Plant growth regulators (PGRs) are thought to play a vital role in the control of plant development. This control is mediated by interplay between the sensitivity of a tissue to a given PGR, and the concentration of the PGR within that tissue. PGR concentration is a function of its biosynthesis and metabolism. Biosynthesis feeds the active pool, but the role of metabolism in PGR homeostasis is more complex. This thesis describes an investigation into the importance of one aspect of metabolism, O-glucosylation, in the control of cytokinin levels in plant tissues. Cytokinins are N<SUP>6</SUP>-substituted adenine derivatives, which occur in many different forms. Glucosylation of "active" forms renders the molecule biologically inactive. It is, however, thought that cytokinins glucosylated at the O-position of the N<SUP>6</SUP> side chain are susceptible to cleavage by β-glucosidases, leading to the release of the active form. Very little work has been carried out on these metabolities, and no accurate technique for their measurement in plant tissues exists. This project therefore involves the development of 3 specific immunoassays for the accurate measurement of (OG)Z, [9R](OG)Z and [9R](OG)DHZ in plant tissues. These assays, were employed, in addition to pre-existing techniques, to assess the importance of cytokinin O-glucosylation in 3 very different systems. Direct ELISAs for 9-riboside and O-glucoside cytokinins were used in conjunction with HPLC to investigate cytokinin metabolism during leaf maturation in <I>Phaseolus vulgaris. </I>Cytokinin O-glucosides were found to increase with leaf age with a concomitant decrease in the ratio of active to conjugated forms. This is consistent with the hypothesis that a decrease in active cytokinins is involved with the onset of foliar senescence. Premature leaf senescence and reduced apical dominance of forest trees are two major symptoms of novel forest decline. Both of these symptoms suggest a perturbation of hormone controlled processes in damaged trees. I therefore investigated the possible inter-relationship between changes in cytokinin physiology and exposure to atmospheric pollution in <I>Picea abies. </I>This was achieved by examining the content of Z-, DHZ- and iP-type cytokinins in a range of metabolic forms, in the needles of trees grown at a polluted and a non-polluted site in central Slovenia. Results indicated that trees exposed to high levels of pollution had elevated needle cytokinin content. This appeared to be due to perturbation of both synthesis and metabolism, as changes were detected in total cytokinin content and the relative proportions of the metabolites measured, in response to pollution stress. The results are discussed with reference to the impact the changes may have on whole plant physiology and the possible relationship between cytokinins and alleviation of environmental stress.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598505 |
| Date | January 1997 |
| Creators | Dent, R. M. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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