2011 Annual Report
1a.Objectives (from AD-416)
Objectives are to (1) identify genes and genome regions controlling key traits in diverse maize germplasm using multiple populations and novel genetic mapping methods; (2) identify and characterize new sources of resistance to southern leaf blight (SLB), gray leaf spot (GLS), northern leaf blight (NLB), and Fusarium ear rot by developing new near-isogenic line sets, identifying QTL conferring resistance to multiple foliar diseases, and evaluating the known genetic diversity among public maize inbreds; and (3) incorporate favorable alleles from exotic maize into adapted maize lines, with particular emphasis on improvement of resistance to Fusarium ear rot and fumonisin contamination.
1b.Approach (from AD-416)
Develop genetic mapping populations appropriate for identification of favorable alleles in exotic maize by identifying genome regions and specific genes controlling the response to photoperiod in multiple tropical maize inbreds. Develop interconnected multiple populations and novel statistical methods to map specific genes controlling quantitative trait variation in diverse maize. Fine-map QTL for SLB, NLB, GLS, Fusarium ear rot and fumonisin contamination resistance. Characterize specific disease resistance QTL using near-isogenic line pairs. Identify QTL conferring resistance to multiple foliar diseases. Complete multiple environment screening of a 302-line population encompassing the known genetic diversity among public maize inbreds for resistance to SLB, NLB and GLS. Identify new sources of resistance to SLB, Fusarium ear rot, and fumonisin contamination from the GEM program and the NCSU tropical maize breeding program. Develop new lines with elite agronomic performance and enhanced resistance to Fusarium ear rot by backcrossing. Test if selection for resistance to Fusarium ear rot results in improved resistance to contamination by the associated mycotoxin fumonisin.
We continue to make progress in fine mapping of a Southern Leaf Blight (SLB) quantitative resistance gene on chromosome 3 and have identified about 65 recombination events that span sub-centiMorgan region that contains the underlying gene. After evaluating the recombinant lines this summer we expect to be able to map the resistance gene to a very small region containing one or a handful of genes.
We have also fine-mapped a SLB resistance gene on chromosome 6 to a region of about a centiMorgan. We have now identified several plants with transposon insertions which appear to disrupt the function of this gene. Analysis of these plants should lead to identification of the underlying gene.
We have continued with a project to develop a robust gene silencing system in maize in collaboration with a scientist at Nobel Foundation, OK. We have identified several lines of maize which can be used for viral induced gene silencing. However the phenotype is transitory and further work will be required to produce a truly robust system.
In collaboration with scientists at Purdue University, we have identified several more loci that affect the strength of the maize defense response using the maize nested association mapping population. We have investigated the temperature dependent nature of a rust resistance gene and have shown that this gene is entirely non-functional above 30oC.
Using a set of near isogenic lines we developed, we have identified several genome regions that confer resistance to multiple leaf pathogens of maize. We identified several loci from teosinte, the wild progenitor of maize, which are associated with resistance to southern leaf blight and gray leaf spot. We have produced segregating populations in order to verify the effects of these loci.
Millions of deoxyribonucleic acid (DNA) variants were tested to identify 50 - 100 associated with the flowering response of maize to long daylengths. Several of these associated variants are in or adjacent to genes that have been shown to control daylength responses in other crops.
Superior backcross-derived lines combining resistance to Fusarium ear rot and hybrid yield potential were identified. 75 advanced lines from the USDA Germplasm Enhancement of Maize (GEM) project and the North Carolina State University maize inbred development programs were tested for resistance to Fusarium ear rot and fumonisin contamination resistance. 100 GEM lines were tested for resistance to SLB.
Genome-wide association analysis identifies genes involved in photoperiod response. Tropical maize harbors tremendous genetic variation that could be used to improve the U.S. corn crop. The use of tropical maize in the USA is hindered by the sensitivity of tropical maize to long daylengths that occur in the summer in the U.S. Corn Belt. ARS researchers in Raleigh, NC studied the response of 5000 lines from the maize Nested Association Mapping population and discovered 13 critical genome regions that control the response of tropical maize to long daylengths. They tested millions of DNA sequence variants and identified a number of variants associated with sensitivity to long daylengths, some of which were in or near genes known to affect the photoperiod response in other crops.
Eller, M.S., Payne, G.A., Holland, J.B. 2010. Selection for Reduced Fusarium Ear Rot and Fumonisin Content in Advanced Backcross Maize Lines and Their Topcross Hybrids. Crop Science. 50:2249-2260.
Kump, K., Bradbury, P., Buckler IV, E.S., Belcher, A., Oropeza-Rosas, M., Wisser, R., Zwonitzer, J., Kresovich, S., McMullen, M.D., Ware, D., Balint Kurti, P.J., Holland, J.B. 2011. Genome-wide association study of quantitative resistance to southern leaf blight in the maize nested association mapping population. Nature Genetics. 43:163-168.
Kump, K.L., Holland, J.B., Jung, M.T., Wolters, P., Balint Kurti, P.J. 2010. Joint Analysis of Near Isogenic and Recombinant Inbred Line Populations Yields Precise Positional Estimates for QTL. The Plant Genome. 3:142-153.
Wisser, R., Kolkman, J., Patzoldt, M., Holland, J.B., Jianming, Y., Krakowsky, M.D., Nelson, R., Balint Kurti, P.J. 2011. Multivariate analysis of maize disease resistances suggests a pleiotropic genetic basis and implicates a glutathione S-transferase gene. Proceedings of the National Academy of Sciences. 108:7339-7344.
Wisser, R., Balint Kurti, P.J., Holland, J.B. 2011. A novel genetic framework for studying response to artificial selection. Plant Genetic Resources. 9:281-283.
Coles, N., Zila, C., Holland, J.B. 2011. Functional Allelic Variation at Key Photoperiod Response QTL in Maize. Crop Science. 51:1036-1049.
Nageri, A., Coles, N., Holland, J.B., Balint Kurti, P.J. 2011. Mapping QTL controlling southern leaf blight resistance by combined analysis of three related recombinant inbred line populations. Crop Science. 51:1571-1579.
Jumbo, M., Weldekidan, T., Holland, J.B., Hawk, J. 2011. Comparison of Conventional, Modified Single Seed Descent, and Doubled Haploid Breeding Methods for Maize Inbred Line Development Using GEM Breeding Crosses. Crop Science. 51:1534-1543.