Exotic 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.

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.

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.

Fusarium verticillioides (Sacc.) Nirenberg (teleomorph Gibberella moniliformis Wineland) is a fungal pathogen of maize that causes ear rots and stalk rots worldwide. In addition, it produces a group of mycotoxins called fumonisins when the fungus colonizes maize and maize-based products. Fumonisin B1 (FB1), the predominant form occurring in nature, can cause detrimental health effects in animals and humans. Several efforts were made to study the host and pathogen factors that contribute to the production of fumonisins. Using the available genomic resources, three genes with a potential role in FB1 regulation and development were identified. The genes are GBP1, GBB1 and GAP1. This research describes molecular characterization of these genes with respect to regulation of FB1 and development in F. verticillioides. GBP1 is a monomeric GTP binding protein with similarity to DRG and Obg sub-classes of G-proteins. GBB1 encodes heterotrimeric GTP binding protein β subunit. GAP1 is a GPI (Glycophosphotidylinositol) anchored protein, which belongs to a family of cell wall proteins. Targeted deletion and complementation studies indicated that GBP1 is negatively associated with FB1 biosynthesis but had no effect on conidiation in F. verticillioides. GBB1 plays an important role in regulation of FB1 biosynthesis, conidiation and hyphal growth, but not virulence. GAP1 is associated with growth, development and conidiation but not in positive regulation of FB1 or pathogenicity. The outcome of this study revealed new molecular genetic components that will help scientists better understand signal transduction pathways that regulate FB1 biosynthesis and conidiation in F. verticillioides.

Exotic 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.

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.

The 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.

Sorghum downy mildew (SDM) of maize is caused by the oomycete Peronosclerospora sorghi (Weston and Uppal) C. G. Shaw. The disease can cause devastating yield losses in maize (Zea mays L.). Quantitative trait loci (QTLs) mediating resistance to SDM were mapped using both restriction fragment length polymorphisms (RFLPs), and simple sequence repeats (SSRs) in 220 F2 individual maize progeny derived from a cross between two extremes; highly susceptible inbred parent SC-TEP5-19-1-3-1-4-1-1 (white) and highly resistant inbred P345C4S2B46-2-2-1-2-B-B-B (yellow). The phenotypic expression was assessed on F2:3 families in a wide range of environments under natural field infection and in a controlled greenhouse screening method. Heritability estimates of disease reaction ranged from 93.3% in Thailand sit 1 to 48% in Thailand sit 2. One hundred and thirty three polymorphic markers were assigned to the ten chromosomes of maize with LOD scores exceeding 4.9 covering about 1265 cM with an average interval length between markers of 9.5 cM. About 90% of the genome was located within a 10 cM distance to the nearest marker. Three putative QTLs were detected in association with resistance to SDM in different environments using composite interval mapping. Despite environmental and symptom differences, one QTL on chromosome 2 bin 9 had a major effect in all trials and explained up to 70% of the phenotypic variation in Thailand where the highest disease pressure was experienced. Two other QTLs on chromosome 3 bin 5 and chromosome 9 bin 2 had a minor effect, each explaining no more than 4% of the phenotypic variation. These results revealed one major gene and two minor genes that control sorghum downy mildew resistance. These markers should be very useful in breeding programs in facilitating the introgression of the resistance genes into commercial varieties. Marker-assisted selection for these loci should be useful in incorporating SDM resistance genes in maize across environments, even in the absence of the pathogen.

Heterosis, the phenotypic superiority of first generation progeny over that of its inbred parents, has been extensively investigated. However, differences in the phenology and dynamic growth patterns between parents and offspring provide challenges in understanding causal factors behind superior trait values. For maize, manipulation of heterosis to increase grain yield has been of primary importance, and the number of spikelets that develop on the female inflorescence is the primary determinant of grain yield. The initial experiment examined heterosis in genetic backgrounds that led to minimal differences in phenology and plant architecture. Growth curves were used to characterize the dynamic expression of heterosis between the hybrid and the inbred parents for a series of vegetative and reproductive traits across stages of development. The second experiment was conducted to determine the effects that stress due to planting density might have on the number of properly developed spikelets, as the first experiment, along with results from the literature, provided evidence to suggest that a proportion of the later forming spikelets found on the distal portion of the female inflorescence were not capable of producing kernels. Results from the initial experiment suggested that expression of heterosis for individual characteristics – such as fresh weight, whose percent mid-parent heterosis was 82% at V4 and declined steadily to 17% at V11 – begins at a high level and decreased during development. On a whole plant level – as determined by a combined analysis of the values and growth rates of the individual characteristics – heterosis increases throughout development until it reaches a steady-state level. Results from the second experiment indicated that increasing plant density did not affect the total number of spikelets per ear but decreased the number of kernels per ear, and it was found that the number of properly developed spikelets per ear was equal to the number of kernels per ear in eight of the nine genotypes tested. Optimal growth stages for more in-depth investigation of transcriptomic changes that may identify causal genetic factors of heterosis for yield were not found, and stress increases the proportion of improperly developed spikelets causing a loss in kernel number. / Natural Sciences and Engineering Research Council of Canada, Ontario Ministry of Agriculture, Food and Rural Affairs, Canadian Foundation for Innovation, and Ontario Innovative Trust

Maize (Zea mays L.) streak virus disease (MSD) is the most important virus disease in Africa but farmers are unaware of its status. A project was initiated to assess the current status of MSD and to breed for its resistance. Four populations comprised of two BC1F1 and two F2 progenies developed by backcrossing and selfing the F1 progenies of two crosses between a donor line (CMl 202) and two susceptible lines (CMl 321 and CMl 384) were developed. Conventional and molecular marker assisted selection (MAS) methods were used to screen for resistance to MSD in each of the four populations. To facilitate unbiased comparison, separate screening nurseries were established for MAS and conventional screening. The objectives of the study were five-fold; 1) to assess the status of MSD in Uganda and understand farmers' preferences and varietal selection criteria for maize using a participatory rural appraisal (PRA), 2) to screen for MSD resistance in early generations of segregating maize populations using conventional method, 3) to screen for resistance to MSD using SSR marker assisted selection , 4) to compare the effectiveness of marker assisted selection and conventional methods for selection for resistance to MSD, and 5) to compare costs associated with MAS and conventional selection methods. Results of PRA showed that unreliable rainfall and insect pests were the dominant constraints to maize productivity in Uganda. Diseases were ranked fifth among the production constraints . Maize streak virus disease was considered the most important disease constraint. Farmers showed common preference for high yielding and early maturing cultivars. However, farmers had other special preferences which were diverse and included large, white and high test density kernels for marketing, and sweet taste, particularly for home consumption. Farmers' research priorities included tolerance to drought, resistance to insect pests and diseases, sweetness, prolificacy, resistance to lodging, and drooping leaves because theyt cover the soil fast and prevent weed growth. Conventional screening for resistance to MSD showed that backcross and selfing populations segregated in 1:1 and 3:1 Mendelian ratios confirming the presence of one major gene with simple inheritance . Severity and incidence of disease were positively correlated suggesting a non-reference by the insects. In the selfing populations, the presence of complete esistance against MSD was suggested because frequency distribution patterns were highly skewed in favour of resistance. There was a decrease in disease severities with selection from BC1F1 to BC2F1 and from F2 to F3 generations indicating that high response to selection was achieved. On the other hand, one marker, umc1917, consistently polymorphic and eo-dominant was selected and used in MAS protocol. Results showed that the observed outcomes fitted the expected ratio of 1:2:1 for a F2 population and 1:1 for a BC1F1 population (X2 not significant). Evaluation of F3 and BC2F1 progeny selected using markers showed low disease severity suggesting that marker assisted selection was effective. However, the study showed that the presence of the O'Tl, was not consistent with symptom expression in the field. Evaluation of lines in three-way crosses identified ten potential lines that were high yielding, highly resistant to MSD and stable across three locations. Both MAS and conventional selection were equally effective in identifying high yielding lines although resistance was higher under MAS. Costs of MAS and conventional method varied depending on the units for comparison. The total costs of conventional method were higher than that of MAS in both first and second selection cycles. Comparing costs per row for conventional and costs per plant or data point for MAS showed that conventional selection was 2.4 times more expensive than costs per sample for MAS. However, costs per plant for MAS were 6.6 times higher than for conventional selection. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2006