Enhancing Biotic Pest Resistance in Corn Germplasm

Corn germplasm will be evaluated to locate new sources of resistance to diseases and insects using field and laboratory screenings. Inbred lines and populations with resistance to Aspergillus flavus, southwestern corn borer, and fall armyworm will be developed. Inheritance of resistance will be investigated using quantitative and molecular genetic approaches. Mechanisms of resistance and the bases of resistance will be investigated in field tests and in laboratory bioassays.
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PROGRESS: 2000/10 TO 2001/09
<ol> <li>What major problem or issue is being resolved and how are you resolving it? The major problem being addressed is contamination of corn grain with aflatoxin. The problem is being resolved by identifying corn germplasm with resistance to infection by Aspergillus flavus, the fungus that produces aflatoxin, and/or resistance to the production of aflatoxin. When potentially useful sources of resistance are identified, germplasm lines are developed and released for use by plant breeders with public institutions and private companies in the production of corn hybrids that will then be available to corn producers. Gene mapping studies are conducted to identify chromosomal regions associated with resistance so that selection strategies using molecular markers can be developed for breeding for aflatoxin resistance. These methods will be used to expedite the production of aflatoxin resistant corn hybrids. Research is also being conducted to determine the infection pathways within developing ears and to compare the infection pathways in susceptible and resistant genotypes. Tissues from ears of resistant and susceptible genotypes are analyzed to identify proteins associated with resistance and to isolate the genes encoding these proteins. Insect damage increases the vulnerability of corn to fungal infection and aflatoxin accumulation; therefore, the development and release of corn germplasm with resistance to feeding by the southwestern corn borer and fall armyworm also help to reduce aflatoxin accumulation. Genes for resistance to these insects are being isolated and chromosomal regions associated with this resistance are being identified. Improved methods for identifying corn germplasm with resistance to insects and diseases are being developed and evaluated.
<li>How serious is the problem? Why does it matter? Contamination of corn grain with aflatoxin is a major food and feed safety problem. Aflatoxin is extremely toxic and a known carcinogen that is harmful to humans, livestock, and wildlife. Corn with aflatoxin levels that exceed 20 parts per billion is limited in use and is banned from interstate and international trade. Although aflatoxin contamination is a sporadic problem in the Midwest, it occurs much more frequently in the South. In 1998, Texas, Louisiana, Arkansas, Mississippi, Alabama, and South Carolina were devastated by high levels of aflatoxin in corn. Depending on the level of aflatoxin present, the grain was either severely discounted or banned from the market altogether. Losses in Texas, Louisiana, and Mississippi were estimated to exceed $85,000,000. Corn producers, grain elevator operators, and consumers suffer from aflatoxin contamination.
<li>How does it relate to the National Program(s) and National Component(s)? The research on identification of corn germplasm with resistance to aflatoxin contamination of corn and identification of genes for resistance fits well into the second component of ARS National Program Plant, Microbial, and Insect Genetic Resources, Genomics, and Genetic Improvement(301) and the third component of ARS National Program Food Safety (108). Reducing the threat from aflatoxin contamination is being approached by developing new knowledge that will improve the management of pests using environmentally safe methods by enhancing the plant's natural processes of defense. The research will provide new basic knowledge on improved stability and security of crop production in the face of environmental stress. The Unit collaborates with ARS scientists in New Orleans (CRIS Project 6435-42000-016-00D) on laboratory evaluation of germplasm for aflatoxin resistance and on research to identify proteins associated with resistance. Collaboration with ARS in Stoneville, MS (CRIS Project 6402-42000-001-00D) provides an additional site for field evaluation of germplasm for resistance to aflatoxin, southwestern corn borer, and fall armyworm.
<li>What were the most significant accomplishments this past year? A. Single Most Significant Accomplishment during FY 2001: Southwestern corn borers and fall armyworms cause substantial reductions in yield and quality of corn produced in the southern USA, and feeding by these insects also increases the likelihood of Aspergillus flavus infection and aflatoxin contamination of Southern corn. Scientists at Mississippi State have used field evaluations of insect damage and conventional breeding methods to identify and develop corn germplasm with resistance to southwestern corn borer and fall armyworm. Corn germplasm line Mp716, which not only exhibits resistance to southwestern corn borer and fall armyworm but also moderate resistance to aflatoxin contamination, was released in 2001. Numerous requests for seed of this germplasm line from public and private research institutions have been answered. This germplasm could be used in developing and producing corn hybrids with higher levels of natural resistance to insect damage and aflatoxin contamination than are currently available to farmers. B. Other Significant Accomplishment(s) if any: The threat of aflatoxin contamination is a serious food and feed safety problem and a major limitation to profitable production of corn in the South, and growing corn hybrids that have genetic resistance to aflatoxin contamination under a range of environmental conditions is widely considered the best defense against the problem. Experimental corn hybrids selected for aflatoxin resistance were grown and evaluated for aflatoxin contamination at locations in Texas, Louisiana, Mississippi, and Georgia in an effort that included scientists with Texas A&M and Louisiana State Universities and ARS scientists at Tifton, GA; Stoneville, MS; and Mississippi State, MS. Crosses between lines selected for aflatoxin resistance at Mississippi State sustained significantly lower levels of aflatoxin contamination at all locations. This indicates that germplasm selected for aflatoxin resistance at Mississippi State by inoculating developing corn ears with Aspergillus flavus spores can be used effectively to produce aflatoxin resistant corn hybrids for a wide range of locations in the South. C. Significant Accomplishments/Activities that Support Special Target Populations:
<li>Describe the major accomplishments over the life of the project including their predicted or actual impact. The release of germplasm line Mp715 in 1999 was a major accomplishment. It, along with earlier germplasm releases, should lead to the eventual commercial availability of aflatoxin resistant hybrids. Hybrids that have been developed by commercial seed companies from this germplasm are now being evaluated in advanced testing programs. Replacement of aflatoxin susceptible hybrids with resistant hybrids represents the greatest opportunity for reducing or eliminating aflatoxin contamination of corn. Combining genes for aflatoxin resistance and genes for insect resistance should result in hybrids with the lowest levels of aflatoxin accumulation. Germplasm lines Mp713 and Mp714 with resistance to southwestern corn borer and fall armyworm were also developed and released. These are being used by seed companies in developing insect resistant hybrids. This resistance is also being combined with resistance from Bacillus thuringiensis to provide better control of some insects, especially the fall armyworm. A gene associated with resistance in corn to southwestern corn borer and fall armyworm has been isolated, sequenced, and patented. A major vegetable seed company is transferring this gene into vegetable crops in an attempt to control insect pests of those crops.
<li>What do you expect to accomplish, year by year, over the next 3 years? This project has been terminated and replaced by bridging CRIS Project 6406-21000-007-00D. Anticipated accomplishments can be found under that project report.
<li>What science and/or technologies have been transferred and to whom? When is the science and/or technology likely to become available to the end user (industry, farmer, other scientists)? What are the constraints if known, to the adoption & durability of the technology product? The most significant transfer of technology from the research unit has been the release of corn germplasm with resistance to southwestern corn borer and fall armyworm and Aspergillus flavus/aflatoxin. Seed of the most recent releases have been requested by plant breeders with both commercial seed corn companies and public institutions. There has also been renewed interest in earlier releases made by the Unit. Using conventional breeding methods to transfer the resistance from this germplasm into elite breeding lines, it could take 10 years or longer to develop and market insect and aflatoxin resistant hybrids. Aflatoxin resistance is controlled by several genes, and environmental effects on the expression of resistance are substantial. This makes transferring the resistance into elite germplasm difficult and expensive. Current techniques for evaluating germplasm for resistance are labor intensive and difficult to employ in large scale breeding programs that are essential in commercial seed corn companies. New techniques for evaluating germplasm for resistance will, if successful, help alleviate some of these problems. The current efforts to map genes for resistance to insects and diseases and to isolate and identify genes for resistance will permit more efficient transfer of resistance into elite breeding lines. Cooperative Research and Development Agreements (CRADA) with two commercial seed companies were established in FY 1999 and extended in FY 2000 and FY 2001 so that the Unit could evaluate hybrids developed by these companies for resistance to aflatoxin. This cooperative research should result in the commercial availability of corn hybrids with aflatoxin resistance and good agronomic qualities within the next two to three years. See CRIS Projects 6406-21000-007-01T and 02T. A patent on a gene encoding a protein associated with insect resistance was granted November 1999, and the technology has been licensed for use in vegetable crops to control insect pests.
<li>List your most important publications in the popular press (no abstracts) and presentations to non-scientific organizations and articles written about your work (NOTE: this does not replace your peer-reviewed publications which are listed below) Selecting the best from Mother Nature. MAFES Research Highlights. Winter 2001. v.64(1). p.8-9. Aflatoxin-resistant corn line released. Iowa Farmer Today. 16 Dec. 2000. p.10. </ol>