Control of Cell Proliferation, Organ Growth, and DNA Damage Response Operate Independently of Dephosphorylation of the Arabidopsis Cdk1 Homolog CDKA;1  

Abstract

Entry into mitosis is universally controlled by cyclin-dependent kinases (CDKs). A key regulatory event in metazoans and fission yeast is CDK activation by the removal of inhibitory phosphate groups in the ATP binding pocket catalyzed by Cdc25 phosphatases. In contrast with other multicellular organisms, we show here that in the flowering plant Arabidopsis thaliana, cell cycle control does not depend on sudden changes in the phosphorylation pattern of the PSTAIRE-containing Cdk1 homolog CDKA;1. Consistently, we found that neither mutants in a previously identified CDC25 candidate gene nor plants in which it is overexpressed display cell cycle defects. Inhibitory phosphorylation of CDKs is also the key event in metazoans to arrest cell cycle progression upon DNA damage. However, we show here that the DNA damage checkpoint in Arabidopsis can also operate independently of the phosphorylation of CDKA;1. These observations reveal a surprising degree of divergence in the circuitry of highly conserved core cell cycle regulators in multicellular organisms. Based on biomathematical simulations, we propose a plant-specific model of how progression through the cell cycle could be wired in Arabidopsis.

The plant‐specific CDKB 1‐ CYCB 1 complex mediates homologous recombination repair in Arabidopsis

“…DE displayed shorter roots than the wild type on plates without DNA damage‐inducing agents (Fig 4A, G, J and M) as well as on media supplemented with BLM (Fig 4C and L). Previously, we found that DE is not sensitive to HU (Dissmeyer et al , 2009) (Fig 4B and K). Taking the root growth ratio into account, we determined that DE was neither more sensitive to HU nor to BLM than the wild type (Fig EV3B).…”

“…Although homozygous cdka;1 mutants exist, they are so severely compromised that an analysis, especially under stress conditions, faces the danger of giving ambiguous results due to pleiotropic and indirect effects (Nowack et al , 2012). We therefore made use of a previously described weak loss‐of‐function allele, designated CDKA;1 T14D;Y15E or short DE (Dissmeyer et al , 2009), to address whether CDKA;1 plays a role in the response to cisplatin. DE displayed shorter roots than the wild type on plates without DNA damage‐inducing agents (Fig 4A, G, J and M) as well as on media supplemented with BLM (Fig 4C and L).…”

“…Here, we have shown that plants with a reduced activity level of CDKA;1 are neither hypersensitive to BLM nor cisplatin. In addition, we have previously found that a weak loss‐of‐function allele of CDKA;1 also does not sensitize plants to HU (Dissmeyer et al , 2009). Although we cannot fully exclude a role of CDKA;1 in DNA repair, our combined data suggest that this kinase is largely not required for a proper DNA damage response in Arabidopsis .…”

“…The reporter line uidA/IC9C was used as described in Molinier et al (2004) and the pCDKB1;1:GUS line as in Boudolf et al (2004). The mutant CDKA;1 T14D;Y15E is published in Dissmeyer et al (2009), the cdkb1;1 cdkb1;2 double mutant in Nowack et al (2012). All genotypes were determined by PCR and primers are indicated in Table EV1.…”

“…Furthermore, even though a homolog of the yeast Wee1 kinase exists in Arabidopsis and other plants, its function appears to be different as Arabidopsis WEE1 was found to act during S phase after hydroxyurea (HU)‐induced replication stress and not in repressing CDK activity during mitosis or blocking cell division after DSB formation (De Schutter et al , 2007; Cools et al , 2011). Moreover, transgenic plants expressing a mutant version of CDKA;1, the Arabidopsis homolog of mammalian Cdk1 and Cdk2, in which the putative WEE1 target sites were replaced with non‐phosphorylatable amino acids, were not hypersensitive to HU indicated that cell‐cycle arrest after DNA damage is differently regulated in plants (Dissmeyer et al , 2009, 2010). …”