Master of Science / Department of Plant Pathology / Karen A. Garrett / Ecological clines often result in gradients of disease pressure in natural plant communities, imposing a gradient of selection on disease resistance genes. We describe the diversity of a resistance gene homolog in natural populations of the dominant tallgrass prairie grass, Andropogon gerardii, across a precipitation gradient ranging from 47.63 cm/year in western Kansas to 104.7 cm/year in central Missouri. Since moisture facilitates infection by foliar bacterial pathogens, plants along this precipitation gradient will tend to experience heavier bacterial disease pressure to the east. In maize, the gene Rxo1 confers resistance to the pathogenic bacterium Burkholderia andropogonis. Rxo1 homologs have been identified in A. gerardii and B. andropogonis is known to infect natural populations of A. gerardii. The spatial genetic structure of A. gerardii was assessed from central Missouri to western Kansas by genotyping with AFLP markers. Samples were also genotyped for Rxo1 homologs by amplifying an 810 base pair region of the leucine-rich repeat and digesting with restriction enzymes. We compared Rxo1 homolog diversity to AFLP diversity across different spatial scales. Genetic dissimilarity based on AFLP markers was lower than would have occurred by chance at distances up to 30 m, and different prairies were more dissimilar than would have occurred by chance, but there was not a longitudinal trend in within-prairie dissimilarity as measured by AFLP markers. Dissimilarity of the Rxo1 homologs was higher in the east suggesting the presence of diversifying selection in the more disease-conducive eastern environments.

Doctor of Philosophy / Department of Agronomy / Kassim Al-Khatib / Mitchell R. Tuinstra / Sorghum acreage is declining throughout the United States because management options and yield have not maintained pace with maize improvements. The most extreme difference has been the absence of herbicide technology development for sorghum over the past twenty years. The objectives of this study were to evaluate the level of resistance, type of inheritance, and causal mutation of wild sorghums that are resistant to either acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides or acetohydroxyacid synthase (AHAS)-inhibiting herbicides. ACCase-inhibiting herbicides used in this study were aryloxyphenoxypropionate (APP) family members fluazifop-P and quizalofop-P along with cyclohexanedione (CHD) family members clethodim and sethoxydim. The level of resistance was very high for APP herbicides but low to nonexistent to CHD herbicides. With genetic resistance to APP herbicides, the resistance factors, the ratio of resistance to susceptible, were greater than 54 to 64 for homozygous individuals and greater than 9 to 20 for heterozygous individuals. Resistance to CHD herbicides was very low with resistance factors ranging from one to about five. Genetic segregation studies indicate a single gene is the cause of resistance to APP herbicides. Sequencing identified a single mutation that results in cysteine replacing tryptophan (Trp-2027-Cys). Trp-2027-Cys has previously been reported to provide resistance to APP but not CHD herbicides. The other wild sorghum evaluated in this study was resistant to AHAS-inhibiting herbicides including imidazolinone (IM) family member, imazapyr, and sulfonylurea (SU) family member, nicosulfuron. Resistance factors in this genotype were very high, greater than 770 for the IM herbicide and greater than 500 for the SU herbicide, for both herbicide chemical families. Genetic segregation studies demonstrate that resistance was controlled by one major locus and two modifier loci. DNA sequencing of the AHAS gene identified two mutations, Val-560-Ile and Trp-574-Leu. Val-560-Ile is of unknown importance, but valine and isoleucine are similar and residue 560 is not conserved. Trp-574 is a conserved residue and Leu-574 is a known mutation that provides strong cross resistance to IM and SU herbicides. The results of these studies suggest that these sources of APP, SU, and IM resistance may provide useful herbicide resistance traits for use in sorghum.

Doctor of Philosophy / Agronomy / Allan K. Fritz / Bikram S. Gill / Gary M. Paulsen / Post-anthesis high temperature stress in wheat (Triticum aestivum L.) is a major cause of yield reduction. This process results in the loss of viable leaf area and a decrease in green leaf duration ultimately causing a yield loss. The objectives of this study were to (i) phenotype a recombinant inbred line population for heat tolerance traits, (ii) understand the genetic basis of heat tolerance by mapping quantitative trait loci (QTL) linked to yield-related traits under high temperature, (iii) model stay-green under high temperature stress and map the QTL linked to stay-green parameters, and (iv) validate the markers linked to QTL under field conditions. A filial6:7 (F6:7) recombinant inbred line (RIL) population was developed by crossing Ventnor, a heat-tolerant white winter wheat with Karl 92, a relatively heat susceptible hard red winter wheat. From 10 DAA to maturity, the treatments of optimum temperature or high temperature stress (30/25°C) were imposed on the RILs. The traits measured included grain filling duration (GFD), kernels per spike, thousand kernel weight (TKW), and grain filling rate (GFR). The stay-green traits calculated were: i) time between 75% and 25% green, ii) maximum rate of senescence, iii) time to maximum rate of senescence, and v) percent green at maximum senescence. Genetic characterization was performed using microsatellite (SSR), amplified fragment length polymorphism (AFLP) and a sequence tag site (STS) markers. GFD was positively correlated with TKW and negatively with GFR and maximum rate of senescence. Principle component analysis (PCA) showed kernels per spike, maximum rate of senescence, and TKW accounted for 98% of total variability among the genotypes for heat tolerance. The most significant QTL for yield traits co-localized with marker Xgwm296 for TKW, Xgwm356 for kernels per spike, and Xksum61 for GFR. The QTL for stay-green traits co-localized with markers P41/M62-107 on Chromosome 2A, Xbarc136 on Chromosome 2D, P58/MC84-146 on Chromosome 3B, P58/M77-343 on Chromosome 6A, and. P58/MC84-406 on Chromosome 6B. These results indicate that increased green leaf area duration has a positive effect on the grain yield under high temperature. Once the kernels per spike are established, GFD and TKW can be used as selection criteria for post-anthesis heat-tolerance.