Search

Whereas our knowledge about the diverse pathways aiding DNA repair upon genome damage is steadily increasing, little is known about the molecular players that adjust the plant cell cycle in response to DNA stress. By a meta-analysis of DNA stress microarray data sets, three family members of the SIAMESE/SIAMESE-RELATED (SIM/SMR) class of cyclin-dependent kinase inhibitors were discovered that react strongly to genotoxicity. Transcriptional reporter constructs corroborated specific and strong activation of the three SIM/SMR genes in the meristems upon DNA stress, whereas overexpression analysis confirmed their cell cycle inhibitory potential. In agreement with being checkpoint regulators, SMR5 and SMR7 knockout plants displayed an impaired checkpoint in leaf cells upon treatment with the replication inhibitory drug hydroxyurea (HU). Surprisingly, HU-induced SMR5/SMR7 expression depends on ATAXIA TELANGIECTASIA MUTATED (ATM) and SUPPRESSOR OF GAMMA RESPONSE1, rather than on the anticipated replication stress-activated ATM AND RAD3-RELATED kinase. This apparent discrepancy was explained by demonstrating that, in addition to its effect on replication, HU triggers the formation of reactive oxygen species (ROS). ROS-dependent transcriptional activation of the SMR genes was confirmed by different ROS-inducing conditions, including high-light treatment. We conclude that the identified SMR genes are part of a signaling cascade that induces a cell cycle checkpoint in response to ROS-induced DNA damage.

Eukaryotic organisms evolved under aerobic conditions subjecting nuclear DNA to damage provoked by reactive oxygen species (ROS). Although ROS are thought to be a major cause of DNA damage, little is known about the molecular mechanisms protecting nuclear DNA from oxidative stress. Here we show that protection of nuclear DNA in plants requires a coordinated function of ROS-scavenging pathways residing in the cytosol and peroxisomes, demonstrating that nuclear ROS scavengers such as peroxiredoxin and glutathione are insufficient to safeguard DNA integrity. Both catalase (CAT2) and cytosolic ascorbate peroxidase (APX1) play a key role in protecting the plant genome against photorespiratory-dependent H(2)O(2)-induced DNA damage. In apx1/cat2 double-mutant plants, a DNA damage response is activated, suppressing growth via a WEE1 kinase-dependent cell-cycle checkpoint. This response is correlated with enhanced tolerance to oxidative stress, DNA stress-causing agents, and inhibited programmed cell death.