ARS | Publication request: Genome-wide nested association mapping of quantitative resistance to northern leaf blight in maize

Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 14, 2011
Publication Date: June 26, 2011
Repository URL: http://www.pnas.org/cgi/doi/10.1073/pnas.1010894108
Citation: Poland, J.A., Bradbury, P., Buckler IV, E.S., Nelson, R. 2011. Genome-wide nested association mapping of quantitative resistance to northern leaf blight in maize. Proceedings of the National Academy of Sciences. 108:6893-6898.

Interpretive Summary: There are two generally recognized forms of disease resistance in plants; quantitative and qualitative. Qualitative resistance is generally conditioned by a single gene of large effect. This type of resistance is very effective and useful in breeding resistant cultivars, however, cultivars developed with a single resistance gene often become susceptible due to mutations in the pathogen which render the resistance gene ineffective. On the other hand, quantitative resistance is generally conditioned by multiple genes of small effect that can be combined to make resistant cultivars that are more robust to pathogen evolution. The mechanisms of quantitative resistance, however, is not well understood. In an effort to better understand quantitative disease resistance, we evaluated and analyzed a large maize population consisting of 5,000 inbred lines for resistance to northern leaf blight (NLB), an economically important disease of maize throughout the world. We found 30 genomic locations that were associated with resistance to NLB and explained most of the differences in resistance. We then used a technique termed nested-association mapping to identify genes associated with resistance and found multiple candidate genes for NLB resistance. These candidate genes were consistent with hypothesized classes of genes for quantitative resistance. Taken together, these results greatly expand the current understanding of quantitative disease resistance in plants.

Technical Abstract: Quantitative resistance to plant pathogens, controlled by multiple loci of small effect, is important for food production, food security, and food safety but is poorly understood. To gain insights into the genetic architecture of quantitative resistance in maize, we evaluated a 5,000 inbred-line nested association mapping (NAM) population for resistance to northern leaf blight, a maize disease of global economic importance and identified 30 different quantitative trait loci (QTL). It was observed that large variation in the resistance phenotype is the result of the accumulation of numerous loci of small additive effects and that most QTL had multiple alleles. This genetic architecture is very similar to that of flowering time in maize, a classic complex trait. Genome-wide nested association mapping, using 1.6 million SNPs, identified multiple candidate genes including several defense-related genes: three receptor-like kinases, a MAP kinase, an ethylene response factor, and an Mlo-like gene. These results are consistent with the hypothesis that quantitative disease resistance is conditioned by a range of mechanisms different than those that control qualitative resistance in plants.