Inbred Mo17 Research Articles

The genetic basis of flecking and its relationship to disease resistance in the IBM maize mapping population.

In this paper, we determine the genetic architecture controlling leaf flecking in maize and investigate its relationship to disease resistance and the defense response. Flecking is defined as a mild, often environmentally dependent lesion phenotype observed on the leaves of several commonly used maize inbred lines. Anecdotal evidence suggests a link between flecking and enhanced broad-spectrum disease resistance. Neither the genetic basis underlying flecking nor its possible relationship to disease resistance has been systematically evaluated. The commonly used maize inbred Mo17 has a mild flecking phenotype. The IBM-advanced intercross mapping population, derived from a cross between Mo17 and another commonly used inbred B73, has been used for mapping a number of traits in maize including several related to disease resistance. In this study, flecking was assessed in the IBM population over 6 environments. Several quantitative trait loci for flecking were identified, with the strongest one located on chromosome 6. Low but moderately significant correlations were observed between stronger flecking and higher disease resistance with respect to two diseases, southern leaf blight and northern leaf blight and between stronger flecking and a stronger defense response.

Transcriptome analysis of maize leaf systemic symptom infected by Bipolaris zeicola.

Bipolaris zeicola is a fungal pathogen that causes Northern corn leaf spot (NCLS), which is a serious foliar disease in maize and one of the most significant pathogens affecting global food security. Here, we report a genome-wide transcriptional profile analysis using next-generation sequencing (NGS) of maize leaf development after inoculation with B. zeicola. We performed High-Throughput Digital Gene Expression analysis to identify differentially expressed genes (DEGs) in resistant inbred Mo17 lines after infection with B. zeicola at four successive disease development stages—CP (contact period), PP (penetration period), IP (incubation period), and DP (disease period); the expression of the genes was compared with those in a CK (mock-treatment) control. In addition, a sensitive maize line (Zheng58) was used for the comparisons with the Mo17. Among all tested genes, 466 differentially expressed genes were identified in all libraries, and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of these genes suggested that they are involved in many biological processes related to systemic symptom development, such as plant hormone signal transduction, starch and sucrose metabolism, phenylpropanoid biosynthesis and photosynthesis. Our systematic analysis provides comprehensive transcriptomic information regarding systemic symptom development in fungal-infected plants. This information will help in furthering our understanding of the detailed mechanisms of plant responses to fungal infection.

Registration of High‐Methionine Versions of Maize Inbreds A632, B73, and Mo17

Maize (Zea mays L.) is a primary energy‐supplying grain for animal feed in the United States. However, it is deficient in the essential amino acid methionine. Of the maize used for animal feed, 20% is fed to poultry, where the methionine requirement is particularly important. Significant increases in maize methionine levels in the lines reported here can be effectively used in poultry nutrition as well as in human diets where corn and bean (Phaseolus L.) play a major role. Eleven inbred lines of high‐methionine maize (58611 Inbred A632 [Reg. No. GP‐557, PI 648423], 58609 A632 (Meth) BC5S4 [Reg. No. GP‐558, PI 648424], 58610 A632 (Meth) BC5S4 [Reg. No. GP‐559, PI 648425], 58612 Inbred B73 [Reg. No. GP‐560, PI 648426], 58613 B73 (Meth) BC5S4 [Reg. No. GP‐561, PI 648427], 58614 B73 (Meth) BC5S4 [Reg. No. GP‐562, PI 648428], 58615 B73 (Meth) BC5S4 [Reg. No. GP‐563, PI 648429], 58801 Inbred Mo17 [Reg. No. GP‐564, PI 648430], 58802 Mo17 (Meth) BCS3 [Reg. No. GP‐565, PI 648431], 58803 Mo17 (Meth) BCS3 [Reg. No. GP‐566, PI 648432], and 58804 Mo17 (Meth) BCS3 [Reg. No. GP‐567, PI 648433]) were developed by the University of Minnesota and released on 26 Oct. 2007 by the Minnesota Agricultural Experimental Station. These materials were produced by crossing BSSS53 with inbred lines A632, B73, and Mo17 and backcrossing to the respective inbred with intermittent selfing. Maize inbred lines A632, B73, and Mo17 are released with methionine elevated as much as 12.5, 25.0, and 50.0%, respectively, above the recurrent parent. The germplasm source of the high methionine trait is a random line isolate from the Iowa Stiff Stalk Synthetic, identified as BSSS53 and later released by the Iowa State Experiment Station as B101 after obtaining the high methionine information from the University of Minnesota.

Distribution of Genes, Recombination, and Repetitive Elements in the Maize Genome

A high‐density genetic map of maize (Zea mays L.), integrated with a high‐resolution bacterial artificial chromosome (BAC) contig map compiled from two inbred lines, B73 and Mo17, was used to determine gene density and recombination frequency along chromosomes and to compare the distribution of these features as a function of physical distance. Recombination frequency is well correlated with gene density for each chromosome (r = 0.767–0.960), although in centromeric regions, recombination is more depressed than a strict correlation with gene density would indicate. In addition, the distribution of major retrotransposon classes and of Mu and Helitron insertion sites were examined. Overall retrotransposon density is equal at all maize chromosomes (r = 0.996), although different classes of retrotransposons display preferential insertion into different chromosome regions. Cinful, Zeon, and Prem are more common near the centromere, while Opie, Ji, and Huck preferentially reside at the subtelomeric segments of chromosomes. Grande does not show preference in distribution relative to physical location on the chromosome. Comparison of the distribution of genic overgo probe hybridization‐positive BACs from maize inbred line B73 to those of the inbred Mo17 indicates that nonshared genic segments, many of which correspond to pseudogenes carried by the Helitron elements, tend to be less common in the centromeric region. As previously reported, Mu transposons seem to preferentially insert in the vicinity of genes, as indicated by their paucity in centromeric regions, and moderate correlation (r = 0.759) with gene density.

Quantitative trait loci for callus initiation and totipotency in maize (Zea mays L.)

Induction of embryogenic callus in culture is an important step in plant transformation procedures, but response is genotype specific and the genetics of the trait are not well understood. Quantitative trait loci (QTL) were mapped in a set of 126 recombinant inbred lines (RILs) of inbred H99 (high Type I callus response) by inbred Mo17 (low Type I callus response) that were evaluated over two years for Type I callus response. QTL were observed in a total of eleven bins on eight chromosomes, including eight QTL with main effects and three epistatic interactions. Many of the QTL were mapped to the same or bordering chromosomal bins as candidate genes for abscisic acid metabolism, indicating a possible role for the hormone in the induction of embryogenic callus, as has previously been indicated in microspore embryo induction. Further examinations of allelic variability for known candidate genes located near the observed QTL could be useful for expanding the understanding of the genetic basis of induction embryogenic callus. The QTL observed herein could also be used in a marker assisted selection (MAS) program to improve the response of agronomically useful inbreds, but only if the resources required for MAS are lower than those required for phenotypic selection.

Construction and characterization of normalized cDNA library of maize inbred MO17 from multiple tissues and developmental stages

Comprehensive complementary DNA (cDNA) library is a valuable resource for functional genomics. In this study, we set up a normalized cDNA library of Mo17 (MONL) by saturation hybridization with genomic DNA, which contained expressed genes of eight tissues and organs from inbred Mo17 of maize (Zea mays L.). In this library, the insert sizes range from 0.4 kb to 4 kb and the average size is 1.18 kb. 10.830 clones were spotted on nylon membrane to make a cDNA microarray. Randomly picked 300 clones from the cDNA library were sequenced. The cDNA microarry was hybridized with pooled tissue mRNA probes or housekeeping gene cDNA probes. The results showed the normalized cDNA library comprehensively includes tissue-specific genes in which 71% are unique ESTs (expressed sequence tags) based on the 300 sequences analyzed. Using BLAST program to compare the sequences against online nucleotide databases, 88% sequences were found in ZmDB or NCBI, and 12% sequences were not found in existing nucleotide databases. More than 73% sequences are of unknown function. The library could be extensively used in developing DNA markers, sequencing ESTs, mining new genes, identifying positional cloning and candidate gene, and developing microarrays in maize genomics research.

Triple Testcross Analysis to Detect Epistasis in Maize

Maize (Zea mays L.) breeders have successfully exploited heterosis by crossing inbred lines to develop hybrids. Epistatic effects can contribute to the expression of heterosis for specific hybrids. The hybrid B73 × Mo17 was a widely grown hybrid with exceptional performance in the central U.S. Corn Belt during the late 1970s and early 1980s. The objective of this study was to determine if epistatic effects contribute significantly to the performance of B73 × Mo17. With use of the triple testcross design, 100 random F2 plants from B73 × Mo17 were testcrossed to B73, Mo17, and their F1. The 300 testcrosses were evaluated at three locations in 1992 and two locations in 1993. Triple testcross analysis suggested that epistatic effects were important for ear length, number of kernel rows, ear height, and flowering traits. In the analysis of variance, additive by additive effects were not significant for grain yield, whereas additive by dominance and dominance by dominance effects were significant. The additive by additive by environment interaction was more important than additive by additive effects per se for grain yield. Epistatic deviations from the comparison of testcross means suggest that B73 had favorable additive by additive effects for grain yield, barren plants, kernel‐row number, ear height, and silk delay. Inbred Mo17 had favorable additive by additive effects for ear length. The presence of significant positive epistatic effects may have contributed to the expression of heterosis and could explain why B73 × Mo17 was an exceptional and widely grown hybrid.

Genetic Mapping of Quantitative Trait Loci in Maize in Stress and Nonstress Environments: I. Grain Yield and Yield Components

The utility of restriction fragment length polymorphisms (RFLPs) in detecting quantitative trait loci (QTL) in maize (Zea mays L.) populations has been established. Conflicting results have been reported on the ability to consistently detect the same QTL across environments. Grain yield is considered to be highly polygenic and strongly influenced by the environment. QTL mapping studies conducted in contrasting environments might increase our understanding genotype × environment interactions and improve selection response in maize breeding. Our objectives were to compare the QTL for grain yield and yield components detected in two diverse environments in an elite, adapted maize population and to investigate the relationship among the genetic factors controlling these traits. A population of 150 F2:3 lines was produced from the cross of elite inbreds Mo17 and H99. The lines per se were evaluated for 2 yr at the same location. The first year had relatively average growing conditions, but the second year with cool, wet growing conditions provided a stress environment. Grain yield and yield components were measured on replicated progeny each year. By using a linkage map of 111 loci, QTL determinations were made in the two environments and in the mean of the two environments. About 50% of all QTL detected across environments were detected in both enviromnents. These QTL were consistent in the magnitude of their effects and parental contribution. Determinations of QTL in the mean of the two environments were most informative in that 74% of all QTL and 82% of the QTL detected in each of the individual environments were identified in the mean environment. Most QTL identified regions associated with two or more yield components in a manner consistent with correlations among traits and parents phenotypes.

Genetic Mapping of Qunatitative Trait Loci in Maize in Stress and Nonstress Environments: II. Plant Height and Flowering

The utility of restriction fragment length polymorphisms (RFLPs) in mapping quantitative trait loci (QTL) has been established in elite maize (Zea mays L.) populations, hut the ability to consistently detect QTL in diverse environments has not been assessed. QTL mapping studies conducted in diverse environments could be a means of locating genes which respond to stress conditions and could provide further understanding of some types of genotype × environment interaction. Our objectives were to investigate the ability to detect QTL for flowering and plant stature traits in two diverse environments and to assess the genetic relationship of QTL for correlated traits. A single‐cross of elite inbreds Mo17 and H99 was used to produce a population of 150 F2:3 lines. Traits were measured on replicated progeny grown in two climatically diverse years at the same location, in which one year was an average environment, and the other year was a stress environment. QTL determinations were made in each environment and the mean of the two environments by using a linkage map of 111 loci. About 50% of all QTL were detected in both environments. The location and parental contribution of these QTL were consistent, hut their effects varied in size. More QTL were identified in the stress environment for anthesis and silk emergence, but the other traits had equivalent numbers of QTL detected in each environment. All QTL identified in the nonstress environment and all QTL identified in the stress environment were also detected in the mean environment. Correlations between traits were generally reflected in the similarity of locations and parental contributions of QTL for these traits. The exception was anthesis and anthesis‐to‐silking interval, which had QTL in common but very poor correlation. The locus of a probe for an1, a plant height mutant, was most closely associated with the QTL having the largest effect on plant height. This finding contributes to the supposition that quantitatively and qualitatively inherited traits share a common set of loci with alleles of varying magnitude of effects.

Molecular-marker-facilitated studies of morphological traits in maize. II: Determination of QTLs for grain yield and yield components

Genetic factors controlling quantitative inheritance of grain yield and its components have not previously been investigated by using replicated lines of an elite maize (Zea mays L.) population. The present study was conducted to identify quantitative trait loci (QTLs) associated with grain yield and grain-yield components by using restriction fragment length polymorphism (RFLP) markers. A population of 150 random F2∶3 lines was derived from the single cross of inbreds Mo17 and H99, which are considered to belong to the Lancaster heterotic group. Trait values were measured in a replicated trial near Ames, Iowa, in 1989. QTLs were located on a linkage map constructed with one morphological and 103 RFLP loci. QTLs were found for grain yield and all yield components. Partial dominance to overdominance was the primary mode of gene action. Only one QTL, accounting for 35% of the phenotypic variation, was identified for grain yield. Two to six QTLs were identified for the other traits. Several regions with pleiotropic or linked effects on several of the yield components were detected.

Repression of the high‐methionine zein gene in the maize inbred line Mo17

SummaryThe methionine content in maize kernels is largely determined by the level of a methionine‐rich 10 kDa protein, encoded by the gene Zps10/(22). In the maize inbred BSSS53 this gene is over‐expressed compared with its normal expression in inbred lines like W64A and W22. This is in part due to the trans‐acting factor Zpr10/(22). However, compared with the normal level of expression, this gene is under‐expressed in the inbred Mo17 and the high‐methionine zein accumulates to very low levels. Therefore, differential expression of this gene between BSSS53 and Mo17 is even larger than in other inbred lines of maize. Repression of 10 kDa zein expression in Mo17 is not due to the translatability of 10 kDa zein mRNA or a cis‐acting mutation in the mRNA and neither is it due to a change in the transcription rate per se. However, steady state levels of mRNA appear to be controlled in trans by narrowing the developmental time frame of mRNA accumulation.

Genetic diversity among progenitors and elite lines from the Iowa Stiff Stalk Synthetic (BSSS) maize population: comparison of allozyme and RFLP data

Data for restriction fragment length polymorphisms (RFLPs) of 144 clone-enzyme combinations and for 22 allozyme loci from 21 U.S. Corn Belt maize (Zea mays L.) inbreds were analyzed. The genetic materials included 14 progenitors of the Iowa Stiff Stalk Synthetic (BSSS) maize population, both parents of one missing BSSS progenitor, four elite inbreds derived from BSSS, and inbred Mo17. Objectives were to characterize the genetic variation among these 21 inbreds for both allozymes and RFLPs, to compare the results from both types of molecular markers, and to estimate the proportion of unique alleles in the BSSS progenitors. Genetic diversity among the 21 inbreds was substantially greater for RFLPs than for allozymes, but the percentages of unique RFLP variants (27%) and unique allozyme alleles (25%) in the BSSS progenitors were similar. Genetic distances between inbreds, estimated as Rogers' distance (RD), were, on average, twice as large for RFLP (0.51) as for allozyme data (0.24). RDs obtained from allozyme and RFLP data for individual line combinations were only poorly correlated (r = 0.23); possible reasons for discrepancies are discussed. Principal component analysis of RFLP data, in contrast to allozyme data, resulted in separate groupings of the ten BSSS progenitors derived from the 'Reid Yellow Dent' population, the four BSSS elite lines, and Mo17. The remaining six BSSS progenitors were genetically rather diverse and contributed a large number of rare alleles to BSSS. The results of this study corroborate the fact that RFLPs are superior to allozymes for characterizing the genetic diversity of maize breeding materials, because of (1) the almost unlimited number of markers available and (2) the greater amount of polymorphisms found. In particular, RFLPs allow related lines and inbreds with common genetic background to be identified, but a large number of probe-enzyme combinations is needed to estimate genetic distances with the precision required.

PHYSIOLOGICAL STUDIES ON THE EFFECTS OF DRYING TEMPERATURES ON CORN SEED QUALITY

The effects of drying temperatures on certain physiological parameters associated with germination of corn (Zea mays L.) seed were determined on two commonly grown inbreds. Seed produced on both maternal inbreds Mo17 (intolerant of high drying temperatures) and A632 (relatively tolerant) leached significantly more sugars into water when dried at 50 °C than at 35 °C. Leaching was most pronounced during the first hour. Electrical conductivity of leachate from maternal inbreds Mo17 and A632 was greater for seed dried at 50 °C than at 35 °C. Seedlings grown from embryos of the maternal inbred Mo17 seed dried at 50 °C had significantly lower shoot and root dry weights and percentage germination than did seedlings grown from embryos of Mo17 seed dried at 35 °C. Histochemical light microscopy of Mo17 and A632 embryos showed that the number of starch grains in the embryonic axis was substantially reduced by the high drying temperature. Embryos from Mo17 seed dried at 50 °C contained a carotenogenic pigment. Almost none of this pigment was produced by A632. Our results suggest that the high drying temperature may result in hydrolysis of starch in the embryonic axis of the seed during the early stages of the drying process. The greater electrical conductivity of leachate from seed dried at 50 °C than those dried at 35 °C may be indicative of membrane damage.Key words: Seed drying, histochemistry, embryo, culture, membrane, Zea mays L.

Heterotic behaviour of the transmembrane electric potential difference in some maize hybrid-parents combinations

In three hybrid-parents combinations the transmembrane (vacuole-exterior) electric potential difference (PD) of cortical cells of maize root is significantly (approx. 15%) higher in the hybrids than in their parental inbreds. To investigate the nature of this PD difference the PD of cortical cells of subapical maize root segments from the inbreds Mo17 and B73 and their hybrid was measured under the following conditions: (1) inhibition of glycolytic and mitochondrial ATP synthesis by AsO 4 3−; (2) inhibition of mitochondrial ATP synthesis and permeabilization of the plasmalemma to H + by carbonylcyanid- p-trifluorometoxyphenylhydrazon (FCCP); (3) activation of active electrogenic H + extrusion by fusicoccin. The results show that: (I) The inhibition of the active PD component by either AsO 4 3− or FCCP lowers the PD-values more in the hybrid than in the inbreds, so that after 30–60 min of treatment no difference is observed any more between the three lines. This suggests that the higher PD of the hybrid depends on a higher rate of operation of the active electrogenic mechanism. (II) The treatment with fusicoccin (FC) increases the PD-values of the three maize lines, but more so in the inbreds than in their hybrid. This may suggest that the potentiality of the electrogenic H + extrusion mechanism is more efficiently expressed in the hybrid.