Browsing by Author "Bedinger, Patricia A., committee member"

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Open Access
Dissecting drought tolerance in winter wheat using phenotypic and genetic analyses of agronomic and spectral traits
(Colorado State University. Libraries, 2015) Grogan, Sarah Marie, author; Byrne, Patrick F., advisor; Bedinger, Patricia A., committee member; Haley, Scott D., committee member; McMaster, Gregory S., committee member
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Open Access
Effects of elevated plant selenium levels on reproduction and root-nematode interactions
(Colorado State University. Libraries, 2011) Prins, Christine N., author; Pilon-Smits, Elizabeth A. H., advisor; Ward, Sarah M., committee member; Bedinger, Patricia A., committee member
Selenium is an important element in soils throughout the western United States, and its presence has important consequences for the ecology of these seleniferous sites. Some plants in seleniferous areas have evolved mechanisms to hyperaccumulate Se to 0.1 - 1% of their dry weight. Other plants accumulate moderate Se levels between 0.01 - 0.1% of their dry weight. In the studies described in this thesis, facets of the evolution of Se accumulation and the associated ecology of Se hyperaccumulators are considered. First, I examined the effect of increasing Se on reproductive parameters of Se accumulators and hyperaccumulators. The reproductive parameters were measured through cross-pollinations of greenhouse-grown accumulator plants receiving different Se concentrations. In the accumulator Brassica juncea , increasing Se concentrations in plant tissues caused decreases in biomass, pollen germination, seed weight, seed production, and seed germination. In some cases, however, interactions of similar Se concentrations in both parents actually proved beneficial to reproduction. The hyperaccumulator Stanleya pinnata showed no effect of increased Se concentration on pollen germination. These data provide interesting insight into the potential reproductive cost of Se accumulation, and the apparent evolution of physiological tolerance mechanisms in hyperaccumulators to avoid these reproductive problems. To further investigate the effect of Se on reproduction, S. pinnata plants were taken to a field site with hives of the European honey bee (Apis mellifera) to examine the effect of Se in floral tissues on potential pollinators. The bees and other pollinators showed no preference for or against Se in the flower and foraged on both high- and low-Se plants equally. Because the bees showed no preference, the honey of bees in seleniferous sites was analyzed for the presence of Se, and there were small amounts (up to 2 mg kg-1 FW) of Se found in this honey. These findings are important for bee keepers in seleniferous areas, as they show no evidence of toxic Se levels in their honey and they may even market their honey as Se-enriched and beneficial for human health. The finding that bees do not discriminate between high and low-Se plants does warrant further studies on the potential health effects of the ingested Se on the pollinators and the movement of Se into the food chain. Next, to further investigate the ecology of Se hyperaccumulators, I examined the interactions of hyperaccumulator roots with root-associated nematodes. Selenium hyperaccumulators S. pinnata and Astragalus bisulcatus growing in the field have root Se concentrations between 100 and 1,500 mg Se kg-1 DW, a toxic concentration to most above-ground herbivores. Therefore, it was expected that with increasing root Se concentrations, there would be reduced levels of nematodes associated with plants. There was no significant negative correlation with increasing Se concentration, and even roots containing >1,000 mg Se kg-1 Se harbored nematode herbivores. However, when nematodes extracted from field-harvested plants were used to inoculate greenhouse-grown S. pinnata , plants treated with Se did harbor significantly fewer nematodes several months later. These findings are of significance, both because they suggest the presence of Se-tolerant and potentially Se-specialist nematodes in seleniferous sites, and for the possible use of Se as a pesticide for nematodes in non-seleniferous sites. Furthermore, the roots of hyperaccumulators were examined for the spatial distribution and speciation of Se using X-ray Absorption Spectroscopy (XAS). The majority of the Se was found in the cortex and epidermis of the root, with lower levels in the wood. Organic Se of the C-Se-C type (Se bonded to two carbon atoms, similar to methyl-selenocysteine) was the predominant form of Se in the hyperaccumulator roots, together with a small fraction of inorganic selenite. The findings presented in this thesis may also have relevance for hyperaccumulators of other elements, such as arsenic, cadmium, nickel or zinc, as these metals may also protect roots from nematodes and other root herbivores, and may have similar effects on reproduction. Further investigations may reveal other herbivores that are deterred by root hyperaccumulation, as well as more evidence of specialist herbivores that have evolved tolerance in response to the hyperaccumulator's elemental defense. Beyond insight into the ecological and co-evolutionary relationships between roots and herbivores, the results presented here also have applications in agriculture. Since Se is both a nutrient and a toxin, depending on its concentration, Se could be used as an alternative to organic pesticides in controlling root nematode and herbivore levels in organic and subsistence farming. With careful monitoring, the resulting plants may be considered Se-fortified food with enhanced nutritional value. Finally, the findings presented here provide a framework for follow-up studies investigating the evolution of plant Se hyperaccumulation and the associated effects of (hyper)accumulated plant Se on ecological interactions in seleniferous habitats.
Open Access
Meiotic recombination and synapsis in wild-type and asynaptic mutants of tomato (Solanum lycopersicum)
(Colorado State University. Libraries, 2010) Qiao, Huanyu, author; Anderson, Lorinda K., advisor; Stack, Stephen M., committee member; Bedinger, Patricia A., committee member; Ranu, Rajinder Singh, committee member
Recombination nodules (RNs) and synaptonemal complexes (SCs) are meiosis-specific structures that play important roles in crossing over. During pachytene, RNs mark crossover sites along SCs. MLH1, a mismatch repair protein, promotes crossing over and is a component of most RNs. In wild-type tomato, each bivalent has one, two or three crossovers (=chiasmata), and the number and distribution of these crossovers is affected by crossover interference (the tendency for one crossover to reduce the likelihood of another crossover nearby). Although the phenomenon of genetic interference was discovered nearly one hundred years ago, its molecular basis is still unknown. SCs occur between pairs of homologous chromosomes (bivalents) during prophase I and consist of two parallel rod-like lateral elements held together by transverse fibers. Each lateral element is associated with the two sister chromatids of one of the homologous chromosomes. Cohesin complexes consisting of four proteins (SMC1, SMC3, SYN1/REC8 and SCC3) are found in lateral elements and link sister chromatids together. My research addressed the question of how synapsis (SC formation) is related to the frequency and control of crossing over using tomato, particularly the as1 meiotic mutant, as a model system. Meiocytes from tomato plants homozygous for the mutation as1 do not complete chromosome synapsis and have few chiasmate bivalents, resulting in unbalanced chromosome segregation and sterility. We found a severe delay of prophase I in the as1 mutant compared to wild-type tomato using an in vivo BrdU labeling method, which may be related to the asynaptic phenotype. The asynapsis and delay in the as1 mutant are not likely to be due to a defect in the early steps of recombination, since the frequency and distribution of early recombination proteins (MRE11, RAD50, and RAD51) are similar in wild-type and in the as1 mutant. EM immunolabeling demonstrated that MLH1, a late recombination protein, is present in a subset of RNs in as1, an observation similar to that in wild-type. However, RNs in as1 are larger than those in wild-type. Previous work by other researchers showed a normal level of crossovers in several genetic intervals of the as1 mutant, which was unexpected based on the high degree of asynapsis observed at the cytological level. To evaluate crossing over in the as1 mutant, we examined the immunolabeling patterns of MLH1 foci that mark crossover sites. In as1 meiocytes, we observed that most MLH1 foci were associated with SC segments between two homologous chromosomes. We found that the number of MLH1 foci per micrometer is higher in the as1 mutant compared to wild-type. In addition, interference between MLH1 foci was lower in the mutant than in wild-type tomato. The weakened genetic interference in the as1 mutant may be due to a defect of the medium of interference, since early events of the recombination pathway in as1 seem normal, and MLH1 foci representing crossovers, the last step of the recombination pathway, are still present in the mutant. A good candidate to transmit interference is the cohesin complex that makes up a part of lateral elements. Compared to wild-type, we observed reduced immunofluorescence for the cohesins SMC1, SYN1, and SCC3, but not SMC3 in the as1 mutant. Although we do not yet know the specific mutation of as1 in tomato, we have shown that the asynaptic phenotype is accompanied by alterations in cohesin proteins in AE/LEs and in the distribution of MLH1 foci compared to wild-type. To our knowledge, this is the first report of an association between cohesin proteins and crossover interference regulation in any organism. This discovery represents a significant advance in our efforts to understand the molecular basis of crossover interference.