2009 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.
New germplasm accessions were obtained through the Germplasm Enhancement of Maize (GEM) project and evaluated for resistance to aflatoxin accumulation in 2008, and the lines exhibiting the lowest levels of aflatoxin were selected for inclusion in the breeding program. Acremonium strictum, an endophyte of corn, did not suppress aflatoxin production in corn kernels when injected into the ears or stalks prior to infection with A. flavus. Aspergillus niger, a common contaminant of corn kernels, limited aflatoxin in corn kernels when the fungus was applied prior to Aspergillus flavus infection. Systemic infection of corn stalks with F. verticillioides had no effect on aflatoxin contamination of corn inoculated with A. flavus. The interactions of F. verticilliodes and A. flavus on hybrids transformed with genes from Bacillus thuringiensis (BT) were monitored in field studies. Hybrids were produced by crossing lines with different levels of susceptibility to A. flavus/aflatoxin to BT and non-BT versions of proprietary lines. These hybrids are currently being evaluated for aflatoxin accumulation at Mississippi State, MS; Tifton, GA; and Raleigh, NC, to determine whether BT tester lines are superior to non-BT testers when screening for resistance to A. flavus/aflatoxin. When ear-feeding insects are present, aflatoxin accumulation is generally lower in BT hybrids than in the non-BT versions of the same hybrids. A panel of 300 diverse lines from U.S. breeding programs and from the International Maize and Wheat Improvement Center (CIMMYT) were chosen for an association mapping project to identify genes associated with reduced aflatoxin accumulation. These lines were crossed with Va35, a susceptible line; the testcrosses are being evaluated for aflatoxin accumulation in field tests conducted in two locations in Mississippi and two locations in Texas under Specific Cooperative Agreements. This investigation should be useful in identifying genes and groups of genes associated with resistance to A. flavus/aflatoxin and in determining which lines possess unique genes for resistance. Two germplasm lines (Mp715, Mp717) developed and released as sources of resistance to A. flavus/aflatoxin also exhibited resistance to F. verticillioides/fumonisin in field tests both as inbred lines per se and in testcrosses. A 3-year study to identify quantitative trait loci (QTL) for resistance to A. flavus/aflatoxin in Mp715 was completed and provides valuable information on molecular markers for resistance to aflatoxin accumulation in corn. Several lines selected from crosses between Mp715 and Va35 exhibited not only high levels of aflatoxin resistance, but also earlier maturity and other desirable agronomic qualities in several environments. A cooperator in Texas also selected highly resistant lines from crosses with Mp715. These lines are currently being evaluated in field tests in both Texas and Mississippi. The lines are also being analyzed to determine whether they possess QTL identified in the analysis of the mapping population. The most promising of the lines developed in Mississippi and Texas will be released with associated molecular marker information.
Corn germplasm developed as a source of resistance to Aspergillus flavus exhibits resistance to Fusarium verticillioides and mycotoxin accumulation. Contamination of corn grain with aflatoxin, produced by the fungus A. flavus, or fumonisin, produced by F. verticillioides, reduce its value and marketability and cause serious food and feed safety problems. Two germplasm lines, Mp715 and Mp717, that were developed and released as sources of resistance to A. flavus infection and aflatoxin accumulation also exhibited resistance in field tests conducted in Mississippi to F. verticillioides infection and fumonisin accumulation. The analysis of a diallel cross of eight parental inbred lines indicated that general combining ability (GCA) effects for reduced fumonisin accumulation were highly significant for Mp715 and Mp717. Both lines had previously exhibited highly significant GCA effects for reduced aflatoxin contamination. Mp715 and Mp717 will be useful in developing lines and hybrids with resistance to both aflatoxin and fumonisin contamination, and breeding methods that maximize the use of GCA should be effective in simultaneously enhancing levels of resistance to both aflatoxin and fumonisin accumulation in superior corn hybrids.
Alwala, S., Kimbeng, C.A., Williams, W.P., Kang, M.S. 2008. Molecular markers associated with resistance to Aspergillus flavus in maize: QTL and discriminant analyses. Journal of New Seeds. 9:1-18.
Williams, W.P., Krakowsky, M.D., Windham, G.L., Balint-Kurti, P.J., Hawkins, L.K., Henry, W.B. 2008. Identifying maize germplasm with resistance to aflatoxin accumulation. Toxin Reviews. 27:319-345.
Windham, G.L., Williams, W.P., Hawkins, L.K., Brooks, T.D. 2009. Effect of Aspergillus flavus inoculation methods and environmental conditions on aflatoxin accumulation in corn hybrids. Toxin Reviews. 28:70-78.
Warburton, M.L., Brooks, T.D., Krakowsky, M.D., Shan, X., Windham, G.L., Williams, W.P. 2009. Identification and mapping of new sources of resistance to aflatoxin accumulation in maize. Crop Science. 49:1403-1408.