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Mapping and Genetic Dissection of Loci Conditioning Disease Resistance in Maize

Chung, Chia-Lin1; Longfellow, Joy1; Walsh Ellie1; Esbroeck, George V.2; Balint-Kurti, Peter2; Nelson, Rebecca J.1
1 Dept. of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, NY 14853, USA
2 USDA-ARS; Dept. of Plant Pathology, North Carolina State University, Raleigh, NC 27695, USA

A range of approaches for QTL analysis have been used to identify, characterize and dissect loci conditioning disease resistance (disease QTLs) in maize. By investigating a set of chromosomal segment substitution lines (CSSLs) derived from B73/Tx303, several QTLs for resistance to northern leaf blight (NLB) were mapped. Two QTLs with large effects, associated with the B73 allele at bin 1.02 and the Tx303 allele at bin 1.06, have been further validated in F2 populations. The effects of the two NLB-QTLs on different stages of pathogenesis is being characterized by detailed phenotyping of the derived NILs. The heterogeneous inbred family (HIF) analysis was explored for targeted QTL mapping and NIL development. Starting with F5 and F6 HIFs derived from B73/CML52 and S11/DK888, we used 74 SSR markers covering 38 bins to identify residual heterozygotes, and generated a series of NIL pairs contrasting for chromosomal regions associated with multiple disease resistance. By testing the NILs for resistance to NLB, gray leaf spot (GLS), southern leaf blight (SLB), anthracnose leaf blight (ALB), anthracnose stalk rot (ASR), common rust, common smut, and Stewart’s wilt, we identified several disease QTLs. Most were effective for single diseases. The DK888 allele at bin 8.06 was associated with race-specific resistance to NLB. Race specificity and map position of this resistance locus suggest that it is Htn1, a major gene that delays lesion development. The CML52 allele at bin 6.05 is associated with resistance to three vascular diseases – NLB, ASR and Stewart’s wilt. Genetic dissection of these disease QTLs is underway.

Selection mapping of loci for quantitative disease resistance in a diverse maize population

Randall J. Wisser*1 Seth C. Murray*, Judith M. Kolkman†, Hernán Ceballos‡, and Rebecca J. Nelson*†,2
*Institute for Genomic Diversity, Department of Plant Breeding and Genetics, Cornell University, Ithaca, NY 14853, USA
†Department of Plant Pathology, Cornell University, Ithaca, NY 14853, USA
‡National University of Colombia and International Center for Tropical Agriculture Palmira, Colombia

The selection response of a diverse maize population improved for quantitative disease resistance to northern leaf blight (NLB) was investigated at the molecular genetic level. A tiered marker analysis with 151 simple sequence repeat (SSR) markers in 90 individuals of the population indicated that on average six alleles per locus were available for selection. An improved test statistic for selection mapping was developed, in which quantitative trait loci (QTL) are identified through the analysis of allele frequency shifts at mapped multi-allelic loci over generations of selection. After correcting for the multiple tests performed, 25 loci showed evidence of selection. Putatively selected loci were dispersed across the genome, which was consistent with the diffuse distribution of previously published QTL for NLB resistance. Compelling evidence for selection was found on maize chromosome 8, where several putatively selected loci co-localized with published NLB QTL and a race-specific resistance gene. Analysis of F2 populations derived from the selection mapping population suggested that multiple linked loci in this chromosomal segment were, in part, responsible for the selection response for quantitative resistance to NLB.
1 Department of Plant Pathology, North Carolina State University, Raleigh,NC 27695, USA

Resistance to Aspergillus flavus colonization in maize

Santiago Mideros and Rebecca Nelson

Aspergillus Ear Rot of maize is caused by the fungal plant pathogens Aspergillus flavus Link:Fr. and A. parasiticus Speare (3). The disease is most prevalent on warm and dry environments and is widely known because the pathogens produce a potent toxic compound to humans and animals known as aflatoxin (3). Aflatoxin is one of the most potent hepatocarcinogens know but this is not the only concern for human and animal consumption. The acute form of aflatoxicosis causes direct liver damage while a chronic form of the disease causes immunologic suppression and nutritional interference. In maize, varietal resistance to infection, to fungal growth and to accumulation of the mycotoxin are distinct and important traits that can contribute to pre-harvest control of aflatoxin contamination (2, 4). Levels of aflatoxin contamination may be influenced by quantitative resistance traits that affect the pathogen at different points in the processes of infection and development in the maize ear. Previous studies (eg. (1)) suggest the hypothesis that there are maize varieties that express mechanisms for resistance to infection and colonization of the fungal pathogen. For this reason, I propose to identify these resistance mechanisms using the following approach: i) development and validation of a quantitative real time PCR (qPCR) technique to estimate A. flavus biomass in infected plant tissues; ii) screening of components of resistance such as sporulation, latent period and colonization (qPCR) in a panel of possible sources of resistance and a population of recombinant inbred lines RIL; and iii) the characterization of plant resistance hallmarks in a selected set of inbreds or NILs. Preliminary results will be presented in the development of the qPCR technique and screening of resistance in the RILs population and the panel of sources of resistance.

Literature Cited
1. Brown R. L., Chen Z., Menkir A., Cleveland T. E., Cardwell K., Kling J., and White D. G. 2001. Resistance to aflatoxin accumulation in kernels of maize inbreeds selected for ear rot resistance in west and central Africa. J. Food Prot. 64 3:396-400.
2. Moreno O. J., and Kang M. S. 1999. Aflatoxins in maize: The problem and genetic solutions. Plant Breeding 118 1:1-16.
3. White D. G., editor. 1999. Compendium of Corn Diseases. 3rd ed. APS Press, St. Paul, Minn.
4. Zhang Y., Kang M. S., and Magari R. 1997. Genetics of resistance to kernel infection by Aspergillus flavus in maize. Plant Breeding 116 2:146-152.

Studies on Cercospora zeae-maydis and C. zeina, two species causing gray leaf spot disease on maize

Lin-Si Hsieh and Rebecca Nelson

Gray leaf spot (GLS) of maize, one of the most significant corn leaf diseases worldwide and in the US Corn Belt, is caused by Cercospora zeae-maydis and C. zeina. The two species, of putative North American and African origin respectively, are similar in microscopic characteristics and the symptoms they elicit, but can be separated by differences in their physiological properties and phylogenetic profiles. A 1998 study indicated that C. zeae-maydis was predominant in US and, based a sample of two isolates from Chemung, NY, identified both species in New York state. We hypothesized that after several years, one species would have outcompeted the other, resulting in only one remaining species. To test this hypotheses, we conducted a systematic sampling, using Chemung as the centre of the semicircle with radius about 50 miles across 10 counties and six watersheds in the southern tier of New York. We sampled two to five diseased maize leaves per field from 34 fields located in low-lying areas. By using species-specific histone H3 primer pairs and Cercospora-specific mating-type primer sets, we screened 77 monoconidial Cercospora isolates from different lesions. Both species were present, and often both were found at same locality. Both MAT1-1-1 and MAT1-2 alleles were identified for each species, indicating likely sexual recombination. According to these results, it appears that C. zeina has successfully established itself in southern NY without eliminating its sibling species. We will conduct AFLP analysis and greenhouse experiments to further assess the genetic variation and the differences in pathogenicity among the New York isolates of the two species.

How do disease QTLs affect the development of northern leaf blight in maize?

Walsh, Ellie1; Longfellow, Joy1; Chung, Chia-Lin1; Poland, Jesse2; Nelson, Rebecca1,2
1 303G Plant Science Bldg., Dept. of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, NY, USA 14853
2 303G Plant Science Bldg., Dept. of Plant Breeding and Genetics, Cornell University, Ithaca, NY, USA 14853

Our general objective is to understand how disease resistance QTLs affect fungal pathogenesis in maize.  Studies on host-pathogen interactions in a range of pathosystems have revealed an array of mechanisms by which plants reduce the efficiency of pathogenesis.  With the aim of relating specific loci with effects on specific stages of fungal development, we have mapped QTL for resistance to northern leaf blight (NLB), caused by Exserohilum turcicum, using  chromosomal segment substitution lines (CSSLs) derived from B73/Tx303.  We identified QTLs for NLB resistance using macroscopic disease components, including incubation period (IP), lesion expansion, and diseased leaf area (DLA).  Near isogenic lines (NILs) capturing specific NLB-QTLs were generated and are being characterized for detailed microscopic components, targeting different stages of pathogenesis.  Preliminary results suggest that the B73 allele at bin 1.02 does not reduce the efficiency of fungal infection, but is effective for reducing secondary hyphal growth surrounding the infection site, as well as inhibiting hyphal growth into the xylem.  The effect of this disease QTL on destructive hyphal growth is under further investigation.