Homologous recombination: ends as the means

Aging-Induced Reduction in Safflower Seed Germination via Impaired Energy Metabolism and Genetic Integrity Is Partially Restored by Sucrose and DA-6 Treatment

Seed storage underpins global agriculture and the seed trade and revealing the mechanisms of seed aging is essential for enhancing seed longevity management. Safflower is a multipurpose oil crop, rich in unsaturated fatty acids that are at high risk of peroxidation as a contributory factor to seed aging. However, the molecular mechanisms responsible for safflower seed viability loss are not yet elucidated. We used controlled deterioration (CDT) conditions of 60% relative humidity and 50 °C to reduce germination in freshly harvested safflower seeds and analyzed aged seeds using biochemical and molecular techniques. While seed malondialdehyde (MDA) and fatty acid content increased significantly during CDT, catalase activity and soluble sugar content decreased. KEGG analysis of gene function and qPCR validation indicated that aging severely impaired several key functional and biosynthetic pathways including glycolysis, fatty acid metabolism, antioxidant activity, and DNA replication and repair. Furthermore, exogenous sucrose and diethyl aminoethyl hexanoate (DA-6) treatment partially promoted germination in aged seeds, further demonstrating the vital role of impaired sugar and fatty acid metabolism during the aging and recovery processes. We concluded that energy metabolism and genetic integrity are impaired during aging, which contributes to the loss of seed vigor. Such energy metabolic pathways as glycolysis, fatty acid degradation, and the tricarboxylic acid cycle (TCA) are impaired, especially fatty acids produced by the hydrolysis of triacylglycerols during aging, as they are not efficiently converted to sucrose via the glyoxylate cycle to provide energy supply for safflower seed germination and seedling growth. At the same time, the reduced capacity for nucleotide synthesis capacity and the deterioration of DNA repair ability further aggravate the damage to DNA, reducing seed vitality.

Employing template-directed CRISPR-based editing of the OsALS gene to create herbicide tolerance in Basmati rice

Rice (Oryza sativa) is one of the primary food crops which contributes major portion of daily calorie intake. It is used as model crop for various genome editing studies. Basmati rice was also explored for establishing non-homologous end joining-based genome editing. But it was not clear whether homology-directed repair (HDR)-based genome editing can be done in Basmati rice. The current study was designed to establish HDR-based genome editing in Basmati rice to develop herbicide tolerance. There is severe weed spread when rice is grown via direct planted rice method in various countries to save labour and water resources. Therefore, the use of herbicides is necessary to control weeds. These herbicides can also affect cultivated rice which creates the need to develop herbicide-tolerant rice. In current study, we introduced a point mutation in Acetolactate Synthase gene to convert tryptophan to leucine at position 548. For this purpose, different constructs for HDR were tested with different RNA scaffold and orientation of repair templates. Out of four different architectures, the one having repair template identical to the target DNA strand precisely edited the target site. We successfully established template-directed CRISPR-Cas9 system in Super Basmati rice by detecting desired substitutions at the target site in Acetolactate Synthase locus. Moreover, this editing of Acetolactate Synthase gene resulted in the production of herbicide tolerance in Super Basmati rice. This study suggests that such type of HDR system can be used to precisely edit other genes for crop improvement.

Fusion of the Cas9 endonuclease and the VirD2 relaxase facilitates homology-directed repair for precise genome engineering in rice

Precise genome editing by systems such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) requires high-efficiency homology-directed repair (HDR). Different technologies have been developed to improve HDR but with limited success. Here, we generated a fusion between the Cas9 endonuclease and the Agrobacterium VirD2 relaxase (Cas9-VirD2). This chimeric protein combines the functions of Cas9, which produces targeted and specific DNA double-strand breaks (DSBs), and the VirD2 relaxase, which brings the repair template in close proximity to the DSBs, to facilitate HDR. We successfully employed our Cas9-VirD2 system for precise ACETOLACTATE SYNTHASE (OsALS) allele modification to generate herbicide-resistant rice (Oryza sativa) plants, CAROTENOID CLEAVAGE DIOXYGENASE-7 (OsCCD7) to engineer plant architecture, and generate in-frame fusions with the HA epitope at HISTONE DEACETYLASE (OsHDT) locus. The Cas9-VirD2 system expands our ability to improve agriculturally important traits in crops and opens new possibilities for precision genome engineering across diverse eukaryotic species.

Fusion of the Cas9 endonuclease and the VirD2 relaxase facilitates homology-directed repair for precise genome engineering in rice

Precise genome editing by systems such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) requires high-efficiency homology-directed repair (HDR). Different technologies have been developed to improve HDR but with limited success. Here, we generated a fusion between the Cas9 endonuclease and the Agrobacterium VirD2 relaxase (Cas9-VirD2). This chimeric protein combines the functions of Cas9, which produces targeted and specific DNA double-strand breaks (DSBs), and the VirD2 relaxase, which brings the repair template in close proximity to the DSBs, to facilitate HDR. We successfully employed our Cas9-VirD2 system for precise ACETOLACTATE SYNTHASE (OsALS) allele modification to generate herbicide-resistant rice (Oryza sativa) plants, CAROTENOID CLEAVAGE DIOXYGENASE-7 (OsCCD7) to engineer plant architecture, and generate in-frame fusions with the HA epitope at HISTONE DEACETYLASE (OsHDT) locus. The Cas9-VirD2 system expands our ability to improve agriculturally important traits in crops and opens new possibilities for precision genome engineering across diverse eukaryotic species.

Ali, Shami, Sedeek et al. generate a fusion between Cas9 and the VirD2 relaxase (Cas9-VirD2), which combines the functions of both proteins in producing targeted and specific double strand breaks and promoting homology-directed repair. They show the utility of their method by producing herbicide resistant rice.

Targeted integration in human cells through single crossover mediated by ZFN or CRISPR/Cas9

BACKGROUND

Targeted DNA integration is widely used in basic research and commercial applications because it eliminates positional effects on transgene expression. Targeted integration in mammalian cells is generally achieved through a double crossover event between the genome and a linear donor containing two homology arms flanking the gene of interest. However, this strategy is generally less efficient at introducing larger DNA fragments. Using the homology-independent NHEJ mechanism has recently been shown to improve efficiency of integrating larger DNA fragments at targeted sites, but integration through this mechanism is direction-independent. Therefore, developing new methods for direction-dependent integration with improved efficiency is desired.

RESULTS

We generated site-specific double-strand breaks using ZFNs or CRISPR/Cas9 in the human CCR5 gene and a donor plasmid containing a 1.6-kb fragment homologous to the CCR5 gene in the genome. These DSBs efficiently drove the direction-dependent integration of 6.4-kb plasmids into the genomes of two human cell lines through single-crossover recombination. The integration was direction-dependent and resulted in the duplication of the homology region in the genome, allowing the integration of another copy of the donor plasmid. The CRISPR/Cas9 system tended to disrupt the sgRNA-binding site within the duplicated homology region, preventing the integration of another plasmid donor. In contrast, ZFNs were less likely to completely disrupt their binding sites, allowing the successive integration of additional plasmid donor copies. This could be useful in promoting multi-copy integration for high-level expression of recombinant proteins. Targeted integration through single crossover recombination was highly efficient (frequency: 33%) as revealed by Southern blot analysis of clonal cells. This is more efficient than a previously described NHEJ-based method (0.17-0.45%) that was used to knock in an approximately 5-kb long DNA fragment.

CONCLUSION

We developed a method for the direction-dependent integration of large DNA fragments through single crossover recombination. We compared and contrasted our method to a previously reported technique for the direction-independent integration of DNA cassettes into the genomes of cultured cells via NHEJ. Our method, due to its directionality and ability to efficiently integrate large fragments, is an attractive strategy for both basic research and industrial application.

Comparison of CRISPR/Cas9 and TALENs on editing an integrated EGFP gene in the genome of HEK293FT cells

Background

Genome editors such as CRISPR/Cas9 and TALENs are at the forefront of research into methodologies for targeted modification of the mammalian genome. To date few comparative studies have been carried out to investigate the difference of genome editing characteristics between CRISPR/Cas9 and TALENs. While the CRISPR/Cas9 system has overtaken TALENs as the tool of choice for most research groups working in this field, we hypothesized that there could be certain applications whereby the application of TALENs would have specific benefits. Here we compare CRISPR/Cas9 and TALEN as tools for introducing site-specific editing events at an integrated EGFP gene in the genome of HEK293FT cells.

Results

Guide RNAs and TALEN pairs were designed to target two loci within the EGFP gene. We found that paired Cas9 nucleases induced targeted genomic deletion more efficiently and precisely than two TALEN pairs. However, when concurrently supplied with a plasmid template spanning the two DNA double-strand breaks (DSBs) within EGFP, TALENs stimulated homology directed repair (HDR) more efficiently than CRISPR/Cas9 and caused fewer targeted genomic deletions.

Conclusions

Our data suggest that the choice of genome editing tool should be determined by the desired genome editing outcome. Such a rational approach is likely to benefit research outputs for groups working in fields as diverse as modification of cell lines, to animal models for disease studies, or gene therapy strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s40064-016-2536-3) contains supplementary material, which is available to authorized users.

CRISPR/Cas9-mediated efficient and heritable targeted mutagenesis in tomato plants in the first and later generations

The CRISPR/Cas9 system has successfully been used in various organisms for precise targeted gene editing. Although it has been demonstrated that CRISPR/Cas9 system can induce mutation in tomato plants, the stability of heredity in later generations and mutant specificity induced by the CRISPR/Cas9 system in tomato plants have not yet been elucidated in detail. In this study, two genes, SlPDS and SlPIF4, were used for testing targeted mutagenesis in tomato plants through an Agrobacterium tumefaciens-mediated transformation method. A high mutation frequency was observed in all tested targets in the T0 transgenic tomato plants, with an average frequency of 83.56%. Clear albino phenotypes were observed for the psd mutants. High frequencies of homozygous and biallelic mutants were detected even in T0 plants. The majority of the detected mutations were 1- to 3-nucleotide deletions, followed by 1-bp insertions. The target mutations in the T0 lines were stably transmitted to the T1 and T2 generations, without new modifications or revision. Off-target activities associated with SlPDS and SlPIF4 were also evaluated by sequencing the putative off-target sites, and no clear off-target events were detected. Our results demonstrate that the CRISPR/Cas9 system is an efficient tool for generating stable and heritable modifications in tomato plants.

Recent Advances in Genome Editing Using CRISPR/Cas9

The CRISPR (clustered regularly interspaced short palindromic repeat)-Cas9 (CRISPR-associated nuclease 9) system is a versatile tool for genome engineering that uses a guide RNA (gRNA) to target Cas9 to a specific sequence. This simple RNA-guided genome-editing technology has become a revolutionary tool in biology and has many innovative applications in different fields. In this review, we briefly introduce the Cas9-mediated genome-editing method, summarize the recent advances in CRISPR/Cas9 technology, and discuss their implications for plant research. To date, targeted gene knockout using the Cas9/gRNA system has been established in many plant species, and the targeting efficiency and capacity of Cas9 has been improved by optimizing its expression and that of its gRNA. The CRISPR/Cas9 system can also be used for sequence-specific mutagenesis/integration and transcriptional control of target genes. We also discuss off-target effects and the constraint that the protospacer-adjacent motif (PAM) puts on CRISPR/Cas9 genome engineering. To address these problems, a number of bioinformatic tools are available to help design specific gRNAs, and new Cas9 variants and orthologs with high fidelity and alternative PAM specificities have been engineered. Owing to these recent efforts, the CRISPR/Cas9 system is becoming a revolutionary and flexible tool for genome engineering. Adoption of the CRISPR/Cas9 technology in plant research would enable the investigation of plant biology at an unprecedented depth and create innovative applications in precise crop breeding.

The Structural Features of Thousands of T-DNA Insertion Sites Are Consistent with a Double-Strand Break Repair-Based Insertion Mechanism

Transformation by Agrobacterium tumefaciens, an important tool in modern plant research, involves the integration of T-DNA initially present on a plasmid in agrobacteria into the genome of plant cells. The process of attachment of the agrobacteria to plant cells and the transport of T-DNA into the cell and further to the nucleus has been well described. However, the exact mechanism of integration into the host's DNA is still unclear, although several models have been proposed. During confirmation of T-DNA insertion alleles from the GABI-Kat collection of Arabidopsis thaliana mutants, we have generated about 34,000 sequences from the junctions between inserted T-DNA and adjacent genome regions. Here, we describe the evaluation of this dataset with regard to existing models for T-DNA integration. The results suggest that integration into the plant genome is mainly mediated by the endogenous plant DNA repair machinery. The observed integration events showed characteristics highly similar to those of repair sites of double-strand breaks with respect to microhomology and deletion sizes. In addition, we describe unexpected integration events, such as large deletions and inversions at the integration site that are relevant for correct interpretation of results from T-DNA insertion mutants in reverse genetics experiments.

Highly efficient targeted chromosome deletions using CRISPR/Cas9

The CRISPR/Cas9 system has emerged as an intriguing new technology for genome engineering. It utilizes the bacterial endonuclease Cas9 which, when delivered to eukaryotic cells in conjunction with a user-specified small guide RNA (gRNA), cleaves the chromosomal DNA at the target site. Here we show that concurrent delivery of gRNAs designed to target two different sites in a human chromosome introduce DNA double-strand breaks in the chromosome and give rise to targeted deletions of the intervening genomic segment. Predetermined genomic DNA segments ranging from several-hundred base pairs to 1 Mbp can be precisely deleted at frequencies of 1-10%, with no apparent correlation between the size of the deleted fragment and the deletion frequency. The high efficiency of this technique holds promise for large genomic deletions that could be useful in generation of cell and animal models with engineered chromosomes.

Towards social acceptance of plant breeding by genome editing

Although genome-editing technologies facilitate efficient plant breeding without introducing a transgene, it is creating indistinct boundaries in the regulation of genetically modified organisms (GMOs). Rapid advances in plant breeding by genome-editing require the establishment of a new global policy for the new biotechnology, while filling the gap between process-based and product-based GMO regulations. In this Opinion article we review recent developments in producing major crops using genome-editing, and we propose a regulatory model that takes into account the various methodologies to achieve genetic modifications as well as the resulting types of mutation. Moreover, we discuss the future integration of genome-editing crops into society, specifically a possible response to the 'Right to Know' movement which demands labeling of food that contains genetically engineered ingredients.

Genome modifications in plant cells by custom-made restriction enzymes

Genome editing, i.e. the ability to mutagenize, insert, delete and replace sequences, in living cells is a powerful and highly desirable method that could potentially revolutionize plant basic research and applied biotechnology. Indeed, various research groups from academia and industry are in a race to devise methods and develop tools that will enable not only site-specific mutagenesis but also controlled foreign DNA integration and replacement of native and transgene sequences by foreign DNA, in living plant cells. In recent years, much of the progress seen in gene targeting in plant cells has been attributed to the development of zinc finger nucleases and other novel restriction enzymes for use as molecular DNA scissors. The induction of double-strand breaks at specific genomic locations by zinc finger nucleases and other novel restriction enzymes results in a wide variety of genetic changes, which range from gene addition to the replacement, deletion and site-specific mutagenesis of endogenous and heterologous genes in living plant cells. In this review, we discuss the principles and tools for restriction enzyme-mediated gene targeting in plant cells, as well as their current and prospective use for gene targeting in model and crop plants.

MRE11 and RAD50, but not NBS1, are essential for gene targeting in the moss Physcomitrella patens

The moss Physcomitrella patens is unique among plant models for the high frequency with which targeted transgene insertion occurs via homologous recombination. Transgene integration is believed to utilize existing machinery for the detection and repair of DNA double-strand breaks (DSBs). We undertook targeted knockout of the Physcomitrella genes encoding components of the principal sensor of DNA DSBs, the MRN complex. Loss of function of PpMRE11 or PpRAD50 strongly and specifically inhibited gene targeting, whilst rates of untargeted transgene integration were relatively unaffected. In contrast, disruption of the PpNBS1 gene retained the wild-type capacity to integrate transforming DNA efficiently at homologous loci. Analysis of the kinetics of DNA-DSB repair in wild-type and mutant plants by single-nucleus agarose gel electrophoresis revealed that bleomycin-induced fragmentation of genomic DNA was repaired at approximately equal rates in each genotype, although both the Ppmre11 and Pprad50 mutants exhibited severely restricted growth and development and enhanced sensitivity to UV-B and bleomycin-induced DNA damage, compared with wild-type and Ppnbs1 plants. This implies that while extensive DNA repair can occur in the absence of a functional MRN complex; this is unsupervised in nature and results in the accumulation of deleterious mutations incompatible with normal growth and development.

Oligo/polynucleotide-based gene modification: strategies and therapeutic potential

Oligonucleotide- and polynucleotide-based gene modification strategies were developed as an alternative to transgene-based and classical gene targeting-based gene therapy approaches for treatment of genetic disorders. Unlike the transgene-based strategies, oligo/polynucleotide gene targeting approaches maintain gene integrity and the relationship between the protein coding and gene-specific regulatory sequences. Oligo/polynucleotide-based gene modification also has several advantages over classical vector-based homologous recombination approaches. These include essentially complete homology to the target sequence and the potential to rapidly engineer patient-specific oligo/polynucleotide gene modification reagents. Several oligo/polynucleotide-based approaches have been shown to successfully mediate sequence-specific modification of genomic DNA in mammalian cells. The strategies involve the use of polynucleotide small DNA fragments, triplex-forming oligonucleotides, and single-stranded oligodeoxynucleotides to mediate homologous exchange. The primary focus of this review will be on the mechanistic aspects of the small fragment homologous replacement, triplex-forming oligonucleotide-mediated, and single-stranded oligodeoxynucleotide-mediated gene modification strategies as it relates to their therapeutic potential.

Have a break: determinants of meiotic DNA double strand break (DSB) formation and processing in plants

Meiosis is an essential process for sexually reproducing organisms, leading to the formation of specialized generative cells. This review intends to highlight current knowledge of early events during meiosis derived from various model organisms, including plants. It will particularly focus on cis- and trans-requirements of meiotic DNA double strand break (DSB) formation, a hallmark event during meiosis and a prerequisite for recombination of genetic traits. Proteins involved in DSB formation in different organisms, emphasizing the known factors from plants, will be introduced and their functions outlined. Recent technical advances in DSB detection and meiotic recombination analysis will be reviewed, as these new tools now allow analysis of early meiotic recombination in plants with incredible accuracy. To anticipate future directions in plant meiosis research, unpublished results will be included wherever possible.

Epigenetic control of Agrobacterium T-DNA integration

To genetically transform plants, Agrobacterium transfers its T-DNA into the host cell and integrates it into the plant genome, resulting in neoplastic growths. Over the past 2 decades, a great deal has been learned about the molecular mechanism by which Agrobacterium produces T-DNA and transports it into the host nucleus. However, T-DNA integration, which is the limiting, hence, the most critical step of the transformation process, largely remains an enigma. Increasing evidence suggests that Agrobacterium utilizes the host DNA repair machinery to facilitate T-DNA integration. Meanwhile, it is well known that chromatin modifications, including the phosphorylation of histone H2AX, play an important role in DNA repair. Thus, by implication, such epigenetic codes in chromatin may also have a considerable impact on T-DNA integration, although the direct evidence to demonstrate this hypothesis is still lacking. In this review, we summarize the recent advances in our understanding of Agrobacterium T-DNA integration and discuss the potential link between this process and the epigenetic information in the host chromatin. This article is part of a Special Issue entitled: Epigenetic Control of cellular and developmental processes in plants.

Association between simple sequence repeat-rich chromosome regions and intergenomic translocation breakpoints in natural populations of allopolyploid wild wheats

BACKGROUND AND AIMS

Repetitive DNA sequences are thought to be involved in the formation of chromosomal rearrangements. The aim of this study was to analyse the distribution of microsatellite clusters in Aegilops biuncialis and Aegilops geniculata, and its relationship with the intergenomic translocations in these allotetraploid species, wild genetic resources for wheat improvement.

METHODS

The chromosomal localization of (ACG)(n) and (GAA)(n) microsatellite sequences in Ae. biuncialis and Ae. geniculata and in their diploid progenitors Aegilops comosa and Aegilops umbellulata was investigated by sequential in situ hybridization with simple sequence repeat (SSR) probes and repeated DNA probes (pSc119·2, Afa family and pTa71) and by dual-colour genomic in situ hybridization (GISH). Thirty-two Ae. biuncialis and 19 Ae. geniculata accessions were screened by GISH for intergenomic translocations, which were further characterized by fluorescence in situ hybridization and GISH.

KEY RESULTS

Single pericentromeric (ACG)(n) signals were localized on most U and on some M genome chromosomes, whereas strong pericentromeric and several intercalary and telomeric (GAA)(n) sites were observed on the Aegilops chromosomes. Three Ae. biuncialis accessions carried 7U(b)-7M(b) reciprocal translocations and one had a 7U(b)-1M(b) rearrangement, while two Ae. geniculata accessions carried 7U(g)-1M(g) or 5U(g)-5M(g) translocations. Conspicuous (ACG)(n) and/or (GAA)(n) clusters were located near the translocation breakpoints in eight of the ten translocated chromosomes analysed, SSR bands and breakpoints being statistically located at the same chromosomal site in six of them.

CONCLUSIONS

Intergenomic translocation breakpoints are frequently mapped to SSR-rich chromosomal regions in the allopolyploid species examined, suggesting that microsatellite repeated DNA sequences might facilitate the formation of those chromosomal rearrangements. The (ACG)(n) and (GAA)(n) SSR motifs serve as additional chromosome markers for the karyotypic analysis of UM genome Aegilops species.

Nontransgenic genome modification in plant cells

Zinc finger nucleases (ZFNs) are a powerful tool for genome editing in eukaryotic cells. ZFNs have been used for targeted mutagenesis in model and crop species. In animal and human cells, transient ZFN expression is often achieved by direct gene transfer into the target cells. Stable transformation, however, is the preferred method for gene expression in plant species, and ZFN-expressing transgenic plants have been used for recovery of mutants that are likely to be classified as transgenic due to the use of direct gene-transfer methods into the target cells. Here we present an alternative, nontransgenic approach for ZFN delivery and production of mutant plants using a novel Tobacco rattle virus (TRV)-based expression system for indirect transient delivery of ZFNs into a variety of tissues and cells of intact plants. TRV systemically infected its hosts and virus ZFN-mediated targeted mutagenesis could be clearly observed in newly developed infected tissues as measured by activation of a mutated reporter transgene in tobacco (Nicotiana tabacum) and petunia (Petunia hybrida) plants. The ability of TRV to move to developing buds and regenerating tissues enabled recovery of mutated tobacco and petunia plants. Sequence analysis and transmission of the mutations to the next generation confirmed the stability of the ZFN-induced genetic changes. Because TRV is an RNA virus that can infect a wide range of plant species, it provides a viable alternative to the production of ZFN-mediated mutants while avoiding the use of direct plant-transformation methods.

Chlorine ions but not sodium ions alter genome stability of Arabidopsis thaliana

Various environmental stresses influence plant genome stability. Most of these stresses, such as ionizing radiation, heavy metals and organic chemicals, represent potent DNA-damaging agents. Here, we show that exposure to NaCl, the stress that is not thought to cause direct DNA damage, results in an increase in the level of strand breaks and homologous recombination rates (RRs) in Arabidopsis thaliana plants. The effect of salt stress on the RR was found to be primarily associated with Cl(-) ions, since exposure of plants to Na(2)SO(4) did not increase the RR, whereas exposure to MgCl(2) resulted in an increase. Changes in the number of strand breaks and in the RR were also paralleled by transcriptional activation of AtRad51 and down-regulation of AtKu70. The progeny of exposed plants exhibited higher RRs, higher expression of AtRad51, lower expression of AtKu70, higher tolerance to salt and methyl methane sulfate (MMS) stresses, as well as a higher increase in RR upon further exposure to stress. Our experiments showed that NaCl is a genotoxic stress that leads to somatic and transgenerational changes in recombination rates, and these changes are primarily triggered by exposure to Cl(-) ions.

Site-directed mutagenesis in Arabidopsis using custom-designed zinc finger nucleases

Site-directed mutagenesis in higher plants remains a significant technical challenge for basic research and molecular breeding. Here, we demonstrate targeted-gene inactivation for an endogenous gene in Arabidopsis using zinc finger nucleases (ZFNs). Engineered ZFNs for a stress-response regulator, the ABA-INSENSITIVE4 (ABI4) gene, cleaved their recognition sequences specifically in vitro, and ZFN genes driven by a heat-shock promoter were introduced into the Arabidopsis genome. After heat-shock induction, gene mutations with deletion and substitution in the ABI4 gene generated via ZFN-mediated cleavage were observed in somatic cells at frequencies as high as 3%. The homozygote mutant line zfn_abi4-1-1 for ABI4 exhibited the expected mutant phenotypes, i.e., ABA and glucose insensitivity. In addition, ZFN-mediated mutagenesis was applied to the DNA repair-deficient mutant plant, atku80. We found that lack of AtKu80, which plays a role in end-protection of dsDNA breaks, increased error-prone rejoining frequency by 2.6-fold, with increased end-degradation. These data demonstrate that an approach using ZFNs can be used for the efficient production of mutant plants for precision reverse genetics.

Targeted chromosomal deletions in human cells using zinc finger nucleases

We present a novel approach for generating targeted deletions of genomic segments in human and other eukaryotic cells using engineered zinc finger nucleases (ZFNs). We found that ZFNs designed to target two different sites in a human chromosome could introduce two concurrent DNA double-strand breaks (DSBs) in the chromosome and give rise to targeted deletions of the genomic segment between the two sites. Using this method in human cells, we were able to delete predetermined genomic DNA segments in the range of several-hundred base pairs (bp) to 15 mega-bp at frequencies of 10(-3) to 10(-1). These high frequencies allowed us to isolate clonal populations of cells, in which the target chromosomal segments were deleted, by limiting dilution. Sequence analysis revealed that many of the deletion junctions contained small insertions or deletions and microhomologies, indicative of DNA repair via nonhomologous end-joining. Unlike other genome engineering tools such as recombinases and meganucleases, ZFNs do not require preinsertion of target sites into the genome and allow precise manipulation of endogenous genomic scripts in animal and plant cells. Thus, ZFN-induced genomic deletions should be broadly useful as a novel method in biomedical research, biotechnology, and gene therapy.

Mutation accumulation in real branches: fitness assays for genomic deleterious mutation rate and effect in large-statured plants

The genomic deleterious mutation rate and mean effect are central to the biology and evolution of all species. Large-statured plants, such as trees, are predicted to have high mutation rates due to mitotic mutation and the absence of a sheltered germ line, but their size and generation time has hindered genetic study. We develop and test approaches for estimating deleterious mutation rates and effects from viability comparisons within the canopy of large-statured plants. Our methods, inspired by E. J. Klekowski, are a modification of the classic Bateman-Mukai mutation-accumulation experiment. Within a canopy, cell lineages accumulate mitotic mutations independently. Gametes or zygotes produced at more distal points by these cell lineages contain more mitotic mutations than those at basal locations, and within-flower selfs contain more homozygous mutations than between-flower selfs. The resulting viability differences allow demonstration of lethal mutation with experiments similar in size to assays of genetic load and allow estimates of the rate and effect of new mutations with moderate precision and bias similar to that of classic mutation-accumulation experiments in small-statured organisms. These methods open up new possibilities with the potential to provide valuable new insights into the evolutionary genetics of plants.

Analysis of the phytochrome gene family in Ceratodon purpureus by gene targeting reveals the primary phytochrome responsible for photo- and polarotropism

Using gene targeting by homologous recombination in Ceratodon purpureus, we were able to knock out four phytochrome photoreceptor genes independently and to analyze their function with respect to red light dependent phototropism, polarotropism, and chlorophyll content. The strongest phenotype was found in knock-out lines of a newly described phytochrome gene termed CpPHY4 lacking photo- and polarotropic responses at moderate fluence rates. Eliminating the atypical phytochrome gene CpPHY1, which is the only known phytochrome-like gene containing a putative C-terminal tyrosine kinase-like domain, affects red light-induced chlorophyll accumulation. This result was surprising, since no light dependent function was ever allocated to this unusual gene.

Impact of the loss of AtMSH2 on double-strand break-induced recombination between highly diverged homeologous sequences in Arabidopsis thaliana germinal tissues

We experimented a novel reporter system to analyze intrachromosomal recombination between homeologous sequences in Arabidopsis germ cell lineages. The recombination substrates used are the BAR and PAT genes which diverge by about 13% at the nucleotide level and confer resistance to the herbicide glufosinate. DNA double-strand breaks (DSBs) were generated by the I-Sce1 endonuclease to induce recombination. Loss of AtMSH2 induces a 3-fold increase of the frequency of recombination events indicating that AtMSH2 is involved in the anti-recombination activity that prevents exchange between highly diverged sequences in Arabidopsis. Molecular analysis of recombined alleles indicates that in wild type plants the single strand annealing (SSA) pathway can process more efficiently homologous 3' ends than 3' ends generated by resection of non-homologous overhangs. The loss of AtMSH2 disturbs this process, leading to a modification of the distribution of the BAR/PAT junctions and therefore showing that the MSH2 function is also involved in determining the structure of the recombined alleles. In addition, conversion tracts were observed in some alleles. They are shorter in MSH2 deficient plants than in wild-type, suggesting that a short-patch mismatch repair, not controlled by MSH2, could exist in Arabidopsis.

Acute exposure to UVB has a more profound effect on plant genome stability than chronic exposure

Environmental factors that damage DNA have various lengths of exposure and intensity levels. Although the results of increasing the intensity of a DNA damaging agent is often predictable, it is not clear whether the stage during development when the exposure is received has any influence on the amount of DNA damage. In this paper we analyzed the influence of UVB on the stability of Arabidopsis thaliana and the Nicotiana tabacum genomes. Our experiments showed that the acute exposure to UVB produces a significantly greater increase in homologous recombination frequency (HRF) and recombination rate (RR) compared with that produced by chronic exposure. The increase in HRF showed a positive correlation with UVB dose and a negative correlation with plant age. In other words, as the UVB dose was increased, there was a concomitant increase in HRF. Conversely, older plants had a lower HRF increase as compared to younger plants. Our experiments suggest that exposure to UVB makes the most significant impact on genome stability during the early stages of plant development.

Double-strand break repair in plants is developmentally regulated

In this study, we analyzed double-strand break (DSB) repair in Arabidopsis (Arabidopsis thaliana) at various developmental stages. To analyze DSB repair, we used a homologous recombination (HR) and point mutation reversion assays based on nonfunctional beta-glucuronidase reporter genes. Activation of the reporter gene through HR or point mutation reversion resulted in the appearance of blue sectors after histochemical staining. Scoring of these sectors at 3-d intervals from 2 to 31 d post germination (dpg) revealed that, although there was a 100-fold increase in the number of genomes per plant, the recombination frequency only increased 30-fold. This translates to a recombination rate at 31 dpg (2.77 x 10(-8)) being only 30% of the recombination rate at 2 dpg (9.14 x 10(-8)). Conversely, the mutation frequency increased nearly 180-fold, resulting in a 1.8-fold increase in mutation rate from 2 to 31 dpg. Additional analysis of DSBs over the early developmental stages revealed a substantial increase in the number of strand breaks per unit of DNA. Furthermore, RNA analysis of Ku70 and Rad51, two key enzymes in two different DSB repair pathways, and further protein analysis of Ku70 revealed an increase in Ku70 levels and a decrease of Rad51 levels in the developing plants. These data suggest that DSB repair mechanisms are developmentally regulated in Arabidopsis, whereby the proportion of breaks repaired via HR substantially decreases as the plants mature.

Homologous recombination in plants is organ specific

In this paper we analysed the genome stability of various Arabidopsis thaliana plant organs using a transgenic recombination system. The system was based on two copies of non-functional GUS (lines #651 and #11) or LUC (line #15D8) reporter genes serving as a recombination substrate. Both reporter assays showed that recombination in flowers or stems were rare events. Most of the recombination sectors were found in leaves and roots, with leaves having over 2-fold greater number of the recombination events per single cell genome as compared to roots. The recombination events per single genome were 9.7-fold more frequent on the lateral half of the leaves than on the medial halves. This correlated with a 2.5-fold higher metabolic activity in the energy source (lateral) versus energy sink (medial) of leaves. Higher metabolic activity was paralleled by a higher anthocyanin production in lateral halves. The level of double strand break (DSB) occurrence was also different among plant organs; the highest level was observed in roots and the lowest in leaves. High level of DSBs strongly positively correlated with the activity of the key repair enzymes, AtKU70 and AtRAD51. The ratio of AtRAD51 to AtKU70 expression was the highest in leaves, supporting the more active involvement of homologous recombination pathway in the repair of DSBs in this organ. Western blot analysis confirmed the real time PCR expression data for AtKU70 gene.

Towards targeted mutagenesis and gene replacement in plants

Advances in the development of biotechnological tools for plant gene disruption and repair have lagged behind the rapid progress made in whole-genome sequencing of many model and crop plant species. Plant DNA-repair machinery predominantly uses non-homologous end-joining (NHEJ), making the homologous recombination (HR)-based methods, which have proved fruitful for gene targeting in non-plant systems, unsuitable for use in plant systems. Two recent reports describe successful targeted mutagenesis and gene targeting in Arabidopsis by either harnessing the plant NHEJ machinery using site-specific induction of double-strand breaks (DSBs), or by activation of a HR pathway through overexpression of a yeast DNA recombination gene in transgenic plants. These reports provide a foundation from which new technologies for site-specific genome alterations in plant species can be developed.

Modification of endogenous natural genes by gene targeting in rice and other higher plants

The capability to modify a genomic sequence into a designed sequence is a powerful tool for biologists and breeders to elucidate the function of an individual gene and its cis-acting elements of multigene families in the genome. Gene targeting refers to the alteration of a specific DNA sequence in an endogenous gene at its original locus in the genome. In higher plants, however, the overwhelming occurrence of the random integration of transgenes by non-homologous end-joining is the main obstacle to develop efficient gene targeting. Two approaches have been undertaken to modify a genomic sequence in higher plants- chimeric RNA/DNA oligonucleotide-directed gene targeting to generate a site-specific base conversion, and homologous recombination-dependent gene targeting to produce either a base change or a gene replacement in a sequence-specific manner. The successful and reproducible targeting of an endogenous gene by homologous recombination, independently of gene-specific selection by employing a strong positive-negative selection, has been demonstrated for the first time in rice, an important staple food and a model plant for other cereal species. This review addresses the current status of targeting of an endogenous natural gene in rice and other higher plants and discusses possible models for Agrobacterium- mediated gene targeting by homologous recombination using a strong positive-negative selection.

Inhibition of non-homologous end joining and integration of DNA upon transformation of Rhizopus oryzae

Site-directed integration of DNA in the fungus Rhizopus has long been problematic because linearized plasmids used for transformation tend to replicate in high-molecular-weight concatenated structures, and rarely integrate into the chromosome. This work examines the methods that might interfere with the multimerization process, select against plasmids that had recircularized, and encourage strand invasion, hopefully leading to plasmid integration. In vitro methods were used to determine if the structure of the double-strand break had any effect on the ability to rejoin plasmid ends. In cell-free extracts, little difference in end-joining activity was found between linearized plasmids with 5' overhangs, 3' overhangs, or blunt ends. In addition, dephosphorylation of ends had no effect. Transformation of plasmids prepared in the same ways confirmed that they were easily religated in vivo, with almost all prototrophic isolates retaining autonomously replicated plasmids. It was possible to block religation by modifying the free ends of the linearized plasmids using oligonucleotide adapters which were blocked at the 3'-OH position and contained phosphorothioate nucleotides to make them nuclease-resistant. However, gene replacement, with repair of the auxotrophic mutation in the host chromosome, was the predominant event observed upon the transformation of these plasmids. The highest rates of integration were obtained with a plasmid containing a truncated, non-functional pyrG gene. Autonomous replication of this plasmid did not support prototrophic growth, but homologous recombination into the chromosome restored the function of the endogenous pyrG gene. All of the transformants obtained with this selective construct were found to have integrated the plasmid, with multicopy insertion being common.

Enhancing gene targeting efficiency in higher plants: rice is on the move

Meeting the challenge of routine gene targeting (GT) in higher plants is of crucial interest to researchers and plant breeders who are currently in need of a powerful tool to specifically modify a given locus in a genome. Higher plants have long been considered the last lineage resistant to targeting technology. However, a recent report described an efficient method of T-DNA-mediated targeted disruption of a non-selectable locus in rice [Terada et al., Nat Biotechnol 20: 1030-1034 (2002)]. Though this study was an obvious breakthrough, further improvement of GT frequencies may derive from a better understanding of the natural mechanisms that control homologous recombination (HR) processes. In this review, we will focus on what is known about HR and the factors which may hamper the development of routine GT by HR in higher plants. We will also present the current strategies envisaged to overcome these limitations, such as expression of recombination proteins and refinements in the design of the transformation vector.

Targeted mutagenesis using zinc-finger nucleases in Arabidopsis

Targeted mutagenesis is an essential tool of reverse genetics that could be used experimentally to investigate basic plant biology or modify crop plants for improvement of important agricultural traits. Although targeted mutagenesis is routine in several model organisms including yeast and mouse, efficient and widely usable methods to generate targeted modifications in plant genes are not currently available. In this study we investigated the efficacy of a targeted-mutagenesis approach based on zinc-finger nucleases (ZFNs). In this procedure, ZFNs are used to generate double-strand breaks at specific genomic sites, and subsequent repair produces mutations at the break site. To determine whether ZFNs can cleave and induce mutations at specific sites within higher plant genomes, we introduced a construct carrying both a ZFN gene, driven by a heat-shock promoter, and its target into the Arabidopsis genome. Induction of ZFN expression by heat shock during seedling development resulted in mutations at the ZFN recognition sequence at frequencies as high as 0.2 mutations per target. Of 106 ZFN-induced mutations characterized, 83 (78%) were simple deletions of 1-52 bp (median of 4 bp), 14 (13%) were simple insertions of 1-4 bp, and 9 (8%) were deletions accompanied by insertions. In 10% of induced individuals, mutants were present in the subsequent generation, thus demonstrating efficient transmission of the ZFN-induced mutations. These data indicate that ZFNs can form the basis of a highly efficient method for targeted mutagenesis of plant genes.

A tale of two integrations, transgene and T-DNA: gene targeting by homologous recombination in rice

The first successful and reproducible gene targeting by homologous recombination, without the concomitant occurrence of ectopic events, has been reported. This will be a powerful approach for the characterization of gene function in rice, an important crop and a model for other cereal species. Models have been proposed to explain gene replacement by homologous recombination, including a possible model for Agrobacterium-mediated gene targeting using a strong positive-negative selection.

Plant gametogenesis: conservation and contrasts in development

Although the process of gamete formation in plants has many unique features, much has been learnt from the comparative analysis between plants and other eukaryotic systems. Plants have a number of factors that have made them desirable for the analysis of gamete development; these include late germline specification, the non-lethality of mutations affecting gamete development and the large size of their chromosomes. The availability of the fully annotatedArabidopsisgenome and comparative analysis using yeast, animal andE. colihas led to the identification and functional characterisation of many genes with roles in gamete development, principally those associated with meiosis, recombination and DNA repair. The advantages that plants give with the use of mutant screens to identify genes associated with gamete formation have also provided access to genes that are difficult to characterise by alternative routes. This has yielded novel information regarding the processes of gamete formation in higher plants. The times may now be changing with the advantages that plants provide serving to advance knowledge of gamete formation in other eukaryotic systems.

DSB repair: the yeast paradigm

Genome stability is of primary importance for the survival and proper functioning of all organisms. Double-strand breaks (DSBs) arise spontaneously during growth, or can be created by external insults. In response to even a single DSB, organisms must trigger a series of events to promote repair of the DNA damage in order to survive and restore chromosomal integrity. In doing so, cells must regulate a fine balance between potentially competing DSB repair pathways. These are generally classified as either homologous recombination (HR) or non-homologous end joining (NHEJ). The yeast Saccharomyces cerevisiae is an ideal model organism for studying these repair processes. Indeed, much of what we know today on the mechanisms of repair in eukaryotes come from studies carried out in budding yeast. Many of the proteins involved in the various repair pathways have been isolated and the details of their mode of action are currently being unraveled at the molecular level. In this review, we focus on exciting new work eminating from yeast research that provides fresh insights into the DSB repair process. This recent work supplements and complements the wealth of classical genetic research that has been performed in yeast systems over the years. Given the conservation of the repair mechanisms and genes throughout evolution, these studies have profound implications for other eukaryotic organisms.

Repair of plasmid DNA used for transformation of Rhizopus oryzae by gene conversion

Techniques for genetic manipulation of the filamentous fungus Rhizopus have been hampered due to a lack of understanding regarding the recombination and replication mechanisms that affect the fate of introduced DNA. The ability to target chromosomal integration of a plasmid has been difficult because DNA transformed into Rhizopus rarely integrates and is autonomously replicated in a high molecular weight concatenated arrangement (i.e., series or chain). Linearization of the plasmid prior to transformation at a site having homology with the genomic DNA yields the highest frequency of integration, but repair of the double-strand break by end-joining is still the predominant event. We recently attempted to circumvent replication of the plasmid by introducing frameshift mutations in pyrG, the R. oryzae orotidine-5'-monophosphate decarboxylase gene used for selection of the vector. It was hypothesized that autonomous replication of the mutated plasmids would be incapable of restoring prototrophic growth, since the genomic pyrG also contained a mutation. However, homologous integration of the plasmid results in duplication of the pyrG gene, which can create a functional copy of pyrG if both the genomic and plasmid mutations are paired on the same duplicate copy. While this event was detected in one of the isolates, it represented less than 8% of the total transformants. The majority of transformants contained plasmid replicating autonomously in a concatenated arrangement. Sequence analysis showed that prototrophic growth was restored by repairing the non-functional pyrG sequence in the plasmid, while the genomic pyrG gene was unaltered. Frequent transfer of the genomic pyrG mutation to the plasmid suggests that gene conversion is likely occurring by recombination pathways involving break-induced replication or synthesis-dependent strand annealing.

Ku80 plays a role in non-homologous recombination but is not required for T-DNA integration in Arabidopsis

Chromosomal breaks are repaired by homologous recombination (HR) or non-homologous end joining (NHEJ) mechanisms. The Ku70/Ku80 heterodimer binds DNA ends and plays roles in NHEJ and telomere maintenance in organisms ranging from yeast to humans. We have previously identified a ku80 mutant of the model plant Arabidopsis thaliana and shown the role of Ku80 in telomere homeostasis in plant cells. We show here that this mutant is hypersensitive to the DNA-damaging agent methyl methane sulphonate and has a reduced capacity to carry out NHEJ recombination. To understand the interplay between HR and NHEJ in plants, we measured HR in the absence of Ku80. We find that the frequency of intrachromosomal HR is not affected by the absence of Ku80. Previous work has clearly implicated the Ku heterodimer in Agrobacterium-mediated T-DNA transformation of yeast. Surprisingly, ku80 mutant plants show no defect in the efficiency of T-DNA transformation of plants with Agrobacterium, showing that an alternative pathway must exist in plants.

Different pathways of homologous recombination are used for the repair of double-strand breaks within tandemly arranged sequences in the plant genome

Different DNA repair pathways that use homologous sequences in close proximity to genomic double-strand breaks (DSBs) result in either an internal deletion or a gene conversion. We determined the efficiency of these pathways in somatic plant cells of transgenic Arabidopsis lines by monitoring the restoration of the beta-glucuronidase (GUS) marker gene. The transgenes contain a recognition site for the restriction endonuclease I-SceI either between direct GUS repeats to detect deletion formation (DGU.US), or within the GUS gene to detect gene conversion using a nearby donor sequence in direct or inverted orientation (DU.GUS and IU.GUS). Without expression of I-SceI, the frequency of homologous recombination (HR) was low and similar for all three constructs. By crossing the different lines with an I-SceI expressing line, DSB repair was induced, and resulted in one to two orders of magnitude higher recombination frequency. The frequencies obtained with the DGU.US construct were about five times higher than those obtained with DU.GUS and IU.GUS, irrespective of the orientation of the donor sequence. Our results indicate that recombination associated with deletions is the most efficient pathway of homologous DSB repair in plants. However, DSB-induced gene conversion seems to be frequent enough to play a significant role in the evolution of tandemly arranged gene families like resistance genes.