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United States Department of Agriculture

Agricultural Research Service


Location: Plant Science Research

2013 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.

3. Progress Report:
Five-Year summary for 6645-21220-013-00D: Millions of 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. A combination of high resolution genetic mapping in multiple populations, association analysis, and gene expression assays confirmed the identity of a gene called ZmCCT as the most important photoperiod response gene in maize. We also defined the ten most important genomic regions containing photoperiod response genes in tropical maize inbreds. We determined the genetic nature of resistance to southern corn leaf blight disease at an unprecedented level of resolution for a complex disease resistance trait. We determined that at least 30 genes are involved in the control of this resistance, and that resistance genes are dispersed across many different maize lines, suggested that targeted breeding could be used to combine them to achieve higher levels of disease resistance. We identified a number of genes closely associated with the observed resistance and have initiated follow-up studies on these genes to determine their effects at a molecular level. We created superior backcross-derived lines combining resistance to Fusarium ear rot and hybrid yield potential and conducted three cycles of selection in a diverse population to combine ear rot resistance with higher yield potential and good standability. For each of the five years of this project, we screened 50 or more advanced lines from the USDA Germplasm Enhancement of Maize (GEM) project and the North Carolina State University maize inbred development programs for resistance to Fusarium ear rot and fumonisin contamination resistance and southern leaf blight. Several loci associated with the maize hypersensitive defense response were identified with extremely high precision using association mapping.

4. Accomplishments
1. Identification of genes associated with Fusarium ear rot resistance. A genome wide association study was performed to detect genes controlling resistance to Fusarium ear rot. This disease is complex, being affected by many genes and influenced strongly by the environment. This complexity hinders breeding for resistance. To surmount this difficulty, we sought to identify specific genes controlling some part of the resistance response in diverse maize. Association analysis was used to identify a gene involved in cell wall composition in seeds that was strongly associated with ear rot resistance. Corn breeders can use this information to target their selections for this gene.

Last Modified: 07/20/2017
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