RESUMO
The WEE1 and ATM AND RAD3-RELATED (ATR) kinases are important regulators of the plant intra-S-phase checkpoint; consequently, WEE1KO and ATRKO 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.
Assuntos
Proteínas de Arabidopsis/genética , Arabidopsis/fisiologia , Replicação do DNA , Proteínas Serina-Treonina Quinases/genética , Proteínas Proto-Oncogênicas c-myb/genética , Transdução de Sinais , Arabidopsis/genética , Proteínas de Arabidopsis/metabolismo , Proteínas Mutadas de Ataxia Telangiectasia/genética , Proteínas Mutadas de Ataxia Telangiectasia/metabolismo , Genoma de Planta , Instabilidade Genômica , Proteínas Serina-Treonina Quinases/metabolismo , Proteínas Proto-Oncogênicas c-myb/metabolismo , Estresse Fisiológico , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismoRESUMO
The anaphase promoting complex/cyclosome (APC/C) controls unidirectional progression through the cell cycle by marking key cell cycle proteins for proteasomal turnover. Its activity is temporally regulated by the docking of different activating subunits, known in plants as CELL DIVISION PROTEIN20 (CDC20) and CELL CYCLE SWITCH52 (CCS52). Despite the importance of the APC/C during cell proliferation, the number of identified targets in the plant cell cycle is limited. Here, we used the growth and meristem phenotypes of Arabidopsis (Arabidopsis thaliana) CCS52A2-deficient plants in a suppressor mutagenesis screen to identify APC/CCCS52A2 substrates or regulators, resulting in the identification of a mutant cyclin CYCA3;4 allele. CYCA3;4 deficiency partially rescues the ccs52a2-1 phenotypes, whereas increased CYCA3;4 levels enhance the scored ccs52a2-1 phenotypes. Furthermore, whereas the CYCA3;4 protein is promptly broken down after prophase in wild-type plants, it remains present in later stages of mitosis in ccs52a2-1 mutant plants, marking it as a putative APC/CCCS52A2 substrate. Strikingly, increased CYCA3;4 levels result in aberrant root meristem and stomatal divisions, mimicking phenotypes of plants with reduced RETINOBLASTOMA-RELATED PROTEIN1 (RBR1) activity. Correspondingly, RBR1 hyperphosphorylation was observed in CYCA3;4 gain-of-function plants. Our data thus demonstrate that an inability to timely destroy CYCA3;4 contributes to disorganized formative divisions, possibly in part caused by the inactivation of RBR1.