CKS1At overexpression in Arabidopsis thaliana inhibits growth by reducing meristem size and inhibiting cell-cycle progression

The SUC1/CKS1 proteins associate with cyclin-dependent kinases (CDKs) and play an essential role in the regulation of the cell cycle. Recently, an Arabidopsis thaliana SUC1/CKS1 homologous gene, designated CKS1At, has been cloned. Here, overexpression of CKS1At in Arabidopsis is shown to reduce leaf size and root growth rates. Reduced root growth resulted primarily from an increase of the cell-cycle duration and a shortening of the meristem. Endoreduplication was unaffected. The increased cell-cycle duration was associated with an equal extension of both the G1 and G2 phases. This inhibition was due to the binding of CDK subunits with CDKs. The reduced growth rates in response to altered cell-cycle gene expression demonstrates a direct dependence of plant growth rates on cell-cycle regulation.     

Evidence for a Role of Arabidopsis CDT1 Proteins in Gametophyte Development and Maintenance of Genome Integrity

Meristems retain the ability to divide throughout the life cycle of plants, which can last for over 1000 years in some species. Furthermore, the germline is not laid down early during embryogenesis but originates from the meristematic cells relatively late during development. Thus, accurate cell cycle regulation is of utmost importance to avoid the accumulation of mutations during vegetative growth and reproduction. The Arabidopsis thaliana genome encodes two homologs of the replication licensing factor CDC10 Target1 (CDT1), and overexpression of CDT1a stimulates DNA replication. Here, we have investigated the respective functions of Arabidopsis CDT1a and CDT1b. We show that CDT1 proteins have partially redundant functions during gametophyte development and are required for the maintenance of genome integrity. Furthermore, CDT1-RNAi plants show endogenous DNA stress, are more tolerant than the wild type to DNA-damaging agents, and show constitutive induction of genes involved in DNA repair. This DNA stress response may be a direct consequence of reduced CDT1 accumulation on DNA repair or may relate to the ability of CDT1 proteins to form complexes with DNA polymerase ε, which functions in DNA replication and in DNA stress checkpoint activation. Taken together, our results provide evidence for a crucial role of Arabidopsis CDT1 proteins in genome stability.     

Arabidopsis E2FA stimulates proliferation and endocycle separately through RBR-bound and RBR-free complexes

Post-embryonic growth in plants depends on the continuous supply of undifferentiated cells within meristems. Proliferating cells maintain their competence for division by active repression of differentiation and the associated endocycle entry. We show by upregulation and downregulation of E2FA that it is required for maintaining proliferation, as well as for endocycle entry. While E2FB-RBR1 (retinoblastoma-related protein 1) complexes are reduced after sucrose addition or at elevated CYCD3;1 levels, E2FA maintains a stable complex with RBR1 in proliferating cells. Chromatin immunoprecipitation shows that RBR1 binds in the proximity of E2F promoter elements in CCS52A1 and CSS52A2 genes, central regulators for the switch from proliferation to endocycles. Overexpression of a truncated E2FA mutant (E2FA(ΔRB)) lacking the RBR1-binding domain interferes with RBR1 recruitment to promoters through E2FA, leading to decreased meristem size in roots, premature cell expansion and hyperactivated endocycle in leaves. E2F target genes, including CCS52A1 and CCS52A2, are upregulated in E2FA(ΔRB) and e2fa knockout lines. These data suggest that E2FA in complex with RBR1 forms a repressor complex in proliferating cells to inhibit premature differentiation and endocycle entry. Thus, E2FA regulates organ growth via two distinct, sequentially operating pathways.     

G2/M-checkpoint activation in fasciata1 rescues an aberrant S-phase checkpoint but causes genome instability

The WEE1 and ATM AND RAD3-RELATED (ATR) kinases are important regulators of the plant intra-S-phase checkpoint; consequently, WEE1^KO and ATR^KO roots are hypersensitive to replication-inhibitory drugs. Here, we report on a loss-of-function mutant allele of the FASCIATA1 (FAS1) subunit of the chromatin assembly factor 1 (CAF-1) complex that suppresses the phenotype of WEE1- or ATR-deficient Arabidopsis (Arabidopsis thaliana) plants. We demonstrate that lack of FAS1 activity results in the activation of an ATAXIA TELANGIECTASIA MUTATED (ATM)- and SUPPRESSOR OF GAMMA-RESPONSE 1 (SOG1)-mediated G2/M-arrest that renders the ATR and WEE1 checkpoint regulators redundant. This ATM activation accounts for the telomere erosion and loss of ribosomal DNA that are described for fas1 plants. Knocking out SOG1 in the fas1 wee1 background restores replication stress sensitivity, demonstrating that SOG1 is an important secondary checkpoint regulator in plants that fail to activate the intra-S-phase checkpoint.     

SIAMESE, a plant-specific cell cycle regulator, controls endoreplication onset in Arabidopsis thaliana

Recessive mutations in the SIAMESE (SIM) gene of Arabidopsis thaliana result in multicellular trichomes harboring individual nuclei with a low ploidy level, a phenotype strikingly different from that of wild-type trichomes, which are single cells with a nuclear DNA content of approximately 16C to 32C. These observations suggested that SIM is required to suppress mitosis as part of the switch to endoreplication in trichomes. Here, we demonstrate that SIM encodes a nuclear-localized 14-kD protein containing a cyclin binding motif and a motif found in ICK/KRP (for Interactors of Cdc2 kinase/Kip-related protein) cell cycle inhibitor proteins. Accordingly, SIM was found to associate with D-type cyclins and CDKA;1. Homologs of SIM were detected in other dicots and in monocots but not in mammals or fungi. SIM proteins are expressed throughout the shoot apical meristem, in leaf primordia, and in the elongation zone of the root and are localized to the nucleus. Plants overexpressing SIM are slow-growing and have narrow leaves and enlarged epidermal cells with an increased DNA content resulting from additional endocycles. We hypothesize that SIM encodes a plant-specific CDK inhibitor with a key function in the mitosis-to-endoreplication transition.     

Genome-wide analysis of the diatom cell cycle unveils a novel type of cyclins involved in environmental signaling

Background

Despite the enormous importance of diatoms in aquatic ecosystems and their broad industrial potential, little is known about their life cycle control. Diatoms typically inhabit rapidly changing and unstable environments, suggesting that cell cycle regulation in diatoms must have evolved to adequately integrate various environmental signals. The recent genome sequencing of Thalassiosira pseudonana and Phaeodactylum tricornutum allows us to explore the molecular conservation of cell cycle regulation in diatoms.

Results

By profile-based annotation of cell cycle genes, counterparts of conserved as well as new regulators were identified in T. pseudonana and P. tricornutum. In particular, the cyclin gene family was found to be expanded extensively compared to that of other eukaryotes and a novel type of cyclins was discovered, the diatom-specific cyclins. We established a synchronization method for P. tricornutum that enabled assignment of the different annotated genes to specific cell cycle phase transitions. The diatom-specific cyclins are predominantly expressed at the G1-to-S transition and some respond to phosphate availability, hinting at a role in connecting cell division to environmental stimuli.

Conclusion

The discovery of highly conserved and new cell cycle regulators suggests the evolution of unique control mechanisms for diatom cell division, probably contributing to their ability to adapt and survive under highly fluctuating environmental conditions.     

A novel aux/IAA28 signaling cascade activates GATA23-dependent specification of lateral root founder cell identity

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Highlights? GATA23 is involved in lateral root founder cell specification ? A coordinated migration of pericycle nuclei precedes the first asymmetric division ? An Aux/IAA28-dependent auxin signaling cascade controls GATA23 expression     

The Arabidopsis thaliana PIN1At gene encodes a single-domain phosphorylation-dependent peptidyl prolyl cis/trans isomerase

A homologue of the human site-specific prolyl cis/trans isomerase PIN1 was identified in Arabidopsis thaliana. The PIN1At gene encodes a protein of 119 amino acids that is 53% identical with the catalytic domain of the human PIN1 parvulin. Steady-state PIN1At mRNA is found in all plant tissues tested. We show by two-dimensional NMR spectroscopy that the PIN1At is a prolyl cis/trans isomerase with specificity for phosphoserine-proline bonds. PIN1At is the first example of an eukaryotic parvulin without N- or C-terminal extensions. The N-terminal WW domain of 40 amino acids, typical of all the phosphorylation-dependent eukaryotic parvulins, is absent. However, triple-resonance NMR experiments showed that PIN1At contained a hydrophobic helix similar to the alpha1 helix observed in PIN1 that could mediate the protein-protein interactions.