

Expression of frutalin, an alpha-D-galactose-binding jacalin-related lectin, in the yeast Pichia pastoris

Frutalin is an alpha-D-galactose-binding lectin expressed in breadfruit seeds. Its isolation from plant is time-consuming and results in a heterogeneous mixture of different lectin isoforms. In order to improve and facilitate the availability of the breadfruit lectin, we cloned an optimised codifying frutalin mature sequence into the pPICZalphaA expression vector. This expression vector, designed for protein expression in the methylotrophic yeast Pichia pastoris, contains the Saccharomyces alpha-factor preprosequence to direct recombinant proteins into the secretory pathway. Soluble recombinant frutalin was detected in the culture supernatants and recognised by native frutalin antibody. Approximately 18-20 mg of recombinant lectin per litre medium was obtained from a typical small scale methanol-induced culture purified by size-exclusion chromatography. SDS-PAGE and Edman degradation analysis revealed that frutalin was expressed as a single chain protein since the four amino-acid linker peptide "T-S-S-N", which connects alpha and beta chains, was not cleaved. In addition, incomplete processing of the signal sequence resulted in recombinant frutalin with one Glu-Ala N-terminal repeat derived from the alpha-factor prosequence. Endoglycosidase treatment and SDS-PAGE analysis revealed that the recombinant frutalin was partly N-glycosylated. Further characterisation of the recombinant lectin revealed that it specifically binds to the monosaccharide Me-alpha-galactose presenting, nevertheless, lesser affinity than the native frutalin. Recombinant frutalin eluted from a size-exclusion chromatography column with a molecular mass of about 62-64 kDa, suggesting a tetrameric structure, however it did not agglutinate rabbit erythrocytes as native frutalin does. This work shows that the galactose-binding jacalin-related lectins four amino-acid linker peptide "T-S-S-N" does not undergo any proteolytic cleavage in the yeast P. pastoris and also that linker cleavage might not be essential for lectin sugar specificity.