Browsing by Subject "maize"
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Item Breeding Maize for Drought Tolerance: Diversity Characterization and Linkage Disequilibrium of Maize Paralogs ZmLOX4 and ZmLOX5(2012-07-16) De La Fuente, GeraldMaize production is limited agronomically by the availability of water and nutrients during the growing season. Of these two limiting factors, water availability is predicted to increase in importance as climate change and the growing urban landscape continue to stress limited supplies of freshwater. Historically, efforts to breed maize for water-limited environments have been extensive; especially in the areas of root architecture and flowering physiology. As progress has been made and new traits have been discovered and selected for, the different responses to drought stress at specific developmental stages of the maize plant have been selected as a whole when drought tolerance is evaluated. Herein we attempt to define the characteristics of the maize drought response during different developmental stages of the maize plant that can be altered through plant breeding. Towards breeding for drought tolerance, 400 inbred lines from a diversity panel were amplified and sequenced at the ZmLOX4 and ZmLOX5 loci in an effort to characterize their linkage disequilibrium and genetic diversity. Understanding these characteristics is essential for an association mapping study that accompanies this project, searching for novel and natural allelic diversity to improve drought tolerance and aflatoxin resistance in maize. This study is among the first to investigate genetic diversity at important gene paralogs ZmLOX4 and ZmLOX5 believed to be highly conserved among all Eukaryotes. We show very little genetic diversity and very low linkage disequilibrium in these genes, but also identified one natural variant line with knocked out ZmLOX5, a variant line missing ZmLOX5, and five line variants with a duplication of ZmLOX5. Tajima's D test suggests that both ZmLOX4 and ZmLOX5 have both been under neutral selection. Further investigation of haplotype data revealed that ZmLOX12, a member of the ZmLOX family, showed strong LD that extends much further than expected in maize. Linkage disequilibrium patterns at these loci of interest are crucial to quantify for future candidate gene association mapping studies. Knockout and copy number variants of ZmLOX5, while not a surprising find, are under further investigation for crop improvement.Item Bt vs. non-Bt corn (Zea mays L.) hybrids: effect on degradation of corn stover in soil(2010-07-14) Salvatore, Herminia T.A billion tons per year of genetically modified corn residues are soil incorporated having both direct and indirect effects on the belowground environment, soil carbon (C) sequestration, and nutrient cycling. If Bt genetic modification has non-target effects on corn stover structural/non-structural carbohydrate and nitrogen (N) concentrations, then the degradation rate of Bt-corn stover may be different than that of non-Bt isolines, possibly influencing soil C storage and N mineralization. Thus, this research focused primarily on the comparison of C and N mineralization of corn stover in soil as affected by Bt-trait, plant portion, water-availability and HFC-trait; and secondarily on the existence of Bt-related variations in the chemical structure of corn residues that might affect the degradation rate of stover in soil and consequently the soil C and N dynamics. A laboratory experiment was conducted under non-limiting N conditions with stover of Bt/non-Bt isogenic pairs of two varieties, a ?high fermentable corn? (HFC) line harvested at Snook, Texas and a non-HFC corn line harvested at the irrigated field of Snook and the non-irrigated field of College Station, Texas. The stover was partitioned into three plant portions, incorporated into a Weswood soil and incubated during 223 days. Results showed that the differences observed in the degradation in soil of Bt vs. non-Bt corn stover were dependent on environmental conditions (irrigated vs. non-irrigated settings) and hybrid variety (HFC vs. non-HFC hybrid lines). The structural composition of corn plants was affected by the Bt-trait, HFC-trait, irrigation and their interactions. Variations in the biomass fractions of the initial stover of Bt and non-Bt hybrids had minimum to non-impact on soil C and N concentrations measured at the end of the 223-day incubation period. Lignin concentration was affected by a Bt-trait*variety interaction. There were no significant differences in lignin concentration between non-Bt/Bt-corn derived stovers of the non-HFC variety irrespective of irrigation regime but Bt-hybrids of the HFC variety contained more than twice as much lignin as the non-Bt isogenic plants. The effects of higher lignin concentration on C mineralization rate appeared to be offset by an increased lignin degradability inherent in HFC-trait. Overall, results indicated that the cultivation of Bt-modified maize lines is not likely to have significant effects on soil C or N dynamics compared with the cropping of non-Bt hybrids.Item Food quality and properties of quality protein maize.(Texas A&M University, 2004-09-30) Leal Diaz, Ana MariaQuality protein maize (QPM), high protein corn (HPC) and food grade maize (FGM) were processed into tortillas and direct expanded extruded snacks. QPM had similar test weight, density and kernel size with 45% more lysine and 38% more tryptophan compared to FGM. HPC had the largest kernel with density and test weight similar to FGM. During alkaline cooking, HPC absorbed water faster than QPM and FGM. White QPM required shorter cooking time and had less dry matter losses compared to FGM. All corn varieties had excellent pericarp removal at the optimum cooking time. Tortillas from QPM had better pliability and rollability after storage compared to FGM and HPC. HPC tortillas had lower rupture force after storage. The use of QPM for tortilla production may reduce energy and sewage cost, and could produce a tortilla with longer shelf stability with improved nutritional value. Decorticated and non-decorticated QPM, FGM and HPC grain were processed into corn meal and direct expanded snacks. A modified short scale dry milling system was used to produce the corn meal. QPM produced more coarse meal with greater fat content compared to FGM. Decortication decreased fiber content and coarse meal yield. Non-decorticated meal had greater protein, fiber and fat content compared to decorticated meal. The modified short flow milling system provides reduced lost fractions for extrusion into nutritionally improved products. Extrusion was performed in a low cost friction extruder. QPM extruded faster than FGM and HPC. FGM required greater specific mechanical energy than QPM. Extrudates from FGM were the most expanded followed by QPM and HPC. Extrudates from the three corn varieties were acceptable to the panelists and decortication did not affect acceptability. The improved nutritional value of QPM, was retained during dry milling and extrusion. Current QPM varieties can be processed into tortillas with longer shelf stability and meal for extrusion into a wide variety of snacks and other foods. These may have application in specialty health foods and in developing countries where maize is a staple food.Item Genomic analysis of 12-oxo-phytodienoic acid reductase genes of Zea mays(Texas A&M University, 2006-04-12) Zhang, JinglanThe 12-oxo-phytodienoic acid reductases (OPRs) are enzymes of the octadecanoid pathway which converts linolenic acid to a phytohormone, jasmonic acid. Bioinformatics analysis of ESTs and genomic sequences from available private and public databases revealed that the maize genome encodes eight different OPR genes. This number of maize OPR genes has been independently confirmed by Southern blot analysis and by mapping of individual OPR genes to maize chromosomes using oat maize chromosome addition lines. Survey of massively parallel signature sequencing (MPSS) assays revealed that transcripts of each OPR gene accumulate differentially in diverse organs of maize plants. This data suggested that individual OPR genes may have a distinct function in development. Similarly, RNA blot analysis revealed that distinct OPR genes are differentially regulated in response to stress hormones, wounding or pathogen infection. ZmOPR1 and ZmOPR2 appear to have important functions in defense responses to pathogens because they are transiently induced by salicylic acid (SA), chitooligosaccharides and by infection with Cochliobolus carbonum, Bipolaris maydis and Fusarium verticillioides and not by wounding. In contrast to these two genes, ZmOPR6 and ZmOPR7/8 are highly induced by wounding and treatments with wound-associated signaling molecules jasmonic acid, ethylene and abscisic acid. ZmOPR6 and ZmOPR7/8 are not induced by SA treatments or pathogen infections suggesting their specific involvement in wound-induced defense responses. Possible functions of specific OPR genes are discussed.Item Genotypic and phenotypic characterization of response to aflatoxin and secondary traits in maize(Texas A&M University, 2006-08-16) Edwards, Melanie LoveOne major problem facing maize producers in the southern US is contamination with the mycotoxin aflatoxin, produced by Aspergillus flavus (Link:fr). Aflatoxin is a serious threat to human and animal health, with no resistant commercial hybrid available. Development of resistance to aflatoxin production has several major limitations. Aflatoxin is highly variable both across and within environments, even under inoculation, requiring several locations and replications for breeding. Additionally, there is no screening method that is reliable, rapid, inexpensive, and allows for high throughput. Several secondary traits, such as kernel texture, kernel integrity, husk cover, and visible ear rot, have previously shown to be related to aflatoxin accumulation. These traits are easily characterized in the field and are candidates for indirect selection if they are correlated to aflatoxin concentration. Root lodging, a plant??s inability to maintain upright stature, is another complex characteristic of root related traits that traditionally is selected for indirectly. It can greatly reduce harvestable yield. It is affected by morphological traits and environmental conditions, but its genetic components are little understood. This dissertation comprises three studies presented in chapters II, III, and IV. Chapter II involved white and yellow hybrid maize trials as well as quality protein maize trials from several years across Texas environments. Data was analyzed both per and across location to determine repeatability of response to aflatoxin. Additionally, aflatoxin levels were correlated to several secondary characteristics (female flowering, endosperm texture, husk cover, and ear rot ratings) to determine usefulness in indirect selection. Chapter III was a phenotypic evaluation of a recombinant inbred line (RIL) mapping population, which was derived from divergent parental inbreds Tx811 and CML176. The trials were conducted in two Texas locations, and phenotypic data for aflatoxin concentration, kernel integrity, endosperm texture, female flowering date, and root lodging was collected. Variance components for these traits and genetic and phenotypic correlations were determined. Chapter IV was a genotypic evaluation of the Tx811/CML176 mapping population using simple sequence repeat markers. Genotypic and phenotypic data were combined to identify quantitative trait loci (QTL) and epistatic interactions for response to aflatoxin and for root lodging.Item Multi-Location Evaluation of Agronomic Traits in Maize Hybrids(2012-12-12) McKee, Michael 1982-Maize (Zea mays L.) is one of the main crops grown in the United States. Genetic improvement over the last century has seen a shift from using open-pollinated varieties to single cross hybrids. This has resulted in major grain yield gains and improved management methodologies. However, there is still concern about reduced genetic diversity in elite corn germplasm and the potential effects this could have on future maize productivity in the presence of numerous abiotic and biotic pressures. One solution to this issue is the incorporation of exotic germplasm into existing maize improvement programs. This exotic material must be evaluated and characterized because too much or poorly matched exotic material can lead to reduced productivity. The use of multiple environments representative to the target improvement area is the best way to determine the true potential of certain material. The objectives of this research were to: i) estimate the responses of hybrids to aflatoxin and their agronomic performance across a range of environments under inoculation with Aspergillus flavus; ii) identify the hybrids within each group that exhibit the lowest levels of contamination; iii) analyze the relationship between agronomic performance and aflatoxin accumulation; and iv) determine how Genotype x Environment interactions affect these traits. Agronomic data was collected in ten Texas environments in 2005 for hybrids created from yellow, white, and Quality Protein Maize material that was crossed with one of two elite temperate inbred testers, LH195 or LH210. Response to aflatoxin was measured in eight of these environments. U.S. commercial hybrids were used as checks. Significant differences between hybrids were observed at different environments for different traits. Overall the experimental hybrids had lower aflatoxin accumulation than the commercial checks. They also yielded lower and had lower test weights and 1000 kernel weights. However, there were some hybrids that were competitive with the commercial checks for these agronomic traits. The incorporation of this material into established U.S. lines could be beneficial with regards to aflatoxin accumulation and kernel quality, which could ultimately translate to higher yields and crop quality.Item Preharvest aflatoxin in maize genotypes under inoculation with Aspergillus flavus(2009-05-15) Mayfield, Kerry L.Pre-harvest aflatoxin contamination is a major limitation to maize production in Texas and the southern United States, causing major economic loss and severe health problems worldwide. Screening for resistance to aflatoxin accumulation is commonly conducted through inoculation with a highly concentrated solution of Aspergillus flavus FR: Link spores, a naturally occurring fungus which infects maize and produces a toxic metabolite (aflatoxin) to humans and animals consuming the grain. No commercial hybrids exist with full resistance to aflatoxin accumulation; however, sources exist to reduce susceptibility. These sources commonly lack good agronomic characteristics for use in commercial hybrids. Exotic germplasm with favorable traits for reduced aflatoxin accumulations are introgressed with temperate and locally adapted lines. This program utilizes only one isolate of A. flavus even though many isolates exist in the environment. The objectives of this thesis are i) to evaluate the progress of the Maize Breeding and Genetics Program?s accomplishments of breeding maize for the reduction in susceptibility of aflatoxin accumulation in yellow inbreds through analysis of hybrid and inbred per se trials and ii) to determine whether interaction exists between geneticallydifferent isolates of A. flavus and several genotypes of maize. Response to aflatoxin accumulation for hybrids and inbreds was measured at up to three environments across Texas. Significant differences were detected for most years and environments. Maize lines CML285, CML288, CML323, CML325, CML326, CML338, Tx601y and lines derived from Population 69 and from Tx772 crosses in hybrid combinations tended to accumulate less aflatoxin than commercial hybrid checks. Significant differences were detected at each environment aflatoxin accumulation was measured for inbred lines per se. Inbreds Tx772, Tx601y, CML289, CML294, CML323 and derived lines from Population 69 show reduced aflatoxin accumulations. Interaction between genetically different isolates of A. flavus and several genotypes of maize were not detected in hybrid or inbred trials at two or three environments, across locations and across years. Introgression of exotic germplasm into locally adapted germplasm has improved agronomic characteristics for use in the Southern U.S. and brought sources for decreased aflatoxin accumulation.Item Price discovery in the wholesale markets for maize and beans in Uganda(Texas A&M University, 2006-08-16) Kuteesa, AnnetteMarket information services established in 1999 were aimed at the promotion of market efficiency through provision of information across the nation. While the responsible bodies have improved the knowledge of prices, information exchange and flow, as a result of competition between markets, is not known and questions of market effectiveness still stand. This study examines market efficiency based upon response to price signals across Ugandan markets. We focus on information exchange for maize and beans among 16 key markets. We study weekly price data from the first week of 2000 to the last week of 2003 from each of the sixteen markets. Each commodity is studied separately using Vector Autoregessions (VARs) and Directed Acyclic Graphs (DAGs). The two techniques are widely used to show market risk and causal relations in time series data. While results are presented individually for each commodity, the markets are comparable. In determining market efficiency, we test for stationarity of the data, explore the magnitude of forecast error decompositions over time across markets, and observe the patterns of communication based on DAGs. We find that markets are more efficient in exchanging information on maize than beans. Communication of data is mostly between markets in eastern, western, and central parts of Uganda. Overall, markets are very slow in reacting to information in the short run.Information from the Mbale and Iganga markets, which are located in areas of high production, is very valuable in the maize trade. However, of the two markets, it is data from the Mbale market, located near the border with Kenya, which is of paramount importance. Specifically, price is discovered in Mbale in the maize trade. Our results also show the Gulu market, which is situated in an insecure zone, to be very responsive to price signals over the long run. In the case of beans, it is the price signals from Tororo and Jinja that cause more disruption in most of the markets. Price is discovered in these two markets. A majority of the markets is more affected by data from Jinja than Tororo. This segmentation in market price discovery suggests an existing market failure. Arua and Gulu are found to be the least responding markets in regards to price signals for beans. We do not find information from the Kampala market to be important in either the maize or beans trade.Item Response to aflatoxin and grain composition of exotic maize germplasm(2009-06-02) Corn, Rebecca JoannExotic germplasm has potential to provide new alleles for disease and insect resistance. US maize (Zea mays L.) currently lacks genetic resistance to Aspergillus flavus, a fungal pathogen that produces aflatoxin in maize kernels. Aflatoxin is one of the main limitations to maize production in hot, dry regions like the Southern US because of the harmful effects on humans and animals and subsequent marketing regulations. Two experiments were conducted to evaluate different exotic maize collections for response to aflatoxin. Exotic adapted maize lines, known as LAMA lines, were found to accumulate less aflatoxin than US hybrids in tests across Southern Texas. Exotic introgression lines developed by The International Center for Maize and Wheat Improvement (CIMMYT) including inbred lines, yellow hybrids, and white hybrids, were more resistant to aflatoxin than US inbred lines and hybrids in field trials in Texas, Georgia, and Mississippi. Another experiment evaluated the grain composition of hybrids with exotic adapted LAMA maize lines and a collection of US hybrids, quality protein maize (QPM) hybrids, and advanced breeding lines using near-infrared spectroscopy. Individual LAMA lines and advanced breeding lines have higher starch content than US hybrid checks. Starch content was the primary grain composition trait of interest as an enhanced-value market has emerged for high starch maize hybrids. Limited germplasm has been analyzed for grain composition because wet chemistry analysis methods required large sample sizes and were time and labor intensive. The near infrared spectroscopy (NIR) method requires a relatively small sample and is a non-destructive analysis method. In this study, NIR was effective at ranking genotypes based on starch, oil, and protein content of the grain.Item Unraveling Reciprocal Lipid-Mediated Communication between Maize Seed and Aspergillus flavus(2014-07-31) Borrego, Eli JamesIt is generally accepted, that the reciprocal exchange of molecules between plants and fungi govern the outcome of their interaction. From a multitude of potential signals, one class of oxidized lipids (oxylipins) has taken central stage in this concept. Synthesized from enzymatic and non-enzymatic peroxidation of fatty acids, oxylipins are a large and diverse group of potent endogenous signaling molecules. Because plant and fungal oxylipins are similar biochemically and structurally, a novel hypothesis has emerged that during plant-fungal interactions, that these metabolites are exchanged, perceived, and affect the behavior of opposing partner. This study sought to explore this hypothesis and assign a role to specific oxylipin-producing enzymes from maize and Aspergillus flavus within the context of the oxylipin-mediated cross-kingdom crosstalk. Maize wild-type and near-isogenic mutants for several lipoxygenase (LOX) and 12-oxophytodienoate reductases (OPR) related to jasmonic acid biosynthesis and nine oxylipin-mutant strains of Aspergillus flavus were utilized to investigate the contribution of oxylipins from the plant and the fungi on fungal ability to colonize the host, sporulate and produce aflatoxin. Phytohormone content and gene expression analyses of infected seed were preformed to explore potential associations of defense hormones with fungal pathogenesis processes. Results showed that several genes involved in JA biosynthesis affect specific fungal processes. Despite belonging to separate subgroups, both LOX3 and LOX7 are involved in defense against colonization. Additionally, LOX3 expression correlates negatively with colonization and lox3 knockout mutants are more susceptible regardless of the fungal genotype indicating a general defensive role for LOX3 against A. flavus colonization. An unexpected major finding from this study is that JA biosynthesis genes appear to promote aflatoxin accumulation. Results provide evidence that the fungal LOX gene is required for normal colonization of seed, while PpoA is required for normal conidia and aflatoxin production. The ratio of ABA/JA correlates positively with aflatoxin accumulation. These findings are expected to expedite studies of seed-fungal interactions, lead to uncovering novel regulators of seed defense, find specific host genes and signals that regulate conidiation and mycotoxin production, and eventually provide the maize industry with genetic targets and biochemical markers for selecting aflatoxin resistant lines.