Efficient CRISPR/Cas9-mediated Targeted Mutagenesis in Populus in the First Generation

Efficient CRISPR/Cas9 Genome Editing of Phytoene desaturase in Cassava

CRISPR/Cas9 has become a powerful genome-editing tool for introducing genetic changes into crop species. In order to develop capacity for CRISPR/Cas9 technology in the tropical staple cassava (Manihot esculenta), the Phytoene desaturase (MePDS) gene was targeted in two cultivars using constructs carrying gRNAs targeting two sequences within MePDS exon 13. After Agrobacterium-mediated delivery of CRISPR/Cas9 reagents into cassava cells, both constructs induced visible albino phenotypes within cotyledon-stage somatic embryos regenerating on selection medium and the plants regenerated therefrom. A total of 58 (cv. 60444) and 25 (cv. TME 204) plant lines were recovered, of which 38 plant lines (19 from each cultivar) were analyzed for mutagenesis. The frequency of plant lines showing albino phenotype was high, ranging from 90 to 100% in cv. TME 204. Observed albino phenotypes were comprised of full albinos devoid of green tissue and chimeras containing a mixture of white and green tissues. Sequence analysis revealed that 38/38 (100%) of the plant lines examined carried mutations at the targeted MePDS site, with insertions, deletions, and substitutions recorded. One putatively mono-allelic homozygous line (1/19) was found from cv. 60444, while 1 (1/19) and 4 (4/19) putatively bi-allelic homozygous lines were found in 60444 and TME204, respectively. The remaining plant lines, comprised mostly of the chimeras, were found to be putatively heterozygous. We observed minor (1 bp) nucleotide substitutions and or deletions upstream of the 5' and or downstream of the 3' targeted MePDS region. The data reported demonstrates that CRISPR/Cas9-mediated genome editing of cassava is highly efficient and relatively simple, generating multi-allelic mutations in both cultivars studied. Modification of MePDS described here generates visually detectable mutated events in a relatively short time frame of 6-8 weeks, and does not require sequencing to confirm editing at the target. It therefore provides a valuable platform to facilitate rapid assessment and optimization of CRISPR/Cas9 and other genome-editing technologies in cassava.

The Power of CRISPR-Cas9-Induced Genome Editing to Speed Up Plant Breeding

Genome editing with engineered nucleases enabling site-directed sequence modifications bears a great potential for advanced plant breeding and crop protection. Remarkably, the RNA-guided endonuclease technology (RGEN) based on the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) is an extremely powerful and easy tool that revolutionizes both basic research and plant breeding. Here, we review the major technical advances and recent applications of the CRISPR-Cas9 system for manipulation of model and crop plant genomes. We also discuss the future prospects of this technology in molecular plant breeding.

Efficient and Heritable Targeted Mutagenesis in Mosses Using the CRISPR/Cas9 System

Targeted genome modification by RNA-guided nucleases derived from the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9) system has seen rapid development in many organisms, including several plant species. In the present study, we succeeded in introducing the CRISPR/Cas9 system into the non-model organism Scopelophila cataractae, a moss that exhibits heavy metal tolerance, and the model organism Physcomitrella patens Utilizing the process by which moss plants regenerate from protoplasts, we conducted targeted mutagenesis by expression of single-chain guide RNA (sgRNA) and Cas9 in protoplasts. Using this method, the acquisition rate of strains exhibiting phenotypic changes associated with the target genes was approximately 45-69%, and strains with phenotypic changes exhibited various insertion and deletion mutations. In addition, we report that our method is capable of multiplex targeted mutagenesis (two independent genes) and also permits the efficient introduction of large deletions (∼3 kbp). These results demonstrate that the CRISPR/Cas9 system can be used to accelerate investigations of bryology and land plant evolution.

Patterns of CRISPR/Cas9 activity in plants, animals and microbes

The CRISPR/Cas9 system and related RNA-guided endonucleases can introduce double-strand breaks (DSBs) at specific sites in the genome, allowing the generation of targeted mutations in one or more genes as well as more complex genomic rearrangements. Modifications of the canonical CRISPR/Cas9 system from Streptococcus pyogenes and the introduction of related systems from other bacteria have increased the diversity of genomic sites that can be targeted, providing greater control over the resolution of DSBs, the targeting efficiency (frequency of on-target mutations), the targeting accuracy (likelihood of off-target mutations) and the type of mutations that are induced. Although much is now known about the principles of CRISPR/Cas9 genome editing, the likelihood of different outcomes is species-dependent and there have been few comparative studies looking at the basis of such diversity. Here we critically analyse the activity of CRISPR/Cas9 and related systems in different plant species and compare the outcomes in animals and microbes to draw broad conclusions about the design principles required for effective genome editing in different organisms. These principles will be important for the commercial development of crops, farm animals, animal disease models and novel microbial strains using CRISPR/Cas9 and other genome-editing tools.

Application of Genomic Technologies to the Breeding of Trees

The recent introduction of next generation sequencing (NGS) technologies represents a major revolution in providing new tools for identifying the genes and/or genomic intervals controlling important traits for selection in breeding programs. In perennial fruit trees with long generation times and large sizes of adult plants, the impact of these techniques is even more important. High-throughput DNA sequencing technologies have provided complete annotated sequences in many important tree species. Most of the high-throughput genotyping platforms described are being used for studies of genetic diversity and population structure. Dissection of complex traits became possible through the availability of genome sequences along with phenotypic variation data, which allow to elucidate the causative genetic differences that give rise to observed phenotypic variation. Association mapping facilitates the association between genetic markers and phenotype in unstructured and complex populations, identifying molecular markers for assisted selection and breeding. Also, genomic data provide in silico identification and characterization of genes and gene families related to important traits, enabling new tools for molecular marker assisted selection in tree breeding. Deep sequencing of transcriptomes is also a powerful tool for the analysis of precise expression levels of each gene in a sample. It consists in quantifying short cDNA reads, obtained by NGS technologies, in order to compare the entire transcriptomes between genotypes and environmental conditions. The miRNAs are non-coding short RNAs involved in the regulation of different physiological processes, which can be identified by high-throughput sequencing of RNA libraries obtained by reverse transcription of purified short RNAs, and by in silico comparison with known miRNAs from other species. All together, NGS techniques and their applications have increased the resources for plant breeding in tree species, closing the former gap of genetic tools between trees and annual species.

Current status of potential applications of repurposed Cas9 for structural and functional genomics of plants

Redesigned Cas9 has emerged as a tool with various applications like gene editing, gene regulation, epigenetic modification and chromosomal imaging. Target specific single guide RNA (sgRNA) can be used with Cas9 for precise gene editing with high efficiency than previously known methods. Further, nuclease-deactivated Cas9 (dCas9) can be fused with activator or repressor for activation (CRISPRa) and repression (CRISPRi) of gene expression, respectively. dCas9 fused with epigenetic modifier like methylase or acetylase further expand the scope of this technique. Fluorescent probes can be tagged to dCas9 to visualize the chromosome. Due to its wide-spread application, simplicity, accessibility, efficacy and universality, this technique is expanding the structural and functional genomic studies of plant and developing CRISPR crops. The present review focuses on current status of using repurposed Cas9 system in these various areas, with major focus on application in plants. Major challenges, concerns and future directions of using this technique are discussed in brief.

Recent Developments in Systems Biology and Metabolic Engineering of Plant-Microbe Interactions

Microorganisms play a crucial role in the sustainability of the various ecosystems. The characterization of various interactions between microorganisms and other biotic factors is a necessary footstep to understand the association and functions of microbial communities. Among the different microbial interactions in an ecosystem, plant-microbe interaction plays an important role to balance the ecosystem. The present review explores plant-microbe interactions using gene editing and system biology tools toward the comprehension in improvement of plant traits. Further, system biology tools like FBA (flux balance analysis), OptKnock, and constraint-based modeling helps in understanding such interactions as a whole. In addition, various gene editing tools have been summarized and a strategy has been hypothesized for the development of disease free plants. Furthermore, we have tried to summarize the predictions through data retrieved from various types of sources such as high throughput sequencing data (e.g., single nucleotide polymorphism detection, RNA-seq, proteomics) and metabolic models have been reconstructed from such sequences for species communities. It is well known fact that systems biology approaches and modeling of biological networks will enable us to learn the insight of such network and will also help further in understanding these interactions.

CRISPR/Cas9-mediated efficient targeted mutagenesis in Chardonnay (Vitis vinifera L.)

The type II clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 system (CRISPR/Cas9) has been successfully applied to edit target genes in multiple plant species. However, it remains unknown whether this system can be used for genome editing in grape. In this study, we described genome editing and targeted gene mutation in ‘Chardonnay’ suspension cells and plants via the CRISPR/Cas9 system. Two single guide RNAs (sgRNAs) were designed to target distinct sites of the L-idonate dehydrogenase gene (IdnDH). CEL I endonuclease assay and sequencing results revealed the expected indel mutations at the target site, and a mutation frequency of 100% was observed in the transgenic cell mass (CM) as well as corresponding regenerated plants with expression of sgRNA1/Cas9. The majority of the detected mutations in transgenic CM were 1-bp insertions, followed by 1- to 3-nucleotide deletions. Off-target activities were also evaluated by sequencing the potential off-target sites, and no obvious off-target events were detected. Our results demonstrated that the CRISPR/Cas9 system is an efficient and specific tool for precise genome editing in grape.

CRISPR/Cas9: a promising way to exploit genetic variation in plants

Creation of variation in existing gene pool of crop plants is the foremost requirement in crop improvement programmes. Genome editing is a tool to produce knock out of target genes either by introduction of insertion or by deletion that disrupts the function of a specific gene. The CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9) system is the most recent addition to the toolbox of sequence-specific nucleases that includes ZFNs and TALENs. The CRISPR/Cas9 system allows targeted cleavage of genomic DNA guided by a small noncoding RNA, resulting in gene modifications by both non-homologous end joining and homology-directed repair mechanisms. Here, we present an overview of mechanisms of CRISPR, its potential roles in creating variation in germplasm and applications of this novel interference pathway in crop improvement. The availability of the CRISPR/Cas9 system holds promise in facilitating both forward and reverse genetics and will enhance research in crops that lack genetic resources.

Efficient Genome Editing in Apple Using a CRISPR/Cas9 system

Genome editing is a powerful technique for genome modification in molecular research and crop breeding, and has the great advantage of imparting novel desired traits to genetic resources. However, the genome editing of fruit tree plantlets remains to be established. In this study, we describe induction of a targeted gene mutation in the endogenous apple phytoene desaturase (PDS) gene using the CRISPR/Cas9 system. Four guide RNAs (gRNAs) were designed and stably transformed with Cas9 separately in apple. Clear and partial albino phenotypes were observed in 31.8% of regenerated plantlets for one gRNA, and bi-allelic mutations in apple PDS were confirmed by DNA sequencing. In addition, an 18-bp gRNA also induced a targeted mutation. These CRIPSR/Cas9 induced-mutations in the apple genome suggest activation of the NHEJ pathway, but with some involvement also of the HR pathway. Our results demonstrate that genome editing can be practically applied to modify the apple genome.

Manipulating the Biosynthesis of Bioactive Compound Alkaloids for Next-Generation Metabolic Engineering in Opium Poppy Using CRISPR-Cas 9 Genome Editing Technology

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated9 (Cas9) endonuclease system is a powerful RNA-guided genome editing tool. CRISPR/Cas9 has been well studied in model plant species for targeted genome editing. However, few studies have been reported on plant species without whole genome sequence information. Currently, no study has been performed to manipulate metabolic pathways using CRISPR/Cas9. In this study, the type II CRISPR/SpCas9 system was used to knock out, via nonhomologous end-joining genome repair, the 4'OMT2 in opium poppy (Papaver somniferum L.), a gene which regulates the biosythesis of benzylisoquinoline alkaloids (BIAs). For sgRNA transcription, viral-based TRV and synthetic binary plasmids were designed and delivered into plant cells with a Cas9 encoding-synthetic vector by Agrobacterium-mediated transformation. InDels formed by CRISPR/Cas9 were detected by sequence analysis. Our results showed that the biosynthesis of BIAs (e.g. morphine, thebaine) was significantly reduced in the transgenic plants suggesting that 4'OMT2 was efficiently knocked-out by our CRISPR-Cas9 genome editing approach. In addition, a novel uncharacterized alkaloid was observed only in CRISPR/Cas9 edited plants. Thus, the applicabilitiy of the CRISPR/Cas9 system was demonstrated for the first time for medicinal aromatic plants by sgRNAs transcribed from both synthetic and viral vectors to regulate BIA metabolism and biosynthesis.

CRISPR/Cas9 Platforms for Genome Editing in Plants: Developments and Applications

The clustered regularly interspaced short palindromic repeat (CRISPR)-associated protein9 (Cas9) genome editing system (CRISPR/Cas9) is adapted from the prokaryotic type II adaptive immunity system. The CRISPR/Cas9 tool surpasses other programmable nucleases, such as ZFNs and TALENs, for its simplicity and high efficiency. Various plant-specific CRISPR/Cas9 vector systems have been established for adaption of this technology to many plant species. In this review, we present an overview of current advances on applications of this technology in plants, emphasizing general considerations for establishment of CRISPR/Cas9 vector platforms, strategies for multiplex editing, methods for analyzing the induced mutations, factors affecting editing efficiency and specificity, and features of the induced mutations and applications of the CRISPR/Cas9 system in plants. In addition, we provide a perspective on the challenges of CRISPR/Cas9 technology and its significance for basic plant research and crop genetic improvement.

Synthetic strategies for plant signalling studies: molecular toolbox and orthogonal platforms

Plants deploy a wide array of signalling networks integrating environmental cues with growth, defence and developmental responses. The high level of complexity, redundancy and connection between several pathways hampers a comprehensive understanding of involved functional and regulatory mechanisms. The implementation of synthetic biology approaches is revolutionizing experimental biology in prokaryotes, yeasts and animal systems and can likewise contribute to a new era in plant biology. This review gives an overview on synthetic biology approaches for the development and implementation of synthetic molecular tools and techniques to interrogate, understand and control signalling events in plants, ranging from strategies for the targeted manipulation of plant genomes up to the spatiotemporally resolved control of gene expression using optogenetic approaches. We also describe strategies based on the partial reconstruction of signalling pathways in orthogonal platforms, like yeast, animal and in vitro systems. This allows a targeted analysis of individual signalling hubs devoid of interconnectivity with endogenous interacting components. Implementation of the interdisciplinary synthetic biology tools and strategies is not exempt of challenges and hardships but simultaneously most rewarding in terms of the advances in basic and applied research. As witnessed in other areas, these original theoretical-experimental avenues will lead to a breakthrough in the ability to study and comprehend plant signalling networks.

CRISPR/Cas9-Based Multiplex Genome Editing in Monocot and Dicot Plants

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-mediated genome targeting system has been applied to a variety of organisms, including plants. Compared to other genome-targeting technologies such as zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs), the CRISPR/Cas9 system is easier to use and has much higher editing efficiency. In addition, multiple "single guide RNAs" (sgRNAs) with different target sequences can be designed to direct the Cas9 protein to multiple genomic sites for simultaneous multiplex editing. Here, we present a procedure for highly efficient multiplex genome targeting in monocot and dicot plants using a versatile and robust CRISPR/Cas9 vector system, emphasizing the construction of binary constructs with multiple sgRNA expression cassettes in one round of cloning using Golden Gate ligation. We also describe the genotyping of targeted mutations in transgenic plants by direct Sanger sequencing followed by decoding of superimposed sequencing chromatograms containing biallelic or heterozygous mutations using the Web-based tool DSDecode. © 2016 by John Wiley & Sons, Inc.

Biotechnology and synthetic biology approaches for metabolic engineering of bioenergy crops

The Green Revolution has fuelled an exponential growth in human population since the mid-20th century. Due to population growth, food and energy demands will soon surpass supply capabilities. To overcome these impending problems, significant improvements in genetic engineering will be needed to complement breeding efforts in order to accelerate the improvement of agronomical traits. The new field of plant synthetic biology has emerged in recent years and is expected to support rapid, precise, and robust engineering of plants. In this review, we present recent advances made in the field of plant synthetic biology, specifically in genome editing, transgene expression regulation, and bioenergy crop engineering, with a focus on traits related to lignocellulose, oil, and soluble sugars. Ultimately, progress and innovation in these fields may facilitate the development of beneficial traits in crop plants to meet society's bioenergy needs.

Applications of CRISPR/Cas9 technology for targeted mutagenesis, gene replacement and stacking of genes in higher plants

Mutagenesis continues to play an essential role for understanding plant gene function and, in some instances, provides an opportunity for plant improvement. The development of gene editing technologies such as TALENs and zinc fingers has revolutionised the targeted mutation specificity that can now be achieved. The CRISPR/Cas9 system is the most recent addition to gene editing technologies and arguably the simplest requiring only two components; a small guide RNA molecule (sgRNA) and Cas9 endonuclease protein which complex to recognise and cleave a specific 20 bp target site present in a genome. Target specificity is determined by complementary base pairing between the sgRNA and target site sequence enabling highly specific, targeted mutation to be readily engineered. Upon target site cleavage, error-prone endogenous repair mechanisms produce small insertion/deletions at the target site usually resulting in loss of gene function. CRISPR/Cas9 gene editing has been rapidly adopted in plants and successfully undertaken in numerous species including major crop species. Its applications are not restricted to mutagenesis and target site cleavage can be exploited to promote sequence insertion or replacement by recombination. The multiple applications of this technology in plants are described.

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.

The Amaryllidaceae alkaloids: biosynthesis and methods for enzyme discovery

Amaryllidaceae alkaloids are an example of the vast diversity of secondary metabolites with great therapeutic promise. The identification of novel compounds in this group with over 300 known structures continues to be an area of active study. The recent identification of norbelladine 4'-O-methyltransferase (N4OMT), an Amaryllidaceae alkaloid biosynthetic enzyme, and the assembly of transcriptomes for Narcissus sp. aff. pseudonarcissus and Lycoris aurea highlight the potential for discovery of Amaryllidaceae alkaloid biosynthetic genes with new technologies. Recent technical advances of interest include those in enzymology, next generation sequencing, genetic modification, nuclear magnetic resonance spectroscopy (NMR), and mass spectrometry (MS).

Optimization of CRISPR/Cas9 genome editing to modify abiotic stress responses in plants

Genome editing using the CRISPR/Cas9 system can be used to modify plant genomes, however, improvements in specificity and applicability are still needed in order for the editing technique to be useful in various plant species. Here, using genome editing mediated by a truncated gRNA (tru-gRNA)/Cas9 combination, we generated new alleles for OST2, a proton pump in Arabidopsis, with no off-target effects. By following expression of Cas9 and the tru-gRNAs, newly generated mutations in CRIPSR/Cas9 transgenic plants were detected with high average mutation rates of up to 32.8% and no off-target effects using constitutive promoter. Reducing nuclear localization signals in Cas9 decreased the mutation rate. In contrast, tru-gRNA Cas9 cassettes driven by meristematic- and reproductive-tissue-specific promoters increased the heritable mutation rate in Arabidopsis, showing that high expression in the germ line can produce bi-allelic mutations. Finally, the new mutant alleles obtained for OST2 exhibited altered stomatal closing in response to environmental conditions. These results suggest further applications in molecular breeding to improve plant function using optimized plant CRISPR/Cas9 systems.

Precision Targeted Mutagenesis via Cas9 Paired Nickases in Rice

Recent reports of CRISPR- (clustered regularly interspaced short palindromic repeats)/Cas9 (CRISPR-associated protein 9) mediated heritable mutagenesis in plants highlight the need for accuracy of the mutagenesis directed by this system. Off-target mutations are an important issue when considering functional gene analysis, as well as the molecular breeding of crop plants with large genome size, i.e. with many duplicated genes, and where the whole-genome sequence is still lacking. In mammals, off-target mutations can be suppressed by using Cas9 paired nickases together with paired guide RNAs (gRNAs). However, the performance of Cas9 paired nickases has not yet been fully assessed in plants. Here, we analyzed on- and off-target mutation frequency in rice calli and regenerated plants using Cas9 nuclease or Cas9 nickase with paired gRNAs. When Cas9 paired nickases were used, off-target mutations were fully suppressed in rice calli and regenerated plants. However, on-target mutation frequency also decreased compared with that induced by the Cas9 paired nucleases system. Since the gRNA sequence determines specific binding of Cas9 protein-gRNA ribonucleoproteins at the targeted sequence, the on-target mutation frequency of Cas9 paired nickases depends on the design of paired gRNAs. Our results suggest that a combination of gRNAs that can induce mutations at high efficiency with Cas9 nuclease should be used together with Cas9 nickase. Furthermore, we confirmed that a combination of gRNAs containing a one nucleotide (1 nt) mismatch toward the target sequence could not induce mutations when expressed with Cas9 nickase. Our results clearly show the effectiveness of Cas9 paired nickases in delivering on-target specific mutations.

New developments in engineering plant metabolic pathways

Plants contain countless metabolic pathways that are responsible for the biosynthesis of complex metabolites. Armed with new tools in sequencing and bioinformatics, the genes that encode these plant biosynthetic pathways have become easier to discover, putting us in an excellent position to fully harness the wealth of compounds and biocatalysts (enzymes) that plants provide. For overproduction and isolation of high-value plant-derived chemicals, plant pathways can be reconstituted in heterologous hosts. Alternatively, plant pathways can be modified in the native producer to confer new properties to the plant, such as better biofuel production or enhanced nutritional value. This perspective highlights a range of examples that demonstrate how the metabolic pathways of plants can be successfully harnessed with a variety of metabolic engineering approaches.

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.

The CRISPR/Cas Genome-Editing Tool: Application in Improvement of Crops

The Clustered Regularly Interspaced Short Palindromic Repeats associated Cas9/sgRNA system is a novel targeted genome-editing technique derived from bacterial immune system. It is an inexpensive, easy, most user friendly and rapidly adopted genome editing tool transforming to revolutionary paradigm. This technique enables precise genomic modifications in many different organisms and tissues. Cas9 protein is an RNA guided endonuclease utilized for creating targeted double-stranded breaks with only a short RNA sequence to confer recognition of the target in animals and plants. Development of genetically edited (GE) crops similar to those developed by conventional or mutation breeding using this potential technique makes it a promising and extremely versatile tool for providing sustainable productive agriculture for better feeding of rapidly growing population in a changing climate. The emerging areas of research for the genome editing in plants include interrogating gene function, rewiring the regulatory signaling networks and sgRNA library for high-throughput loss-of-function screening. In this review, we have described the broad applicability of the Cas9 nuclease mediated targeted plant genome editing for development of designer crops. The regulatory uncertainty and social acceptance of plant breeding by Cas9 genome editing have also been described. With this powerful and innovative technique the designer GE non-GM plants could further advance climate resilient and sustainable agriculture in the future and maximizing yield by combating abiotic and biotic stresses.

Advances and perspectives on the use of CRISPR/Cas9 systems in plant genomics research

Genome editing with site-specific nucleases has become a powerful tool for functional characterization of plant genes and genetic improvement of agricultural crops. Among the various site-specific nuclease-based technologies available for genome editing, the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) systems have shown the greatest potential for rapid and efficient editing of genomes in plant species. This article reviews the current status of application of CRISPR/Cas9 to plant genomics research, with a focus on loss-of-function and gain-of-function analysis of individual genes in the context of perennial plants and the potential application of CRISPR/Cas9 to perturbation of gene expression, and identification and analysis of gene modules as part of an accelerated domestication and synthetic biology effort.

An alternative strategy for targeted gene replacement in plants using a dual-sgRNA/Cas9 design

Precision DNA/gene replacement is a promising genome-editing tool that is highly desirable for molecular engineering and breeding by design. Although the CRISPR/Cas9 system works well as a tool for gene knockout in plants, gene replacement has rarely been reported. Towards this end, we first designed a combinatory dual-sgRNA/Cas9 vector (construct #1) that successfully deleted miRNA gene regions (MIR169a and MIR827a). The deletions were confirmed by PCR and subsequent sequencing, yielding deletion efficiencies of 20% and 24% on MIR169a and MIR827a loci, respectively. We designed a second structure (construct #2) that contains sites homologous to Arabidopsis TERMINAL FLOWER 1 (TFL1) for homology-directed repair (HDR) with regions corresponding to the two sgRNAs on the modified construct #1. The two constructs were co-transformed into Arabidopsis plants to provide both targeted deletion and donor repair for targeted gene replacement by HDR. Four of 500 stably transformed T0 transgenic plants (0.8%) contained replaced fragments. The presence of the expected recombination sites was further confirmed by sequencing. Therefore, we successfully established a gene deletion/replacement system in stably transformed plants that can potentially be utilized to introduce genes of interest for targeted crop improvement.

CRISPR/Cas9: an advanced tool for editing plant genomes

To meet current challenges in agriculture, genome editing using sequence-specific nucleases (SSNs) is a powerful tool for basic and applied plant biology research. Here, we describe the principle and application of available genome editing tools, including zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and the clustered regularly interspaced short palindromic repeat associated CRISPR/Cas9 system. Among these SSNs, CRISPR/Cas9 is the most recently characterized and rapidly developing genome editing technology, and has been successfully utilized in a wide variety of organisms. This review specifically illustrates the power of CRISPR/Cas9 as a tool for plant genome engineering, and describes the strengths and weaknesses of the CRISPR/Cas9 technology compared to two well-established genome editing tools, ZFNs and TALENs.

Genome Editing of Structural Variations: Modeling and Gene Correction

The analysis of chromosomal structural variations (SVs), such as inversions and translocations, was made possible by the completion of the human genome project and the development of genome-wide sequencing technologies. SVs contribute to genetic diversity and evolution, although some SVs can cause diseases such as hemophilia A in humans. Genome engineering technology using programmable nucleases (e.g., ZFNs, TALENs, and CRISPR/Cas9) has been rapidly developed, enabling precise and efficient genome editing for SV research. Here, we review advances in modeling and gene correction of SVs, focusing on inversion, translocation, and nucleotide repeat expansion.

Targeted Gene Manipulation in Plants Using the CRISPR/Cas Technology

The CRISPR/Cas technology is emerging as a revolutionary genome editing tool in diverse organisms including plants, and has quickly evolved into a suite of versatile tools for sequence-specific gene manipulations beyond genome editing. Here, we review the most recent applications of the CRISPR/Cas toolkit in plants and also discuss key factors for improving CRISPR/Cas performance and strategies for reducing the off-target effects. Novel technical breakthroughs in mammalian research regarding the CRISPR/Cas toolkit will also be incorporated into this review in hope to stimulate prospective users from the plant research community to fully explore the potential of these technologies.

Exploiting the CRISPR/Cas9 System for Targeted Genome Mutagenesis in Petunia

Recently, CRISPR/Cas9 technology has emerged as a powerful approach for targeted genome modification in eukaryotic organisms from yeast to human cell lines. Its successful application in several plant species promises enormous potential for basic and applied plant research. However, extensive studies are still needed to assess this system in other important plant species, to broaden its fields of application and to improve methods. Here we showed that the CRISPR/Cas9 system is efficient in petunia (Petunia hybrid), an important ornamental plant and a model for comparative research. When PDS was used as target gene, transgenic shoot lines with albino phenotype accounted for 55.6%–87.5% of the total regenerated T0 Basta-resistant lines. A homozygous deletion close to 1 kb in length can be readily generated and identified in the first generation. A sequential transformation strategy—introducing Cas9 and sgRNA expression cassettes sequentially into petunia—can be used to make targeted mutations with short indels or chromosomal fragment deletions. Our results present a new plant species amenable to CRIPR/Cas9 technology and provide an alternative procedure for its exploitation.

Multi-gene engineering in plants with RNA-guided Cas9 nuclease

The use of RNA-guided Cas9 endonuclease for the concurrent engineering of multiple genes has been demonstrated in a number of plant species. Although Cas9 is a large monomeric protein, the single guide RNA (sgRNA) that directs it to a specific DNA target sequence is small and easy to reprogram. It is therefore relatively simple to produce numerous sgRNAs to target multiple endogenous sequences. Several approaches to express multiple sgRNAs and Cas9 in plants for the purpose of simultaneous editing or transcriptional regulation of many genes have recently been reported.

Reassessment of the Four Yield-related Genes Gn1a, DEP1, GS3, and IPA1 in Rice Using a CRISPR/Cas9 System

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-associated (Cas) systems have been successfully used as efficient tools for genome editing in a variety of species. We used the CRISPR/Cas9 system to mutate the Gn1a (Os01g0197700), DEP1 (Os09g0441900), GS3 (Os03g0407400), and IPA1 (Os08g0509600) genes of rice cultivar Zhonghua 11, genes which have been reported to function as regulators of grain number, panicle architecture, grain size and plant architecture, respectively. Analysis of the phenotypes and frequencies of edited genes in the first generation of transformed plants (T0) showed that the CRISPR/Cas9 system was highly efficient in inducing targeted gene editing, with the desired genes being edited in 42.5% (Gn1a), 67.5% (DEP1), 57.5% (GS3), and 27.5% (IPA1) of the transformed plants. The T2 generation of the gn1a, dep1, and gs3 mutants featured enhanced grain number, dense erect panicles, and larger grain size, respectively. Furthermore, semi-dwarf, and grain with long awn, phenotypes were observed in dep1 and gs3 mutants, respectively. The ipa1 mutants showed two contrasting phenotypes, having either fewer tillers or more tillers, depending on the changes induced in the OsmiR156 target region. In addition, we found that mutants with deletions occurred more frequently than previous reports had indicated and that off-targeting had taken place in highly similar target sequences. These results proved that multiple regulators of important traits can be modified in a single cultivar by CRISPR/Cas9, and thus facilitate the dissection of complex gene regulatory networks in the same genomic background and the stacking of important traits in cultivated varieties.

The CRISPR-Cas9 technology: Closer to the ultimate toolkit for targeted genome editing

The first period of plant genome editing was based on Agrobacterium; chemical mutagenesis by EMS (ethyl methanesulfonate) and ionizing radiations; each of these technologies led to randomly distributed genome modifications. The second period is associated with the discoveries of homing and meganuclease enzymes during the 80s and 90s, which were then engineered to provide efficient tools for targeted editing. From 2006 to 2012, a few crop plants were successfully and precisely modified using zinc-finger nucleases. A third wave of improvement in genome editing, which led to a dramatic decrease in off-target events, was achieved in 2009-2011 with the TALEN technology. The latest revolution surfaced in 2013 with the CRISPR-Cas9 system, whose high efficiency and technical ease of use is really impressive; scientists can use in-house kits or commercially available kits; the only two requirements are to carefully choose the location of the DNA double strand breaks to be induced and then to order an oligonucleotide. While this close-to- ultimate toolkit for targeted editing of genomes represents dramatic scientific progress which allows the development of more complex useful agronomic traits through synthetic biology, the social acceptance of genome editing remains regularly questioned by anti-GMO citizens and organizations.

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.

Biomass traits and candidate genes for bioenergy revealed through association genetics in coppiced European Populus nigra (L.)

BACKGROUND

Second generation (2G) bioenergy from lignocellulosic feedstocks has the potential to develop as a sustainable source of renewable energy; however, significant hurdles still remain for large-scale commercialisation. Populus is considered as a promising 2G feedstock and understanding the genetic basis of biomass yield and feedstock quality are a research priority in this model tree species.

RESULTS

We report the first coppiced biomass study for 714 members of a wide population of European black poplar (Populus nigra L.), a native European tree, selected from 20 river populations ranging in latitude and longitude between 40.5 and 52.1°N and 1.0 and 16.4°E, respectively. When grown at a single site in southern UK, significant Site of Origin (SO) effects were seen for 14 of the 15 directly measured or derived traits including biomass yield, leaf area and stomatal index. There was significant correlation (p < 0.001) between biomass yield traits over 3 years of harvest which identified leaf size and cell production as strong predictors of biomass yield. A 12 K Illumina genotyping array (constructed from 10,331 SNPs in 14 QTL regions and 4648 genes) highlighted significant population genetic structure with pairwise FST showing strong differentiation (p < 0.001) between the Spanish and Italian subpopulations. Robust associations reaching genome-wide significance are reported for main stem height and cell number per leaf; two traits tightly linked to biomass yield. These genotyping and phenotypic data were also used to show the presence of significant isolation by distance (IBD) and isolation by adaption (IBA) within this population.

CONCLUSIONS

The three associations identified reaching genome-wide significance at p < 0.05 include a transcription factor; a putative stress response gene and a gene of unknown function. None of them have been previously linked to bioenergy yield; were shown to be differentially expressed in a panel of three selected genotypes from the collection and represent exciting, novel candidates for further study in a bioenergy tree native to Europe and Euro-Asia. A further 26 markers (22 genes) were found to reach putative significance and are also of interest for biomass yield, leaf area, epidermal cell expansion and stomatal patterning. This research on European P. nigra provides an important foundation for the development of commercial native trees for bioenergy and for advanced, molecular breeding in these species.

Clustered Regularly Interspaced Short Palindromic Repeats/Cas9 Genetic Engineering: Robotic Genetic Surgery

As a novel technology that utilizes the endogenous immune defense system in bacteria, CRISPR/Cas9 has transcended DNA engineering into a more pragmatic and clinically efficacious field. Using programmable sgRNA sequences and nucleases, the system effectively introduces double strand breaks in target genes within an entire organism. The applications of CRISPR range from biomedicine to drug development and epigenetic modification. Studies have demonstrated CRISPR mediated targeting of various tumorigenic genes and effector proteins known to be involved in colon carcinomas. This technology significantly expands the scope of gene manipulation and allows for an enhanced modeling of colon cancers, as well as various other malignancies.

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.

CRISPRing into the woods

The CRISPR/Cas9 technology is a welcome breakthrough for genome editing, owing to its precision, efficiency, versatility and ease of adoption. We recently reported the first application of CRISPR/Cas9 for biallelic mutations in stably transformed Populus, extending the species range of this powerful technology to woody perennials. An underappreciated obstacle in genome editing of outcrossing species is the frequent occurrence of sequence polymorphisms that can render CRISPR/Cas9 unproductive. We discuss experimental evidence as well as genome-wide computational analysis to demonstrate the sensitivity of CRISPR/Cas9 to allelic heterozygosity, and highlight tools and strategies that can help deal with such sequence polymorphisms. With its specificity, CRISPR/Cas9 offers a less equivocal means than previous approaches for discerning functional redundancy of paralogous genes that are prevalent in plant genomes. Continuing improvements of the CRISPR/Cas9 system for multiplex genome engineering should facilitate these efforts. The paradigm shift brought about by CRISPR/Cas9 promises to accelerate not only basic research but also applied crop improvement progress.

CRISPRing into the woods

The CRISPR/Cas9 technology is a welcome breakthrough for genome editing, owing to its precision, efficiency, versatility and ease of adoption. We recently reported the first application of CRISPR/Cas9 for biallelic mutations in stably transformed Populus, extending the species range of this powerful technology to woody perennials. An underappreciated obstacle in genome editing of outcrossing species is the frequent occurrence of sequence polymorphisms that can render CRISPR/Cas9 unproductive. We discuss experimental evidence as well as genome-wide computational analysis to demonstrate the sensitivity of CRISPR/Cas9 to allelic heterozygosity, and highlight tools and strategies that can help deal with such sequence polymorphisms. With its specificity, CRISPR/Cas9 offers a less equivocal means than previous approaches for discerning functional redundancy of paralogous genes that are prevalent in plant genomes. Continuing improvements of the CRISPR/Cas9 system for multiplex genome engineering should facilitate these efforts. The paradigm shift brought about by CRISPR/Cas9 promises to accelerate not only basic research but also applied crop improvement progress.