2012 Annual Report
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.
Trichothecenes are fungal toxins that block protein synthesis and are harmful to human and animal health. Trichothecenes are classified as Type A or B based on differences in their chemical structures with A trichothecenes being more toxic to animals. The toxins produced by a selection of Fusarium species were characterized and two genes were sequenced to determine how changes in these genes affect the type of toxin produced. We found B trichothecenes in four genetically distinct groups of Fusarium, including one previously thought to produce only A trichothecenes. These findings advance efforts to predict the toxin potential of different Fusarium species and provide valuable information on the relative risk that Fusarium species pose to food safety. Contamination of wheat with the B trichothecene deoxynivalenol (DON) is a food safety problem worldwide. Trichothecene contamination results from infection with Fusarium graminearum, a fungus that causes head scab. In the United States and Canada, the predominant Fusarium graminearum strains causing wheat head scab produces 15-acetyl DON in culture. Recent surveys indicate that new Fusarium strains that produce 3-acetyl DON are emerging in North America and may be more aggressive or result in higher amounts of toxin in contaminated grain. In order to determine if modifications in the trichothecene produced impact the severity of the disease or the amount of toxin produced in grain, we produced and chemically characterized sets of Fusarium graminearum mutant strains that differ only in the genes that control which toxins are produced. We tested the ability of these mutants to cause disease and produce toxin in greenhouse grown wheat plants. Our data demonstrate that the type of DON produced was not correlated with production of higher amounts of toxin, and suggested that toxin type alone was not responsible for differences in aggressiveness toward wheat. Information from these studies will be used to develop control strategies for these emerging strains, reduce DON contamination of wheat, and improve the safety of food and feed derived from cereals. A way to limit head scab and its associated toxin-contamination of grain is to introduce into plants genes that can detoxify the DON produced by Fusarium during infection and thereby limit the spread of the disease. We collaborated with University of Wisconsin researchers to characterize DON detoxification genes. In addition, we collaborated with scientists from project 3620-42000-043-00D to screen the ARS Culture Collection for microbes that can detoxify trichothecenes and found yeast that metabolize T-2 toxin, including yeast species that convert T-2 toxin into a sugar derivative. This sugar derivative has recently been found in contaminated grain but is considered a masked mycotoxin because it escapes detection with the analytical methods that are currently used to keep mycotoxins out of food and feed. The yeast system for efficiently producing trichothecene sugar derivatives will facilitate the development of new methods to detect masked mycotoxins and assessments of their toxicity to animals and role in plant-pathogen interactions.
Genetic control of toxins in emerging fungal pathogens of cereal crops. Fusarium head blight is a devastating disease of cereal crops. It is a concern for food and feed safety because the fungus causing the disease also contaminates the grain with a mycotoxin, deoxynivalenol (DON). In the U.S., most of the Fusarium strains that cause this disease make one type of DON. New Fusarium strains that produce a different form of DON, and higher amounts of the toxin, have been spreading across North America. Scientists in the Agricultural Research Service (ARS) Bacterial Foodborne Pathogens and Mycology Research Unit at the National Center for Agricultural Utilization Research in Peoria, IL, used modern molecular and genetic techniques to design mutant strains that differ only in the single gene that determines which form of DON is made. Greenhouse tests found that the form of DON produced was not correlated with the amount of toxin made in infected grain and that other factors need to be considered. This information is needed by food safety scientists and plant breeders to develop strategies to limit toxin contamination of wheat, and to maintain the quality and safety of food and feed derived from cereals.
Bin Umer, M.A., McLaughlin, J., Basu, D., McCormick, S.P., Tumer, N.E. 2011. Trichothecene mycotoxins inhibit mitochondrial translation - implication for the mechanism of toxicity. Toxins. 3(12):1484-1501.
Malmierca, M.G., Cardoza, R.E., Alexander, N.J., McCormick, S.P., Hermosa, R., Monte, E., Gutierrez, S. 2012. Involvement of Trichoderma trichothecenes in the biocontrol activity and in the induction of plant defense related genes. Applied and Environmental Microbiology. 78(14):4856-4868.