ATR Regulates a G2-Phase Cell-Cycle Checkpoint in Arabidopsis thaliana

Ataxia telangiectasia-mutated and Rad3-related (ATR) plays a central role in cell-cycle regulation, transmitting DNA damage signals to downstream effectors of cell-cycle progression. In animals, ATR is an essential gene. Here, we find that Arabidopsis (Arabidopsis thaliana) atr-/- mutants were viable, fertile, and phenotypically wild-type in the absence of exogenous DNA damaging agents but exhibit altered expression of AtRNR1 (ribonucleotide reductase large subunit) and alteration of some damage-induced cell-cycle checkpoints. atr mutants were hypersensitive to hydroxyurea (HU), aphidicolin, and UV-B light but only mildly sensitive to gamma-radiation. G2 arrest was observed in response to gamma-irradiation in both wild-type and atr plants, albeit with slightly different kinetics, suggesting that ATR plays a secondary role in response to double-strand breaks. G2 arrest also was observed in wild-type plants in response to aphidicolin but was defective in atr mutants, resulting in compaction of nuclei and subsequent cell death. By contrast, HU-treated wild-type and atr plants arrested in G1 and showed no obvious signs of cell death. We propose that, in plants, HU invokes a novel checkpoint responsive to low levels of deoxynucleotide triphosphates. These results demonstrate the important role of cell-cycle checkpoints in the ability of plant cells to sense and cope with problems associated with DNA replication.