Matthew D. Krakowsky

Multivariate analysis of maize disease resistances suggests a pleiotropic genetic basis and implicates a GST gene

Plants are attacked by pathogens representing diverse taxonomic groups, such that genes providing multiple disease resistance (MDR) are expected to be under positive selection pressure. To address the hypothesis that naturally occurring allelic variation conditions MDR, we extended the framework of structured association mapping to allow for the analysis of correlated complex traits and the identification of pleiotropic genes. The multivariate analytical approach used here is directly applicable to any species and set of traits exhibiting correlation. From our analysis of a diverse panel of maize inbred lines, we discovered high positive genetic correlations between resistances to three globally threatening fungal diseases. The maize panel studied exhibits rapidly decaying linkage disequilibrium that generally occurs within 1 or 2 kb, which is less than the average length of a maize gene. The positive correlations therefore suggested that functional allelic variation at specific genes for MDR exists in maize. Using a multivariate test statistic, a glutathione S-transferase (GST) gene was found to be associated with modest levels of resistance to all three diseases. Resequencing analysis pinpointed the association to a histidine (basic amino acid) for aspartic acid (acidic amino acid) substitution in the encoded protein domain that defines GST substrate specificity and biochemical activity. The known functions of GSTs suggested that variability in detoxification pathways underlie natural variation in maize MDR.

Mapping Resistance Quantitative Trait Loci for Three Foliar Diseases in a Maize Recombinant Inbred Line Population—Evidence for Multiple Disease Resistance?

Southern leaf blight (SLB), gray leaf spot (GLS), and northern leaf blight (NLB) are all important foliar diseases impacting maize production. The objectives of this study were to identify quantitative trait loci (QTL) for resistance to these diseases in a maize recombinant inbred line (RIL) population derived from a cross between maize lines Ki14 and B73, and to evaluate the evidence for the presence genes or loci conferring multiple disease resistance (MDR). Each disease was scored in multiple separate trials. Highly significant correlations between the resistances and the three diseases were found. The highest correlation was identified between SLB and GLS resistance (r = 0.62). Correlations between resistance to each of the diseases and time to flowering were also highly significant. Nine, eight, and six QTL were identified for SLB, GLS, and NLB resistance, respectively. QTL for all three diseases colocalized in bin 1.06, while QTL colocalizing for two of the three diseases were identified in bins 1.08 to 1.09, 2.02/2.03, 3.04/3.05, 8.05, and 10.05. QTL for time to flowering were also identified at four of these six loci (bins 1.06, 3.04/3.05, 8.05, and 10.05). No disease resistance QTL was identified at the largest-effect QTL for flowering time in bin 10.03.

Preharvest aflatoxin contamination of corn and other grain crops grown on the U.S. Southeastern Coastal Plain

Preharvest aflatoxin contamination of grain grown on the U.S. Southeastern Coastal Plain is provoked and aggravated by both biotic and abiotic stress factors that influence infection by the Asperigillus group. Asperigillus flavus, Link ex Fr., is one of the principal toxigenic fungi of summer grains grown in the region, and the hot, humid weather patterns along with suboptimal summer rainfall favor the development of this organism. An array of arthropod species also contributes to the dispersal of this fungus as they attack and feed on the developing grain. Research on summer grains grown on the Coastal Plain has the expressed goal of reducing, and perhaps eliminating aflatoxin contamination in adapted germplasm using classical crop improvement methods to deploy host plant resistance. This research is complimented and enhanced by molecular techniques that have proven invaluable in the identification and development of superior germplasm. It also emphasizes the need to fully understand the biological interactions between fungus, arthropods, crops, and the environmental conditions that govern the aflatoxin contamination. Alternative cropping systems that avoid contamination are also integrated into this summary of this research progress.

IDENTIFYING MAIZE GERMPLASM WITH RESISTANCE TO AFLATOXIN ACCUMULATION

Contamination of maize grain, Zea mays L., with aflatoxin, a toxin produced by the fungus Aspergillus flavus, reduces its value and marketability. Growing hybrids with resistance is generally considered a highly desirable way to reduce A. flavus infection and aflatoxin accumulation. Identifying maize germplasm with resistance is critical to the development and production of such hybrids. USDA-ARS scientists at Mississippi State, Mississippi; Tifton, Georgia; and Raleigh, North Carolina; have engaged in a multilocation approach to germplasm screening. A major component of this has been the evaluation of accessions obtained from the Germplasm Enhancement of Maize (GEM) project at both Mississippi State and Tifton. Selections from GEM accessions 250_01_XL370A_S11_F2S4_9214_Blk21/00-# and 2250_02_XL370A_S11_F2S4_3363_Blk03/00-# exhibited the highest levels of resistance both as lines per se and in testcrosses. Lines developed at the International Maize and Wheat Improvement Center (CIMMYT) and North Carolina State University also exhibited reduced levels of aflatoxin contamination. CML348, NC388, NC400, NC408, and NC458 were among those with low levels of aflatoxin contamination. The lines that displayed low levels of contamination should be useful in maize breeding programs for developing parental inbred lines and aflatoxin-resistant maize hybrids.

Identifying and developing maize germplasm with resistance to accumulation of aflatoxins

Efforts to identify maize germplasm with resistance to Aspergillus flavus infection and subsequent accumulation of aflatoxins were initiated by the US Department of Agriculture, Agricultural Research Service at several locations in the late 1970s and early 1980s. Research units at four locations in the south-eastern USA are currently engaged in identification and development of maize germplasm with resistance to A. flavus infection and accumulation of aflatoxins. The Corn Host Plant Resistance Research Unit, Mississippi State, MS, developed procedures for screening germplasm for resistance to A. flavus infection and accumulation of aflatoxins. Mp313E, released in 1990, was the first line released as a source of resistance to A. flavus infection. Subsequently, germplasm lines Mp420, Mp715, Mp717, Mp718, and Mp719 were released as additional sources of resistance. Quantitative trait loci associated with resistance have also been identified in four bi-parental populations. The Crop Protection and Management ...

Identification of resistance to the Ga1-m gametophyte factor in maize

Due to maize’s wind-driven pollination, non-target pollen contamination is problematic for producers and breeders. Maize gametophyte factors, specifically gametophyte factor 1 (ga1), have long been used to produce selectively pollinating phenotypes. The use of these factors is a cornerstone of commercial popcorn production, and they are used for a large range of other purposes, including preventing contamination by genetically modified pollen in organic production. However this system is at great risk from another allele at the ga1 locus, Ga1-m, which overcomes the selectively pollinating phenotypes. To further complicate this problem, the risk posed by this allele has been under-assessed. Here we reinterpret the key study on Ga1-s and report genetic resistance to the Ga1-m allele in maize lines that carry dominant gametophyte factors. We identified genetic resistance to the allele segregating in lines derived from four landraces, showed the resistance is heritable, and that it acts in full-strength and attenuated versions. Additionally, we have suggested the validity of evolutionary-based inquiry into our plant genetic resources, and provided some validation of this effort. Our results provide the first report of effective genetic resistance to pollination by the Ga1-m allele, providing an option to continue the use of genetic barriers to non-target pollination. A source of resistance to the Ga1-m allele allows research to be conducted about the allele itself, allowing for research into the possible existence of multiple versions of the allele and their distributions. We anticipate our research will be a starting point for identification of additional sources of resistance to the Ga1-m allele, specifically in popcorn production, where it is most immediately needed to prevent pollen contamination, as well as the eventual localization and mapping of the resistance alleles. We also believe the suggestion of evolutionary-based inquiry into plant genetic resources will provide a highly effective method for identification of specific traits, but will need more extensive validation.

Identification of maize-derived dominant gametophyte factors

Abstract The use of gametophyte factors to protect specialty-type maize has long been advocated, but as of yet, they have made very little impact on preventing pollen contamination due to the complications associated with breeding with these materials, mainly the additive nature of the alleles. A dominant gametophyte factor (DGF) overcomes this problem, allowing for less time consuming production of gametophytic hybrids, but effectively utilized sources do not exist. Tcb1-s, a known DGF, is a teosinte introgression into maize and the leading candidate for utilization, however, it has several issues that limit its effective use in expediting the breeding process for gametophytic hybrids. The use of maize for a source of DGFs may overcome this problem; with the idea years of selection by farmers would likely have minimized any segregation for yield associated with these alleles, making their use for production of gametophytic hybrids an appealing option for modern breeders. Through screening and backcrossing selected maize accessions, we identified DGFs in seven accessions from race Maiz Dulce, which we document here as a starting point for identification of additional maize-derived DGFs. These accessions did not appear to segregate for yield, a marked improvement over existing DGFs. Additionally, we assessed the compatibility of identified maize-derived DGFs from one accession, and showed that, while lines are generally compatible, they are not obligately so since a single accession may segregate for multiple gametophyte factors. There is, therefore, a need to consider the compatibility of pairs of DGFs early in the inbreeding process. Maize-derived DGFs provide a more effective method of producing gametophytic hybrids, making their production economical enough to be brought to market. The use of DGFs has wider potential to benefit any producers interested in preventing pollen contamination with gametophytic hybrids through the same benefits provided to breeders for organic and other specialty systems. In combination with Ga1-m resistance, maize-derived DGFs provide a long-term gametophytic solution to pollen contamination, in a more expeditious way.