Developmental expression of two methyl -recycling enzymes in Arabidopsis thaliana.

摘要

The research described in this thesis investigates the expression of two methyl-recycling enzymes, adenosine kinase (ADK) and S-adenosylhomocysteine hydrolase (SAHH), during the development of Arabidopsis thaliana. ADK and SAHH transcript and protein levels, along with their enzyme activities, were monitored in leaves, stems, buds, siliques and roots of Arabidopsis . In all but roots, the transcript abundance of ADK and SAHH fluctuated coordinately, matching changes in their protein and enzymes activities. ADK and SAHH expression patterns were consistent with those of the methyl recycling and utilization enzymes, S-adenosylmethionine synthetase (SAMS) and a methyltransferase involved in phospholipid synthesis. A positive correlation between lignin accumulation and ADK and SAHH protein and transcript levels was observed, even at the tissue specific level, where ADK and SAHH transcripts and proteins were highly associated with lignifying cells. Thus, the results point to a coordinated and likely transcriptional regulation of these genes in most organs of Arabidopsis, coincident with their role in maintaining SAM-dependent methylation activities.;The second part of this work involves an examination of the expression of ADK and SAHH in flower, fruit, seeds, vegetative and inflorescence meristems of Arabidopsis. With the exception of the seed coat, the expression of ADK and SAHH was coordinated during fruit development. An association of ADK and SAHH in the ena endocarp layer of the pericarp with the lignification of the adjacent enb layer as well with a programmed cell death process is discussed. Moreover, we have found that ADK-deficiency results in a delay of silique development, likely due to inability of these plants to keep up with the methylation needs associated with fruit development.;The third section of this thesis describes experiments investigating the relationship between ADK-deficiency and pectin methylation in cell walls of Arabidopsis. Using immunofluorescence and immunogold techniques, and monoclonal antibodies specific for homogalacturonan epitopes with different degrees of pectin methyl-esterification, leaf pectins in wild-type plants were observed to have higher levels of methyl-esterification than ADK-deficient plants.;Finally, using immunogold labeling, the subcellular localization of ADK and SAM was studied. Contrary to the current belief that ADK and SAM are cytosolic enzymes, gold particles for both proteins were observed not only in the cytosol, but also in chloroplasts and nuclei of developing bud cells. (Abstract shortened by UMI.).