The intermated B73 3 Mo17 (IBM) population, an advanced intercross recombinant inbred line population derived from a cross between the maize lines B73 (susceptible) and Mo17 (resistant), was evaluated in four environments for resistance to southern leaf blight (SLB) disease caused by Cochliobolus heterostrophus race O. Two environments were artificially inoculated, while two were not inoculated and consequently had substantially lower disease pressure. Four common SLB resistance quantitative trait loci (QTL) were identified in all environments, two in bin 3.04 and one each in bins 1.10 and 8.02/3. There was no significant correlation between disease resistance and days to anthesis. A direct comparison was made between SLB QTL detected in two populations, independently derived from the same parental cross: the IBM advanced intercross population and a conventional recombinant inbred line population. Several QTL for SLB resistance were detected in both populations, with the IBM providing between 5 and, in one case, 50 times greater mapping resolution.

A set of 89 near-isogenic lines (NILs) of maize was created using marker-assisted selection. Nineteen genomic regions, identified by restriction fragment length polymorphism loci and chosen to represent portions of all ten maize chromosomes, were introgressed by backcrossing three generations from donor line Tx303 into the B73 genetic background. NILs were genotyped at an additional 128 simple sequence repeat loci to estimate the size of introgressions and the amount of background introgression. Tx303 introgressions ranged in size from 10 to 150 cM, with an average of 60 cM. Across all NILs, 89% of the Tx303 genome is represented in targeted and background introgressions. The average proportion of background introgression was 2.5% (range 0-15%), significantly lower than the expected value of 9.4% for third backcross generation lines developed without marker-assisted selection. The NILs were grown in replicated field evaluations in two years to map QTLs for flowering time traits. A parallel experiment of testcrosses of each NIL to the unrelated inbred, Mo17, was conducted in the same environments to map QTLs in NIL testcross hybrids. QTLs affecting days to anthesis, days to silking, and anthesis-silk interval were detected in both inbreds and hybrids in both environments. The testing environments differed dramatically for drought stress, and different sets of QTLs were detected across environments. Furthermore, QTLs detected in inbreds were typically different from QTLs detected in hybrids, demonstrating the genetic complexity of flowering time. NILs can serve as a valuable genetic mapping resource for maize breeders and geneticists.

BackgroundMaize is a major cereal crop widely consumed in developing countries, which have a high prevalence of iron (Fe) deficiency anemia. The major cause of Fe deficiency in these countries is inadequate intake of bioavailable Fe, where poverty is a major factor. Therefore, biofortification of maize by increasing Fe concentration and or bioavailability has great potential to alleviate this deficiency. Maize is also a model system for genomic research and thus allows the opportunity for gene discovery. Here we describe an integrated genetic and physiological analysis of Fe nutrition in maize kernels, to identify loci that influence grain Fe concentration and bioavailability.MethodologyQuantitative trait locus (QTL) analysis was used to dissect grain Fe concentration (FeGC) and Fe bioavailability (FeGB) from the Intermated B73 × Mo17 (IBM) recombinant inbred (RI) population. FeGC was determined by ion coupled argon plasma emission spectroscopy (ICP). FeGB was determined by an in vitro digestion/Caco-2 cell line bioassay.ConclusionsThree modest QTL for FeGC were detected, in spite of high heritability. This suggests that FeGC is controlled by many small QTL, which may make it a challenging trait to improve by marker assisted breeding. Ten QTL for FeGB were identified and explained 54% of the variance observed in samples from a single year/location. Three of the largest FeGB QTL were isolated in sister derived lines and their effect was observed in three subsequent seasons in New York. Single season evaluations were also made at six other sites around North America, suggesting the enhancement of FeGB was not specific to our farm site. FeGB was not correlated with FeGC or phytic acid, suggesting that novel regulators of Fe nutrition are responsible for the differences observed. Our results indicate that iron biofortification of maize grain is achievable using specialized phenotyping tools and conventional plant breeding techniques.

Two compounds, the C-glycosyl flavone maysin and the phenylpropanoid product chlorogenic acid (CGA), have been implicated in corn earworm (Helicoverpa zea Boddie) resistance in maize (Zea mays L.). Previous quantitative trait locus (QTL) analyses identified the pericarp color (p) locus, which encodes a transcription factor, as the major QTL for maysin and CGA. QTL analysis has also implicated the dihydroflavanol reductase (DFR; E.C. no. 1.1.1.219) locus anthocyaninless1 (a1) and the duplicate chalcone synthase (CHS; E.C. no. 2.3.1.74) loci colorless2 (c2) and white pollen1 (whp1) as genes underlying QTL for maysin and/or CGA synthesis. Epistatic interactions between p and a1 and between p and c2 were also defined. CHS catalyzes the first step in the flavonoid pathway and represents one of the first enzyme steps following the branch off the general phenylpropanoid pathway towards CGA synthesis. In maize, the reduction of dihydroflavanol to leucoanthocyanin by DFR immediately follows the pathway branch leading to C-glycosyl flavone production. The detection of QTLs for maysin and CGA concentration at loci encoding enzyme steps following the pathway branch points implicates alterations in the flow of biochemical intermediates as the biological basis of the QTL effects. To examine if sequence variation among alleles of a1, c2, and whp1 affect maysin and CGA synthesis in maize silks, we performed an association analysis. Because the p locus has often been a major QTL for maysin and CGA and has exhibited epistatic interactions with a1, c2, and whp1, association analysis was conditioned on the p genotype. A highly significant association of two sequence polymorphisms in the promoter of a1 with maysin synthesis was demonstrated. Additional conditioning on the genotype of the significant a1 polymorphism allowed the detection of a significant polymorphism within the whp1 promoter. Our analyses demonstrate that conditioning for epistatic factors greatly increases the power of association testing.