| Summary: | The modulation of cell division is a fundamental feature of higher plant development. Control of plant cell division is exercised during the G1 phase of the cell cycle as part of developmental programmes. Cyclin proteins play an important role in the mechanism which commits cells to the process of division during G1 phase, and then-role has been conserved throughout eukaryotic evolution. The work described in this thesis was carried out with the objective of identifying and characterising cyclins which serve this function in higher plants. Two approaches to the isolation of plant homologues were taken based on the conservation of structure and function of cyclins in eukaryotes. Low stringency hybridisations using DNA encoding fungal cyclins were used to identify similar plant sequences; and the complementation of a yeast mutant deficient in cyclin function with plant cDNAs was used to identify functionally equivalent plant genes. Plant genes so isolated have been characterised on the basis of DNA sequence and their expression during the cell cycle in synchronised cells of Arabidopsis thaliana. A system was developed for the synchronisation of A. thaliana cells in suspension culture, using cycloheximide. Neither DNA sequence nor expression pattern of the genes isolated provided evidence for a role in the control of cell division. The expression patterns of A. thaliana cyclins D1, D2, D3, CYC1, CYC2, and CYC3 were also examined within this system. The expression of D2 and D3 was suggestive of functional differences and supported the hypothesis that D-type cyclins function as part of the cellular machinery which integrates diverse signals impinging upon commitment to cell division. The expression of CYC1, 2, and 3 confirms their classification as mitotic cyclins and suggests that CYC3 functions earlier in the cell cycle than CYC1 or 2.
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