Gerda Segers

The Arabidopsis Cks1At protein binds the cyclin-dependent kinases Cdc2aAt and Cdc2bAt

In Arabidopsis, two cyclin‐dependent kinases (CDK), Cdc2aAt and Cdc2bAt, have been described. Here, we have used the yeast two‐hybrid system to identify Arabidopsis proteins interacting with Cdc2aAt. Three different clones were isolated, one of which encodes a Suc1/Cks1 homologue. The functionality of the Arabidopsis Suc1/Cks1 homologue, designed Cks1At, was demonstrated by its ability to rescue the temperature‐sensitive cdc2‐L7 strain of fission yeast at low and intermediate expression levels. In contrast, high cks1At expression levels inhibited cell division in both mutant and wild‐type yeast strains. Cks1At binds both Cdc2aAt and Cdc2bAt in vivo and in vitro. Furthermore, we demonstrate that the fission yeast Suc1 binds Cdc2aAt but only weakly Cdc2bAt, whereas the human CksHs1 associated exclusively with Cdc2aAt.

Expression of CKS1At in Arabidopsis thaliana indicates a role for the protein in both the mitotic and the endoreduplication cycle.

Abstract. Although endoreduplication is common in plants, little is known about the mechanisms regulating this process. Here, we report the patterns of endoreduplication at the cellular level in the shoot apex of Arabidopsis thaliana L. Heynh. plants grown under short-day conditions. We show that polyploidy is developmentally established in the pith, maturing leaves, and stipules. To investigate the role of the cell cycle genes CDC2aAt, CDC2bAt, CYCB1;1, and CKS1At in the process of endoreduplication, in-situ hybridizations were performed on the vegetative shoot apices. Expression of CDC2aAt, CDC2bAt, and CYCB1;1 was restricted to mitotically dividing cells. In contrast, CKS1At expression was present in both mitotic and endoreduplicating tissues. Our data indicate that CDC2aAt, CDC2bAt, and CYCB1;1 only operate during mitotic divisions, whereas CKS1At may play a role in both the mitotic and endoreduplication cycle.

Arabidopsis thaliana cyclin-dependent kinase genes cdc2aAt and cdc2bAt : cell cycle regulated transcription and kinase activity

At the turn of the last century, the field of heredity included embryology: the ‘entwicklungsmechanik’ of His, Roux and Driesch obviously contained genetic components. The split between genetics and embryology gradually emerged afterwards. It was formalized in ‘The Theory of the Gene’ (1926) by T. H. Morgan and paradoxically even more in his ‘Embryology and Genetics’ (1934), a joint textbook instead of an attempt to unify the fields. Eminent embryologists such as H. Spemann and F. R. Lillie ignored genetics. The path from Experimental Embryology to Developmental Genetics was first traveled by the former Spemann’s graduate student Salome Gluecksohn-Schoenheimer. This happened soon after she met the American geneticist Leslie C. Dunn who was studying a mouse strain with a dominant mutation responsible for tail shortening, Brachyury (T) detected in 1927 by N. Dobrovolskaia-Zavadskaia. Homozygous condition resulted in spontaneous abortion at 11 days in utero correlated with missing of the posterior half of the embryo body. Since it soon appeared that T mutation was involved in axial determination, Salome Gluecksohn-Schoenheimer found that she might be working on a gene responsible for the posterior organizing substance of the mammalian embryo. Her first paper on that subject in which she coined the expression ‘developmental genetics’ was published in 1938 (GluecksohnSchoenheimer, 1938). She then pursued a research program linking embryonic organizers and specific genes in the mouse mainly the now well documented T-locus genes, a ‘super-gene complex’ (Bennett, 1975). Developmental genetics story thus started with the genetics of induction, the gene acting as an organizer (Gilbert, 1991). Aside from GluecksohnSchoenheimer and Dunn, only a few others were studying the developmental action of genes. One of the few ‘chemical embryologists’, Conrad Hal Waddington was frustrated in his analysis of the Spemann’s organizer. He thus started applying to Drosophila, the genetic organism par excellence, the same type of developmental genetics that Salome Gluecksohn-Schoenheimer had pioneered on mice. From his early analysis of alleles causing deformation of the wing he concluded that wing ‘appears favorable for investigation on the developmental action of genes’ (Waddington, 1939). The initial goal of developmental geneticists was thus to draw conclusions on the nature of the experiments ‘performed’ by the mutated genes but, as already stated in 1945 by Salome Gluecksohn herself, their final goal is the analysis of the action of genes. This goal would have to await the techniques of molecular biology but the initial one is far to be abandoned. The classical way to identify a gene by a mutated phenotype remains indeed obviously the most efficient one. This explains why so many genes essential for embryonic pattern formation were identified in Drosophila by virtue of their loss-offunction. A powerful genetic technique called ‘saturation screening’, which allows the whole genome to be scanned for developmentally important genes, has been pioneered in the early 1970s by Christiane Nüsslein-Volhard and Eric Wieschaus, two of the recent Nobel prize-winners. It has been estimated that among the 6000 ‘essential’ genes from which 5000 mutate to lethality and about 1000 to sterility, less than 200 genes play specific roles in embryonic development and pattern formation. This may however be underestimated since the screens might not have been suitable to identify all of them. As stated by C. Nüsslein-Volhard (1994) ‘Drosophila

The Molecular Basis of Cell Cycle Control in Arabidopsis thaliana

Cell division is an integral part of growth and development. Although the basic mechanisms of cell cycle control are apparently conserved in all eukaryotes examined until now, its fine regulation may differ according to the diverse developmental programs of each organism. Plants have several unique properties which distinguish their development from that of animals. Due to the absence of cell migration (plant cells are surrounded by a rigid cell wall) most cells in the embryo have to undergo morphogenesis in the correct position. Furthermore, most cell divisions are taking place in specialized regions, the meristems, which mostly continue to produce new organs throughout the entire plant’s life. Most non-dividing cells retain the potency to divide, allowing them to dedifferentiate and to produce, in suitable tissue culture conditions, new plants. The patterns of cell division are not only determined by intrinsic developmental programs, but they are also influenced by external factors such as light, wounding, gravity, etc. Plant hormones such as auxins and cytokinins, have outspoken effects on cell division and differentiation. The underlying mechanism is yet almost entirely unknown.

Cell cycle control in Arabidopsis thaliana

Summary The crucifer Arabidopsis thaliana is an excellent model plant to study the molecular regulation of cell division. Here, we describe the state of the art of cell cycle research in this organism. The expression of the key regulatory gene, cdc2, has been shown to be correlated with cell division and with the cellular competence to divide. Furthermore, plant hormones, such as auxins and cytokinins, have intriguing cell—specific effects on the expression of the cdc2 gene. The results are discussed in view of future developments.