László Bakó

Molecular Biology of Somatic Embryogenesis

Similar to other higher eukaryotic organisms, in flowering plants, embryo development is the consequence of fertilization events. Union of gametes as the male sperm nucleus and the female egg results in the zygote which later develops into an embryo within the ovule. During the sexual reproductive cycle, the egg cell is prepared for initiation of the embryogenic development that is triggered by signals after sperm-egg contact. In vivo the gametophytic and sporophytic cell differentiation is separated and the haploid gametes are specialized for sexual fusion and the fertilized egg has the potential to develop into a new organism. In most higher eukaryotes, the differentiation of totipotent embryogenic cells is controlled by a pre-set developmental program and terminally differentiated cells are formed. In early embryos, the cells have rapid division cycles and the chromatin becomes transcriptionally active after variable number of division cycles during embryo development.

Developmental regulation of the maize Zm-p60.1 gene encoding a β-glucosidase located to plastids.

Abstract A β-glucosidase that cleaves the biologically inactive hormone conjugates cytokinin-O- and kinetin-N3-glucosides is encoded by the maize Zm-p60.1 gene. The expression of the Zm-p60.1 gene was analyzed by Northern blot analysis and in-situ hybridization. It was found that the expression levels of the Zm-p60.1-specific mRNA changed after pollination of carpellate inflorescences. The Zm-p60.1 cDNA was expressed in E. coli and antibodies were raised against this protein. An antibody was used to determine the tissue-specific localization of this protein. By in situ immunolocalization experiments, this protein was found to be located in cell layers below the epidermis and around the vascular bundles of the coleoptile. In the primary leaf, the Zm-p60.1 protein was detected in cells of the outermost cell layer and around the vascular tissue. In floral tissue, Zm-p60.1 was present in the glumes, the carpels and in the outer cell layer of the style. In coleoptiles, as determined by immuno-electronmicroscopy, the Zm-p60.1 protein was located exclusively in the plastids.

Key components of cell cycle control during auxin-induced cell division

This review article provides a comprehensive summary of the basic molecular and cellular events underlying the induction of the cell division cycle in auxin-treated somatic plant cells. Various pathways of signal transduction chains are discussed as mediators between auxin receptors and alteration of the gene expression pattern. The central role of calcium as a second messenger is analyzed in relation to its interaction with calmodulin and a variety of protein kinases. Experimental data indicate that the control of the cell cycle in higher plants involves several key elements and regulatory mechanisms common to other eukaryotic cells. Recent results show a complex formation between p34cdc2 kinase and cyclin-like proteins. Furthermore, the cell cycle-dependent changes in the p34cdc2 kinase activity which peak at S- and G2/M-phases suggest functional roles for S- and M-forms of the p34cdc2 or related kinases. The homologues of cdc2and cyclin genes have been cloned from different plant species. The expression of plant cdc2 genes is under transcriptional control in auxin-reactivated cells while high constitutive expression of this gene was found in fast cycling cells grown in suspension culture.

RNAPII: A Specific Target for the Cell Cycle Kinase Complex

Transcription in plants, as in other eukaryotes, is catalyzed by three RNA polymerases (RNAPs). Catalytically active forms of RNAPs were first isolated by Roeder and Rutter (1969) and designated as RNAPI(A), II(B) and III(C). RNAPI transcribes rRNA genes, RNAPII synthesizes the precursors of mRNAs and RNAPIII is involved in the transcription of 5S RNA and tRNA genes. In contrast to prokaryotes in which a single RNA polymerase, consisting of ββ’α2 subunits and associated σ factors (Yura and Ishihama 1979; Helmann and Chamberlin 1988), is sufficient for promoter recognition, the assembly of transcriptionally active initiation complexes in eukaryotes requires specific interactions of RNAPs with multiple transcription factors (TFs) and promoter-specific activator proteins. Studies of the regulation of transcription were started by characterization of the subunit composition of RNAPs.