Plant genome editing by novel tools: TALEN and other sequence specific nucleases

Progress in gene editing tools, implications and success in plants: a review

Genetic modifications are made through diverse mutagenesis techniques for crop improvement programs. Among these mutagenesis tools, the traditional methods involve chemical and radiation-induced mutagenesis, resulting in off-target and unintended mutations in the genome. However, recent advances have introduced site-directed nucleases (SDNs) for gene editing, significantly reducing off-target changes in the genome compared to induced mutagenesis and naturally occurring mutations in breeding populations. SDNs have revolutionized genetic engineering, enabling precise gene editing in recent decades. One widely used method, homology-directed repair (HDR), has been effective for accurate base substitution and gene alterations in some plant species. However, its application has been limited due to the inefficiency of HDR in plant cells and the prevalence of the error-prone repair pathway known as non-homologous end joining (NHEJ). The discovery of CRISPR-Cas has been a game-changer in this field. This system induces mutations by creating double-strand breaks (DSBs) in the genome and repairing them through associated repair pathways like NHEJ. As a result, the CRISPR-Cas system has been extensively used to transform plants for gene function analysis and to enhance desirable traits. Researchers have made significant progress in genetic engineering in recent years, particularly in understanding the CRISPR-Cas mechanism. This has led to various CRISPR-Cas variants, including CRISPR-Cas13, CRISPR interference, CRISPR activation, base editors, primes editors, and CRASPASE, a new CRISPR-Cas system for genetic engineering that cleaves proteins. Moreover, gene editing technologies like the prime editor and base editor approaches offer excellent opportunities for plant genome engineering. These cutting-edge tools have opened up new avenues for rapidly manipulating plant genomes. This review article provides a comprehensive overview of the current state of plant genetic engineering, focusing on recently developed tools for gene alteration and their potential applications in plant research.

From Petri Dish to Field: Plant Tissue Culture and Genetic Engineering of Oats for Improved Agricultural Outcomes

Oats (Avena sativa) hold immense economic and nutritional value as a versatile crop. They have long been recognized as an exceptional choice for human consumption and animal feed. Oats' unique components, including proteins, starches, and β-glucans, have led to its widespread use in various food products such as bread, noodles, flakes, and milk. The popularity of oat milk as a vegan alternative to dairy milk has soared due to the increasing number of vegetarians/vegans and growing environmental awareness. Oat milk offers a sustainable option with reduced greenhouse gas emissions during its production, rendering it an appropriate choice for individuals who are lactose-intolerant or have dairy allergies. To ensure improved adaptability and enhanced nutrition, the development of new oat varieties is crucial, considering factors like cultivation, climate, and growing conditions. Plant cell culture plays a crucial role in both traditional and contemporary breeding methods. In classical breeding, plant cell culture facilitates the rapid production of double haploid plants, which can be employed to accelerate the breeding process. In modern breeding methods, it enables genetic manipulation and precise genome editing at the cellular level. This review delves into the importance of oats and their diverse applications, highlighting the advantages of plant cell culture in both classical and modern breeding methods. Specifically, it provides an overview of plant tissue culture, encompassing genetic transformation, haploid technology, protoplast technology, and genome editing.

Role of BraRGL1 in regulation of Brassica rapa bolting and flowering

Gibberellin (GA) plays a major role in controlling Brassica rapa stalk development. As an essential negative regulator of GA signal transduction, DELLA proteins may exert significant effects on stalk development. However, the regulatory mechanisms underlying this regulation remain unclear. In this study, we report highly efficient and inheritable mutagenesis using the CRISPR/Cas9 gene editing system in BraPDS (phytoene desaturase) and BraRGL1 (key DELLA protein) genes. We observed a loss-of-function mutation in BraRGL1 due to two amino acids in GRAS domain. The flower bud differentiation and bolting time of BraRGL1 mutants were significantly advanced. The expression of GA-regulatory protein (BraGASA6), flowering related genes (BraSOC1, BraLFY), expansion protein (BraEXPA11) and xyloglucan endotransferase (BraXTH3) genes was also significantly upregulated in these mutants. BraRGL1-overexpressing plants displayed the contrasting phenotypes. BraRGL1 mutants were more sensitive to GA signaling. BraRGL1 interacted with BraSOC1, and the interaction intensity decreased after GA3 treatment. In addition, BraRGL1 inhibited the transcription-activation ability of BraSOC1 for BraXTH3 and BraLFY genes, but the presence of GA3 enhanced the activation ability of BraSOC1, suggesting that the BraRGL1-BraSOC1 module regulates bolting and flowering of B. rapa through GA signal transduction. Thus, we hypothesized that BraRGL1 is degraded, and BraSOC1 is released in the presence of GA3, which promotes the expression of BraXTH3 and BraLFY, thereby inducing stalk development in B. rapa. Further, the BraRGL1-M mutant promoted the flower bud differentiation without affecting the stalk quality. Thus, BraRGL1 can serve as a valuable target for the molecular breeding of early maturing varieties.

An overview of genome engineering in plants, including its scope, technologies, progress and grand challenges

Genome editing is a useful, adaptable, and favored technique for both functional genomics and crop enhancement. Over the years, rapidly evolving genome editing technologies, including clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas), transcription activator-like effector nucleases (TALENs), and zinc finger nucleases (ZFNs), have shown broad application prospects in gene function research and improvement of critical agronomic traits in many crops. These technologies have also opened up opportunities for plant breeding. These techniques provide excellent chances for the quick modification of crops and the advancement of plant science in the future. The current review describes various genome editing techniques and how they function, particularly CRISPR/Cas9 systems, which can contribute significantly to the most accurate characterization of genomic rearrangement and plant gene functions as well as the enhancement of critical traits in field crops. To accelerate the use of gene-editing technologies for crop enhancement, the speed editing strategy of gene-family members was designed. As it permits genome editing in numerous biological systems, the CRISPR technology provides a valuable edge in this regard that particularly captures the attention of scientists.

Petroleum Hydrocarbon Catabolic Pathways as Targets for Metabolic Engineering Strategies for Enhanced Bioremediation of Crude-Oil-Contaminated Environments

Anthropogenic activities and industrial effluents are the major sources of petroleum hydrocarbon contamination in different environments. Microbe-based remediation techniques are known to be effective, inexpensive, and environmentally safe. In this review, the metabolic-target-specific pathway engineering processes used for improving the bioremediation of hydrocarbon-contaminated environments have been described. The microbiomes are characterised using environmental genomics approaches that can provide a means to determine the unique structural, functional, and metabolic pathways used by the microbial community for the degradation of contaminants. The bacterial metabolism of aromatic hydrocarbons has been explained via peripheral pathways by the catabolic actions of enzymes, such as dehydrogenases, hydrolases, oxygenases, and isomerases. We proposed that by using microbiome engineering techniques, specific pathways in an environment can be detected and manipulated as targets. Using the combination of metabolic engineering with synthetic biology, systemic biology, and evolutionary engineering approaches, highly efficient microbial strains may be utilised to facilitate the target-dependent bioprocessing and degradation of petroleum hydrocarbons. Moreover, the use of CRISPR-cas and genetic engineering methods for editing metabolic genes and modifying degradation pathways leads to the selection of recombinants that have improved degradation abilities. The idea of growing metabolically engineered microbial communities, which play a crucial role in breaking down a range of pollutants, has also been explained. However, the limitations of the in-situ implementation of genetically modified organisms pose a challenge that needs to be addressed in future research.

Japanese Regulatory Framework and Approach for Genome-edited Foods Based on Latest Scientific Findings

The food supply system is facing important challenges and its sustainability has to be considered. Genome-editing technology, which accelerates the development of new variety, could be used to achieve sustainable development goals, thereby protecting the environment and ensuring the stable production of food for an increasing global population. The most widely used genome-editing tool, CRISPR/Cas9, is easy to use, affordable, and versatile. Foods produced by genome-editing technologies have been developed worldwide to create novel traits. In the first half of the review, the latest scientific findings on genome-editing technologies are summarized, and the technical challenge in genome sequence analysis are clarified. CRISPR/Cas9 has versatile alternative techniques, such as base editor and prime editor. Genome sequencing technology has developed rapidly in recent years. However, it is still difficult to detect large deletions and structural variations. Long-read sequencing technology would solve this challenge. In the second part, regulatory framework and approach for genome-edited foods is introduced. The four government ministries, including the Ministry of Environment, the Ministry of Agriculture, Forestry and Fisheries, and the Ministry of Health, Labour and Welfare (MHLW), started to discuss how the regulation should be implemented in 2019. The SDN-1 technique is excluded from the current genetically modified organism (GMO) regulation. The Japanese regulatory framework includes pre-submission consultation and submission of notification form. In the last part of this review, transparency of regulatory framework and consumer confidence were described. Since maintaining consumer trust is vital, transparency of regulatory framework is a key to consumers. The information of notification process on approved genome-edited foods is made public immediately. This review will help regulators build regulatory frameworks, and lead to harmonization of the framework between the countries.

Biotechnological Advances to Improve Abiotic Stress Tolerance in Crops

The major challenges that agriculture is facing in the twenty-first century are increasing droughts, water scarcity, flooding, poorer soils, and extreme temperatures due to climate change. However, most crops are not tolerant to extreme climatic environments. The aim in the near future, in a world with hunger and an increasing population, is to breed and/or engineer crops to tolerate abiotic stress with a higher yield. Some crop varieties display a certain degree of tolerance, which has been exploited by plant breeders to develop varieties that thrive under stress conditions. Moreover, a long list of genes involved in abiotic stress tolerance have been identified and characterized by molecular techniques and overexpressed individually in plant transformation experiments. Nevertheless, stress tolerance phenotypes are polygenetic traits, which current genomic tools are dissecting to exploit their use by accelerating genetic introgression using molecular markers or site-directed mutagenesis such as CRISPR-Cas9. In this review, we describe plant mechanisms to sense and tolerate adverse climate conditions and examine and discuss classic and new molecular tools to select and improve abiotic stress tolerance in major crops.

Potential of Industrial Hemp for Phytoremediation of Heavy Metals

The accumulation of anthropogenic heavy metals in soil is a major form of pollution. Such potentially toxic elements are nonbiodegradable and persist for many years as threats to human and environmental health. Traditional forms of remediation are costly and potentially damaging to the land. An alternative strategy is phytoremediation, where plants are used to capture metals from the environment. Industrial hemp (Cannabis sativa) is a promising candidate for phytoremediation. Hemp has deep roots and is tolerant to the accumulation of different metals. In addition, the crop biomass has many potential commercial uses after harvesting is completed. Furthermore, the recent availability of an annotated genome sequence provides a powerful tool for the bioengineering of C. sativa for better phytoremediation.

Genome editing for plant research and crop improvement

The advent of clustered regularly interspaced short palindromic repeat (CRISPR) has had a profound impact on plant biology, and crop improvement. In this review, we summarize the state-of-the-art development of CRISPR technologies and their applications in plants, from the initial introduction of random small indel (insertion or deletion) mutations at target genomic loci to precision editing such as base editing, prime editing and gene targeting. We describe advances in the use of class 2, types II, V, and VI systems for gene disruption as well as for precise sequence alterations, gene transcription, and epigenome control.

Targeted lipid nanoparticles for RNA therapeutics and immunomodulation in leukocytes

Abnormalities in leukocytes' function are associated with many immune related disorders, such as cancer, autoimmunity and susceptibility to infectious diseases. Recent developments in Genome-wide-association-studies give rise to new opportunities for novel therapeutics. RNA-based modalities, that allow a selective genetic manipulation in vivo, are powerful tools for personalized medicine, enabling downregulation or expression of relevant proteins. Yet, RNA-based therapeutics requires a delivery modality to facilitate the stability, uptake and intracellular release of the RNA molecules. The use of lipid nanoparticles as a drug delivery approach improves the payloads' stability, pharmacokinetics, bio-distribution and therapeutic benefit while reducing side effects. Moreover, a wide variety of targeting moieties allow a precise and modular manipulation of gene expression, together with the ability to identify and selectively affect disease-relevant leukocytes-subsets. Altogether, RNA-based therapeutics, targeting leukocytes subsets, is believed to be one of the most promising therapeutic concepts of the near future, addressing pressing issues in cancer and inflammation heterogeneity.

Conventional and Molecular Techniques from Simple Breeding to Speed Breeding in Crop Plants: Recent Advances and Future Outlook

In most crop breeding programs, the rate of yield increment is insufficient to cope with the increased food demand caused by a rapidly expanding global population. In plant breeding, the development of improved crop varieties is limited by the very long crop duration. Given the many phases of crossing, selection, and testing involved in the production of new plant varieties, it can take one or two decades to create a new cultivar. One possible way of alleviating food scarcity problems and increasing food security is to develop improved plant varieties rapidly. Traditional farming methods practiced since quite some time have decreased the genetic variability of crops. To improve agronomic traits associated with yield, quality, and resistance to biotic and abiotic stresses in crop plants, several conventional and molecular approaches have been used, including genetic selection, mutagenic breeding, somaclonal variations, whole-genome sequence-based approaches, physical maps, and functional genomic tools. However, recent advances in genome editing technology using programmable nucleases, clustered regularly interspaced short palindromic repeats (CRISPR), and CRISPR-associated (Cas) proteins have opened the door to a new plant breeding era. Therefore, to increase the efficiency of crop breeding, plant breeders and researchers around the world are using novel strategies such as speed breeding, genome editing tools, and high-throughput phenotyping. In this review, we summarize recent findings on several aspects of crop breeding to describe the evolution of plant breeding practices, from traditional to modern speed breeding combined with genome editing tools, which aim to produce crop generations with desired traits annually.

Mutagenesis Approaches and Their Role in Crop Improvement

Induced mutagenesis is one of the most efficient tools that has been utilized extensively to create genetic variation as well as for identification of key regulatory genes for economically important traits toward the crop improvement. Mutations can be induced by several techniques such as physical, chemical, and insertional mutagen treatments; however, these methods are not preferred because of cost and tedious process. Nonetheless, with the advancements in next-generation sequencing (NGS) techniques, millions of mutations can be detected in a very short period of time and, therefore, considered as convenient and cost efficient. Furthermore, induced mutagenesis coupled with whole-genome sequencing has provided a robust platform for forward and reverse genetic applications. Moreover, the availability of whole-genome sequence information for large number of crops has enabled target-specific genome editing techniques as a preferable method to engineer desired mutations. The available genome editing approaches such as ZFNs (Zinc Finger Nucleases), transcription activator like effector nucleases (TALENS), and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated9 (Cas9) endonuclease have been utilized to perform site-specific mutations in several plant species. In particular, the CRISPR/Cas9 has transformed the genome editing because of its simplicity and robustness, therefore, have been utilized to enhance biotic and abiotic stress resistance. The Special Issue of Plants highlights the efforts by the scientific community utilizing mutagenesis techniques for the identification of novel genes toward crop improvement.

Functional genomic approaches to improve crop plant heat stress tolerance

Heat stress as a yield limiting issue has become a major threat for food security as global warming progresses. Being sessile, plants cannot avoid heat stress. They respond to heat stress by activating complex molecular networks, such as signal transduction, metabolite production and expressions of heat stress-associated genes. Some plants have developed an intricate signalling network to respond and adapt it. Heat stress tolerance is a polygenic trait, which is regulated by various genes, transcriptional factors, proteins and hormones. Therefore, to improve heat stress tolerance, a sound knowledge of various mechanisms involved in the response to heat stress is required. The classical breeding methods employed to enhance heat stress tolerance has had limited success. In this era of genomics, next generation sequencing techniques, availability of genome sequences and advanced biotechnological tools open several windows of opportunities to improve heat stress tolerance in crop plants. This review discusses the potential of various functional genomic approaches, such as genome wide association studies, microarray, and suppression subtractive hybridization, in the process of discovering novel genes related to heat stress, and their functional validation using both reverse and forward genetic approaches. This review also discusses how these functionally validated genes can be used to improve heat stress tolerance through plant breeding, transgenics and genome editing approaches.

Redox Balance-DDR-miRNA Triangle: Relevance in Genome Stability and Stress Responses in Plants

Plants are continuously faced with complex environmental conditions which can affect the oxidative metabolism and photosynthetic efficiency, thus leading to the over-production of reactive oxygen species (ROS). Over a certain threshold, ROS can damage DNA. DNA damage, unless repaired, can affect genome stability, thus interfering with cell survival and severely reducing crop productivity. A complex network of pathways involved in DNA damage response (DDR) needs to be activated in order to maintain genome integrity. The expression of specific genes belonging to these pathways can be used as indicators of oxidative DNA damage and effective DNA repair in plants subjected to stress conditions. Managing ROS levels by modulating their production and scavenging systems shifts the role of these compounds from toxic molecules to key messengers involved in plant tolerance acquisition. Oxidative and anti-oxidative signals normally move among the different cell compartments, including the nucleus, cytosol, and organelles. Nuclei are dynamically equipped with different redox systems, such as glutathione (GSH), thiol reductases, and redox regulated transcription factors (TFs). The nuclear redox network participates in the regulation of the DNA metabolism, in terms of transcriptional events, replication, and repair mechanisms. This mainly occurs through redox-dependent regulatory mechanisms comprising redox buffering and post-translational modifications, such as the thiol-disulphide switch, glutathionylation, and S-nitrosylation. The regulatory role of microRNAs (miRNAs) is also emerging for the maintenance of genome stability and the modulation of antioxidative machinery under adverse environmental conditions. In fact, redox systems and DDR pathways can be controlled at a post-transcriptional level by miRNAs. This review reports on the interconnections between the DDR pathways and redox balancing systems. It presents a new dynamic picture by taking into account the shared regulatory mechanism mediated by miRNAs in plant defense responses to stress.

CRISPR/Cas9-mediated multiple gene editing in Brassica oleracea var. capitata using the endogenous tRNA-processing system

Cabbage (Brassica oleracea var. capitata) is a biennial plant with strong self-incompatibility and an obligate requirement for prolonged vernalization by exposure to low temperatures to induce flowering. These characteristics significantly increase the difficulty of exploiting novel germplasm induced by physical or chemical mutagens. In this study, we report a CRISPR/Cas9 gene-editing system based on endogenous tRNA processing to induce high efficiency and inheritable mutagenesis in cabbage. Using the phytoene desaturase gene BoPDS, the S-receptor kinase gene BoSRK, and the male-sterility-associated gene BoMS1 as the target genes, multisite and multiple gene mutations were achieved using a construct with tandemly arrayed tRNA-sgRNA architecture to express multiple sgRNAs. The BoSRK3 gene mutation suppressed self-incompatibility completely, converting the self-incompatible line into a self-compatible line. In addition, the BoMS1 gene mutation produced a completely male-sterile mutant, which was highly cross compatible with its nonmutant isoline at the flowering stage as a result of a simultaneous BoSRK3 gene mutation, enabling the economic propagation of the male-sterile line through bee-mediated cross-pollination. Interestingly, higher site mutation efficiency was detected when a guide sequence was inserted into a location in the tandemly arrayed tRNA-sgRNA architecture that was distal from the upstream Pol III promoter. In addition, mutation sites were also detected in the paralogous genes of the BoPDS and BoSRK genes that had fully consistent sequences or base mismatches but beyond the "seed" region in the spacer sequence compared with the target sgRNAs. Collectively, our results demonstrate that the CRISPR/Cas9 system, coupled with an endogenous tRNA-processing system, is an efficient tool to improve cabbage traits.

Can the world's favorite fruit, tomato, provide an effective biosynthetic chassis for high-value metabolites?

Tomato has a relatively short growth cycle (fruit ready to pick within 65-85 days from planting) and a relatively high yield (the average for globe tomatoes is 3-9 kg fruit per plant rising to as much as 40 kg fruit per plant). Tomatoes also produce large amounts of important primary and secondary metabolites which can serve as intermediates or substrates for producing valuable new compounds. As a model crop, tomato already has a broad range of tools and resources available for biotechnological applications, either increased nutrients for health-promoting biofortified foods or as a production system for high-value compounds. These advantages make tomato an excellent chassis for the production of important metabolites. We summarize recent achievements in metabolic engineering of tomato and suggest new candidate metabolites which could be targets for metabolic engineering. We offer a scheme for how to establish tomato as a chassis for industrial-scale production of high-value metabolites.

Next-Generation Plant Metabolic Engineering, Inspired by an Ancient Chinese Irrigation System

Specialized secondary metabolites serve not only to protect plants against abiotic and biotic challenges, but have also been used extensively by humans to combat diseases. Due to the great importance of medicinal plants for health, we need to find new and sustainable ways to improve the production of the specialized metabolites. In addition to direct extraction, recent progress in metabolic engineering of plants offers an alternative supply option. We argue that metabolic engineering for producing the secondary metabolites in plants may have distinct advantages over microbial production platforms, and thus propose new approaches of plant metabolic engineering, which are inspired by an ancient Chinese irrigation system. Metabolic engineering strategies work at three levels: introducing biosynthetic genes, using transcription factors, and improving metabolic flux including increasing the supply of precursors, energy, and reducing power. In addition, recent progress in biotechnology contributes markedly to better engineering, such as the use of specific promoters and the deletion of competing branch pathways. We propose that next-generation plant metabolic engineering will improve current engineering strategies, for the purpose of producing valuable metabolites in plants on industrial scales.

Development of Wheat With Hypoimmunogenic Gluten Obstructed by the Gene Editing Policy in Europe

Coeliac Disease (CD) is an auto-immune reaction to gluten in 1-2% of the human population. A gluten-free (GF) diet, excluding wheat, barley, and rye, is the only remedy. This diet is difficult to adhere to, partly because wheat gluten is added to many processed products for their viscoelastic properties. In addition, GF products are less healthy and expensive. Wheat products containing only hypoimmunogenic gluten proteins would be a desirable option. Various gluten peptides that trigger CD have been characterized. A single wheat variety contains around hundred gluten genes, producing proteins with varying numbers of epitopes. Gene editing using CRISPR/Cas9 can precisely remove or modify the DNA sequences coding for immunogenic peptides. Wheat with hypoimmunogenic gluten thus exemplifies the potential of gene editing for improving crops for human consumption where conventional breeding cannot succeed. We describe here, in relation to breeding hypoimmunogenic wheat varieties, the inconsistencies of applying GM regulation in Europe for gene-edited plants while mutation breeding-derived plants are exempted. We explain that healthy products derived from this new technology may become available in the United States, Canada, Argentina and other countries but not in Europe, because of strict regulation of unintended GM risk at the expense of reduction the existing immunogenicity risks of patients. We argue that regulation of gene-edited plants should be based on scientific evidence. Therefore, we strongly recommend implementing the innovation principle. Responsible Research and Innovation, involving stakeholders including CD patient societies in the development of gene-editing products, will enable progress toward healthy products and encourage public acceptance.

Advancing Agrobacterium-Based Crop Transformation and Genome Modification Technology for Agricultural Biotechnology

The last decade has seen significant strides in Agrobacterium-mediated plant transformation technology. This has not only expanded the number of crop species that can be transformed by Agrobacterium, but has also made it possible to routinely transform several recalcitrant crop species including cereals (e.g., maize, sorghum, and wheat). However, the technology is limited by the random nature of DNA insertions, genotype dependency, low frequency of quality events, and variation in gene expression arising from genomic insertion sites. A majority of these deficiencies have now been addressed by improving the frequency of quality events, developing genotype-independent transformation capability in maize, developing an Agrobacterium-based site-specific integration technology for precise gene targeting, and adopting Agrobacterium-delivered CRISPR-Cas genes for gene editing. These improved transformation technologies are discussed in detail in this chapter.

DNA-Free Genome Editing: Past, Present and Future

Genome Editing using engineered endonuclease (GEEN) systems rapidly took over the field of plant science and plant breeding. So far, Genome Editing techniques have been applied in more than fifty different plants; including model species like Arabidopsis; main crops like rice, maize or wheat as well as economically less important crops like strawberry, peanut and cucumber. These techniques have been used for basic research as proof-of-concept or to investigate gene functions in most of its applications. However, several market-oriented traits have been addressed including enhanced agronomic characteristics, improved food and feed quality, increased tolerance to abiotic and biotic stress and herbicide tolerance. These technologies are evolving at a tearing pace and especially the field of CRISPR based Genome Editing is advancing incredibly fast. CRISPR-Systems derived from a multitude of bacterial species are being used for targeted Gene Editing and many modifications have already been applied to the existing CRISPR-Systems such as (i) alter their protospacer adjacent motif (ii) increase their specificity (iii) alter their ability to cut DNA and (iv) fuse them with additional proteins. Besides, the classical transformation system using Agrobacteria tumefaciens or Rhizobium rhizogenes, other transformation technologies have become available and additional methods are on its way to the plant sector. Some of them are utilizing solely proteins or protein-RNA complexes for transformation, making it possible to alter the genome without the use of recombinant DNA. Due to this, it is impossible that foreign DNA is being incorporated into the host genome. In this review we will present the recent developments and techniques in the field of DNA-free Genome Editing, its advantages and pitfalls and give a perspective on technologies which might be available in the future for targeted Genome Editing in plants. Furthermore, we will discuss these techniques in the light of existing- and potential future regulations.

Trait stacking in modern agriculture: application of genome editing tools

Advances in plant transgenic technology in the 20th century overcame the major hurdle for transfer of genetic material between species. This not only enabled fundamental insights into plant biology, but also revolutionized commercial agriculture. Adoption of transgenic plants in industrial agriculture has reduced pesticide application, while bringing significant increase in crop yields and farmers' profits. The progress made in transgenic technology over the last three decades paved the way mainly for simple single-gene insect and herbicide tolerance (HT) trait products. Modern agriculture demands stacking and pyramiding of complex traits that provide broad-spectrum insect and HT with other agronomic traits. In addition, more recent developments in genome editing provide unique opportunities to create precise on-demand genome modifications to enhance crop productivity. The major challenge for the plant biotech industry therefore remains to combine multiple forms of traits needed to create commercially viable stacked product. This review provides a historical perspective of conventional breeding stacks, current status of molecular stacks and future developments needed to enable genome-editing technology for trait stacking.

Use of Natural Diversity and Biotechnology to Increase the Quality and Nutritional Content of Tomato and Grape

Improving fruit quality has become a major goal in plant breeding. Direct approaches to tackling fruit quality traits specifically linked to consumer preferences and environmental friendliness, such as improved flavor, nutraceutical compounds, and sustainability, have slowly been added to a breeder priority list that already includes traits like productivity, efficiency, and, especially, pest and disease control. Breeders already use molecular genetic tools to improve fruit quality although most advances have been made in producer and industrial quality standards. Furthermore, progress has largely been limited to simple agronomic traits easy-to-observe, whereas the vast majority of quality attributes, specifically those relating to flavor and nutrition, are complex and have mostly been neglected. Fortunately, wild germplasm, which is used for resistance against/tolerance of environmental stresses (including pathogens), is still available and harbors significant genetic variation for taste and health-promoting traits. Similarly, heirloom/traditional varieties could be used to identify which genes contribute to flavor and health quality and, at the same time, serve as a good source of the best alleles for organoleptic quality improvement. Grape (Vitis vinifera L.) and tomato (Solanum lycopersicum L.) produce fleshy, berry-type fruits, among the most consumed in the world. Both have undergone important domestication and selection processes, that have dramatically reduced their genetic variability, and strongly standardized fruit traits. Moreover, more and more consumers are asking for sustainable production, incompatible with the wide range of chemical inputs. In the present paper, we review the genetic resources available to tomato/grape breeders, and the recent technological progresses that facilitate the identification of genes/alleles of interest within the natural or generated variability gene pool. These technologies include omics, high-throughput phenotyping/phenomics, and biotech approaches. Our review also covers a range of technologies used to transfer to tomato and grape those alleles considered of interest for fruit quality. These include traditional breeding, TILLING (Targeting Induced Local Lesions in Genomes), genetic engineering, or NPBT (New Plant Breeding Technologies). Altogether, the combined exploitation of genetic variability and innovative biotechnological tools may facilitate breeders to improve fruit quality tacking more into account the consumer standards and the needs to move forward into more sustainable farming practices.

Protecting DNA from errors and damage: an overview of DNA repair mechanisms in plants compared to mammals

The genome integrity of all organisms is constantly threatened by replication errors and DNA damage arising from endogenous and exogenous sources. Such base pair anomalies must be accurately repaired to prevent mutagenesis and/or lethality. Thus, it is not surprising that cells have evolved multiple and partially overlapping DNA repair pathways to correct specific types of DNA errors and lesions. Great progress in unraveling these repair mechanisms at the molecular level has been made by several talented researchers, among them Tomas Lindahl, Aziz Sancar, and Paul Modrich, all three Nobel laureates in Chemistry for 2015. Much of this knowledge comes from studies performed in bacteria, yeast, and mammals and has impacted research in plant systems. Two plant features should be mentioned. Plants differ from higher eukaryotes in that they lack a reserve germline and cannot avoid environmental stresses. Therefore, plants have evolved different strategies to sustain genome fidelity through generations and continuous exposure to genotoxic stresses. These strategies include the presence of unique or multiple paralogous genes with partially overlapping DNA repair activities. Yet, in spite (or because) of these differences, plants, especially Arabidopsis thaliana, can be used as a model organism for functional studies. Some advantages of this model system are worth mentioning: short life cycle, availability of both homozygous and heterozygous lines for many genes, plant transformation techniques, tissue culture methods and reporter systems for gene expression and function studies. Here, I provide a current understanding of DNA repair genes in plants, with a special focus on A. thaliana. It is expected that this review will be a valuable resource for future functional studies in the DNA repair field, both in plants and animals.

Targeted modification of plant genomes for precision crop breeding

The development of gene targeting and gene editing techniques based on programmable site-directed nucleases (SDNs) has increased the precision of genome modification and made the outcomes more predictable and controllable. These approaches have achieved rapid advances in plant biotechnology, particularly the development of improved crop varieties. Here, we review the range of alterations which have already been implemented in plant genomes, and summarize the reported efficiencies of precise genome modification. Many crop varieties are being developed using SDN technologies and although their regulatory status in the USA is clear there is still a decision pending in the EU. DNA-free genome editing strategies are briefly discussed because they also present a unique regulatory challenge. The potential applications of genome editing in plant breeding and crop improvement are highlighted by drawing examples from the recent literature.

TALEN-mediated genome-editing approaches in the liverwort Marchantia polymorpha yield high efficiencies for targeted mutagenesis

BACKGROUND

The liverwort Marchantia polymorpha occupies a crucial position in land plant evolution and provides the opportunity to investigate adaptations to a terrestrial plant life style. Marchantia reverse genetic analyses have thus far been conducted by employing a homologous recombination approach, which yields an efficiency of around 3%. Availability of the characterized and suitable endogenous MpEF1α promoter prompted us to establish the TALEN gene targeting technique for Marchantia.

RESULTS

Here, two different TALEN techniques, using custom and self-assembled TALEN constructs, were applied and compared. The MpNOP1 gene was selected as a candidate gene, as the respective knockout mutant has been shown to lack air chamber formation, representing an easily traceable phenotype. We demonstrate that both TALEN approaches are successful in Marchantia yielding high gene targeting efficiencies of over 20%. Investigation of selected G1 up to G4 generations proved the stability of the knockout mutants. In 392 analyzed T1 plants, no additional phenotypes were observed and only one chimeric knockout plant was detected after an extended cultivation period. Interestingly, two out of the 24 sequenced mutants harbored indels causing in-frame mutations and revealed novel Mpnop1-related phenotypes. This demonstrates the potential to detect crucial amino acids and motives of targeted proteins, which is of special interest for essential genes where full knockouts are lethal. The FastTALE™ TALEN assembly kit enables the rapid assembly and ligation of the TALEN arms within half a day. For transformations, custom and assembled constructs were subcloned into Marchantia binary vectors possessing the MpEF1α promoter.

CONCLUSION

Considering time, costs and practicability, the assembly TALEN approach represents a rapid and highly efficient gene targeting system to generate Marchantia knockout mutants, which can be further adapted for future advanced genome-editing applications.

Genotypes, Networks, Phenotypes: Moving Toward Plant Systems Genetics

One of the central goals in biology is to understand how and how much of the phenotype of an organism is encoded in its genome. Although many genes that are crucial for organismal processes have been identified, much less is known about the genetic bases underlying quantitative phenotypic differences in natural populations. We discuss the fundamental gap between the large body of knowledge generated over the past decades by experimental genetics in the laboratory and what is needed to understand the genotype-to-phenotype problem on a broader scale. We argue that systems genetics, a combination of systems biology and the study of natural variation using quantitative genetics, will help to address this problem. We present major advances in these two mostly disconnected areas that have increased our understanding of the developmental processes of flowering time control and root growth. We conclude by illustrating and discussing the efforts that have been made toward systems genetics specifically in plants.

Exploring Genetic Diversity in Plants Using High-Throughput Sequencing Techniques

Food security has emerged as an urgent concern because of the rising world population. To meet the food demands of the near future, it is required to improve the productivity of various crops, not just of staple food crops. The genetic diversity among plant populations in a given species allows the plants to adapt to various environmental conditions. Such diversity could therefore yield valuable traits that could overcome the food-security challenges. To explore genetic diversity comprehensively and to rapidly identify useful genes and/or allele, advanced high-throughput sequencing techniques, also called next-generation sequencing (NGS) technologies, have been developed. These provide practical solutions to the challenges in crop genomics. Here, we review various sources of genetic diversity in plants, newly developed genetic diversity-mining tools synergized with NGS techniques, and related genetic approaches such as quantitative trait locus analysis and genome-wide association study.

Progress of targeted genome modification approaches in higher plants

Transgene integration in plants is based on illegitimate recombination between non-homologous sequences. The low control of integration site and number of (trans/cis)gene copies might have negative consequences on the expression of transferred genes and their insertion within endogenous coding sequences. The first experiments conducted to use precise homologous recombination for gene integration commenced soon after the first demonstration that transgenic plants could be produced. Modern transgene targeting categories used in plant biology are: (a) homologous recombination-dependent gene targeting; (b) recombinase-mediated site-specific gene integration; (c) oligonucleotide-directed mutagenesis; (d) nuclease-mediated site-specific genome modifications. New tools enable precise gene replacement or stacking with exogenous sequences and targeted mutagenesis of endogeneous sequences. The possibility to engineer chimeric designer nucleases, which are able to target virtually any genomic site, and use them for inducing double-strand breaks in host DNA create new opportunities for both applied plant breeding and functional genomics. CRISPR is the most recent technology available for precise genome editing. Its rapid adoption in biological research is based on its inherent simplicity and efficacy. Its utilization, however, depends on available sequence information, especially for genome-wide analysis. We will review the approaches used for genome modification, specifically those for affecting gene integration and modification in higher plants. For each approach, the advantages and limitations will be noted. We also will speculate on how their actual commercial development and implementation in plant breeding will be affected by governmental regulations.

The expanding footprint of CRISPR/Cas9 in the plant sciences

CRISPR/Cas9 has evolved and transformed the field of biology at an unprecedented pace. From the initial purpose of introducing a site specific mutation within a genome of choice, this technology has morphed into enabling a wide array of molecular applications, including site-specific transgene insertion and multiplexing for the simultaneous induction of multiple cleavage events. Efficiency, specificity, and flexibility are key attributes that have solidified CRISPR/Cas9 as the genome-editing tool of choice by scientists from all areas of biology. Within the field of plant biology, several CRISPR/Cas9 technologies, developed in other biological systems, have been successfully implemented to probe plant gene function and to modify specific crop traits. It is anticipated that this trend will persist and lead to the development of new applications and modifications of the CRISPR technology, adding to an ever-expanding collection of genome-editing tools. We envision that these tools will bestow plant researchers with new utilities to alter genome complexity, engineer site-specific integration events, control gene expression, generate transgene-free edited crops, and prevent or cure plant viral disease. The successful implementation of such utilities will represent a new frontier in plant biotechnology.

Geminiviruses for biotechnology: the art of parasite taming

Viruses are intracellular pathogens that have evolved efficient strategies for replication and expression of their proteins in the host cells. Geminiviruses - plant viruses with small circular single-stranded DNA genomes - effectively manipulate plant cell processes for viral functions, entailing great potential for biotechnological applications. This potentiality has been realized in the form of protein expression and gene-silencing vectors, and, more recently, vectors for genome editing - a technology that these viruses seem particularly well-suited to facilitate. This insight offers an overview of the biological properties of geminiviruses, with emphasis on those leveraging development of geminivirus-based replicons. It illustrates the basis for engineering geminivirus-based replicons and their applications. Furthermore, it discusses the reported use and future perspectives of geminivirus-based replicons for genome editing.

Next generation breeding

The genomic revolution of the past decade has greatly improved our understanding of the genetic make-up of living organisms. The sequencing of crop genomes has completely changed our vision and interpretation of genome organization and evolution. Re-sequencing allows the identification of an unlimited number of markers as well as the analysis of germplasm allelic diversity based on allele mining approaches. High throughput marker technologies coupled with advanced phenotyping platforms provide new opportunities for discovering marker-trait associations which can sustain genomic-assisted breeding. The availability of genome sequencing information is enabling genome editing (site-specific mutagenesis), to obtain gene sequences desired by breeders. This review illustrates how next generation sequencing-derived information can be used to tailor genomic tools for different breeders' needs to revolutionize crop improvement.

CRISPR-Cas9: Tool for Qualitative and Quantitative Plant Genome Editing

Recent developments in genome editing techniques have aroused substantial excitement among agricultural scientists. These techniques offer new opportunities for developing improved plant lines with addition of important traits or removal of undesirable traits. Increased adoption of genome editing has been geared by swiftly developing Clustered regularly interspaced short palindromic repeats (CRISPR). This is appearing as driving force for innovative utilization in diverse branches of plant biology. CRISPR-Cas9 mediated genome editing is being used for rapid, easy and efficient alteration of genes among diverse plant species. With approximate completion of conceptual work about CRISPR-Cas9, plant scientists are applying this genome editing tool for crop attributes enhancement. The capability of this system for performing targeted and efficient modifications in genome sequence as well as gene expression will certainly spur novel developments not only in model plants but in crop and ornamental plants as well. Additionally, due to non-involvement of foreign DNA, this technique may help alleviating regulatory issues associated with genetically modified plants. We expect that prevailing challenges in plant science like genomic region manipulation, crop specific vectors etc. will be addressed along with sustained growth of this genome editing tool. In this review, recent progress of CRISPR-Cas9 technology in plants has been summarized and discussed. We reviewed significance of CRISPR-Cas9 for specific and non-traditional aspects of plant life. It also covers strengths of this technique in comparison with other genome editing techniques, e.g., Zinc finger nucleases, Transcription activator-like effector nucleases and potential challenges in coming decades have been described.

Generation of artificial sequence-specific nucleases via a preassembled inert-template

Sequence specific nucleases are important tools for processing nucleic acids in a predictable way. Herein, we demonstrate a conceptually new approach for generating sequence-specific nucleases via a preassembled inert-template (PAIT). A fairly stable DNase I/inert-DNA complex was prepared with a customized sequence specificity for a target DNA which contains a sequence complementary to the inert-DNA template. The complex could efficiently cleave the targeted sequence within either a long double-stranded DNA or a single-stranded DNA without affecting other unrelated DNA strands. The discrimination factor against single-base mismatch strands within a 14 nt target region was as high as 25.3. The strategy was also successfully applied to RNase A. Our findings may hold great potential for the development of a number of new powerful enzymatic tools.

CRISPR/Cas9-mediated genome editing and gene replacement in plants: Transitioning from lab to field

The CRISPR/Cas9 genome engineering system has ignited and swept through the scientific community like wildfire. Owing largely to its efficiency, specificity, and flexibility, the CRISPR/Cas9 system has quickly become the preferred genome-editing tool of plant scientists. In plants, much of the early CRISPR/Cas9 work has been limited to proof of concept and functional studies in model systems. These studies, along with those in other fields of biology, have led to the development of several utilities of CRISPR/Cas9 beyond single gene editing. Such utilities include multiplexing for inducing multiple cleavage events, controlling gene expression, and site specific transgene insertion. With much of the conceptual CRISPR/Cas9 work nearly complete, plant researchers are beginning to apply this gene editing technology for crop trait improvement. Before rational strategies can be designed to implement this technology to engineer a wide array of crops there is a need to expand the availability of crop-specific vectors, genome resources, and transformation protocols. We anticipate that these challenges will be met along with the continued evolution of the CRISPR/Cas9 system particularly in the areas of manipulation of large genomic regions, transgene-free genetic modification, development of breeding resources, discovery of gene function, and improvements upon CRISPR/Cas9 components. The CRISPR/Cas9 editing system appears poised to transform crop trait improvement.