Arabidopsis has emerged as an important model system for molecular plant biology. The extensive resources available for Arabidopsis make it an attractive system to study the molecular mechanisms involved in early seed development. During the early stages of seed development Arabidopsis endosperm is syncytial and proliferates rapidly through repeated rounds of mitosis without cytokinesis. This stage of endosperm development is both important in determining final seed size and is a model for studying various aspects of cellular and molecular biology, such as the cell cycle and genomic imprinting. However, the small size of Arabidopsis seed, the syncytial nature of the proliferative endosperm, and the surrounding maternal tissues make high throughput molecular analysis of the early endosperm technically difficult. To get around this we used laser capture microdissection to enable transcript analysis of the early proliferative endosperm of Arabidopsis at 4 days after pollination (DAP). Microarray results identified several thousand genes with endosperm expression, including many that were endosperm preferred. A number of genes were validated by relative quantification PCR and were consistent with the findings of the microarray. Meta analysis of the endosperm transcriptome revealed a developmental program dominated by mitosis and under the influence of several phytohormones, predominated by cytokinin signaling. The list of endosperm-preferred genes included all characterised imprinted genes in Arabidopsis. Imprinting is an epigenetic phenomenon by which genes are expressed predominantly from either their paternal or their maternal allele and very few imprinted genes have been identified in plants. The mono-allelic expression of the characterised imprinted genes appears to be limited to the endosperm where they provide important regulatory controls for seed development via direct effects on endosperm development. Genes from the endosperm-preferred list were screened for mono-allelic expression using sequence polymorphisms between the Colombia and Landsberg erecta ecotypes. We generated PCR products that spanned the polymorphisms of 67 genes from template obtained by laser capture of endosperm tissue from hybrid seed. Sequence analysis revealed three genes which gave strong allelic bias toward the maternal allele (At2g32460, At1g55550 and At2g21420) and one biased for the paternal allele (At1g47840). In summary, laser capture microdissection has enabled high-resolution transcript analysis of the proliferative stage of Arabidopsis endosperm development. The data generated provides a useful resource providing novel insight into early seed development, facilitating both identification of endosperm expressed and novel imprinted genes.

Thesis (Ph.D.)--York University, 2003. Graduate Programme in Biology. / Typescript. Includes bibliographical references. Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pNQ99195

The present investigation can be divided into two main sections: the first dealing with the post-shedding embryogenesis of Encephalartos natalensis and the second concerned with the cell wall properties of immature and mature embryos of this species. Development of the embryo of E. natalensis from a rudimentary meristematic structure approximately 700 μm in length, extends over six months after the seed is shed from the strobilus. Throughout its development the embryo remains attached to a long suspensor. Differentiation of the shoot meristem flanked by two cotyledonary protuberances occurs over the first two months, during which peripheral tannin channels become apparent. Tannins, apparently elaborated by the endoplasmic reticulum, first accumulate in the large central vacuole and ultimately fill the channel. By the fourth month of development the root meristem is apparent and procambial tissue forming discrete vascular bundles can be discerned in the elongating cotyledons. Between four and six months, mucilage ducts differentiate, and, after six months when the seed becomes germinable, the embryo is characterised by cotyledons far longer than the axis. Shoot and root meristem cells remain ultrastructurally similar throughout embryo ontogeny, containing small vacuoles, many welldifferentiated mitochondria and ER profiles, abundant polysomes, plastids containing small starch deposits and Golgi bodies. Unusually however, Golgi bodies are infrequent in other cells including those elaborating mucilage which is accumulated in distended ER and apparently secreted into the duct lumen directly by ER-derived vesicles. The nonmeristematic cells accumulate massive starch deposits to the exclusion of any protein bodies, and only very sparse lipid, features which are considered in terms of the prolonged period of embryo development and the high atmospheric oxygen content of the Carboniferous Period, when cycads are suggested to have originated. With regard to plant cell walls, the present investigation employed immunofluorescence microscopy and immunocytochemistry to characterise the cell walls of immature and mature embryos of the recalcitrant-seeded E. natalensis to determine wall composition and potential changes with development. These techniques, together with cryo-scanning- and transmissionelectron microscopy (TEM) were used to analyse potential changes in the cell walls of mature embryos upon desiccation. Immature cell walls appeared to be composed of low- and high methyl esterified epitopes of pectin, rhamnogalacturonan-associated arabinan, and the hemicellulose xyloglucan, while partially-esterified epitopes of pectin appear to have a punctuate distribution in the wall. Arabinogalactan protein recognised by the LM2 antibody, along with rhamnogalacturonan-associated galactan and the hemicellulose xylan, were not positively localised using immunological probes, suggesting that the embryo of the current species does not possess these epitopes. Interestingly, mature embryos appeared to be identical to immature ones with respect to the cell wall components investigated, implying that these may not change during the protracted post-shedding embryogeny of this species. Analysis of the monosaccharide composition of the walls by gas liquid chromatography complemented the immuno-labelling work. However, there appeared to be abnormally high levels of glucose (Glc), which may indicate the presence of Glc-rich polymers not accounted for by the antibodies used in the current study. Preliminary Glc-normalised data revealed that there may be considerable quantities of arabinose polymers in the wall comparable to that found in desiccation tolerant plants. Drying appeared to induce some degree of cell wall folding in mature embryos, correlating with their possession of wall plasticisers such as arabinose polymers, but this was limited, due to the abundance of amyloplasts, which filled the cytoplasmic space. From the results of this study, it is proposed that the embryo cell walls of E. natalensis are constitutively prepared for the flexibility required during cell growth and expansion, which may facilitate the observed moderate cell wall folding in mature embryos upon drying. This, together with an abundant supply of amyloplasts in the cytomatrix may provide sufficient mechanical stabilisation during desiccation even though the seeds of this species are highly desiccation sensitive. Overall, this study has been a relatively comprehensive coverage of histological and ultrastructural aspects of embryogenesis in E. natalensis. This work will form a pivotal basis for future studies, which may ultimately lead to the successful germplasm cryopreservation and in vitro production on a commercial scale of these, and other, endangered cycad species. Furthermore, the work on cell walls in this investigation has provided improved comprehension of the responses of seed cell walls to dehydration. / Ph.D. University of KwaZulu-Natal, Durban 2013.

Thesis (Ph. D.)--University of Chicago, Dept. of Biology. / Includes bibliographical references. Also available on the Internet.

Thesis (doctoral)--Universität Zürich, 1906. / Includes bibliographical references (p. 90-93).

This study describes the effect of the root hemi-parasitic angiosperm Rhinanthus minor on the structure of the communities in which it lives and seeks to elucidate a mechanism through which the parasite acts to effect these changes in the community. Field manipulations reveal that R. minor suppressed the growth of grasses and legumes in a newly sown meadow whilst promoting the forbs within one growing season. In contrast the removal of R. minor from mature meadow plots did not influence their composition. After an additional growing season the parasite did not further influence the composition of the new meadows but removal did begin to benefit the biomass of mature plots. In isolation the parasite caused most damage to grasses whilst leaving legumes and forbs undamaged. Moreover, the parasite performed worst in terms of growth and photosynthesis when attached to the forbs. Consequently the parasite was able to moderate intra-specific competition between grasses and forbs. I thus hypothesised that forbs were able to prevent the parasite form abstracting resources where as grasses could not. Tracer experiments using isotopically e5N) labelled potassium nitrate confirmed this hypothesis showing that more of the resources taken up by the host were stolen by the parasite from grasses than from forbs. There was much variability in the translocation of resources from the legume studied. The reasons underlying the differential uptake of resources were highlighted using histological studies which showed that all of the forbs possessed successful resistance mechanisms to the parasite whilst no successful resistance was observed in the grasses or legumes. Two different resistance mechanisms were observed in the forbs; hypersensitive cell-death at the host-parasite interface and host lignification. I therefore propose that differential host resistance may underlie this parasite's community level effects as forbs possess a resistance capacity that other potential hosts do not.

Studies in the 1970's reporting the occurrence of fossil pollen types in the Cretaceous, coupled with surveys of extant pollen morphology of primitive flowering plants, laid the foundation for proposing a Lower Cretaceous origin of angiosperms. Over the last 30 years, morphological, ultrastructural, and ontogenetic studies of both extant and fossil pollen have provided an array of new characters, as well as greater resolution in defining character polarities. Moreover, a range of fossil pollen types exhibiting angiosperm characters occur in low frequency within Triassic and Jurassic sediments. The pollen data provide evidence of a pre-Cretaceous origin of angiosperms. Speciation and extinction rates were likely equal during the Triassic and Jurassic, resulting in the paucity of angiosperm pollen types from different geographic areas in the Atlantic - South American/African rift zone. It was not until the Lower Cretaceous that origination rates exceed extinction rates, resulting in the subsequent diversification of angiosperms and the origin of the eudicots.

No description available.

Genome size (GS) is a fundamental trait influencing cellular, developmental and ecological parameters, and varies c. 2400- fold in angiosperms. This astonishing range has the potential to influence a plant's nutrient demands, since nucleic acids are amongst the most phosphate and nitrogen demanding cellular biomolecules, and hence its ability to grow and compete in environments where macronutrients are limited. Angiosperm GS are strongly skewed towards small genomes, despite the prevalence of polyploidy in the ancestry of most if not all angiosperm lineages. This thesis examines the hypothesis that large genome sizes are costly to build and maintain and that angiosperm species with large GS are constrained by nitrogen and phosphate limitation. It untangles the interactions between GS, polyploidy and competition in plant communities, and examines how herbivory and GS play a role in plant productivity, measured as above-ground biomass. The hypothesis that large GS are costly was approached by analysing: 1) plant communities growing under different macronutrient conditions at the Park Grass Experiment (Rothamsted, UK); 2) plant communities under different conditions of macronutrient limitation and insect, mollusc, and rabbit herbivory at Nash's Field in Silwood Park (UK); and, 3) Ellenberg's indicator values which represent the realised niche of a species in terms light, water, and soil fertility. Support for the hypothesis was found in all experiments. The range of analyses show that angiosperm plants with large genomes (e.g. 1C-value > 5 pg) are indeed under greater macronutrient limitation in comparison to plants with small genomes, and that it is polyploid plants with large GS which are the most competitive when macronutrient resources are plentiful. In terms of herbivory, the key finding is a highly significant negative association between GS and rabbit herbivory. A species' realised niche for soil fertility was found to show a positive association with its GS. Overall the thesis shows that angiosperm GS plays a central role in plant community composition and responses to macronutrient conditions, and potentially on higher ecosystem processes through associations at different trophic levels.

An apoplastic pathway, the so-called bypass flow, is important for Na+ uptake in rice under saline conditions. The primary aim of this thesis was to identify the point of entry for bypass flow into rice roots subjected to salinity. Investigations using lateral rootless mutants (lrt1, lrt2), a crown rootless mutant (crl1), their wild types (Oochikara, Nipponbare and Taichung 65, respectively) and seedlings of rice cv. IR36 showed that the entry point, quantified using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), was not at the sites of lateral root emergence. However, PTS was identified in the vascular tissue of lateral roots using both epifluorescence microscopy and confocal laser scanning microscopy. Cryo-scanning electron microscopy and epifluorescence microscopy of sections stained with berberine-aniline blue and Fluorol Yellow 088 revealed that an exodermis was absent in the lateral roots, suggesting that the lack of the exodermis allowed PTS to pass through the cortical layers, enter the stele and be transported to the shoot via the transpiration stream. These findings suggest a role for the lateral roots of rice in bypass flow. The addition of polyethylene glycol (PEG) and silicon (Si) to the culture solution significantly reduced Na+ uptake to the shoot by reducing bypass flow through the lateral roots. PEG was found to be more effective than Si. It was also shown that changing the relative humidity in the air around the shoots had a significant effect on the magnitude of bypass flow and the flux of water across the roots: the greater the flux of water through the roots, the greater the Na+ uptake and bypass flow. Furthermore, results showed that recombinant inbred lines of rice with low Na+ transport possessed low magnitudes of bypass flow, whereas lines with high Na+ transport had a high degree of bypass flow, indicating that bypass flow could be used as a criterion for screening salt resistance in rice varieties.