2013 Annual Report
1a.Objectives (from AD-416):
To identify corn germplasm with resistance to Aspergillus flavus infection and aflatoxin accumulation and to identify genes associated with resistance.
1b.Approach (from AD-416):
A panel of 300 corn inbred lines have been crossed with a common tester line. The resulting testcrosses will be evaluated for aflatoxin accumulation at locations in Texas and Mississippi. An association mapping analysis of genotypic and phenotypic data will be conducted to identify alleles associated with resistance to Aspergillus flavus infection and aflatoxin accumulation.
Aflatoxin contamination in corn (Zea mays L.) grain is caused by the fungus Aspergillus flavus. Because aflatoxin is carcinogenic and poses severe risks to animal and human health, it is federally regulated in the U.S. This important regulation limits marketability of grain and causes disproportionate economic losses to corn producers in the southern United States. Most commercial corn hybrids are highly susceptible to Aspergillus flavus infection and aflatoxin accumulation. Plant breeding for decreased susceptibility to aflatoxin accumulation is the most economic and practical method of reducing losses. While a number of germplasm lines with substantial and significant reductions in aflatoxin have been developed and released, no lines found to date have complete resistance. Many have lower yields than the best commercial hybrids and other undesirable agronomic traits as well. The primary objectives were (1) to develop new lines from several different germplasm sources and evaluate both the new lines and their corresponding testcrosses for resistance to aflatoxin accumulation in multiple environments, (2) evaluate the testcross hybrids for yield, and (3) make crosses to pyramid different sources of quantitative resistance into a one or a few breeding populations from which future selections can be made. Yield and agronomic data were collected on 743 experimental corn hybrids in College Station, 389 hybrids in Weslaco, 100 hybrids in Ganado, and 100 hybrids in Halfway, Texas. In College Station, Weslaco, and Ganado multiple tests were planted at the standard planting date and grown under well-watered conditions. Planting of a subset of hybrids was delayed to increase stress and aflatoxin pressure. At College Station and Weslaco, corn kernels colonized by toxin producing Aspergillus flavus isolates were distributed between rows at silk emergence for yield trials of hybrids confirmed in previous years to have acceptable agronomic characteristics and yield. Similar to the technique used by growers to deliver non-toxin producing Aspergillus flavus strains as a bio-control method, this method ensured the immediate availability of conidial inoculum for colonization of silks. Subsamples of grain from each hybrid were collected at maturity for agronomic evaluation and aflatoxin analysis. Quantification of aflatoxin was conducted with monoclonal antibody affinity columns and fluorescence determination. Aflatoxin levels at both College Station and Weslaco in these trials were unusually low (0-100ppb) because of the favorable (non-stress) conditions in 2012. Only a few hundred samples were analyzed for aflatoxin, and no significant differences among hybrids were detected. Sixty-eight hybrids out-yielded at least one commercial check, most of these were derived from lines with known resistance to aflatoxin contamination in their pedigrees. Seven experimental hybrids from the Texas A&M University breeding program were included in a South East Regional Aflatoxin Test (SERAT) trial of 29 hybrids and checks planted for yield and/or aflatoxin evaluation at seven locations: College Station, TX; Lubbock, TX; Mississippi State, MS; Tifton, GA; Cairo, GA; and two locations in North Carolina. Cooperators included several entries in order to obtain better understanding of the environmental interactions among their best germplasm. One entry from Texas was among the best: it yielded well in most locations and was second lowest in aflatoxin accumulation at Tifton and Mississippi State and third lowest, at College Station. An additional aflatoxin-only cooperative trial of inbred lines was planted at Weslaco and College Station. In the summer breeding nursery at College Station and the winter nursery at Weslaco, 288 lines that exhibited good yields in past trials were crossed to one or more of 14 elite lines chosen as testers. Sufficient seed of 707 unique hybrids were produced for evaluation in the next growing season. Many outstanding hybrids should be identified from this objective. Accomplishing this objective is critical to the successful transfer of resistance to aflatoxin contamination into commercial hybrids and into growers’ fields. A number of new breeding crosses focused on germplasm that has been routinely proven resistant in various Texas and regional studies. Two-way crosses among Mp313E, Mp715, Tx772, Tx740, Tx739, and GT602 were made in the summer nursery, and four-way crosses were made in the winter nursery. Also, in the winter nursery a number of other germplasm lines including A6, CML69, CML108, CML311, CML348, Ki3, Hi27, NC334, NC356, Mp313E, Mp715, Tzi8, Tzi18, and Tx772 that exhibited resistance to aflatoxin accumulation in a large association mapping study led by ARS scientists at Mississippi State, MS, were crossed. These single crosses will be inter-mated to develop a synthetic population from which various investigators can select lines with resistance to aflatoxin contamination. In the coming year, germplasm with known sources of resistance will be inter-mated to pyramid resistance to aflatoxin contamination from different sources into breeding populations. The number of inbred entries screened in the cooperative aflatoxin trial will be increased during the coming year.