Location: Corn Host Plant Resistance Research2008 Annual Report
1a. Objectives (from AD-416)
Identify and develop corn germplasm with resistance to Aspergillus flavus infection/aflatoxin contamination and ear-feeding insects and release this germplasm together with information on molecular markers and methodology that will expedite its deployment into commercially available corn hybrids. Specific objectives include the following: (1) determine the effects of indigenous fungal species and ear-feeding insects on A. flavus infection and aflatoxin accumulation in corn grain; (2) identify new sources of corn germplasm with resistance to A. flavus infection and aflatoxin accumulation and/or resistance to damage by southwestern corn borer, fall armyworm, and corn earworm; (3) identify quantitative trait loci, genes, and proteins associated with resistance in corn to A. flavus infection, aflatoxin accumulation, and insect damage; and (4) enhance corn germplasm with resistance to A. flavus infection, aflatoxin accumulation, and insect damage and release germplasm lines as sources of resistance.
1b. Approach (from AD-416)
Objective 1. Determine the effects of indigenous fungal species and ear-feeding insects on A. flavus infection and aflatoxin accumulation in corn grain. Colonization of corn grain is rarely by a single fungal species, but rather a mixture of fungi. Fusarium verticillioides (syn. F. moniliforme) is the most commonly reported fungus infecting corn in the USA, and it is frequently found together with A. flavus. Acremonium zeae is a common contaminant of preharvest corn in the Southeast. It has been reported to suppress growth of both A. flavus and F. verticillioides in laboratory experiments. The interactions of these fungi will be investigated to determine whether F. verticillioides and A. zeae affect A. flavus infection of corn grain and the subsequent accumulation of aflatoxin, and if so, whether these fungi are impediments to the identification of aflatoxin-resistant corn germplasm. The association between insect damage and aflatoxin accumulation in different corn genotypes will be investigated and the extent to which resistance to damage by southwestern corn borer, Diatraea grandiosella; fall armyworm, Spodoptera frugiperda; or corn earworm, Helicoverpa zea, reduces aflatoxin contamination will be determined. Objective 2. Identify new sources of corn germplasm with resistance to A. flavus infection and aflatoxin accumulation and/or resistance to damage by southwestern corn borer, fall armyworm, and corn earworm. Corn germplasm from diverse backgrounds will be screened for resistance to A. flavus/aflatoxin, southwestern corn borer, fall armyworm, and corn earworm. Information on the effects of other fungi or insects on A. flavus/aflatoxin accumulation (Objective 1) will be used to refine and improve techniques for evaluating germplasm for resistance. Newly identified sources of resistance will be used to pursue Objectives 3 and 4. Objective 3. Identify quantitative trait loci, genes, and proteins associated with resistance in corn to A. flavus infection, aflatoxin accumulation, and insect damage. Populations of F2:3 families and recombinant inbred lines derived from crosses between aflatoxin or insect resistant inbred lines and susceptible lines will be used to identify quantitative trait loci (QTL) associated with resistance. Resistant and susceptible corn inbred lines and recombinant inbred lines will be used in complementary investigations to identify candidate genes and proteins associated with resistance. Molecular markers identified in these investigations will be used in developing improved germplasm lines (Objective 4). Objective 4. Enhance corn germplasm with resistance to A. flavus infection, aflatoxin accumulation, and insect damage and release germplasm lines as sources of resistance. Both breeding methods based on phenotypic performance and those based on molecular markers will be used to enhance germplasm with resistance to aflatoxin contamination and insect damage. The effectiveness of molecular markers based on QTL, genes, and proteins identified in Objective 3 in transferring resistance to A. flavus/aflatoxin and insect damage into germplasm lines with desirable agronomic qualities will be determined.
3. Progress Report
This project replaces 6406-21000-009-00D. Primary focus of management unit's(MU) research continues to be identification and development of corn germplasm with resistance to Aspergillus flavus infection and aflatoxin accumulation. The entry and infection of developing ears by Acremonium zeae and the subsequent effects have been determined in field studies. A. zeae was introduced into corn plants via systemic stalk infection, via the silks, or injected directly into the ear. The systemic movement of Aspergillus parasiticus (NOR mutant) has been studied in the field and in the greenhouse. The movement of the NOR mutant through the stalk into developing ears of corn in the field was documented. Also, a greenhouse test was conducted to compare systemic infection of the NOR mutant when seed were infected prior to planting, at planting, or after emergence. The NOR mutant was recovered from stalk and leaf samples from plants infected with each inoculation treatment. Ear shank tissues tested positive for the NOR mutant when plants were inoculated by inserting infected toothpicks into the stalk. MU continues to screen new germplasm accessions obtained from the Germplasm Enhancement of Maize(GEM) project for resistance to A. flavus/aflatoxin. Testcrosses of lines selected and developed from GEM germplasm received in previous years were evaluated at the three locations in 2007, and some of these testcrosses exhibited excellent resistance at all three locations. The agreement between A. flavus/aflatoxin resistance exhibited by testcrosses and that exhibited by lines per se was excellent. Several lines selected from crosses between Mp715, a line released from the MU in 1999, and Va35 exhibited not only high levels of aflatoxin resistance, but also earlier maturity and other desirable agronomic qualities at several locations in 2007. If these lines continue to perform well in 2008, the best of them will be considered for release. A cooperator(6406-21000-011-07S) also selected highly resistant lines from crosses with Mp715. A 3-year study to identify quantitative trait loci(QTL) in Mp715 for resistance to A. flavus/aflatoxin has been completed. These results should provide valuable information on molecular markers that can be used in breeding for resistance to aflatoxin accumulation in corn. Germplasm lines selected for resistance to A. flavus/aflatoxin were obtained from several ARS and state corn breeding programs and are included in a panel of lines for an association mapping project for identifying genes or groups of genes associated for resistance to aflatoxin contamination. Resistance to insects (southwestern corn borer, fall armyworm, and corn earworm) is highly effective in reducing aflatoxin accumulation where those insects are pests. Combining resistance to those insects with resistance to A. flavus should be highly effective in reducing aflatoxin contamination. Crosses are currently being made between A. flavus/aflatoxin resistant lines and proprietary transgenic lines with genes for insect resistance from Bacillus thuringiensis (BT) so that the combined effects of these genes on aflatoxin accumulation can be determined. NP 301,Comp 3.3c.
1. Inheritance of resistance to aflatoxin contamination: The analysis of a diallel cross of 10 parental inbred lines indicated that both general (GCA) and specific (SCA) combining ability were significant sources of variation in the inheritance of resistance to aflatoxin contamination. GCA effects were highly significant for four lines developed and released as sources of resistance: Mp313E, Mp494, Mp715, and Mp717. The GCA effect for reduced aflatoxin contamination was also significant for two other lines, Mo18W and NC408. These lines should be useful in developing lines and hybrids with resistance to aflatoxin contamination. Breeding methods that maximize the use of GCA should be effective in enhancing resistance to aflatoxin accumulation when using these germplasm lines. National Program 301: Plant Genetic Resources, Genomics, and Genetic Improvement; Component 3: Genetic Improvement of Crops; Problem Area 3c: Germplasm Enhancement/Release of Improved Genetic Resources and Varieties.
2. Development of germplasm lines with resistance to aflatoxin contamination: Lines selected from crosses between resistant lines, Mp715 and Mp313E, with the susceptible inbred line Va35 exhibited levels of resistance comparable to that of the resistant parent, but with much better agronomic quality (earlier maturity, resistance to lodging, reduced plant eight) than the resistant parent. The improved agronomic quality of these lines will make them more useful in plant breeding programs with the goal of producing superior corn hybrids with resistance to aflatoxin contamination. National Program 301: Plant Genetic Resources, Genomics, and Genetic Improvement; Component 3: Genetic Improvement of Crops; Problem Area 3c: Germplasm Enhancement/Release of Improved Genetic Resources and Varieties.
5. Significant Activities that Support Special Target Populations
Baird, R., Abbas, H.K., Windham, G.L., Williams, W.P., Baird, S., Ma, P., Kelley, R., Hawkins, L.K., Scruggs, M. 2008. Identification of select fumonisin forming Fusarium spp. using PCR applications of the polyketide syntase gene and its relationship to fumonisin production in vitro. International Journal of Molecular Sciences. 9:554-570.