| Popup Text: | "And yet it turns - The cell cycle with and without the central cyclin-dependent kinase CDKA;1"
ABSTRACT
The core of the eukaryotic cell cycle machinery is controlled by heterodimers of Cyclins and Cyclin-Dependent Kinases (CDKs). In multicellular organisms, families with partly redundant members of these regulators have evolved to take over specific functions. However, it appears to be a universal theme that at least one CDK containing the conserved Cyclin binding PSTAIRE motif is essential for basic cell cycle progression and thus survival of the organism. The Arabidopsis genome contains one single PSTAIRE-domain CDK, CDKA;1, which is able to complement the cdc2/cdc28 mutants in yeast. We and others previously described cdka;1 T-DNA insertion mutants without identifying homozygous individuals segregating from heterozygous parents. Yet, closer analyses revealed the existence of tiny, rootless plant
s, viable only in cell culture conditions. These homozygous cdka;1 mutants stem from embryos of the same size as wild type embryos, but they contain only a fraction of the usual cell number. After germination, the shoot apical meristem becomes active and produces very small few-celled leaves and stout shoots, while the root meristem is not functional. How can plants, in contrast to all other eukaryotes investigated so far, survive without a PSTAIRE-domain CDK? Prime candidates for the backup system revealed in cdka;1 mutants are the plant-specific CDKBs with a PPTALRE/PPTTLRE cyclin-binding domain. Current experiments show that a proCDKA;1::CDKB construct is able to partly rescue the cdka;1 phenotype and that cdka;1;cdkb double mutants are not viable. These results underline the key position of CDKA;1 for cell cycle control, but also indicate that during their evolutionary adaptation, the CDKBs retained a function to drive both entry into S and M phase.
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