Inhibition of Aspergillus flavus Growth and Aflatoxin Production in Transgenic Maize Expressing the α-amylase Inhibitor from Lablab purpureus L.

Summary

Here we present a protocol to analyze Aspergillus flavus growth and aflatoxin production in maize kernels expressing an antifungal protein.  Using a GFP-expressing A. flavus strain we monitored the infection and spread of the fungus in mature kernels in real time. The assay is rapid, reliable, and reproducible.

Abstract

Aflatoxin contamination in food and feed crops is a major challenge worldwide. Aflatoxins, produced by the fungus Aspergillus flavus (A. flavus) are potent carcinogens that substantially reduce crop value in maize and other oil rich crops like peanut besides posing serious threat to human and animal health. Different approaches, including traditional breeding, transgenic expression of resistance associated proteins, and RNA interference (RNAi)-based host-induced gene silencing of critical A. flavus gene targets, are being evaluated to increase aflatoxin resistance in susceptible crops. Past studies have shown an important role of α-amylase in A. flavus pathogenesis and aflatoxin production, suggesting this gene/enzyme is a potential target to reduce both A. flavus growth and aflatoxin production. In this regard, the current study was undertaken to evaluate heterologous expression (under control of the constitutive CaMV 35S promoter) of a Lablab purpureus L. α-amylase inhibitor-like protein (AILP) in maize against A. flavus. AILP is a 36-kDa protein, which is a competitive inhibitor of A. flavus α-amylase enzyme and belongs to the lectin–arcelin–α-amylase inhibitor protein family in common bean. In vitro studies prior to the current work had demonstrated the role of AILP in inhibition of A. flavus α-amylase activity and fungal growth. Fungal growth and aflatoxin production in mature kernels were monitored in real time using a GFP-expressing A. flavus strain. This kernel screening assay (KSA) is very simple to set up and provides reliable and reproducible data on infection and the extent of spread that could be quantified for evaluation of germplasm and transgenic lines. The fluorescence from the GFP strain is closely correlated to fungal growth and, by extension, it is well-correlated to aflatoxin values.  The goal of the current work was to implement this previous knowledge in a commercially important crop like maize to increase aflatoxin resistance. Our results show a 35%–72% reduction in A. flavus growth in AILP-expressing transgenic maize kernels which, in turn, translated into a 62%–88% reduction in aflatoxin levels.

Introduction

Mycotoxin contamination by the fungal genera, Aspergillus, Fusarium, Penicillium, and Alternaria is a major problem of food and feed crops grown worldwide^1,2,3. Among these phytopathogenic fungi, Aspergillus has the highest adverse impact on crop value and human and animal health. Aspergillus flavus (A. flavus) is an opportunistic plant pathogen that infects oil rich crops such as maize, cottonseed and peanut and produces the potent carcinogens, aflatoxins, as well as numerous toxic secondary metabolites (SMs). Ma....

Protocol

1. Plasmid constructs and maize transformation

1. PCR amplify Lablab AILP insert using the primers 5’-TATCTAGAACTAGTGATTACCATGGCTCC-3’ and 5'-ATACTGCAGGATTGCATGCAGAGTAGTACTG-3'. The PCR conditions include an initial denaturation step at 98 °C for 30 s (step 1), followed by denaturation at 98 °C for 10 s (step 2), annealing at 55 °C for 30 s (step 3), elongation at 72 °C for 20 s (step 4), 31 cycles of step 2 to step 4, and a final elongation step at 72 °C for 5 min. Clone the PCR product into a modified pCAMBIA 1,300 vector using XbaI and PstI restriction sites. Sequence the final plant d....

Results

Maize transformation and molecular screening of transgenic plants

Immature embryos of maize Hi-II lines were transformed using Agrobacterium tumefaciens EHA101 strain containing the final plant destination vector expressing the Lablab purpureus AILP gene under the control of CaMV 35S promoter. Five independently transformed maize lines were advanced to the T6 generation for su.......

Discussion

Yield losses in agricultural crops due to pathogens and pests is a global problem²⁰. Currently, application of synthetic fungicides and pesticides is the predominant means for controlling plant pathogens and pests, but residual toxicity of these biochemicals in food and feed can pose serious threat to human and animal health²¹. Considering the economic importance of maize as a food and feed crop, reduction in or elimination of aflatoxin contamination is of utmost importance.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Agar	Caisson
Amazing Marine Goop	Eclectic Products
C1000 Touch CFX96 Real-Time System	Bio-Rad
Corning Falcon Tissue Culture Dishes, 60 mm	Fisher Scientific	08-772F
Eppendorf 5424 Microcentrifuge	Fisher Scientific
Erlenmeyer flask with stopper, 50 mL	Ace Glass	6999-10
Ethanol
FluoroQuant Afla	Romer Labs	COKFA1010
Fluted Qualitative Filter Paper Circles, 15 cm	Fisher Scientific	09-790-14E
Force Air Oven	VWR
FQ-Reader	Romer Labs	EQFFM3010
Geno/Grinder 2010	OPS Diagnostics	SP 2010-115
Improved PolyVinylAlcohol Organic Solvent Resistant Gloves	Ansell	012-15-554
Innova 44 Incubator Shaker	Brunswick Scientific
iScript cDNA Synthesis Kit	Bio-Rad	1708890
liquid Nitrogen
Low Form Griffin Beakers, 100 mL	DKW Life Sciences	14000-100
Methanol
Methylene Chloride
Nexttec 1-step DNA Isolation Kit for Plants	Nexttec	47N
Nikon Eclipse E600 microscope with Nikon DS-Qi1 camera	Nikon
Nikon SMZ25 stereomicroscope with C-HGFI Episcopic Illuminator and Andor Zyla 4.2 sCMOS camera	Nikon
Nunc Square BioAssay Dishes	ThermoFisher Scientific	240835
Phire Plant Direct PCR Kit	ThermoFisher Scientific	F130WH
Polycarbonate Vials, 15 ml	OPS Diagnostics	PCRV 15-100-23
Potato Dextrose Broth
Snap Cap, 22 mm	DKW Life Sciences	242612
Sodium Phosphate dibasic heptahydrate	Sigma-Aldrich
Sodium Phosphate monobasic	Sigma-Aldrich
Spectrum Plant Total RNA Kit	Sigma-Aldrich	STRN50
Stainless Steel Grinding Balls, 3/8''	OPS Diagnostics	GBSS 375-1000-02
Stir Plate
Synergy 4 Fluorometer	Biotek
T100 Thermal Cycler	Bio-Rad
Triton X-100	Sigma-Aldrich	T-9284
V8 juice	Campbell's
Whatman Qualitative Grade Plain Sheets, Grade 3	Fisher Scientific	09-820P
Wrist-Action Shaker	Burrell Scientific