Project Number: 6435-41420-005-00
Project Type: Appropriated

Start Date: Dec 28, 2005
End Date: Dec 27, 2010

Objective:
(1) Understand the molecular basis for fungal responses to conditions encountered during invasion of crops in order to identify and modify the factors in corn and other crops that could induce or impede aflatoxin formation or fungal development. Accomplishment of objective 1 could lead to generic approaches to enhance resistance in all crops vulnerable to aflatoxin contamination. (2) Determine, by genetic and physiological studies of diverse aflatoxin producing species, whether aflatoxin production provides an adaptive advantage for fungal survival and invasion of crops, particularly because many natural isolates of A. flavus do not produce aflatoxins. (3) Determine the molecular responses of aflatoxin producing fungi to stress factors, particularly with regard to developing an understanding of the ability of the fungi to adapt and produce toxins. (4) Undertake and utilize newly available sequences of genomic DNA from Aspergillus species to develop rapid and highly sensitive PCR based tests to identify aflatoxigenic fungi and their toxins in contaminated food products.

Approach:
(1) Microarray experiments will be used to identify candidate genes in A. flavus that are turned on or off during a variety of environmental and nutritional conditions that are known to alter gene expression affecting aflatoxin biosynthesis. The DNA microarrays will exploit the available genomic resources of A. flavus ESTs and the whole genome sequences, combined with statistical analysis of up and down regulated signals on the chips. The candidate genes identified from microarrays (both A. flavus EST and whole genome) will be verified or confirmed through RT-Q- PCR or other well established methods. For further analysis, to identify specific genes, targeted gene mutagenesis will be necessary to determine their biological function. Comparison of the gene expression data under these aflatoxin-producing and non-producing conditions will allow us to identify the specific genes expressed under aflatoxin-producing conditions. (2) The adaptive advantage of aflatoxin production in certain environments will be determined by genetic and physiological studies. It will be ascertained if gene cluster imparts some fitness advantage in some environmental niches, particularly when aflatoxin production does not appear to be a virulence factor for crop infestation. Isogenic isolates will be developed in A. flavus for this experiment. Fungal viability will be measured for up to 12 months with these isolates. The measure of longevity proposed in the present study as a measure of fitness will be an important determinant for understanding aflatoxin production in Aspergillus populations in crop soils. Comparisons of genomic sequences will be made between toxigenic A. flavus found in agricultural fields and other Aspergillus species (non-toxigenic and domesticated) to determine what genes are involved in fungal virulence and toxin production of A. flavus, as well as its ability to survive in the field. (3) Stress cues that change the activity of proteins in the biosynthetic pathway and gene transcription will be evaluated. The chromatin structure of the gene cluster and adjacent regions will be studied after exposure to aflatoxin inducing conditions. Parameters such as DNA methylation patterns, that alter chromatin structure, will be explored in these adjacent regions to determine how aflatoxin biosynthesis is turned on in the fungus. Understanding environmental stress cues affecting aflatoxin gene expression may help to develop strategies to reduce aflatoxin contamination of corn under field conditions. It will be determined if the drought tolerant varieties have lower levels of aflatoxin contamination because the host-fungus interaction somehow alters the ability of the fungus to initiate toxin production at the DNA level. (4) The use of PCR to quantify the level of toxigenic fungi in foods through the use of Taqman primer-probe assay will be assessed to provide information on fungal bioburden, but not the level of aflatoxin contamination. The test will be designed to measure the potential of the commodity to become severely contaminated with the fungus (and consequently toxin) if stored under conditions suitable for subsequent growth of the fungus.