The histone variant H2AX is rapidly phosphorylated at the sites of DNA double-strand breaks (DSBs). This phosphorylated H2AX (␥-H2AX) is involved in the retention of repair and signaling factor complexes at sites of DNA damage. The dependency of this phosphorylation on the various PI3K-related protein kinases (in mammals, ataxia telangiectasia mutated and Rad3-related [ATR], ataxia telangiectasia mutated [ATM], and DNA-PKCs) has been a subject of debate; it has been suggested that ATM is required for the induction of foci at DSBs, whereas ATR is involved in the recognition of stalled replication forks. In this study, using Arabidopsis as a model system, we investigated the ATR and ATM dependency of the formation of ␥ -H2AX foci in M-phase cells exposed to ionizing radiation (IR). We find that although the majority of these foci are ATM-dependent, ϳ10% of IR-induced ␥-H2AX foci require, instead, functional ATR. This indicates that even in the absence of DNA replication, a distinct subset of IR-induced damage is recognized by ATR. In addition, we find that in plants, ␥-H2AX foci are induced at only one-third the rate observed in yeasts and mammals. This result may partly account for the relatively high radioresistance of plants versus yeast and mammals. INTRODUCTIONThe induction of DNA double-strand breaks (DSBs) in eukaryotes triggers a number of protective responses including the upregulation of repair pathways, initiation of cell cycle arrest, and, in some organisms, the induction of programmed cell death. DSBs in actively dividing cells are particularly dangerous. Repair to form translocations and deletions can lead to loss of heterozygosity, which in turn leads to carcinogenesis in mammals or lethality in haploid yeast. For this reason all living things possess the ability to detect the presence of DSBs and relay this information to the cell cycle.Two important protein kinases involved in sensing and signaling DNA damage in eukaryotes are ataxia telangiectasia mutated (ATM) and ataxia telangiectasia mutated and rad3-related (ATR; Abraham, 2001;Sancar et al., 2004). In mammals, ATM is critical for responses to DSBs and signals downstream cell cycle checkpoint regulators including p53 and Chk2 to coordinate apoptotic responses and/or cell cycle arrest (Fernandez-Capetillo et al., 2002). In addition to checkpoint regulation, ATM responds to DSBs by interacting with proteins intimately involved in DNA repair such as the Mre11-Rad50-Nbs1 (M-R-N) complex (Gatei et al., 2000;van den Bosch et al., 2003) and RAD51 (Chen et al., 1999). In comparison, ATR, in a complex with the ATR interacting protein (ATRIP), is thought to respond primarily to agents that block replication, recognizing stalled replication forks and then signaling to Chk1 and p53 to induce cell cycle arrest, replication restart, and apoptosis (Abraham, 2001). In striking contrast to ATM, ATR is an essential gene in mammals; defects in the murine homolog cause early embryonic lethality and loss of ATR in conditional knock-out embryonic stem cells rapidly...

SummaryDouble-strand break (DSB) repair pathways catalyze the rejoining of broken chromosomes and the integration of transforming DNAs. These processes have been well characterized in bacteria, fungi, and animals. Plants are generally thought primarily to utilize a non-homologous end joining (NHEJ) pathway to repair DSBs and integrate transgenes, as transforming DNAs with large tracts of homology to the chromosome are integrated at random. In order to test the hypothesis that NHEJ is an important pathway for the repair of DSBs in plants, we isolated T-DNA insertion mutations in the Arabidopsis homologs of the Ku80 and DNA ligase IV genes, required for the initiation and completion, respectively, of NHEJ. Both mutants were hypersensitive to the cytostatic effects of gamma radiation, suggesting that NHEJ is indeed a critical pathway for the repair of DSBs. T-DNA insertion rates were also decreased in the mutants, indicating that Ku80 and DNA ligase IV play an important role in either the mechanism or the regulation of T-DNA integration in Arabidopsis.

Summary922I.Introduction and a brief survey of 54 033 Arabidopsis publications922II.Arabidopsis reverse genetics: paving the way for gene function studies922III.Arabidopsis stock centers925IV.Databases and online tools925V.Cell biology926VI.Development927VII.Hormones928VIII.The plant immune system and Arabidopsis research929IX.Signaling in response to abiotic stress930X.Pumps, channels, transporters and the like931XI.Cell walls, starch and lipids932XII.Epigenetics and epigenomics: from genotype to phenotype933XIII.Natural variation and genome‐wide association studies934XIV.Gene regulatory networks934XV.Modeling, bioinformatics, systems biology935XVI.Synthetic biology936XVII.Conclusions and outlook937Acknowledgements937References937 Summary The year 2014 marked the 25th International Conference on Arabidopsis Research. In the 50 yr since the first International Conference on Arabidopsis Research, held in 1965 in Göttingen, Germany, > 54 000 papers that mention Arabidopsis thaliana in the title, abstract or keywords have been published. We present herein a citational network analysis of these papers, and touch on some of the important discoveries in plant biology that have been made in this powerful model system, and highlight how these discoveries have then had an impact in crop species. We also look to the future, highlighting some outstanding questions that can be readily addressed in Arabidopsis. Topics that are discussed include Arabidopsis reverse genetic resources, stock centers, databases and online tools, cell biology, development, hormones, plant immunity, signaling in response to abiotic stress, transporters, biosynthesis of cells walls and macromolecules such as starch and lipids, epigenetics and epigenomics, genome‐wide association studies and natural variation, gene regulatory networks, modeling and systems biology, and synthetic biology.

In the absence of the telomerase, telomeres undergo progressive shortening and are ultimately recruited into end-to-end chromosome fusions via the non-homologous end joining (NHEJ) double-strand break repair pathway. Previously, we showed that fusion of critically shortened telomeres in Arabidopsis proceeds with approximately the same efficiency in the presence or absence of KU70, a key component of NHEJ. Here we report that DNA ligase IV (LIG4) is also not essential for telomere joining. We observed only a modest decrease (3-fold) in the frequency of chromosome fusions in triple tert ku70 lig4 mutants versus tert ku70 or tert. Sequence analysis revealed that, relative to tert ku70, chromosome fusion junctions in tert ku70 lig4 mutants contained less microhomology and less telomeric DNA. These findings argue that the KU-LIG4 independent end-joining pathway is less efficient and mechanistically distinct from KU-independent NHEJ. Strikingly, in all the genetic backgrounds we tested, chromosome fusions are initiated when the shortest telomere in the population reaches ∼1 kb, implying that this size represents a critical threshold that heralds a detrimental structural transition. These data reveal the transitory nature of telomere stability, and the robust and flexible nature of DNA repair mechanisms elicited by telomere dysfunction.