1a. Objectives (from AD-416)
The overall goal of this research project is to enhance food safety by developing methods to reduce levels of trichothecenes and other mycotoxins that occur in grain crops as a result of infection by Fusarium graminearum (sexual stage, Gibberella zeae) and related trichothecene-producing species of Fusarium. FHB is a world-wide threat to grain producers and consumers, due to the loss in yield and to the presence of trichothecenes and other mycotoxins in the grain. As the world’s population continues to increase, the need to reduce mycotoxins in grain will increase. Development of methods to reduce mycotoxin contamination in grain will be enhanced through elucidation of the molecular genetic mechanisms that control mycotoxin production in F. graminearum and related fusaria, that control plant-fungal interactions, and that detoxify or otherwise modify mycotoxins. The objectives and proposed research are as follows: Objective 1: Identify and characterize mycotoxin detoxification genes as a mechanism to reduce/eliminate the toxins in grain-based food and feed; Objective 2: Determine the genetic bases and ecological significance of variation in types of trichothecene mycotoxins produced by Fusarium; Objective 3: Identify and characterize plant genes that affect biosynthesis of trichothecenes and other mycotoxins produced by Fusarium.
1b. Approach (from AD-416)
With a growing world population, access to safe food for all consumers, both domestic and international, will continue to be a global priority. In recent years, the world has experienced an increase in mycotoxin contamination of grains due to climatic and agronomic changes that encourage fungal growth during cultivation. One approach to reduce mycotoxin contamination of food and feed is to prevent preharvest infection of crop plants by mycotoxin-producing fungi. An alternative approach is to modify mycotoxins present in crops in order to render them nontoxic and safe for consumption by humans and animals. Fusarium head blight (FHB) is one of the most important diseases of wheat and other cereal grains worldwide. It reduces yield and quality and results in contamination of grain with trichothecene mycotoxins. The disease is caused by Fusarium graminearum as well as other trichothecene-producing species of Fusarium. The primary goal of the proposed research is to reduce levels of trichothecenes and other mycotoxins through studies that reveal how plants, the fungus Fusarium, and mycotoxins interact during infection. We expect to identify novel genes that modify, detoxify, or otherwise confer resistance to mycotoxins and to study the physiological and molecular role of mycotoxin production on the ability of Fusarium to infect wheat and other crops. We also will examine the genetic bases and ecological significance of variation in types of mycotoxins produced by Fusarium. Knowledge from these studies will contribute to development of strategies to control FHB, thereby protecting our food supply from mycotoxins. This technology will ultimately benefit other scientists, small grain breeders, stakeholders in the food and feed industry, and regulatory agencies such as the Center for Disease Control, U.S. Food and Drug Administration, Federal Grain Inspection Service, and Food Safety Inspection Service.
3. Progress Report
Contamination of wheat with deoxynivalenol (DON) and related trichothecenes is a food safety problem worldwide. Trichothecenes inhibits protein synthesis and thereby cause multiple health problems in humans and animals. Trichothecene contamination results from infection of wheat with Fusarium graminearum, a fungus that causes head scab, one of the most destructive wheat diseases worldwide. Deoxynivalenol is toxic to plants and is a virulence factor in wheat head scab. We initiated a study to determine if another mycotoxin produced by Fusarium, culmorin, may work in concert with DON to increase the severity of Fusarium wheat head scab and thereby increase the contamination of wheat with mycotoxins. We have identified highly virulent Fusarium strains that make both DON and culmorin. We found that culmorin acts synergistically with DON in a model plant system test for phytotoxicity. Using molecular techniques, mutant strains have been generated that produce varying amounts of culmorin and/or DON. These strains are being tested in the greenhouse in order to assess the role of culmorin in wheat head scab and to measure DON and culmorin contamination of the grain. In the United States, the predominant Fusarium species causing wheat head scab produces the mycotoxins 15-acetyldeoxynivalenol (15-ADON) in culture. Recent surveys indicate that new Fusarium strains that produce the 3-acetyldeoxynivalenol (3-ADON) are emerging in North America and may be more aggressive. In collaboration with researchers at Agriculture and Agri-Food Canada, we completed a study to determine the levels of toxins in wheat infected with 3-ADON or 15-ADON strains and found more toxin from wheat infected with 15-ADON strains. We used modern genetic and rigorous chemical techniques to determine the genetic basis for 15-ADON and 3-ADON production and identified changes to a single mycotoxin biosynthetic gene that determined which toxin was produced. Using molecular techniques, sets of mutant strains were generated that differ only in whether 3-ADON or 15-ADON is produced. These strains are being tested on greenhouse grown wheat plants in order to assess if the specific toxin produced in culture has an impact on virulence. Although Fusarium strains produce 3-ADON or 15-ADON in culture, wheat infected with these strains is contaminated with DON. We initiated a search of expressed sequence tag (EST) databases for genes that are expressed during Fusarium infection of wheat and identified and ranked several to find genes that may control the conversion of 3-ADON or 15-ADON into DON. We initiated a study of microorganisms that can chemically modify and detoxify trichothecenes. Microorganisms from soil collected from a wheat field in Minnesota were grown in media containing DON or other mycotoxins. We found mixed cultures of aerobic microorganisms from this soil metabolized DON and converted it into a product that was less toxic in a model plant system test for phytotoxicity. We found that the mixed enriched cultures also metabolized culmorin and other trichothecenes.
1. Identification of a fungal metabolite that increases the toxicity of fungal toxins toward plants. Fusarium head blight is a devastating disease of cereal crops. The fungus causing the disease produces a trichothecene toxin, deoxynivalenol (DON), which is toxic to plants and helps the fungus spread in the plant tissue. DON is also toxic to humans and animals, and grain contaminated with DON is a significant food safety problem. Agricultural Research Service (ARS) Bacterial Foodborne Pathogens & Mycology Unit scientists at the National Center for Agricultural Utilization Research (NCAUR) in Peoria, IL, found that some Fusarium strains produce another metabolite, culmorin, that enhances the toxicity of DON to plants, and are using modern molecular and genetic techniques to determine if Fusarium strains that make both DON and culmorin are more virulent on cereal crops. This research produced an improved understanding of the methods used by Fusarium to invade plant tissue and contaminate grain with toxins, and provides new targets for disease control and toxin reduction programs aimed at ensuring the quality and safety of the U.S. food supply.