CRISPR ribonucleoprotein-mediated genetic engineering in plants

Genome editing of a recalcitrant wine grape genotype by lipofectamine-mediated delivery of CRISPR/Cas9 ribonucleoproteins to protoplasts

The main bottleneck in the application of biotechnological breeding methods to woody species is due to the in vitro regeneration recalcitrance shown by several genotypes. On the other side, woody species, especially grapevine (Vitis vinifera L.), use most of the pesticides and other expensive inputs in agriculture, making the development of efficient approaches of genetic improvement absolutely urgent. Genome editing is an extremely promising technique particularly for wine grape genotypes, as it allows to modify the desired gene in a single step, preserving all the quality traits selected and appreciated in elite varieties. A genome editing and regeneration protocol for the production of transgene-free grapevine plants, exploiting the lipofectamine-mediated direct delivery of CRISPR-Cas9 ribonucleoproteins (RNPs) to target the phytoene desaturase gene, is reported. We focused on Nebbiolo (V. vinifera), an extremely in vitro recalcitrant wine genotype used to produce outstanding wines, such as Barolo and Barbaresco. The use of the PEG-mediated editing method available in literature and employed for highly embryogenic grapevine genotypes did not allow the proper embryo development in the recalcitrant Nebbiolo. Lipofectamines, on the contrary, did not have a negative impact on protoplast viability and plant regeneration, leading to the obtainment of fully developed edited plants after about 5 months from the transfection. Our work represents one of the first examples of lipofectamine use for delivering editing reagents in plant protoplasts. The important result achieved for the wine grape genotype breeding could be extended to other important wine grape varieties and recalcitrant woody species.

High performance TadA-8e derived cytosine and dual base editors with undetectable off-target effects in plants

Cytosine base editors (CBEs) and adenine base editors (ABEs) enable precise C-to-T and A-to-G edits. Recently, ABE8e, derived from TadA-8e, enhances A-to-G edits in mammalian cells and plants. Interestingly, TadA-8e can also be evolved to confer C-to-T editing. This study compares engineered CBEs derived from TadA-8e in rice and tomato cells, identifying TadCBEa, TadCBEd, and TadCBEd_V106W as efficient CBEs with high purity and a narrow editing window. A dual base editor, TadDE, promotes simultaneous C-to-T and A-to-G editing. Multiplexed base editing with TadCBEa and TadDE is demonstrated in transgenic rice, with no off-target effects detected by whole genome and transcriptome sequencing, indicating high specificity. Finally, two crop engineering applications using TadDE are shown: introducing herbicide resistance alleles in OsALS and creating synonymous mutations in OsSPL14 to resist OsMIR156-mediated degradation. Together, this study presents TadA-8e derived CBEs and a dual base editor as valuable additions to the plant editing toolbox.

Optimized protoplast isolation and transfection with a breakpoint: accelerating Cas9/sgRNA cleavage efficiency validation in monocot and dicot

The CRISPR-Cas genome editing tools are revolutionizing agriculture and basic biology with their simplicity and precision ability to modify target genomic loci. Software-predicted guide RNAs (gRNAs) often fail to induce efficient cleavage at target loci. Many target loci are inaccessible due to complex chromatin structure. Currently, there is no suitable tool available to predict the architecture of genomic target sites and their accessibility. Hence, significant time and resources are spent on performing editing experiments with inefficient guides. Although in vitro-cleavage assay could provide a rough assessment of gRNA efficiency, it largely excludes the interference of native genomic context. Transient in-vivo testing gives a proper assessment of the cleavage ability of editing reagents in a native genomic context. Here, we developed a modified protocol that offers highly efficient protoplast isolation from rice, Arabidopsis, and chickpea, using a sucrose gradient, transfection using PEG (polyethylene glycol), and validation of single guide RNAs (sgRNAs) cleavage efficiency of CRISPR-Cas9. We have optimized various parameters for PEG-mediated protoplast transfection and achieved high transfection efficiency using our protocol in both monocots and dicots. We introduced plasmid vectors containing Cas9 and sgRNAs targeting genes in rice, Arabidopsis, and chickpea protoplasts. Using dual sgRNAs, our CRISPR-deletion strategy offers straightforward detection of genome editing success by simple agarose gel electrophoresis. Sanger sequencing of PCR products confirmed the editing efficiency of specific sgRNAs. Notably, we demonstrated that isolated protoplasts can be stored for up to 24/48 h with little loss of viability, allowing a pause between isolation and transfection. This high-efficiency protocol for protoplast isolation and transfection enables rapid (less than 7 days) validation of sgRNA cleavage efficiency before proceeding with stable transformation. The isolation and transfection method can also be utilized for rapid validation of editing strategies, evaluating diverse editing reagents, regenerating plants from transfected protoplasts, gene expression studies, protein localization and functional analysis, and other applications.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-024-00139-7.

Exploiting viral vectors to deliver genome editing reagents in plants

Genome editing holds great promise for the molecular breeding of plants, yet its application is hindered by the shortage of simple and effective means of delivering genome editing reagents into plants. Conventional plant transformation-based methods for delivery of genome editing reagents into plants often involve prolonged tissue culture, a labor-intensive and technically challenging process for many elite crop cultivars. In this review, we describe various virus-based methods that have been employed to deliver genome editing reagents, including components of the CRISPR/Cas machinery and donor DNA for precision editing in plants. We update the progress in these methods with recent successful examples of genome editing achieved through virus-based delivery in different plant species, highlight the advantages and limitations of these delivery approaches, and discuss the remaining challenges.

DNA- and Selectable-Marker-Free Genome-Editing System Using Zygotes from Recalcitrant Maize Inbred B73

Genome-editing tools such as the clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9) system have become essential tools for increasing the efficiency and accuracy of plant breeding. Using such genome-editing tools on maize, one of the most important cereal crops of the world, will greatly benefit the agriculture and the mankind. Conventional genome-editing methods typically used for maize involve insertion of a Cas9-guide RNA expression cassette and a selectable marker in the genome DNA; however, using such methods, it is essential to eliminate the inserted DNA cassettes to avoid legislative concerns on gene-modified organisms. Another major hurdle for establishing an efficient and broadly applicable DNA-free genome-editing system for maize is presented by recalcitrant genotypes/cultivars, since cell/tissue culture and its subsequent regeneration into plantlets are crucial for producing transgenic and/or genome-edited maize. In this study, to establish a DNA-free genome-editing system for recalcitrant maize genotypes/cultivars, Cas9-gRNA ribonucleoproteins were directly delivered into zygotes isolated from the pollinated flowers of the maize-B73 cultivar. The zygotes successfully developed and were regenerated into genome-edited plantlets by co-culture with phytosulfokine, a peptide phytohormone. The method developed herein made it possible to obtain DNA- and selectable-marker-free genome-edited recalcitrant maize genotypes/cultivars with high efficiency. This method can advance the molecular breeding of maize and other important cereals, regardless of their recalcitrant characteristics.

Advances in genomics and genome editing for improving strawberry (Fragaria ×ananassa)

The cultivated strawberry, Fragaria ×ananassa, is a recently domesticated fruit species of economic interest worldwide. As such, there is significant interest in continuous varietal improvement. Genomics-assisted improvement, including the use of DNA markers and genomic selection have facilitated significant improvements of numerous key traits during strawberry breeding. CRISPR/Cas-mediated genome editing allows targeted mutations and precision nucleotide substitutions in the target genome, revolutionizing functional genomics and crop improvement. Genome editing is beginning to gain traction in the more challenging polyploid crops, including allo-octoploid strawberry. The release of high-quality reference genomes and comprehensive subgenome-specific genotyping and gene expression profiling data in octoploid strawberry will lead to a surge in trait discovery and modification by using CRISPR/Cas. Genome editing has already been successfully applied for modification of several strawberry genes, including anthocyanin content, fruit firmness and tolerance to post-harvest disease. However, reports on many other important breeding characteristics associated with fruit quality and production are still lacking, indicating a need for streamlined genome editing approaches and tools in Fragaria ×ananassa. In this review, we present an overview of the latest advancements in knowledge and breeding efforts involving CRISPR/Cas genome editing for the enhancement of strawberry varieties. Furthermore, we explore potential applications of this technology for improving other Rosaceous plant species.

Recent Advances in Tomato Gene Editing

The use of gene-editing tools, such as zinc finger nucleases, TALEN, and CRISPR/Cas, allows for the modification of physiological, morphological, and other characteristics in a wide range of crops to mitigate the negative effects of stress caused by anthropogenic climate change or biotic stresses. Importantly, these tools have the potential to improve crop resilience and increase yields in response to challenging environmental conditions. This review provides an overview of gene-editing techniques used in plants, focusing on the cultivated tomatoes. Several dozen genes that have been successfully edited with the CRISPR/Cas system were selected for inclusion to illustrate the possibilities of this technology in improving fruit yield and quality, tolerance to pathogens, or responses to drought and soil salinity, among other factors. Examples are also given of how the domestication of wild species can be accelerated using CRISPR/Cas to generate new crops that are better adapted to the new climatic situation or suited to use in indoor agriculture.

Efficient regeneration of protoplasts from Solanum lycopersicum cultivar Micro-Tom

Protoplast regeneration has become a key platform for genetic and genome engineering. However, we lack reliable and reproducible methods for efficient protoplast regeneration for tomato (Solanum lycopersicum) cultivars. Here, we optimized cell and tissue culture methods for protoplast isolation, microcallus proliferation, shoot regeneration, and plantlet establishment of the tomato cultivar Micro-Tom. A thin layer of alginate was applied to protoplasts isolated from third to fourth true leaves and cultured at an optimal density of 1 × 105 protoplasts/ml. We determined the optimal culture media for protoplast proliferation, callus formation, de novo shoot regeneration, and root regeneration. Regenerated plantlets exhibited morphologically normal growth and sexual reproduction. The entire regeneration process, from protoplasts to flowering plants, was accomplished within 5 months. The optimized protoplast regeneration platform enables biotechnological applications, such as genome engineering, as well as basic research on plant regeneration in Solanaceae species.

CRISPR/Cas9-mediated knockout of NYC1 gene enhances chlorophyll retention and reduces tillering in Zoysia matrella (L.) Merrill

KEY MESSAGE

Genome editing by CRISPR/Cas9 can be applied to Z. matrella 'Wakaba', and knockout mutants of ZmNYC1 gene exhibited stay-green phenotype and reduced tillering. Zoysia matrella is a widely used C4 warm-season turfgrass for landscaping, golf courses, and sports fields. Here, we used the CRISPR/Cas9 system to target the Non-Yellow Coloring1 (ZmNYC1) gene in the highly heterozygous allotetraploid Z. matrella 'Wakaba', aiming to generate a novel stay-green variety. Of 441 Agrobacterium-infected calli, 22 (5.0%) were transformed, and 14 of these (63.6%) showed targeted mutations through cleaved amplified polymorphic sequences analysis. Sequencing analysis revealed mutations mostly consisting of 1 or 2 bp indels, occurring 2 to 4 bp upstream of the PAM sequence. Regenerated plants exhibited five ZmNYC1 target locus genotypes, including homozygous mutants with a complete knockout of all four alleles in the T0 generation. Under dark treatment, ZmNYC1-mutated plants displayed suppressed chlorophyll b (Chl b) degradation, leading to higher chlorophyll content and Chl b, with a lower chlorophyll a/chlorophyll b ratio compared to the wild type (WT). However, the ZmNYC1 mutation also inhibited plant growth in homozygous mutant genotypes, exhibiting reduced tillering compared to WT. Additionally, during winter simulation, mutant with a complete knockout retained greenness longer than the WT. This is the first successful use of CRISPR/Cas9 genome editing in zoysiagrass. The mutants of the ZmNYC1 gene would serve as valuable breeding material for developing improved zoysiagrass varieties that can maintain their green color for longer periods, even during winter dormancy.

An efficient protoplast-based genome editing protocol for Vitis species

CRISPR-Cas technologies allow for precise modifications in plant genomes and promise to revolutionize agriculture. These technologies depend on the delivery of editing components into plant cells and the regeneration of fully edited plants. In vegetatively propagated plants, such as grape, protoplast culture provides one of the best avenues for producing non-chimeric and transgene-free genome-edited plants. However, poor regeneration of plants from protoplasts has hindered their implementation for genome editing. Here, we report an efficient protocol for regenerating plants from protoplasts from multiple grape varieties. By encapsulating the protoplasts in calcium alginate beads and co-culturing them with feeder cultures, the protoplasts divide to form callus colonies that regenerate into embryos and ultimately plants. This protocol worked successfully in wine and table grape (Vitis vinifera) varieties, as well as grape rootstocks and the grapevine wild relative Vitis arizonica. Moreover, by transfecting protoplasts with CRISPR-plasmid or ribonucleoprotein (RNP) complexes, we regenerated albino plants with edits in VvPHYTOENE DESATURASE gene in three varieties and in V. arizonica. The results reveal the potential of this platform to facilitate genome editing in Vitis species.

Genome editing for healthy crops: traits, tools and impacts

Crop cultivars in commercial use have often been selected because they show high levels of resistance to pathogens. However, widespread cultivation of these crops for many years in the environments favorable to a pathogen requires durable forms of resistance to maintain "healthy crops". Breeding of new varieties tolerant/resistant to biotic stresses by incorporating genetic components related to durable resistance, developing new breeding methods and new active molecules, and improving the Integrated Pest Management strategies have been of great value, but their effectiveness is being challenged by the newly emerging diseases and the rapid change of pathogens due to climatic changes. Genome editing has provided new tools and methods to characterize defense-related genes in crops and improve crop resilience to disease pathogens providing improved food security and future sustainable agricultural systems. In this review, we discuss the principal traits, tools and impacts of utilizing genome editing techniques for achieving of durable resilience and a "healthy plants" concept.

CRISPR/Cas-mediated plant genome editing: outstanding challenges a decade after implementation

The discovery of the CRISPR/Cas genome-editing system has revolutionized our understanding of the plant genome. CRISPR/Cas has been used for over a decade to modify plant genomes for the study of specific genes and biosynthetic pathways as well as to speed up breeding in many plant species, including both model and non-model crops. Although the CRISPR/Cas system is very efficient for genome editing, many bottlenecks and challenges slow down further improvement and applications. In this review we discuss the challenges that can occur during tissue culture, transformation, regeneration, and mutant detection. We also review the opportunities provided by new CRISPR platforms and specific applications related to gene regulation, abiotic and biotic stress response improvement, and de novo domestication of plants.

Establishment of targeted mutagenesis in soybean protoplasts using CRISPR/Cas9 RNP delivery via electro-transfection

The soybean (Glycine max L.) is an important crop with high agronomic value. The improvement of agronomic traits through gene editing techniques has broad application prospects in soybean. The polyethylene glycol (PEG)-mediated cell transfection has been successfully used to deliver the CRISPR/Cas9-based ribonucleoprotein (RNP) into soybean protoplasts. However, several downstream analyses or further cell regeneration protocols might be hampered by PEG contamination within the samples. Here in this study, we attempted to transfect CRISPR/Cas9 RNPs into trifoliate leaf-derived soybean protoplasts using Neon electroporation to overcome the need for PEG transfection for the first time. We investigated different electroporation parameters including pulsing voltage (V), strength and duration of pulses regarding protoplast morphology, viability, and delivery of CRISPR/Cas9. Electroporation at various pulsing voltages with 3 pulses and 10 ms per pulse was found optimal for protoplast electro-transfection. Following electro-transfection at various pulsing voltages (500 V, 700 V, 1,000 V, and 1,300 V), intact protoplasts were observed at all treatments. However, the relative frequency of cell viability and initial cell divisions decreased with increasing voltages. Confocal laser scanning microscopy (CLSM) confirmed that the green fluorescent protein (GFP)-tagged Cas9 was successfully internalized into the protoplasts. Targeted deep sequencing results revealed that on-target insertion/deletion (InDel) frequencies were increased with increasing voltages in protoplasts electro-transfected with CRISPR/Cas9 RNPs targeting constitutive pathogen response 5 (CPR5). InDel patterns ranged from +1 bp to -6 bp at three different target sites in CPR5 locus with frequencies ranging from 3.8% to 8.1% following electro-transfection at 1,300 V and 2.1% to 3.8% for 700 V and 1,000 V, respectively. Taken together, our results demonstrate that the CRISPR/Cas9 RNP system can be delivered into soybean protoplasts by the Neon electroporation system for efficient and effective gene editing. The electro-transfection system developed in this study would also further facilitate and serve as an alternative delivery method for DNA-free genome editing of soybean and other related species for genetic screens and potential trait improvement.

Efficient Agrobacterium-mediated transformation and genome editing of Fagopyrum tataricum

Fagopyrum tataricum (L.) Gaertn. is an exceptional crop known for its remarkable health benefits, high levels of beneficial polyphenols and gluten-free properties, making it highly sought-after as a functional food. Its self-fertilisation capability and adaptability to challenging environments further contribute to its potential as a sustainable agricultural option. To harness its unique traits, genetic transformation in F. tataricum is crucial. In this study, we optimised the Agrobacterium-mediated transformation protocol for F. tataricum callus, resulting in a transformation rate of regenerated plants of approximately 20%. The protocol's effectiveness was confirmed through successful GUS staining, GFP expression, and the generation of albino plants via FtPDS gene inactivation. These results validate the feasibility of genetic manipulation and highlight the potential for trait enhancement in F. tataricum.

The sonication-assisted whisker method enables CRISPR-Cas9 ribonucleoprotein delivery to induce genome editing in rice

CRISPR/Cas9-based genome editing represents an unprecedented potential for plant breeding. Unlike animal cells, plant cells contain a rigid cell wall, genome editing tool delivery into plant cells is thus challenging. In particular, the delivery of the Cas9-gRNA ribonucleoprotein (RNP) into plant cells is desired since the transgene insertion into the genome should be avoided for industrial applications in plants. In this study, we present a novel RNP delivery approach in rice. We applied the sonication-assisted whisker method, conventionally developed for DNA delivery in plants, for RNP delivery in rice. Combined with marker gene delivery, we successfully isolated OsLCYβ genome-edited lines generated by RNPs. The calli and regenerated shoot of the OsLCYβ mutant showed abnormal carotenoid accumulation. In addition, we also detected, although at a low frequency, genome editing events in rice calli cells by RNP delivery using the sonication-assisted whisker method without any additional. Therefore, the sonication-assisted whisker method could be an attractive way to create RNP-based genome-edited lines in plants.

Strategies for delivery of CRISPR/Cas-mediated genome editing to obtain edited plants directly without transgene integration

Increased understanding of plant genetics and the development of powerful and easier-to-use gene editing tools over the past century have revolutionized humankind's ability to deliver precise genotypes in crops. Plant transformation techniques are well developed for making transgenic varieties in certain crops and model organisms, yet reagent delivery and plant regeneration remain key bottlenecks to applying the technology of gene editing to most crops. Typical plant transformation protocols to produce transgenic, genetically modified (GM) varieties rely on transgenes, chemical selection, and tissue culture. Typical protocols to make gene edited (GE) varieties also use transgenes, even though these may be undesirable in the final crop product. In some crops, the transgenes are routinely segregated away during meiosis by performing crosses, and thus only a minor concern. In other crops, particularly those propagated vegetatively, complex hybrids, or crops with long generation times, such crosses are impractical or impossible. This review highlights diverse strategies to deliver CRISPR/Cas gene editing reagents to regenerable plant cells and to recover edited plants without unwanted integration of transgenes. Some examples include delivering DNA-free gene editing reagents such as ribonucleoproteins or mRNA, relying on reagent expression from non-integrated DNA, using novel delivery mechanisms such as viruses or nanoparticles, using unconventional selection methods to avoid integration of transgenes, and/or avoiding tissue culture altogether. These methods are advancing rapidly and already enabling crop scientists to make use of the precision of CRISPR gene editing tools.

Multi-allelic gene editing in an apomictic, tetraploid turf and forage grass (Paspalum notatum Flüggé) using CRISPR/Cas9

Polyploidy is common among grasses (Poaceae) and poses challenges for conventional breeding. Genome editing technology circumvents crossing and selfing, enabling targeted modifications to multiple gene copies in a single generation while maintaining the heterozygous context of many polyploid genomes. Bahiagrass (Paspalum notatum Flüggé; 2n=4x=40) is an apomictic, tetraploid C4 species that is widely grown in the southeastern United States as forage in beef cattle production and utility turf. The chlorophyll biosynthesis gene magnesium chelatase (MgCh) was selected as a rapid readout target for establishing genome editing in tetraploid bahiagrass. Vectors containing sgRNAs, Cas9 and nptII were delivered to callus cultures by biolistics. Edited plants were characterized through PCR-based assays and DNA sequencing, and mutagenesis frequencies as high as 99% of Illumina reads were observed. Sequencing of wild type (WT) bahiagrass revealed a high level of sequence variation in MgCh likely due to the presence of at least two copies with possibly eight different alleles, including pseudogenes. MgCh mutants exhibited visible chlorophyll depletion with up to 82% reductions in leaf greenness. Two lines displayed progression of editing over time which was linked to somatic editing. Apomictic progeny of a chimeric MgCh editing event were obtained and allowed identification of uniformly edited progeny plants among a range of chlorophyll depletion phenotypes. Sanger sequencing of a highly edited mutant revealed elevated frequency of a WT allele, probably due to frequent homology-directed repair (HDR). To our knowledge these experiments comprise the first report of genome editing applied in perennial, warm-season turf or forage grasses. This technology will accelerate bahiagrass cultivar development.

Establishment of an efficient transformation system and its application in regulatory mechanism analysis of biological macromolecules in tea plants

Tea (Camellia sinensis), one of the most important beverage crops originated from China and is now cultivated worldwide, provides numerous secondary metabolites that account for its health benefits and rich flavor. However, the lack of an efficient and reliable genetic transformation system has seriously hindered the gene function investigation and precise breeding of C. sinensis. In this study, we established a highly efficient, labor-saving, and cost-effective Agrobacterium rhizogenes-mediated hairy roots genetic transformation system for C. sinensis, which can be used for gene overexpression and genome editing. The established transformation system was simple to operate, bypassing tissue culture and antibiotic screening, and only took two months to complete. We used this system to conduct function analysis of transcription factor CsMYB73 and found that CsMYB73 negatively regulates L-theanine synthesis in tea plant. Additionally, callus formation was successfully induced using transgenic roots, and the transgenic callus exhibited normal chlorophyll production, enabling the study of the corresponding biological functions. Furthermore, this genetic transformation system was effective for multiple C. sinensis varieties and other woody plant species. By overcoming technical obstacles such as low efficiency, long experimental periods, and high costs, this genetic transformation will be a valuable tool for routine gene investigation and precise breeding in tea plants.

Overcoming roadblocks for in vitro nurseries in plants: induction of meiosis

Efforts to increase genetic gains in breeding programs of flowering plants depend on making genetic crosses. Time to flowering, which can take months to decades depending on the species, can be a limiting factor in such breeding programs. It has been proposed that the rate of genetic gain can be increased by reducing the time between generations by circumventing flowering through the in vitro induction of meiosis. In this review, we assess technologies and approaches that may offer a path towards meiosis induction, the largest current bottleneck for in vitro plant breeding. Studies in non-plant, eukaryotic organisms indicate that the in vitro switch from mitotic cell division to meiosis is inefficient and occurs at very low rates. Yet, this has been achieved with mammalian cells by the manipulation of a limited number of genes. Therefore, to experimentally identify factors that switch mitosis to meiosis in plants, it is necessary to develop a high-throughput system to evaluate a large number of candidate genes and treatments, each using large numbers of cells, few of which may gain the ability to induce meiosis.

Site-specific gene knock-in and bacterial phytase gene expression in Chlamydomonas reinhardtii via Cas9 RNP-mediated HDR

In the present study, we applied the HDR (homology-directed DNA repair) CRISPR-Cas9-mediated knock-in system to accurately insert an optimized foreign bacterial phytase gene at a specific site of the nitrate reductase (NR) gene (exon 2) to achieve homologous recombination with the stability of the transgene and reduce insertion site effects or gene silencing. To this end, we successfully knocked-in the targeted NR gene of Chlamydomonas reinhardtii using the bacterial phytase gene cassette through direct delivery of the CRISPR/Cas9 system as the ribonucleoprotein (RNP) complex consisting of Cas9 protein and the specific single guide RNAs (sgRNAs). The NR insertion site editing was confirmed by PCR and sequencing of the transgene positive clones. Moreover, 24 clones with correct editing were obtained, where the phytase gene cassette was located in exon 2 of the NR gene, and the editing efficiency was determined to be 14.81%. Additionally, site-specific gene expression was analyzed and confirmed using RT-qPCR. Cultivation of the positive knocked-in colonies on the selective media during 10 generations indicated the stability of the correct editing without gene silencing or negative insertion site effects. Our results demonstrated that CRISPR-Cas9-mediated knock-in could be applied for nuclear expression of the heterologous gene of interest, and also confirmed its efficacy as an effective tool for site-specific gene knock-in, avoiding nuclear positional effects and gene silencing in C. reinhardtii. These findings could also provide a new perspective on the advantageous application of RNP-CRISPR/Cas9 gene-editing to accelerate the commercial production of complex recombinant proteins in the food-grade organism "C. reinhardtii".

Recent Trends and Advancements in CRISPR-Based Tools for Enhancing Resistance against Plant Pathogens

Targeted genome editing technologies are becoming the most important and widely used genetic tools in studies of phytopathology. The "clustered regularly interspaced short palindromic repeats (CRISPR)" and its accompanying proteins (Cas) have been first identified as a natural system associated with the adaptive immunity of prokaryotes that have been successfully used in various genome-editing techniques because of its flexibility, simplicity, and high efficiency in recent years. In this review, we have provided a general idea about different CRISPR/Cas systems and their uses in phytopathology. This review focuses on the benefits of knock-down technologies for targeting important genes involved in the susceptibility and gaining resistance against viral, bacterial, and fungal pathogens by targeting the negative regulators of defense pathways of hosts in crop plants via different CRISPR/Cas systems. Moreover, the possible strategies to employ CRISPR/Cas system for improving pathogen resistance in plants and studying plant-pathogen interactions have been discussed.

Biotechnological interventions in reducing losses of tropical fruits and vegetables

Tropical fruits and vegetables are predominantly cultivated in warm climate zones, resulting in cultivar diversity in terms of structure, features, and physiology. These constitute a variety of bioactive ingredients such as vitamins, minerals, phenolic acids, anthocyanins, flavonoids, fatty acids, fiber, and their distinctive appearances attract customers across the world. The global production of fruit and vegetables has been attained a tremendous increase for the last few decades. However, huge losses at pre- and postharvest levels are major constraints in their judicious use. Traditional breeding strategies were used to minimize these losses, but their functionality is limited due to their time and labor intensiveness. Recent biotechnological, computational, and multiomics approaches not only address the losses concern but also aid in boosting crop productivity and nutritional values. This article emphasizes molecular tools that have been used to reduce losses of tropical fruits and vegetables at pre- and postharvest levels.

CRISPR technology: A decade of genome editing is only the beginning

The advent of clustered regularly interspaced short palindromic repeat (CRISPR) genome editing, coupled with advances in computing and imaging capabilities, has initiated a new era in which genetic diseases and individual disease susceptibilities are both predictable and actionable. Likewise, genes responsible for plant traits can be identified and altered quickly, transforming the pace of agricultural research and plant breeding. In this Review, we discuss the current state of CRISPR-mediated genetic manipulation in human cells, animals, and plants along with relevant successes and challenges and present a roadmap for the future of this technology.

Optimization of highly efficient exogenous-DNA-free Cas9-ribonucleoprotein mediated gene editing in disease susceptibility loci in wheat (Triticum aestivum L.)

The advancement of precision engineering for crop trait improvement is important in the face of rapid population growth, climate change, and disease. To this end, targeted double-stranded break technology using RNA-guided Cas9 has been adopted widely for genome editing in plants. Agrobacterium or particle bombardment-based delivery of plasmids encoding Cas9 and guide RNA (gRNA) is common, but requires optimization of expression and often results in random integration of plasmid DNA into the plant genome. Recent advances have described gene editing by the delivery of Cas9 and gRNA as pre-assembled ribonucleoproteins (RNPs) into various plant tissues, but with moderate efficiency in resulting regenerated plants. In this report we describe significant improvements to Cas9-RNP mediated gene editing in wheat. We demonstrate that Cas9-RNP assays in protoplasts are a fast and effective tool for rational selection of optimal gRNAs for gene editing in regenerable immature embryos (IEs), and that high temperature treatment enhances gene editing rates in both tissue types. We also show that Cas9-mediated editing persists for at least 14 days in gold particle bombarded wheat IEs. The regenerated edited wheat plants in this work are recovered at high rates in the absence of exogenous DNA and selection. With this method, we produce knockouts of a set of three homoeologous genes and two pathogenic effector susceptibility genes, engineering insensitivity to corresponding necrotrophic effectors produced by Parastagonospora nodorum. The establishment of highly efficient, exogenous DNA-free gene editing technology holds promise for accelerated trait diversity production in an expansive array of crops.

Automated, High-Throughput Protoplast Transfection for Gene Editing and Transgene Expression Studies

In an era of cost-efficient gene synthesis and high-throughput construct assembly, the onus of scientific experimentation is on the rate of in vivo testing for the identification of top performing candidates or designs. Assay platforms that are relevant to the species of interest and in the tissue of choice are highly desirable. A protoplast isolation and transfection method that is compatible with a large repertoire of species and tissues would be the platform of choice. A necessary aspect of this high-throughput screening approach is the need to handle many delicate protoplast samples at the same time, which is a bottleneck for manual operation. Such bottlenecks can be mitigated with the use of automated liquid handlers for the execution of protoplast transfection steps. The method described within this chapter utilizes a 96-well head for simultaneous, high-throughput initiation of transfection. While initially developed and optimized for use with etiolated maize leaf protoplasts, the automated protocol has also been demonstrated to be compatible with other established protoplast systems, such as soybean immature embryo derived protoplast, similarly described within. This chapter also includes instructions for a sample randomization design to reduce the impact of edge effects, which might be present when microplates are used for fluorescence readout following transfection. We also describe a streamlined, expedient, and cost-effective protocol for determining gene editing efficiencies using the T7E1 endonuclease cleavage assay with a publicly available image analysis tool.

Application of CRISPR/Cas9-mediated gene editing for abiotic stress management in crop plants

Abiotic stresses, including drought, salinity, cold, heat, and heavy metals, extensively reducing global agricultural production. Traditional breeding approaches and transgenic technology have been widely used to mitigate the risks of these environmental stresses. The discovery of engineered nucleases as genetic scissors to carry out precise manipulation in crop stress-responsive genes and associated molecular network has paved the way for sustainable management of abiotic stress conditions. In this context, the clustered regularly interspaced short palindromic repeat-Cas (CRISPR/Cas)-based gene-editing tool has revolutionized due to its simplicity, accessibility, adaptability, flexibility, and wide applicability. This system has great potential to build up crop varieties with enhanced tolerance against abiotic stresses. In this review, we summarize the latest findings on understanding the mechanism of abiotic stress response in plants and the application of CRISPR/Cas-mediated gene-editing system towards enhanced tolerance to a multitude of stresses including drought, salinity, cold, heat, and heavy metals. We provide mechanistic insights on the CRISPR/Cas9-based genome editing technology. We also discuss applications of evolving genome editing techniques such as prime editing and base editing, mutant library production, transgene free and multiplexing to rapidly deliver modern crop cultivars adapted to abiotic stress conditions.

CRISPR/Cas9 Technology for Potato Functional Genomics and Breeding

Cultivated potato (Solanum tuberosum L.) is one of the most important staple food crops worldwide. Its tetraploid and highly heterozygous nature poses a great challenge to its basic research and trait improvement through traditional mutagenesis and/or crossbreeding. The establishment of the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (Cas9) as a gene editing tool has allowed the alteration of specific gene sequences and their concomitant gene function, providing powerful technology for potato gene functional analysis and improvement of elite cultivars. This technology relies on a short RNA molecule called single guide RNA (sgRNA) that directs the Cas9 nuclease to induce a site-specific double-stranded break (DSB). Further, repair of the DSB by the error-prone non-homologous end joining (NHEJ) mechanism leads to the introduction of targeted mutations, which can be used to produce the loss of function of specific gene(s). In this chapter, we describe experimental procedures to apply the CRISPR/Cas9 technology for potato genome editing. First, we provide strategies for target selection and sgRNA design and describe a Golden Gate-based cloning system to obtain a sgRNA/Cas9-encoding binary vector. We also describe an optimized protocol for ribonucleoprotein (RNP) complex assembly. The binary vector can be used for both Agrobacterium-mediated transformation and transient expression in potato protoplasts, while the RNP complexes are intended to obtain edited potato lines through protoplast transfection and plant regeneration. Finally, we describe procedures to identify the gene-edited potato lines. The methods described here are suitable for potato gene functional analysis and breeding.

Genome Editing in Chlamydomonas reinhardtii Using Cas9-gRNA Ribonucleoprotein Complex: A Step-by-Step Guide

Genome editing technologies have provided opportunities to manipulate literally any genomic location, opening new avenues for reverse genetics-based improvements. Among them, CRISPR/Cas9 is the most versatile tool for genome editing applications in prokaryotes and eukaryotes. Here, we provide a guide to successfully carry out high-efficiency genome editing in Chlamydomonas reinhardtii using preassembled CRISPR/Cas9-gRNA ribonucleoprotein (RNP) complexes.

Ribonucleoprotein (RNP)-Mediated Allele Replacement in Barley (Hordeum vulgare L.) Leaves

Varietal differences within a species with agronomic importance are often based on minor changes in the genomic sequence. For example, fungus-resistant and fungus-susceptible wheat varieties may vary in only one amino acid. The situation is similar with the reporter genes Gfp and Yfp where two base pairs cause a shift in the emission spectrum from green to yellow. Methods of targeted double-strand break induction now allow this exchange precisely with the simultaneous transfer of the desired repair template. However, these changes rarely lead to a selective advantage that can be used in generating such mutant plants. The protocol presented here allows a corresponding allele replacement at the cellular level using ribonucleoprotein complexes in combination with an appropriate repair template. The efficiencies achieved are comparable to other methods with direct DNA transfer or integration of the corresponding building blocks in the host genome. They are in the range of 35 percent, considering one allele in a diploid organism as barley and using Cas9 RNP complexes.

Genome editing for improving nutritional quality, post-harvest shelf life and stress tolerance of fruits, vegetables, and ornamentals

Agricultural production relies on horticultural crops, including vegetables, fruits, and ornamental plants, which sustain human life. With an alarming increase in human population and the consequential need for more food, it has become necessary for increased production to maintain food security. Conventional breeding has subsidized the development of improved verities but to enhance crop production, new breeding techniques need to be acquired. CRISPR-Cas9 system is a unique and powerful genome manipulation tool that can change the DNA in a precise way. Based on the bacterial adaptive immune system, this technique uses an endonuclease that creates double-stranded breaks (DSBs) at the target loci under the guidance of a single guide RNA. These DSBs can be repaired by a cellular repair mechanism that installs small insertion and deletion (indels) at the cut sites. When equated to alternate editing tools like ZFN, TALENs, and meganucleases, CRISPR- The cas-based editing tool has quickly gained fast-forward for its simplicity, ease to use, and low off-target effect. In numerous horticultural and industrial crops, the CRISPR technology has been successfully used to enhance stress tolerance, self-life, nutritional improvements, flavor, and metabolites. The CRISPR-based tool is the most appropriate one with the prospective goal of generating non-transgenic yields and avoiding the regulatory hurdles to release the modified crops into the market. Although several challenges for editing horticultural, industrial, and ornamental crops remain, this new novel nuclease, with its crop-specific application, makes it a dynamic tool for crop improvement.

CRISPR/Cas9 genome-editing applied to MdPGT1 in apple results in reduced foliar phloridzin without impacting plant growth

Phloridzin is the most abundant polyphenolic compound in apple (Malus × domestica Borkh.), which results from the action of a key phloretin-specific UDP-2'-O-glucosyltransferase (MdPGT1). Here, we simultaneously assessed the effects of targeting MdPGT1 by conventional transgenesis and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated genome editing. To this end, we conducted transcriptomic and metabolic analyses of MdPGT1 RNA interference knockdown and genome-edited lines. Knockdown lines exhibited characteristic impairment of plant growth and leaf morphology, whereas genome-edited lines exhibited normal growth despite reduced foliar phloridzin. RNA-sequencing analysis identified a common core of regulated genes, involved in phenylpropanoid and flavonoid pathways. However, we identified genes and processes differentially modulated in stunted and genome-edited lines, including key transcription factors and genes involved in phytohormone signalling. Therefore, we conducted a phytohormone profiling to obtain insight into their role in the phenotypes observed. We found that salicylic and jasmonic acid were increased in dwarf lines, whereas auxin and ABA showed no correlation with the growth phenotype. Furthermore, bioactive brassinosteroids were commonly up-regulated, whereas gibberellin GA₄ was distinctively altered, showing a sharp decrease in RNA interference knockdown lines. Expression analysis by reverse transcriptase-quantitative polymerase chain reaction expression analysis further confirmed transcriptional regulation of key factors involved in brassinosteroid and gibberellin interaction. These findings suggest that a differential modulation of phytohormones may be involved in the contrasting effects on growth following phloridzin reduction. The present study also illustrates how CRISPR/Cas9 genome editing can be applied to dissect the contribution of genes involved in phloridzin biosynthesis in apple.

Ribonucleoprotein (RNP)-Mediated Targeted Mutagenesis in Barley (Hordeum vulgare L.)

The crop species barley is a genetic model for the small grain temperate cereals. Thanks to the availability of whole genome sequence and the development of customizable endonucleases, site-directed genome modification has recently revolutionized genetic engineering. Several platforms have been established in plants, with the most flexible one offered by the clustered regularly interspaced short palindromic repeats (CRISPR) technology. In this protocol, commercially available synthetic guide RNAs (gRNAs), Cas enzymes, or custom-generated reagents are used for targeted mutagenesis in barley. The protocol has been successfully used with immature embryo explants to generate site-specific mutations in regenerants. As the double-strand break-inducing reagents are customizable and can be efficiently delivered, pre-assembled ribonucleoprotein (RNP) complexes allow efficient generation of genome-modified plants.

Metabolic pathway genes for editing to enhance multiple disease resistance in plants

Diseases are one of the major constraints in commercial crop production. Genetic diversity in varieties is the best option to manage diseases. Molecular marker-assisted breeding has produced hundreds of varieties with good yields, but the resistance level is not satisfactory. With the advent of whole genome sequencing, genome editing is emerging as an excellent option to improve the inadequate traits in these varieties. Plants produce thousands of antimicrobial secondary metabolites, which as polymers and conjugates are deposited to reinforce the secondary cell walls to contain the pathogen to an initial infection area. The resistance metabolites or the structures produced from them by plants are either constitutive (CR) or induced (IR), following pathogen invasion. The production of each resistance metabolite is controlled by a network of biosynthetic R genes, which are regulated by a hierarchy of R genes. A commercial variety also has most of these R genes, as in resistant, but a few may be mutated (SNPs/InDels). A few mutated genes, in one or more metabolic pathways, depending on the host-pathogen interaction, can be edited, and stacked to increase resistance metabolites or structures produced by them, to achieve required levels of multiple pathogen resistance under field conditions.

Plant biomacromolecule delivery methods in the 21st century

The 21st century witnessed a boom in plant genomics and gene characterization studies through RNA interference and site-directed mutagenesis. Specifically, the last 15 years marked a rapid increase in discovering and implementing different genome editing techniques. Methods to deliver gene editing reagents have also attempted to keep pace with the discovery and implementation of gene editing tools in plants. As a result, various transient/stable, quick/lengthy, expensive (requiring specialized equipment)/inexpensive, and versatile/specific (species, developmental stage, or tissue) methods were developed. A brief account of these methods with emphasis on recent developments is provided in this review article. Additionally, the strengths and limitations of each method are listed to allow the reader to select the most appropriate method for their specific studies. Finally, a perspective for future developments and needs in this research area is presented.

CRISPR for accelerating genetic gains in under-utilized crops of the drylands: Progress and prospects

Technologies and innovations are critical for addressing the future food system needs where genetic resources are an essential component of the change process. Advanced breeding tools like "genome editing" are vital for modernizing crop breeding to provide game-changing solutions to some of the "must needed" traits in agriculture. CRISPR/Cas-based tools have been rapidly repurposed for editing applications based on their improved efficiency, specificity and reduced off-target effects. Additionally, precise gene-editing tools such as base editing, prime editing, and multiplexing provide precision in stacking of multiple traits in an elite variety, and facilitating specific and targeted crop improvement. This has helped in advancing research and delivery of products in a short time span, thereby enhancing the rate of genetic gains. A special focus has been on food security in the drylands through crops including millets, teff, fonio, quinoa, Bambara groundnut, pigeonpea and cassava. While these crops contribute significantly to the agricultural economy and resilience of the dryland, improvement of several traits including increased stress tolerance, nutritional value, and yields are urgently required. Although CRISPR has potential to deliver disruptive innovations, prioritization of traits should consider breeding product profiles and market segments for designing and accelerating delivery of locally adapted and preferred crop varieties for the drylands. In this context, the scope of regulatory environment has been stated, implying the dire impacts of unreasonable scrutiny of genome-edited plants on the evolution and progress of much-needed technological advances.

Potato virus X-delivered CRISPR activation programs lead to strong endogenous gene induction and transient metabolic reprogramming in Nicotiana benthamiana

Programmable transcriptional regulators based on CRISPR architecture are promising tools for the induction of plant gene expression. In plants, CRISPR gene activation is effective with respect to modulating development processes, such as the flowering time or customizing biochemical composition. The most widely used method for delivering CRISPR components into the plant is Agrobacterium tumefaciens-mediated genetic transformation, either transient or stable. However, as a result of their versatility and their ability to move, virus-derived systems have emerged as an interesting alternative for supplying the CRISPR components to the plant, in particular guide RNA (gRNA), which represents the variable component in CRISPR strategies. In the present study, we describe a Potato virus X-derived vector that, upon agroinfection in Nicotiana benthamiana, serves as a vehicle for delivery of gRNAs, producing highly specific virus-induced gene activation. The system works in combination with a N. benthamiana transgenic line carrying the remaining complementary CRISPR gene activation components, specifically the dCasEV2.1 cassette, which has been shown previously to mediate strong programmable transcriptional activation in plants. Using an easily scalable, non-invasive spraying method, we show that gRNA-mediated activation programs move locally and systemically, generating a strong activation response in different target genes. Furthermore, by activating three different endogenous MYB transcription factors, we demonstrate that this Potato virus X-based virus-induced gene reprogramming strategy results in program-specific metabolic fingerprints in N. benthamiana leaves characterized by distinctive phenylpropanoid-enriched metabolite profiles.

Optimization of Prime Editing in Rice, Peanut, Chickpea, and Cowpea Protoplasts by Restoration of GFP Activity

Precise editing of the plant genome has long been desired for functional genomic research and crop breeding. Prime editing is a newly developed precise editing technology based on CRISPR-Cas9, which uses an engineered reverse transcriptase (RT), a catalytically impaired Cas9 endonuclease (nCas9), and a prime editing guide RNA (pegRNA). In addition, prime editing has a wider range of editing types than base editing and can produce nearly all types of edits. Although prime editing was first established in human cells, it has recently been applied to plants. As a relatively new technique, optimization will be needed to increase the editing efficiency in different crops. In this study, we successfully edited a mutant GFP in rice, peanut, chickpea, and cowpea protoplasts. In rice, up to 16 times higher editing efficiency was achieved with a dual pegRNA than the single pegRNA containing vectors. Edited-mutant GFP protoplasts have also been obtained in peanut, chickpea, and cowpea after transformation with the dual pegRNA vectors, albeit with much lower editing efficiency than in rice, ranging from 0.2% to 0.5%. These initial results promise to expedite the application of prime editing in legume breeding programs to accelerate crop improvement.

CRISPR/Cas- and Topical RNAi-Based Technologies for Crop Management and Improvement: Reviewing the Risk Assessment and Challenges Towards a More Sustainable Agriculture

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated gene (Cas) system and RNA interference (RNAi)-based non-transgenic approaches are powerful technologies capable of revolutionizing plant research and breeding. In recent years, the use of these modern technologies has been explored in various sectors of agriculture, introducing or improving important agronomic traits in plant crops, such as increased yield, nutritional quality, abiotic- and, mostly, biotic-stress resistance. However, the limitations of each technique, public perception, and regulatory aspects are hindering its wide adoption for the development of new crop varieties or products. In an attempt to reverse these mishaps, scientists have been researching alternatives to increase the specificity, uptake, and stability of the CRISPR and RNAi system components in the target organism, as well as to reduce the chance of toxicity in nontarget organisms to minimize environmental risk, health problems, and regulatory issues. In this review, we discuss several aspects related to risk assessment, toxicity, and advances in the use of CRISPR/Cas and topical RNAi-based technologies in crop management and breeding. The present study also highlights the advantages and possible drawbacks of each technology, provides a brief overview of how to circumvent the off-target occurrence, the strategies to increase on-target specificity, the harm/benefits of association with nanotechnology, the public perception of the available techniques, worldwide regulatory frameworks regarding topical RNAi and CRISPR technologies, and, lastly, presents successful case studies of biotechnological solutions derived from both technologies, raising potential challenges to reach the market and being social and environmentally safe.

Green Revolution to Gene Revolution: Technological Advances in Agriculture to Feed the World

Technological applications in agriculture have evolved substantially to increase crop yields and quality to meet global food demand. Conventional techniques, such as seed saving, selective breeding, and mutation breeding (variation breeding), have dramatically increased crop production, especially during the ‘Green Revolution’ in the 1990s. However, newer issues, such as limited arable lands, climate change, and ever-increasing food demand, pose challenges to agricultural production and threaten food security. In the following ‘Gene Revolution’ era, rapid innovations in the biotechnology field provide alternative strategies to further improve crop yield, quality, and resilience towards biotic and abiotic stresses. These innovations include the introduction of DNA recombinant technology and applications of genome editing techniques, such as transcription activator-like effector (TALEN), zinc-finger nucleases (ZFN), and clustered regularly interspaced short palindromic repeats/CRISPR associated (CRISPR/Cas) systems. However, the acceptance and future of these modern tools rely on the regulatory frameworks governing their development and production in various countries. Herein, we examine the evolution of technological applications in agriculture, focusing on the motivations for their introduction, technical challenges, possible benefits and concerns, and regulatory frameworks governing genetically engineered product development and production.

Tissue Culture and Somatic Embryogenesis in Warm-Season Grasses—Current Status and Its Applications: A Review

Warm-season grasses are C4 plants and have a high capacity for biomass productivity. These grasses are utilized in many agricultural production systems with their greatest value as feeds for livestock, bioethanol, and turf. However, many important warm-season perennial grasses multiply either by vegetative propagation or form their seeds by an asexual mode of reproduction called apomixis. Therefore, the improvement of these grasses by conventional breeding is difficult and is dependent on the availability of natural genetic variation and its manipulation through breeding and selection. Recent studies have indicated that plant tissue culture system through somatic embryogenesis complements and could further develop conventional breeding programs by micropropagation, somaclonal variation, somatic hybridization, genetic transformation, and genome editing. This review summarizes the tissue culture and somatic embryogenesis in warm-season grasses and focus on current status and above applications including the author’s progress.

Advances in gene editing without residual transgenes in plants

Transgene residuals in edited plants affect genetic analysis, pose off-target risks, and cause regulatory concerns. Several strategies have been developed to efficiently edit target genes without leaving any transgenes in plants. Some approaches directly address this issue by editing plant genomes with DNA-free reagents. On the other hand, DNA-based techniques require another step for ensuring plants are transgene-free. Fluorescent markers, pigments, and chemical treatments have all been employed as tools to distinguish transgenic plants from transgene-free plants quickly and easily. Moreover, suicide genes have been used to trigger self-elimination of transgenic plants, greatly improving the efficiency of isolating the desired transgene-free plants. Transgenes can also be excised from plant genomes using site-specific recombination, transposition or gene editing nucleases, providing a strategy for editing asexually produced plants. Finally, haploid induction coupled with gene editing may make it feasible to edit plants that are recalcitrant to transformation. Here, we evaluate the strengths and weaknesses of recently developed approaches for obtaining edited plants without transgene residuals.

Highly Efficient Genome Editing in Plant Protoplasts by Ribonucleoprotein Delivery of CRISPR-Cas12a Nucleases

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) mediated genome editing is a powerful approach for crop improvement. Traditional transformation methods based on plasmid delivery pose concerns associated with transgene integration and off-target effects. CRISPR delivered as ribonucleoproteins (RNPs) can prevent exogenous DNA integration, minimize off-target effects, and reduce cellular toxicity. Although RNP delivered CRISPR genome editing has been demonstrated in many plant species, optimization strategies that yield high editing efficiencies have not been thoroughly investigated. Using rice and citrus protoplast systems we demonstrated highly efficient genome editing using Cas12a delivered as RNPs. Four Cas12a variants, including LbCas12a, LbCas12a-E795L, AsCas12a, and AsCas12a Ultra, were investigated. Nearly 100% editing efficiency was observed for three out of four target sites by LbCas12a, LbCas12a-E795L, and AsCas12a Ultra, as measured by restriction fragment length polymorphism (RFLP) and verified by next generation sequencing of PCR amplicons. RNP delivery resulted in higher editing efficiencies than plasmid delivery at 32°C and 25°C. LbCas12a and LbCas12a-E795L demonstrated increased editing efficiencies in comparison to AsCas12a and AsCas12a Ultra, especially when used at lower RNP concentrations. In addition, we discovered that a 1:1 Cas12a:crRNA molar ratio is sufficient to achieve efficient genome editing. Nuclear localization signals (NLSs) are essential for efficient RNP-based genome editing. However, the different crRNA modifications tested did not significantly improve genome editing efficiency. Finally, we applied the Cas12a RNP system in citrus protoplasts and obtained similarly high editing efficiencies at the target site. Our study provides a comprehensive guideline for Cas12a-mediated genome editing using RNP delivery in plant cells, setting the foundation for the generation of transgene-free genome edited plants.

Advances in Delivery Mechanisms of CRISPR Gene-Editing Reagents in Plants

Gene-editing by CRISPR/Cas systems has revolutionized plant biology by serving as a functional genomics tool. It has tremendously advanced plant breeding and crop improvement by accelerating the development of improved cultivars, creating genetic variability, and aiding in domestication of wild and orphan crops. Gene-editing is a rapidly evolving field. Several advancements include development of different Cas effectors with increased target range, efficacy, and enhanced capacity for precise DNA modifications with base editing and prime editing. The existing toolbox of various CRISPR reagents facilitate gene knockouts, targeted gene insertions, precise base substitutions, and multiplexing. However, the major challenge in plant genome-editing remains the efficient delivery of these reagents into plant cells. Plants have larger and more complex genome structures compared to other living systems due to the common occurrence of polyploidy and other genome re-arrangements. Further, rigid cell walls surrounding plant cells deter the entry of any foreign biomolecules. Unfortunately, genetic transformation to deliver gene-editing reagents has been established only in a limited number of plant species. Recently, there has been significant progress in CRISPR reagents delivery in plants. This review focuses on exploring these delivery mechanisms categorized into Agrobacterium-mediated delivery and breakthroughs, particle bombardment-based delivery of biomolecules and recent improvements, and protoplasts, a versatile system for gene-editing and regeneration in plants. The ultimate goal in plant gene-editing is to establish highly efficient and genotype-independent reagent delivery mechanisms for editing multiple targets simultaneously and achieve DNA-free gene-edited plants at scale.

From Genome Sequencing to CRISPR-Based Genome Editing for Climate-Resilient Forest Trees

Due to the economic and ecological importance of forest trees, modern breeding and genetic manipulation of forest trees have become increasingly prevalent. The CRISPR-based technology provides a versatile, powerful, and widely accepted tool for analyzing gene function and precise genetic modification in virtually any species but remains largely unexplored in forest species. Rapidly accumulating genetic and genomic resources for forest trees enabled the identification of numerous genes and biological processes that are associated with important traits such as wood quality, drought, or pest resistance, facilitating the selection of suitable gene editing targets. Here, we introduce and discuss the latest progress, opportunities, and challenges of genome sequencing and editing for improving forest sustainability.

Efficiency, Specificity and Temperature Sensitivity of Cas9 and Cas12a RNPs for DNA-free Genome Editing in Plants

Delivery of genome editing reagents using CRISPR-Cas ribonucleoproteins (RNPs) transfection offers several advantages over plasmid DNA-based delivery methods, including reduced off-target editing effects, mitigation of random integration of non-native DNA fragments, independence of vector constructions, and less regulatory restrictions. Compared to the use in animal systems, RNP-mediated genome editing is still at the early development stage in plants. In this study, we established an efficient and simplified protoplast-based genome editing platform for CRISPR-Cas RNP delivery, and then evaluated the efficiency, specificity, and temperature sensitivity of six Cas9 and Cas12a proteins. Our results demonstrated that Cas9 and Cas12a RNP delivery resulted in genome editing frequencies (8.7-41.2%) at various temperature conditions, 22°C, 26°C, and 37°C, with no significant temperature sensitivity. LbCas12a often exhibited the highest activities, while AsCas12a demonstrated higher sequence specificity. The high activities of CRISPR-Cas RNPs at 22° and 26°C, the temperature preferred by plant transformation and tissue culture, led to high mutagenesis efficiencies (34.0-85.2%) in the protoplast-regenerated calli and plants with the heritable mutants recovered in the next generation. This RNP delivery approach was further extended to pennycress (Thlaspi arvense), soybean (Glycine max) and Setaria viridis with up to 70.2% mutagenesis frequency. Together, this study sheds light on the choice of RNP reagents to achieve efficient transgene-free genome editing in plants.

A Versatile and Efficient Plant Protoplast Platform for Genome Editing by Cas9 RNPs

The ultimate goal of technology development in genome editing is to enable precisely targeted genomic changes in any cells or organisms. Here we describe protoplast systems for precise and efficient DNA sequence changes with preassembled Cas9 ribonucleoprotein (RNP) complexes in Arabidopsis thaliana, Nicotiana benthamiana, Brassica rapa, and Camelina sativa. Cas9 RNP-mediated gene disruption with dual gRNAs could reach ∼90% indels in Arabidopsis protoplasts. To facilitate facile testing of any Cas9 RNP designs, we developed two GFP reporter genes, which led to sensitive detection of nonhomologous end joining (NHEJ) and homology-directed repair (HDR), with editing efficiency up to 85 and 50%, respectively. When co-transfected with an optimal single-stranded oligodeoxynucleotide (ssODN) donor, precise editing of the AtALS gene via HDR reached 7% by RNPs. Significantly, precise mutagenesis mediated by preassembled primer editor (PE) RNPs led to 50% GFP reporter gene recovery in protoplasts and up to 4.6% editing frequency for the specific AtPDS mutation in the genome. The rapid, versatile and efficient gene editing by CRISPR RNP variants in protoplasts provides a valuable platform for development, evaluation and optimization of new designs and tools in gene and genomic manipulation and is applicable in diverse plant species.

Highly Efficient Generation of Canker-Resistant Sweet Orange Enabled by an Improved CRISPR/Cas9 System

Sweet orange (Citrus sinensis) is the most economically important species for the citrus industry. However, it is susceptible to many diseases including citrus bacterial canker caused by Xanthomonas citri subsp. citri (Xcc) that triggers devastating effects on citrus production. Conventional breeding has not met the challenge to improve disease resistance of sweet orange due to the long juvenility and other limitations. CRISPR-mediated genome editing has shown promising potentials for genetic improvements of plants. Generation of biallelic/homozygous mutants remains difficult for sweet orange due to low transformation rate, existence of heterozygous alleles for target genes, and low biallelic editing efficacy using the CRISPR technology. Here, we report improvements in the CRISPR/Cas9 system for citrus gene editing. Based on the improvements we made previously [dicot codon optimized Cas9, tRNA for multiplexing, a modified sgRNA scaffold with high efficiency, citrus U6 (CsU6) to drive sgRNA expression], we further improved our CRISPR/Cas9 system by choosing superior promoters [Cestrum yellow leaf curling virus (CmYLCV) or Citrus sinensis ubiquitin (CsUbi) promoter] to drive Cas9 and optimizing culture temperature. This system was able to generate a biallelic mutation rate of up to 89% for Carrizo citrange and 79% for Hamlin sweet orange. Consequently, this system was used to generate canker-resistant Hamlin sweet orange by mutating the effector binding element (EBE) of canker susceptibility gene CsLOB1, which is required for causing canker symptoms by Xcc. Six biallelic Hamlin sweet orange mutant lines in the EBE were generated. The biallelic mutants are resistant to Xcc. Biallelic mutation of the EBE region abolishes the induction of CsLOB1 by Xcc. This study represents a significant improvement in sweet orange gene editing efficacy and generating disease-resistant varieties via CRISPR-mediated genome editing. This improvement in citrus genome editing makes genetic studies and manipulations of sweet orange more feasible.