Molecular cloning and developmental expression of AtGR1, a new growth-related Arabidopsis gene strongly induced by ionizing radiation

Disruption of REC8 in Meiosis I led to watermelon seedless

In triploid watermelon (Citrullus lanatus), the homologous chromosomes of germ cells are disorder during meiosis, resulting in the failure of seeds formation and producing seedless fruit. Therefore, mutating the genes specifically functioning in meiosis may be an alternative way to achieve seedless watermelon. REC8, as a key component of the cohesin complex in meiosis, is dramatically essential for sister chromatid cohesion and chromosome segregation. However, the role of REC8 in meiosis has not yet been characterized in watermelon. Here, we identified ClREC8 as a member of RAD21/REC8 family with a high expression in male and female flowers of watermelon. In situ hybridization analysis showed that ClREC8 was highly expressed at the early stage of meiosis during pollen formation. Knocking out ClREC8 in watermelon led to decline of pollen vitality. After pollinating with foreign normal pollen, the ovaries of ClREC8 knockout lines could inflate normally but failed to form seeds. We further compared the meiosis chromosomes of pollen mother cells in different stages between the knockout lines and the corresponding wild type. The results indicated that ClREC8 was required for the monopolar orientation of the sister kinetochores in Meiosis I. Additionally, transcriptome sequencing (RNA-seq) analysis between WT and the knockout lines revealed that the disruption of ClREC8 caused the expression levels of mitosis-related genes and meiosis-related genes to decrease. Our results demonstrated ClREC8 has a specific role in Meiosis I of watermelon germ cells, and loss-of-function of the ClREC8 led to seedless fruit, which may provide an alternative strategy to breed cultivars with seedless watermelon.

Maize miRNA and target regulation in response to hormone depletion and light exposure during somatic embryogenesis

Maize somatic embryogenesis (SE) is induced from the immature zygotic embryo in darkness and under the appropriate hormones' levels. Small RNA expression is reprogrammed and certain miRNAs become particularly enriched during induction while others, characteristic to the zygotic embryo, decrease. To explore the impact of different environmental cues on miRNA regulation in maize SE, we tested specific miRNA abundance and their target gene expression in response to photoperiod and hormone depletion for two different maize cultivars (VS-535 and H-565). The expression levels of miR156, miR159, miR164, miR168, miR397, miR398, miR408, miR528, and some predicted targets (SBP23, GA-MYB, CUC2, AGO1c, LAC2, SOD9, GR1, SOD1A, PLC) were examined upon staged hormone depletion in the presence of light photoperiod or darkness. Almost all examined miRNA, except miR159, increased upon hormone depletion, regardless photoperiod absence/presence. miR528, miR408, and miR398 changed the most. On the other hand, expression of miRNA target genes was strongly regulated by the photoperiod exposure. Stress-related miRNA targets showed greater differences between cultivars than development-related targets. miRNA/target inverse relationship was more frequently observed in darkness than light. Interestingly, miR528, but not miR159, miR168 or miR398, was located on polyribosome fractions suggesting a role for this miRNA at the level of translation. Overall our results demonstrate that hormone depletion exerts a great influence on specific miRNA expression during plant regeneration independently of light. However, their targets are additionally influenced by the presence of photoperiod. The reproducibility or differences observed for particular miRNA-target regulation between two different highly embryogenic genotypes provide clues for conserved miRNA roles within the SE process.

The AtRAD21.1 and AtRAD21.3 Arabidopsis cohesins play a synergistic role in somatic DNA double strand break damage repair

BACKGROUND

The RAD21 cohesin plays, besides its well-recognised role in chromatid cohesion, a role in DNA double strand break (dsb) repair. In Arabidopsis there are three RAD21 paralog genes (AtRAD21.1, AtRAD21.2 and AtRAD21.3), yet only AtRAD21.1 has been shown to be required for DNA dsb damage repair. Further investigation of the role of cohesins in DNA dsb repair was carried out and is here reported.

RESULTS

We show for the first time that not only AtRAD21.1 but also AtRAD21.3 play a role in somatic DNA dsb repair. Comet data shows that the lack of either cohesins induces a similar high basal level of DNA dsb in the nuclei and a slower DNA dsb repair kinetics in both cohesin mutants. The observed AtRAD21.3 transcriptional response to DNA dsb induction reinforces further the role of this cohesin in DNA dsb repair. The importance of AtRAD21.3 in DNA dsb damage repair, after exposure to DNA dsb damage inducing agents, is notorious and recognisably evident at the phenotypical level, particularly when the AtRAD21.1 gene is also disrupted.

CONCLUSIONS

Our data demonstrates that both Arabidopsis cohesin (AtRAD21.1 and AtRAD21.3) play a role in somatic DNA dsb repair. Furthermore, the phenotypical data from the atrad21.1 atrad21.3 double mutant indicates that these two cohesins function synergistically in DNA dsb repair. The implications of this data are discussed.

Identification of MAIN, a factor involved in genome stability in the meristems of Arabidopsis thaliana

Stem cells in the root and shoot apical meristem provide the descendant cells required for growth and development throughout the lifecycle of a plant. We found that mutations in the Arabidopsis MAINTENANCE OF MERISTEMS (MAIN) gene led to plants with distorted stem cell niches in which stem cells are not maintained and undergo premature differentiation or cell death. The malfunction of main meristems leads to short roots, mis-shaped leaves, reduced fertility and partial fasciation of stems. MAIN encodes a nuclear-localized protein and is a member of a so far uncharacterized plant-specific gene family. As main mutant plants are hypersensitive to DNA-damaging agents, expression of genes involved in DNA repair is induced and dead cells with damaged DNA accumulate in the mutant meristems, we propose that MAIN is required for meristem maintenance by sustaining genome integrity in stem cells and their descendants cells.

The Arabidopsis MERISTEM DISORGANIZATION 1 gene is required for the maintenance of stem cells through the reduction of DNA damage

In plants, stem cells reside in apical meristems, and provide the descendants required for post-embryonic growth and development throughout the life of a plant. To identify a novel factor required for the maintenance of stem cells, we isolated an Arabidopsis mutant, named meristem disorganization 1-1 (mdo1-1), that exhibits several developmental defects, such as abnormal phyllotaxy and plastochron, stem fasciation and retarded root growth. We found that the mutant plants fail to maintain stem cells, resulting in the differentiation or death of stem cells. The mutant plants also showed several phenotypes related to DNA damage, suggesting that the mutant cells are exposed constitutively to DNA damage even without external genotoxic stress. The growth defect and the hypersensitivity to DNA-damaging agents of mdo1-1 were enhanced significantly when combined with a lesion of the ATAXIA-TELANGIECTASIA MUTATED (ATM) gene, but not of the ATM/RAD3-RELATED (ATR) gene, suggesting that the function of the MDO1 gene is closely related to that of ATM kinase. The MDO1 gene encodes an unknown protein that is conserved in a wide variety of land plants. The results thus suggested that the MDO1 gene product is required for the maintenance of stem cells through a reduction in DNA damage.

Condensin II alleviates DNA damage and is essential for tolerance of boron overload stress in Arabidopsis

Although excess boron (B) is known to negatively affect plant growth, its molecular mechanism of toxicity is unknown. We previously isolated two Arabidopsis thaliana mutants, hypersensitive to excess B (heb1-1 and heb2-1). In this study, we found that HEB1 and HEB2 encode the CAP-G2 and CAP-H2 subunits, respectively, of the condensin II protein complex, which functions in the maintenance of chromosome structure. Growth of Arabidopsis seedlings in medium containing excess B induced expression of condensin II subunit genes. Simultaneous treatment with zeocin, which induces DNA double-strand breaks (DSBs), and aphidicolin, which blocks DNA replication, mimicked the effect of excess B on root growth in the heb mutants. Both excess B and the heb mutations upregulated DSBs and DSB-inducible gene transcription, suggesting that DSBs are a cause of B toxicity and that condensin II reduces the incidence of DSBs. The Arabidopsis T-DNA insertion mutant atr-2, which is sensitive to replication-blocking reagents, was also sensitive to excess B. Taken together, these data suggest that the B toxicity mechanism in plants involves DSBs and possibly replication blocks and that plant condensin II plays a role in DNA damage repair or in protecting the genome from certain genotoxic stressors, particularly excess B.

Gene expression profiling of Arabidopsis meiocytes

Meiosis is a special type of cell division present in all organisms that reproduce by sexual reproduction. It ensures the transition between the sporophytic and gametophytic state and allows gamete production through meiotic recombination and chromosome number reduction. In this paper, we describe a technique for the isolation of Arabidopsis thaliana male meiocytes. From this cellular material, it was then possible to develop large-scale transcriptome studies using CATMA microarrays and thus to obtain an overview of genes expressed during Arabidopsis meiosis. The expression profiles were studied with either stringent statistical criteria or by performing clustering. Both methods resulted in gene clusters enriched in meiosis-specific genes (from 14- to 55-fold). Analysis of these data provided a unique set of genes that will be pivotal to further analysis aimed at understanding the meiotic process.

Molecular characterization of the role of the Schizosaccharomyces pombe nip1+/ctp1+ gene in DNA double-strand break repair in association with the Mre11-Rad50-Nbs1 complex

The Schizosaccharomyces pombe nip1(+)/ctp1(+) gene was previously identified as an slr (synthetically lethal with rad2) mutant. Epistasis analysis indicated that Nip1/Ctp1 functions in Rhp51-dependent recombinational repair, together with the Rad32 (spMre11)-Rad50-Nbs1 complex, which plays important roles in the early steps of DNA double-strand break repair. Nip1/Ctp1 was phosphorylated in asynchronous, exponentially growing cells and further phosphorylated in response to bleomycin treatment. Overproduction of Nip1/Ctp1 suppressed the DNA repair defect of an nbs1-s10 mutant, which carries a mutation in the FHA phosphopeptide-binding domain of Nbs1, but not of an nbs1 null mutant. Meiotic DNA double-strand breaks accumulated in the nip1/ctp1 mutant. The DNA repair phenotypes and epistasis relationships of nip1/ctp1 are very similar to those of the Saccharomyces cerevisiae sae2/com1 mutant, suggesting that Nip1/Ctp1 is a functional homologue of Sae2/Com1, although the sequence similarity between the proteins is limited to the C-terminal region containing the RHR motif. We found that the RxxL and CxxC motifs are conserved in Schizosaccharomyces species and in vertebrate CtIP, originally identified as a cofactor of the transcriptional corepressor CtBP. However, these two motifs are not found in other fungi, including Saccharomyces and Aspergillus species. We propose that Nip1/Ctp1 is a functional counterpart of Sae2/Com1 and CtIP.

Ctp1 is a cell-cycle-regulated protein that functions with Mre11 complex to control double-strand break repair by homologous recombination

The Mre11-Rad50-Nbs1 (MRN) complex is a primary sensor of DNA double-strand breaks (DSBs). Upon recruitment to DSBs, it plays a critical role in catalyzing 5' --> 3' single-strand resection that is required for repair by homologous recombination (HR). Unknown mechanisms repress HR in G1 phase of the cell cycle during which nonhomologous end-joining (NHEJ) is the favored mode of DSB repair. Here we describe fission yeast Ctp1, so-named because it shares conserved domains with the mammalian tumor suppressor CtIP. Ctp1 is recruited to DSBs where it is essential for repair by HR. Ctp1 is required for efficient formation of RPA-coated single-strand DNA adjacent to DSBs, indicating that it functions with the MRN complex in 5' --> 3' resection. Transcription of ctp1(+) is periodic during the cell cycle, with the onset of its expression coinciding with the start of DNA replication. These data suggest that regulation of Ctp1 underlies cell-cycle control of HR.

A novel plant gene essential for meiosis is related to the human CtIP and the yeast COM1/SAE2 gene

Obligatory homologous recombination (HR) is required for chiasma formation and chromosome segregation in meiosis I. Meiotic HR is initiated by DNA double-strand breaks (DSBs), generated by Spo11, a homologue of the archaebacterial topoisomerase subunit Top6A. In Saccharomyces cerevisiae, Rad50, Mre11 and Com1/Sae2 are essential to process an intermediate of the cleavage reaction consisting of Spo11 covalently linked to the 5' termini of DNA. While Rad50 and Mre11 also confer genome stability to vegetative cells and are well conserved in evolution, Com1/Sae2 was believed to be fungal-specific. Here, we identify COM1/SAE2 homologues in all eukaryotic kingdoms. Arabidopsis thaliana Com1/Sae2 mutants are sterile, accumulate AtSPO11-1 during meiotic prophase and fail to form AtRAd51 foci despite the presence of unrepaired DSBs. Furthermore, DNA fragmentation in AtCom1 is suppressed by eliminating AtSPO11-1. In addition, AtCOM1 is specifically required for mitomycin C resistance. Interestingly, we identified CtIP, an essential protein interacting with the DNA repair machinery, as the mammalian homologue of Com1/Sae2, with important implications for the molecular role of CtIP.

Transcriptomic Profile of Arabidopsis Rosette Leaves during the Reproductive Stage after Exposure to Ionizing Radiation

We attempted to obtain a transcriptomic profile of ionizing radiation-responsive genes in Arabidopsis plants using Affymetrix ATH1 whole-genome microarrays. The Arabidopsis plants were irradiated with 200 Gy gamma rays at the early reproduction stage, 33 days after sowing. Rosette leaves were harvested during the postirradiation period from 36 to 49 days after sowing and used for the microarray analysis. The most remarkable changes in the genome-wide expression were observed at 42 days after sowing (9 days after the irradiation). We identified 2165 genes as gamma-ray inducible and 1735 genes as gamma-ray repressible. These numbers of affected genes were almost two to seven times higher than those at other times. In a comparison of the control and irradiated groups, we also identified 354 differentially expressed genes as significant by applying Welch's t test and fold change analysis. The gene ontology analysis showed that radiation up-regulated defense/ stress responses but down-regulated rhythm/growth responses. Specific expression patterns of 10 genes for antioxidant enzymes, photosynthesis or chlorophyll synthesis after irradiation were also obtained using real-time quantitative PCR analysis. We discuss physiological and genetic alterations in the antioxidative defense system, photosynthesis and chlorophyll metabolism after irradiation at the reproductive stage.

ATM-mediated transcriptional and developmental responses to gamma-rays in Arabidopsis

ATM (Ataxia Telangiectasia Mutated) is an essential checkpoint kinase that signals DNA double-strand breaks in eukaryotes. Its depletion causes meiotic and somatic defects in Arabidopsis and progressive motor impairment accompanied by several cell deficiencies in patients with ataxia telangiectasia (AT). To obtain a comprehensive view of the ATM pathway in plants, we performed a time-course analysis of seedling responses by combining confocal laser scanning microscopy studies of root development and genome-wide expression profiling of wild-type (WT) and homozygous ATM-deficient mutants challenged with a dose of γ-rays (IR) that is sublethal for WT plants. Early morphologic defects in meristematic stem cells indicated that AtATM, an Arabidopsis homolog of the human ATM gene, is essential for maintaining the quiescent center and controlling the differentiation of initial cells after exposure to IR. Results of several microarray experiments performed with whole seedlings and roots up to 5 h post-IR were compiled in a single table, which was used to import gene information and extract gene sets. Sequence and function homology searches; import of spatio-temporal, cell cycling, and mutant-constitutive expression characteristics; and a simplified functional classification system were used to identify novel genes in all functional classes. The hundreds of radiomodulated genes identified were not a random collection, but belonged to functional pathways such as those of the cell cycle; cell death and repair; DNA replication, repair, and recombination; and transcription; translation; and signaling, indicating the strong cell reprogramming and double-strand break abrogation functions of ATM checkpoints. Accordingly, genes in all functional classes were either down or up-regulated concomitantly with downregulation of chromatin deacetylases or upregulation of acetylases and methylases, respectively. Determining the early transcriptional indicators of prolonged S-G2 phases that coincided with cell proliferation delay, or an anticipated subsequent auxin increase, accelerated cell differentiation or death, was used to link IR-regulated hallmark functions and tissue phenotypes after IR. The transcription burst was almost exclusively AtATM-dependent or weakly AtATR-dependent, and followed two major trends of expression in atm: (i)-loss or severe attenuation and delay, and (ii)-inverse and/or stochastic, as well as specific, enabling one to distinguish IR/ATM pathway constituents. Our data provide a large resource for studies on the interaction between plant checkpoints of the cell cycle, development, hormone response, and DNA repair functions, because IR-induced transcriptional changes partially overlap with the response to environmental stress. Putative connections of ATM to stem cell maintenance pathways after IR are also discussed.

Arabidopsis TEBICHI, with helicase and DNA polymerase domains, is required for regulated cell division and differentiation in meristems

In plant meristems, each cell divides and differentiates in a spatially and temporally regulated manner, and continuous organogenesis occurs using cells derived from the meristem. We report the identification of the Arabidopsis thaliana TEBICHI (TEB) gene, which is required for regulated cell division and differentiation in meristems. The teb mutants show morphological defects, such as short roots, serrated leaves, and fasciation, as well as defective patterns of cell division and differentiation in the meristem. The TEB gene encodes a homolog of Drosophila MUS308 and mammalian DNA polymerase theta, which prevent spontaneous or DNA damage-induced production of DNA double strand breaks. As expected from the function of animal homologs, teb mutants show constitutively activated DNA damage responses. Unlike other fasciation mutants with activated DNA damage responses, however, teb mutants do not activate transcriptionally silenced genes. teb shows an accumulation of cells expressing cyclinB1;1:GUS in meristems, suggesting that constitutively activated DNA damage responses in teb lead to a defect in G2/M cell cycle progression. Furthermore, other fasciation mutants, such as fasciata2 and tonsoku/mgoun3/brushy1, also show an accumulation of cells expressing cyclinB1;1:GUS in meristems. These results suggest that cell cycle progression at G2/M is important for the regulation of the pattern of cell division and of differentiation during plant development.

Characterization of the three Arabidopsis thaliana RAD21 cohesins reveals differential responses to ionizing radiation

The RAD21/REC8 gene family has been implicated in sister chromatid cohesion and DNA repair in several organisms. Unlike most eukaryotes, Arabidopsis thaliana has three RAD21 gene homologues, and their cloning and characterization are reported here. All three genes, AtRAD21.1, AtRAD21.2, and AtRAD21.3, are expressed in tissues rich in cells undergoing cell division, and AtRAD21.3 shows the highest relative level of expression. An increase in steady-state levels of AtRAD21.1 transcript was also observed, specifically after the induction of DNA damage. Phenotypic analysis of the atrad21.1 and atrad21.3 mutants revealed that neither of the single mutants was lethal, probably due to the redundancy in function of the AtRAD21 genes. However, AtRAD21.1 plays a critical role in recovery from DNA damage during seed imbibition, prior to germination, as atrad21.1 mutant seeds are hypersensitive to radiation damage.

CtIP, a candidate tumor susceptibility gene is a team player with luminaries

CtIP is a nuclear protein conserved among vertebrates that was discovered as a cofactor of the transcriptional corepressor CtBP. CtIP also interacts with the tumor suppressors such as BRCA1 and the pRb family members through binding sites that are frequently mutated in human cancers. CtIP is a target for BRCA1-dependent phosphorylation by the ATM kinase induced by DNA double strand breakage. CtIP plays a role in DNA-damage-induced cell cycle checkpoint control at the G2/M transition. Homozygous inactivation of the Ctip gene causes very early embryonic lethality during mouse development. The Ctip(-/-) embryo cells are arrested in G1 and do not enter S phase. Depletion of Ctip in established mouse embryo fibroblasts arrests cells in G1 and results in an accumulation of hypophosphorylated Rb and the Cdk inhibitor p21, suggesting that CtIP is also a critical regulator of G1/S transition of the cell cycle. The Ctip gene contains a mononucleotide (A9) repeat and one of the alleles is mutated at a high frequency in colon cancers with microsatellite instability. The Ctip(+/-) mice develop multiple types of tumors suggesting that haploid insufficiency of Ctip leads to tumorigenesis. Among the various tumor types observed in Ctip(+/-) heterozygous mice, large lymphomas are prevalent. Recent studies raise the possibility that Ctip may itself be a tumor susceptibility gene and suggest that it might be important for the activities of tumor suppressors BRCA1, pRb family proteins and Ikaros family members.

Low-level radiocaesium exposure alters gene expression in roots of Arabidopsis

Radiocaesium is one of the main anthropogenic sources of internal and external exposure to beta- and gamma-radiation (e.g. from global fallout of atmospheric atomic bomb testing and from the Chernobyl reactor accident). Here we investigated gene expression by suppression subtractive hybridization (SSH) and reverse transcription-polymerase chain reaction (RT-PCR) in Arabidopsis thaliana, which was induced by the root uptake of 134Cs. SSH analysis resulted in the isolation of 46 clones that were differentially expressed at 30 Bq cm(-3) 134Cs. Most of the expressed sequence tags identified belonged to genes encoding proteins that were involved in cell growth, cell division and the development of plants, and in proteins controlling translation, general metabolism and stress defence, including a DNA excision repair protein. The accumulation of caesium in plant material was measured in plants grown for 5 wk on agar contaminated by up to 60 Bq cm(-3) 134Cs. 134Cs was found to accumulate, in particular, in leaf rosettes and was dependent on the activity concentration in the growth media. The data indicate that low-level ionizing radiation influences important cellular responses, resulting in a changed gene-expression profile.

A versatile promoter for the expression of proteins in glandular and non-glandular trichomes from a variety of plants

A DNA regulatory fragment was isolated from the promoter region of the OASA1 gene, encoding the cytosolic O-acetylserine(thiol)lyase enzyme that is highly expressed in Arabidopsis thaliana trichomes. This DNA fragment has been named an ATP fragment and comprises 1435 bp of the genomic region upstream of the OASA1 gene and 375 bp of the transcriptional initiation start site containing the first intron of the gene. The ATP fragment, fused to the green fluorescent protein (GFP) and beta-glucuronidase (GUS) reporter genes, is able to drive high-level gene expression in A. thaliana trichomes. Deletion analysis of the ATP fragment determined that the region from -266 to -66 contains regulatory elements required for trichome expression. In addition, the region from +112 to +375, comprising the first intronic region of the gene, is also essential for trichome gene expression. Expression of the full-length ATP fragment in tobacco and peppermint shows that this fragment is also able to drive expression in glandular trichomes and suggests additional biotechnological applications for this promoter.

Stress induces the expression of AtNADK-1, a gene encoding a NAD(H) kinase in Arabidopsis thaliana

A novel Arabidopsis thaliana gene (AtNADK-1) was identified based on its response to radiation and oxidative stress. Levels of AtNADK-1 mRNA increase eight-fold following exposure to ionising radiation and are enhanced three-fold by treatment with hydrogen peroxide. The gene also appears to be differentially regulated during compatible and incompatible plant-pathogen interactions in response to Pseudomonas syringae pv. tomato. The full-length AtNADK-1 cDNA encodes a 58-kDa protein that shows high sequence homology to the recently defined family of NAD(H) kinases. Recombinant AtNADK-1 utilises ATP to phosphorylate both NAD and NADH, showing a two-fold preference for NADH. Using reverse genetics, we demonstrate that AtNADK-1 deficient plants display enhanced sensitivity to gamma irradiation and to paraquat-induced oxidative stress. Our results indicate that this novel NAD(H) kinase may contribute to the maintenance of redox status in Arabidopsis thaliana.

Systemic movement of a tobamovirus requires host cell pectin methylesterase

Systemic movement of plant viruses through the host vasculature, one of the central events of the infection process, is essential for maximal viral accumulation and development of disease symptoms. The host plant proteins involved in this transport, however, remain unknown. Here, we examined whether or not pectin methylesterase (PME), one of the few cellular proteins known to be involved in local, cell-to-cell movement of tobacco mosaic virus (TMV), is also required for the systemic spread of viral infection through the plant vascular system. In a reverse genetics approach, PME levels were reduced in tobacco plants using antisense suppression. The resulting PME antisense plants displayed a significant degree of PME suppression in their vascular tissues but retained the wild-type pattern of phloem loading and unloading of a fluorescent solute. Systemic transport of TMV in these plants, however, was substantially delayed as compared to the wild-type tobacco, suggesting a role for PME in TMV systemic infection. Our analysis of virus distribution in the PME antisense plants suggested that TMV systemic movement may be a polar process in which the virions enter and exit the vascular system by two different mechanisms, and it is the viral exit out of the vascular system that involves PME.

AtATM is essential for meiosis and the somatic response to DNA damage in plants

In contrast to yeast or mammalian cells, little is known about the signaling responses to DNA damage in plants. We previously characterized AtATM, an Arabidopsis homolog of the human ATM gene, which is mutated in ataxia telangiectasia, a chromosome instability disorder. The Atm protein is a protein kinase whose activity is induced by DNA damage, particularly DNA double-strand breaks. The phosphorylation targets of Atm include proteins involved in DNA repair, cell cycle control, and apoptosis. Here, we describe the isolation and functional characterization of two Arabidopsis mutants carrying a T-DNA insertion in AtATM. Arabidopsis atm mutants are hypersensitive to gamma-radiation and methylmethane sulfonate but not to UV-B light. In correlation with the radiation sensitivity, atm mutants failed to induce the transcription of genes involved in the repair and/or detection of DNA breaks upon irradiation. In addition, atm mutants are partially sterile, and we show that this effect is attributable to abundant chromosomal fragmentation during meiosis. Interestingly, the transcription of DNA recombination genes during meiosis was not dependent on AtATM, and meiotic recombination occurred at the same rate as in wild-type plants, raising questions about the function of AtAtm during meiosis in plants. Our results demonstrate that AtATM plays a central role in the response to both stress-induced and developmentally programmed DNA damage.