A General G1/S-Phase Cell-Cycle Control Module in the Flowering Plant Arabidopsis thaliana

Abstract

The decision to replicate its DNA is of crucial importance for every cell and, in many organisms, is decisive for the progression through the entire cell cycle. A comparison of animals versus yeast has shown that, although most of the involved cell-cycle regulators are divergent in both clades, they fulfill a similar role and the overall network topology of G1/S regulation is highly conserved. Using germline development as a model system, we identified a regulatory cascade controlling entry into S phase in the flowering plant Arabidopsis thaliana, which, as a member of the Plantae supergroup, is phylogenetically only distantly related to Opisthokonts such as yeast and animals. This module comprises the Arabidopsis homologs of the animal transcription factor E2F, the plant homolog of the animal transcriptional repressor Retinoblastoma (Rb)-related 1 (RBR1), the plant-specific F-box protein F-BOX-LIKE 17 (FBL17), the plant specific cyclin-dependent kinase (CDK) inhibitors KRPs, as well as CDKA;1, the plant homolog of the yeast and animal Cdc2+/Cdk1 kinases. Our data show that the principle of a double negative wiring of Rb proteins is highly conserved, likely representing a universal mechanism in eukaryotic cell-cycle control. However, this negative feedback of Rb proteins is differently implemented in plants as it is brought about through a quadruple negative regulation centered around the F-box protein FBL17 that mediates the degradation of CDK inhibitors but is itself directly repressed by Rb. Biomathematical simulations and subsequent experimental confirmation of computational predictions revealed that this regulatory circuit can give rise to hysteresis highlighting the here identified dosage sensitivity of CDK inhibitors in this network.

An EAR-Dependent Regulatory Module Promotes Male Germ Cell Division and Sperm Fertility in Arabidopsis

“…Whereas a robust molecular framework is emerging for the control of G1/S-phase transition in plants (Zhao et al, 2012), less is known about the regulatory modules promoting G2/Mphase transition (Berckmans and De Veylder, 2009). Our study provides insights into how cell type-specific mechanisms drive G2/M-phase transition, and we propose that the DUO1-DAZ1/ DAZ2 regulatory module controls mitotic division in the male germline ( Figure 9).…”

RETINOBLASTOMA RELATED1 Regulates Asymmetric Cell Divisions in Arabidopsis

“…FBL17, MCM5, and PCNA1 Zhao et al, 2012). Fragments further upstream or downstream of the known RBR1 binding site could not be amplified.…”

“…ChIP was performed as previously described (Berr et al, 2010;Bouyer et al, 2011;Zhao et al, 2012). Two-week-old seedlings of Pro RBR1 :RBR1: mRFP growing on 0.5 MS plates were used.…”

“…CDKA;1 controls entry into M phase and especially entry into S phase by phosphorylating RETINOBLASTOMA RELATED1 (RBR1), the Arabidopsis homolog of the human tumor suppressor Retinoblastoma . In the nonphosphorylated state, RBR1 represses the action of the transcription factor E2F that activates the expression of many genes required for DNA replication, such as F-BOX PROTEIN17 (FBL17), PROLIFERATING CELL NUCLEAR ANTIGEN (PCNA), and MINICHROMOSOME MAINTENANCE PROTEIN5 (MCM5) (Gutzat et al, 2012;Zhao et al, 2012). CDKA;1 action for S phase entry is backed up by the two redundantly acting CDKB1s (CDKB1;1 and CDKB1;2) .…”

Why do plants need so many cyclin-dependent kinase inhibitors?

“…redundancy in both CKI gene families, suggesting that divergence in biochemical function plays a minor role in maintaining the number of genes. 18,20,21 Of particular note, Kumar and coworkers found that 10 different SMRs, including an SMR from the bryophyte Physcomitrella patens, can complement the trichome cell division phenotype of Arabidopsis sim mutants. 13 Additionally, individual KRP and the SMR genes show distinct patterns of tissue-specific and stress-regulated expression, consistent with the individual genes playing roles in integrating the cell cycle with a wide variety of signals.…”