Increased efficiency of targeted mutagenesis by CRISPR/Cas9 in plants using heat stress

Generation of CRISPR-edited birch plants without DNA integration using Agrobacterium-mediated transformation technology

CRISPR/Cas9 system has emerged as a powerful tool in genome editing; however, generation of CRISPR-edited DNA-free plants is still challenging. In this study, Betula platyphylla (birch) was used to build a method to generate CRISPR-edited plant without foreign DNA integration using Agrobacterium-mediated transformation (CPDAT method). This technique utilizes transient genetic transformation to introduce T-DNA coding gRNA and Cas9 into birch cells, and T-DNA will express to synthesize gRNA and Cas9 protein, which will form a complex to cleave the target DNA site. The genome may be mutated due to DNA repair, and these mutations will be preserved and accumulated not dependent on whether T-DNA is integrated into the genome or not. After transient transformation, birch plants were cut into explants to induce adventitious buds without antibiotic selection pressure. Each adventitious bud can be considered as an independent potentially CRISPR-edited line for mutation detection. CRISPR-edited birch plants without foreign DNA integration are further selected by screening CRISPR-edited lines without T-DNA integration. Among 65 randomly chosen independent lines, the mutation rate was 80.00% including 40.00% of lines with both alleles mutated. In addition, 5 lines out of 65 studied lines (7.69%) were CRISPR-edited birch plants without DNA integration. In conclusion, this innovative method presents a novel strategy for generating CRISPR-edited birch plants, thereby significantly enhancing the efficiency of generating common CRISPR-edited plants. These findings offer considerable potential to develop plant genome editing techniques further.

Multiplexed gene editing in citrus by using a multi-intron containing Cas9 gene

Several expression systems have been developed in clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (CRISPR/Cas9) framework allowing for gene editing of disease-associated genes across diverse citrus varieties. In this study, we present a new approach employing a multi-intron containing Cas9 gene plus multiple gRNAs separated with tRNA sequences to target the phytoene desaturase gene in both 'Carrizo' citrange and 'Duncan' grapefruit. Notably, using this unified vector significantly boosted editing efficiency in both citrus varieties, showcasing mutations in all three designated targets. The implementation of this multiplex gene editing system with a multi-intron-containing Cas9 plus a gRNA-tRNA array demonstrates a promising avenue for efficient citrus genome editing, equipping us with potent tools in the ongoing battle against several diseases such as canker and huanglongbing.

The microtubule-nucleating factor MACERATOR tethers AUGMIN7 to microtubules and governs phragmoplast architecture

The plant cytokinetic microtubule array, called the phragmoplast, exhibits higher microtubule dynamics in its center (midzone) than at the periphery (distal zone). This behavior is known as the axial asymmetry. Despite being a major characteristic of the phragmoplast, little is known about regulators of this phenomenon. Here we address the role of microtubule nucleation in axial asymmetry by characterizing MACERATOR (MACET) proteins in Arabidopsis thaliana and Nicotiana benthamiana with a combination of genetic, biochemical, and live-cell imaging assays, using photo-convertible microtubule probes, and modeling. MACET paralogs accumulate at the shrinking microtubule ends and decrease the tubulin OFF rate. Loss of MACET4 and MACET5 function abrogates axial asymmetry by suppressing microtubule dynamicity in the midzone. MACET4 also narrows the microtubule nucleation angle at the phragmoplast leading edge and functions as a microtubule tethering factor for AUGMIN COMPLEX SUBUNIT 7 (AUG7). The macet4 macet5 double mutant shows diminished clustering of AUG7 in the phragmoplast distal zone. Knockout of AUG7 does not affect MACET4 localization, axial asymmetry, or microtubule nucleation angle, but increases phragmoplast length and slows down phragmoplast expansion. The mce4-1 mce5 aug7-1 triple knockout is not viable. Experimental data and modeling demonstrate that microtubule nucleation factors regulate phragmoplast architecture and axial asymmetry directly by generating new microtubules and indirectly by modulating the abundance of free tubulin.

Generation of transgene-free canker-resistant Citrus sinensis cv. Hamlin in the T0 generation through Cas12a/CBE co-editing

Citrus canker disease affects citrus production. This disease is caused by Xanthomonas citri subsp. citri (Xcc). Previous studies confirmed that during Xcc infection, PthA4, a transcriptional activator like effector (TALE), is translocated from the pathogen to host plant cells. PthA4 binds to the effector binding elements (EBEs) in the promoter region of canker susceptibility gene LOB1 (EBEPthA4-LOBP) to activate its expression and subsequently cause canker symptoms. Previously, the Cas12a/CBE co-editing method was employed to disrupt EBEPthA4-LOBP of pummelo, which is highly homozygous. However, most commercial citrus cultivars are heterozygous hybrids and more difficult to generate homozygous/biallelic mutants. Here, we employed Cas12a/CBE co-editing method to edit EBEPthA4-LOBP of Hamlin (Citrus sinensis), a commercial heterozygous hybrid citrus cultivar grown worldwide. Binary vector GFP-p1380N-ttLbCas12a:LOBP1-mPBE:ALS2:ALS1 was constructed and shown to be functional via Xcc-facilitated agroinfiltration in Hamlin leaves. This construct allows the selection of transgene-free regenerants via GFP, edits ALS to generate chlorsulfuron-resistant regenerants as a selection marker for genome editing resulting from transient expression of the T-DNA via nCas9-mPBE:ALS2:ALS1, and edits gene(s) of interest (i.e., EBEPthA4-LOBP in this study) through ttLbCas12a, thus creating transgene-free citrus. Totally, 77 plantlets were produced. Among them, 8 plantlets were transgenic plants (#HamGFP1 - #HamGFP8), 4 plantlets were transgene-free (#HamNoGFP1 - #HamNoGFP4), and the rest were wild type. Among 4 transgene-free plantlets, three lines (#HamNoGFP1, #HamNoGFP2 and #HamNoGFP3) contained biallelic mutations in EBEpthA4, and one line (#HamNoGFP4) had homozygous mutations in EBEpthA4. We achieved 5.2% transgene-free homozygous/biallelic mutation efficiency for EBEPthA4-LOBP in C. sinensis cv. Hamlin, compared to 1.9% mutation efficiency for pummelo in a previous study. Importantly, the four transgene-free plantlets and 3 transgenic plantlets that survived were resistant against citrus canker. Taken together, Cas12a/CBE co-editing method has been successfully used to generate transgene-free canker-resistant C. sinensis cv. Hamlin in the T0 generation via biallelic/homozygous editing of EBEpthA4 of the canker susceptibility gene LOB1.

Development of a CRISPR/Cas9-mediated gene-editing method to isolate a mutant of the unicellular green alga Parachlorella kessleri strain NIES-2152 with improved lipid productivity

BACKGROUND

Previously, we isolated a mutant of Parachlorella kessleri named strain PK4 that accumulated higher concentrations of lipids than the wild-type strain. Resequencing of the PK4 genome identified mutations in three genes which may be associated with the high-lipid phenotype. The first gene, named CDMT1, encodes a protein with a calcium-dependent membrane association domain; the second gene, named DMAN1, encodes endo-1,4-β-mannanase, while the third gene, named AATPL1, encodes a plastidic ATP/ADP antiporter-like protein.

RESULTS

To determine which of these mutant genes are directly responsible for the phenotype of strain PK4, we delivered Cas9-gRNA ribonucleoproteins targeting each of the three genes into the wild-type cells by electroporation and successfully disrupted these three genes separately. The lipid productivity in the disruptants of CDMT1 and DMAN1 was similar to and lower than that in the wild-type strain, while the disruptants of AATPL1 exhibited > 30% higher lipid productivity than the wild-type strain under diurnal conditions.

CONCLUSIONS

We succeeded in improving the lipid productivity of P. kessleri by CRISPR/Cas9-mediated gene disruption of AATPL1. The effective gene-editing method established in this study will be useful to improve Parachlorella strains for industrial applications.

CRISPR-M: Predicting sgRNA off-target effect using a multi-view deep learning network

Using the CRISPR-Cas9 system to perform base substitutions at the target site is a typical technique for genome editing with the potential for applications in gene therapy and agricultural productivity. When the CRISPR-Cas9 system uses guide RNA to direct the Cas9 endonuclease to the target site, it may misdirect it to a potential off-target site, resulting in an unintended genome editing. Although several computational methods have been proposed to predict off-target effects, there is still room for improvement in the off-target effect prediction capability. In this paper, we present an effective approach called CRISPR-M with a new encoding scheme and a novel multi-view deep learning model to predict the sgRNA off-target effects for target sites containing indels and mismatches. CRISPR-M takes advantage of convolutional neural networks and bidirectional long short-term memory recurrent neural networks to construct a three-branch network towards multi-views. Compared with existing methods, CRISPR-M demonstrates significant performance advantages running on real-world datasets. Furthermore, experimental analysis of CRISPR-M under multiple metrics reveals its capability to extract features and validates its superiority on sgRNA off-target effect predictions.

Extreme restructuring of cis-regulatory regions controlling a deeply conserved plant stem cell regulator

A striking paradox is that genes with conserved protein sequence, function and expression pattern over deep time often exhibit extremely divergent cis-regulatory sequences. It remains unclear how such drastic cis-regulatory evolution across species allows preservation of gene function, and to what extent these differences influence how cis-regulatory variation arising within species impacts phenotypic change. Here, we investigated these questions using a plant stem cell regulator conserved in expression pattern and function over ~125 million years. Using in-vivo genome editing in two distantly related models, Arabidopsis thaliana (Arabidopsis) and Solanum lycopersicum (tomato), we generated over 70 deletion alleles in the upstream and downstream regions of the stem cell repressor gene CLAVATA3 (CLV3) and compared their individual and combined effects on a shared phenotype, the number of carpels that make fruits. We found that sequences upstream of tomato CLV3 are highly sensitive to even small perturbations compared to its downstream region. In contrast, Arabidopsis CLV3 function is tolerant to severe disruptions both upstream and downstream of the coding sequence. Combining upstream and downstream deletions also revealed a different regulatory outcome. Whereas phenotypic enhancement from adding downstream mutations was predominantly weak and additive in tomato, mutating both regions of Arabidopsis CLV3 caused substantial and synergistic effects, demonstrating distinct distribution and redundancy of functional cis-regulatory sequences. Our results demonstrate remarkable malleability in cis-regulatory structural organization of a deeply conserved plant stem cell regulator and suggest that major reconfiguration of cis-regulatory sequence space is a common yet cryptic evolutionary force altering genotype-to-phenotype relationships from regulatory variation in conserved genes. Finally, our findings underscore the need for lineage-specific dissection of the spatial architecture of cis-regulation to effectively engineer trait variation from conserved productivity genes in crops.

A CRISPR/Cas9-based vector system enables the fast breeding of selection-marker-free canola with Rcr1-rendered clubroot resistance

Breeding for disease resistance in major crops is of crucial importance for global food security and sustainability. However, common biotechnologies such as traditional transgenesis or genome editing do not provide an ideal solution, whereas transgenic crops free of selection markers such as cisgenic/intragenic crops might be suitable. In this study, after cloning and functional verification of the Rcr1 gene for resistance to clubroot (Plasmodiophora brassicae), we confirmed that the genes Rcr1, Rcr2, Rcr4, and CRa from Brassica rapa crops and the resistance gene from B. napus oilseed rape cv. 'Mendel' on chromosome A03 were identical in their coding regions. We also determined that Rcr1 has a wide distribution in Brassica breeding materials and renders potent resistance against multiple representative clubroot strains in Canada. We then modified a CRISPR/Cas9-based cisgenic vector system and found that it enabled the fast breeding of selection-marker-free transgenic crops with add-on traits, with selection-marker-free canola (B. napus) germplasms with Rcr1-rendered stable resistance to clubroot disease being successfully developed within 2 years. In the B. napus background, the intragenic vector system was able to remove unwanted residue sequences from the final product with high editing efficiency, and off-target mutations were not detected. Our study demonstrates the potential of applying this breeding strategy to other crops that can be transformed by Agrobacterium. Following the streamlined working procedure, intragenic germplasms can be developed within two generations, which could significantly reduce the breeding time and labor compared to traditional introgression whilst still achieving comparable or even better breeding results.

CRISPR/Cas system: A revolutionary tool for crop improvement

World's population is elevating at an alarming rate thus, the rising demands of producing crops with better adaptability to biotic and abiotic stresses, superior nutritional as well as morphological qualities, and generation of high-yielding varieties have led to encourage the development of new plant breeding technologies. The availability and easy accessibility of genome sequences for a number of crop plants as well as the development of various genome editing technologies such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) has opened up possibilities to develop new varieties of crop plants with superior desirable traits. However, these approaches has limitation of being more expensive as well as having complex steps and time-consuming. The CRISPR/Cas genome editing system has been intensively studied for allowing versatile target-specific modifications of crop genome that fruitfully aid in the generation of novel varieties. It is an advanced and promising technology with the potential to meet hunger needs and contribute to food production for the ever-growing human population. This review summarizes the usage of novel CRISPR/Cas genome editing tool for targeted crop improvement in stress resistance, yield, quality and nutritional traits in the desired crop plants.

A high-efficient protoplast transient system for screening gene editing elements in Salvia miltiorrhiza

KEY MESSAGE

A high-efficiency protoplast transient system was devised to screen genome editing elements in Salvia miltiorrhiza. Medicinal plants with high-value pharmaceutical ingredients have attracted research attention due to their beneficial effects on human health. Cell wall-free protoplasts of plants can be used to evaluate the efficiency of genome editing mutagenesis. The capabilities of gene editing in medicinal plants remain to be fully explored owing to their complex genetic background and shortfall of suitable transformation. Here, we took the Salvia miltiorrhiza as a representative example for developing a method to screen favorable gene editing elements with high editing efficiency in medical plants by a PEG-mediated protoplast transformation. Results indicated that using the endogenous SmU6.1 of S. miltiorrhiza to drive sgRNA and the plant codon-optimized Cas9 driven by the promoter SlEF1α can enhance the efficiency of editing. In summary, we uncover an efficacious transient method for screening editing elements and shed new light on increasing gene editing efficiency in medicinal plants.

Clustered Regularly Interspaced Short Palindromic Repeat/CRISPR-Associated Protein and Its Utility All at Sea: Status, Challenges, and Prospects

Initially discovered over 35 years ago in the bacterium Escherichia coli as a defense system against invasion of viral (or other exogenous) DNA into the genome, CRISPR/Cas has ushered in a new era of functional genetics and served as a versatile genetic tool in all branches of life science. CRISPR/Cas has revolutionized the methodology of gene knockout with simplicity and rapidity, but it is also powerful for gene knock-in and gene modification. In the field of marine biology and ecology, this tool has been instrumental in the functional characterization of 'dark' genes and the documentation of the functional differentiation of gene paralogs. Powerful as it is, challenges exist that have hindered the advances in functional genetics in some important lineages. This review examines the status of applications of CRISPR/Cas in marine research and assesses the prospect of quickly expanding the deployment of this powerful tool to address the myriad fundamental marine biology and biological oceanography questions.

Creating saponin-free yellow pea seeds by CRISPR/Cas9-enabled mutagenesis on β-amyrin synthase

Dry pea (Pisum sativum) seeds are valuable sources of plant protein, dietary fiber, and starch, but their uses in food products are restricted to some extent due to several off-flavor compounds. Saponins are glycosylated triterpenoids and are a major source of bitter, astringent, and metallic off-flavors in pea products. β-amyrin synthase (BAS) is the entry point enzyme for saponin biosynthesis in pea and therefore is an ideal target for knock-out using CRISPR/Cas9 genome editing to produce saponin deficient pea varieties. Here, in an elite yellow pea cultivar (CDC Inca), LC/MS analysis identified embryo tissue, not seed coat, as the main location of saponin storage in pea seeds. Differential expression analysis determined that PsBAS1 was preferentially expressed in embryo tissue relative to seed coat and was selected for CRISPR/Cas9 genome editing. The efficiency of CRISPR/Cas9 genome editing of PsBAS1 was systematically optimized in pea hairy roots. From these optimization procedures, the AtU6-26 promoter was found to be superior to the CaMV35S promoter for gRNA expression, and the use of 37°C was determined to increase the efficiency of CRISPR/Cas9 genome editing. These promoter and culture conditions were then applied to stable transformations. As a result, a bi-allelic mutation (deletion and inversion mutations) was generated in the PsBAS1 coding sequence in a T1 plant, and the segregated psbas1 plants from the T2 population showed a 99.8% reduction of saponins in their seeds. Interestingly, a small but statistically significant increase (~12%) in protein content with a slight decrease (~5%) in starch content was observed in the psbas1 mutants under phytotron growth conditions. This work demonstrated that flavor-improved traits can be readily introduced in any pea cultivar of interest using CRISPR/Cas9. Further field trials and sensory tests for improved flavor are necessary to assess the practical implications of the saponin-free pea seeds in food applications.

Development of virus-induced genome editing methods in Solanaceous crops

Genome editing (GE) using CRISPR/Cas systems has revolutionized plant mutagenesis. However, conventional transgene-mediated GE methods have limitations due to the time-consuming generation of stable transgenic lines expressing the Cas9/single guide RNA (sgRNA) module through tissue cultures. Virus-induced genome editing (VIGE) systems have been successfully employed in model plants, such as Arabidopsis thaliana and Nicotiana spp. In this study, we developed two VIGE methods for Solanaceous plants. First, we used the tobacco rattle virus (TRV) vector to deliver sgRNAs into a transgenic tomato (Solanum lycopersicum) line of cultivar Micro-Tom expressing Cas9. Second, we devised a transgene-free GE method based on a potato virus X (PVX) vector to deliver Cas9 and sgRNAs. We designed and cloned sgRNAs targeting Phytoene desaturase in the VIGE vectors and determined optimal conditions for VIGE. We evaluated VIGE efficiency through deep sequencing of the target gene after viral vector inoculation, detecting 40.3% and 36.5% mutation rates for TRV- and PVX-mediated GE, respectively. To improve editing efficiency, we applied a 37°C heat treatment, which increased the editing efficiency by 33% to 46% and 56% to 76% for TRV- and PVX-mediated VIGE, respectively. To obtain edited plants, we subjected inoculated cotyledons to tissue culture, yielding successful editing events. We also demonstrated that PVX-mediated GE can be applied to other Solanaceous crops, such as potato (Solanum tuberosum) and eggplant (Solanum melongena). These simple and highly efficient VIGE methods have great potential for generating genome-edited plants in Solanaceous crops.

Construction of Host Plant Insect-Resistance Mutant Library by High-Throughput CRISPR/Cas9 System and Identification of A Broad-Spectrum Insect Resistance Gene

Insects pose significant challenges in cotton-producing regions. Here, they describe a high-throughput CRISPR/Cas9-mediated large-scale mutagenesis library targeting endogenous insect-resistance-related genes in cotton. This library targeted 502 previously identified genes using 968 sgRNAs, generated ≈2000 T0 plants and achieved 97.29% genome editing with efficient heredity, reaching upto 84.78%. Several potential resistance-related mutants (10% of 200 lines) their identified that may contribute to cotton-insect molecular interaction. Among these, they selected 139 and 144 lines showing decreased resistance to pest infestation and targeting major latex-like protein 423 (GhMLP423) for in-depth study. Overexpression of GhMLP423 enhanced insect resistance by activating the plant systemic acquired resistance (SAR) of salicylic acid (SA) and pathogenesis-related (PR) genes. This activation is induced by an elevation of cytosolic calcium [Ca2+ ]cyt flux eliciting reactive oxygen species (ROS), which their demoted in GhMLP423 knockout (CR) plants. Protein-protein interaction assays revealed that GhMLP423 interacted with a human epidermal growth factor receptor substrate15 (EPS15) protein at the cell membrane. Together, they regulated the systemically propagating waves of Ca2+ and ROS, which in turn induced SAR. Collectively, this large-scale mutagenesis library provides an efficient strategy for functional genomics research of polyploid plant species and serves as a solid platform for genetic engineering of insect resistance.

CRISPR/Cas9 mutated p-coumaroyl shikimate 3'-hydroxylase 3 gene in Populus tomentosa reveals lignin functioning on supporting tree upright

The lignin plays one of the most important roles in plant secondary metabolism. However, it is still unclear how lignin can contribute to the impressive height of wood growth. In this study, C3'H, a rate-limiting enzyme of the lignin pathway, was used as the target gene. C3'H3 was knocked out by CRISPR/Cas9 in Populus tomentosa. Compared with wild-type popular trees, c3'h3 mutants exhibited dwarf phenotypes, collapsed xylem vessels, weakened phloem thickening, decreased hydraulic conductivity and photosynthetic efficiency, and reduced auxin content, except for reduced total lignin content and significantly increased H-subunit lignin. In the c3'h3 mutant, the flavonoid biosynthesis genes CHS, CHI, F3H, DFR, ANR, and LAR were upregulated, and flavonoid metabolite accumulations were detected, indicating that decreasing the lignin biosynthesis pathway enhanced flavonoid metabolic flux. Furthermore, flavonoid metabolites, such as naringenin and hesperetin, were largely increased, while higher hesperetin content suppressed plant cell division. Thus, studying the c3'h3 mutant allows us to deduce that lignin deficiency suppresses tree growth and leads to the dwarf phenotype due to collapsed xylem and thickened phloem, limiting material exchanges and transport.

Extreme restructuring of cis -regulatory regions controlling a deeply conserved plant stem cell regulator

A striking paradox is that genes with conserved protein sequence, function and expression pattern over deep time often exhibit extremely divergent cis -regulatory sequences. It remains unclear how such drastic cis -regulatory evolution across species allows preservation of gene function, and to what extent these differences influence how cis- regulatory variation arising within species impacts phenotypic change. Here, we investigated these questions using a plant stem cell regulator conserved in expression pattern and function over ∼125 million years. Using in-vivo genome editing in two distantly related models, Arabidopsis thaliana (Arabidopsis) and Solanum lycopersicum (tomato), we generated over 70 deletion alleles in the upstream and downstream regions of the stem cell repressor gene CLAVATA3 ( CLV3 ) and compared their individual and combined effects on a shared phenotype, the number of carpels that make fruits. We found that sequences upstream of tomato CLV3 are highly sensitive to even small perturbations compared to its downstream region. In contrast, Arabidopsis CLV3 function is tolerant to severe disruptions both upstream and downstream of the coding sequence. Combining upstream and downstream deletions also revealed a different regulatory outcome. Whereas phenotypic enhancement from adding downstream mutations was predominantly weak and additive in tomato, mutating both regions of Arabidopsis CLV3 caused substantial and synergistic effects, demonstrating distinct distribution and redundancy of functional cis -regulatory sequences. Our results demonstrate remarkable malleability in cis -regulatory structural organization of a deeply conserved plant stem cell regulator and suggest that major reconfiguration of cis -regulatory sequence space is a common yet cryptic evolutionary force altering genotype-to-phenotype relationships from regulatory variation in conserved genes. Finally, our findings underscore the need for lineage-specific dissection of the spatial architecture of cis -regulation to effectively engineer trait variation from conserved productivity genes in crops.

Author summary

We investigated the evolution of cis -regulatory elements (CREs) and their interactions in the regulation of a plant stem cell regulator gene, CLAVATA3 (CLV3) , in Arabidopsis and tomato. Despite diverging ∼125 million years ago, the function and expression of CLV3 is conserved in these species; however, cis -regulatory sequences upstream and downstream have drastically diverged, preventing identification of conserved non-coding sequences between them. We used CRISPR-Cas9 to engineer dozens of mutations within the cis -regulatory regions of Arabidopsis and tomato CLV3. In tomato, our results show that tomato CLV3 function primarily relies on interactions among CREs in the 5' non-coding region, unlike Arabidopsis CLV3 , which depends on a more balanced distribution of functional CREs between the 5' and 3' regions. Therefore, despite a high degree of functional conservation, our study demonstrates divergent regulatory strategies between two distantly related CLV3 orthologs, with substantial alterations in regulatory sequences, their spatial arrangement, and their relative effects on CLV3 regulation. These results suggest that regulatory regions are not only extremely robust to mutagenesis, but also that the sequences underlying this robustness can be lineage-specific for conserved genes, due to the complex and often redundant interactions among CREs that ensure proper gene function amidst large-scale sequence turnover.

Germline Editing of Drosophila Using CRISPR-Cas9-Based Cytosine and Adenine Base Editors

Target-AID, BE3, and ABE7.10 base editors fused to the catalytically modified Cas9 and xCas9(3.7) were tested for germline editing of the fruit fly Drosophila melanogaster. We developed a guide RNA-expressing construct, white-4gRNA, targeting splice sites in the white gene, an X-chromosome located gene. Using white-4gRNA flies and transgenic lines expressing Target-AID, BE3, and ABE7.10 base editors, we tested the efficiency of stable germline gene editing at three different temperatures. Classical Cas9 generating insertions/deletions by non-homologous end joining served as a reference. Our data indicate that gene editing is most efficient at 28°C, the highest temperature suitable for fruit flies. Finally, we created a new allele of the core circadian clock gene timeless using Target-AID. This base edited mutant allele timSS308-9FL had a disrupted circadian clock with a period of ∼29 h. The white-4gRNA expressing fly can be used to test new generations of base editors for future applications in Drosophila.

Microparticle-mediated CRISPR DNA delivery for genome editing in poplar

The use of CRISPR/Cas9 is currently the method of choice for precise genome engineering in plants, including in the biomass crop poplar. The most commonly used method for delivering CRISPR/Cas9 and its components in poplar is via Agrobacterium-mediated transformation, that besides the desired gene-editing event also results in stable T-DNA integration. Here we explore the delivery of the gene-editing reagents via DNA-coated microparticle bombardment into the model tree Populus tremula x P. alba to evaluate its potential for developing transgene-free, gene-edited trees, as well as its potential for integrating donor DNA at specific target sites. Using an optimized transformation method, which favors the regeneration of plants that transiently express the genes on the delivered donor DNA, we regenerated gene-edited plants that are free of the Cas9 and the antibiotic resistance-encoding transgenes. In addition, we report the frequent integration of donor DNA fragments at the Cas9-induced double-strand break, opening opportunities toward targeted gene insertions.

A Variant of LbCas12a and Elevated Incubation Temperatures Enhance the Rate of Gene Editing in the Oomycete Phytophthora infestans

CRISPR-Cas editing systems have proved to be powerful tools for functional genomics research, but their effectiveness in many non-model species remains limited. In the potato and tomato pathogen Phytophthora infestans, an editing system was previously developed that expresses the Lachnospiracae bacterium Cas12a endonuclease (LbCas12a) and guide RNA from a DNA vector. However, the method works at low efficiency. Based on a hypothesis that editing is constrained by a mismatch between the optimal temperatures for P. infestans growth and endonuclease catalysis, we tested two strategies that increased the frequency of editing of two target genes by about 10-fold. First, we found that editing was boosted by a mutation in LbCas12a (D156R) that had been reported to expand its catalytic activity over a broader temperature range. Second, we observed that editing was enhanced by transiently incubating transformed tissue at a higher temperature. These modifications should make CRISPR-Cas12a more useful for interrogating gene and protein function in P. infestans and its relatives, especially species that grow optimally at lower temperatures. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Efficient plant genome engineering using a probiotic sourced CRISPR-Cas9 system

Among CRISPR-Cas genome editing systems, Streptococcus pyogenes Cas9 (SpCas9), sourced from a human pathogen, is the most widely used. Here, through in silico data mining, we have established an efficient plant genome engineering system using CRISPR-Cas9 from probiotic Lactobacillus rhamnosus. We have confirmed the predicted 5'-NGAAA-3' PAM via a bacterial PAM depletion assay and showcased its exceptional editing efficiency in rice, wheat, tomato, and Larix cells, surpassing LbCas12a, SpCas9-NG, and SpRY when targeting the identical sequences. In stable rice lines, LrCas9 facilitates multiplexed gene knockout through coding sequence editing and achieves gene knockdown via targeted promoter deletion, demonstrating high specificity. We have also developed LrCas9-derived cytosine and adenine base editors, expanding base editing capabilities. Finally, by harnessing LrCas9's A/T-rich PAM targeting preference, we have created efficient CRISPR interference and activation systems in plants. Together, our work establishes CRISPR-LrCas9 as an efficient and user-friendly genome engineering tool for diverse applications in crops and beyond.

Optimizing genome editing efficiency in wheat: Effects of heat treatments and different promoters for single guide RNA expression

Genome editing is a promising method for simultaneously mutagenizing homoeologs in the three subgenomes of wheat (Triticum aestivum L.). However, the mutation rate via genome editing must be improved in order to analyze gene function and to quickly modify agronomic traits in wheat. Here, we examined the Cas9-induced mutation rates in wheat plants using two promoters for single guide RNA (sgRNA) expression and applying heat treatment during Agrobacterium tumefaciens-mediated transformation. Using the TaU6 promoter instead of the OsU6 promoter from rice (Oryza sativa L.) to drive sgRNA expression greatly improved the Cas9-induced mutation rate. Moreover, a heat treatment of 30°C for 1 day during tissue culture increased the Cas9-induced mutation rate and the variety of mutations obtained compared to tissue culture at the normal temperature (25°C). The same heat treatment did not affect the regeneration rates of transgenic plants but tended to increase the number of transgene integration sites in each transgenic plant. These results lay the foundation for improving the Cas9-induced mutation rate in wheat to enhance research on gene function and crop improvement.

The circadian clock regulates PIF3 protein stability in parallel to red light

The circadian clock is an endogenous oscillator, but its importance lies in its ability to impart rhythmicity on downstream biological processes or outputs. Focus has been placed on understanding the core transcription factors of the circadian clock and how they connect to outputs through regulated gene transcription. However, far less is known about posttranslational mechanisms that tether clocks to output processes through protein regulation. Here, we identify a protein degradation mechanism that tethers the clock to photomorphogenic growth. By performing a reverse genetic screen, we identify a clock-regulated F-box type E3 ubiquitin ligase, CLOCK-REGULATED F-BOX WITH A LONG HYPOCOTYL 1 ( CFH1 ), that controls hypocotyl length. We then show that CFH1 functions in parallel to red light signaling to target the transcription factor PIF3 for degradation. This work demonstrates that the circadian clock is tethered to photomorphogenesis through the ubiquitin proteasome system and that PIF3 protein stability acts as a hub to integrate information from multiple environmental signals.

Chromatin remodeling of histone H3 variants by DDM1 underlies epigenetic inheritance of DNA methylation

Nucleosomes block access to DNA methyltransferase, unless they are remodeled by DECREASE in DNA METHYLATION 1 (DDM1LSH/HELLS), a Snf2-like master regulator of epigenetic inheritance. We show that DDM1 promotes replacement of histone variant H3.3 by H3.1. In ddm1 mutants, DNA methylation is partly restored by loss of the H3.3 chaperone HIRA, while the H3.1 chaperone CAF-1 becomes essential. The single-particle cryo-EM structure at 3.2 Å of DDM1 with a variant nucleosome reveals engagement with histone H3.3 near residues required for assembly and with the unmodified H4 tail. An N-terminal autoinhibitory domain inhibits activity, while a disulfide bond in the helicase domain supports activity. DDM1 co-localizes with H3.1 and H3.3 during the cell cycle, and with the DNA methyltransferase MET1Dnmt1, but is blocked by H4K16 acetylation. The male germline H3.3 variant MGH3/HTR10 is resistant to remodeling by DDM1 and acts as a placeholder nucleosome in sperm cells for epigenetic inheritance.

Temporally gene knockout using heat shock-inducible genome-editing system in plants

Clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 (Cas9) has emerged as a powerful tool to generate targeted loss-of-function mutations for functional genomic studies. As a next step, tools to generate genome modifications in a spatially and temporally precise manner will enable researchers to further dissect gene function. Here, we present two heat shock-inducible genome-editing (IGE) systems that efficiently edit target genes when the system is induced, thus allowing us to target specific developmental stages. For this conditional editing system, we chose the natural heat-inducible promoter from heat-shock protein 18.2 (HSP18.2) from Arabidopsis thaliana and the synthetic heat-inducible promoter heat shock-response element HSE-COR15A to drive the expression of Cas9. We tested these two IGE systems in Arabidopsis using cyclic or continuous heat-shock treatments at the seedling and bolting stages. A real-time quantitative polymerase chain reaction analysis revealed that the HSP18.2 IGE system exhibited higher Cas9 expression levels than the HSE-COR15A IGE system upon both cyclic and continuous treatments. By targeting brassinosteroid-insensitive 1 (BRI1) and phytoene desaturase (PDS), we demonstrate that both cyclic and continuous heat inductions successfully activated the HSP18.2 IGE system at the two developmental stages, resulting in highly efficient targeted mutagenesis and clear phenotypic outcomes. By contrast, the HSE-COR15A IGE system was only induced at the seedling stage and was less effective than the HSP18.2 IGE system in terms of mutagenesis frequencies. The presented heat shock-IGE systems can be conditionally induced to efficiently inactivate genes at any developmental stage and are uniquely suited for the dissection and systematic characterization of essential genes.

Chromatin remodeling of histone H3 variants underlies epigenetic inheritance of DNA methylation

Epigenetic inheritance refers to the faithful replication of DNA methylation and histone modification independent of DNA sequence. Nucleosomes block access to DNA methyltransferases, unless they are remodeled by DECREASE IN DNA METHYLATION1 (DDM1 Lsh/HELLS ), a Snf2-like master regulator of epigenetic inheritance. We show that DDM1 activity results in replacement of the transcriptional histone variant H3.3 for the replicative variant H3.1 during the cell cycle. In ddm1 mutants, DNA methylation can be restored by loss of the H3.3 chaperone HIRA, while the H3.1 chaperone CAF-1 becomes essential. The single-particle cryo-EM structure at 3.2 Å of DDM1 with a variant nucleosome reveals direct engagement at SHL2 with histone H3.3 at or near variant residues required for assembly, as well as with the deacetylated H4 tail. An N-terminal autoinhibitory domain binds H2A variants to allow remodeling, while a disulfide bond in the helicase domain is essential for activity in vivo and in vitro . We show that differential remodeling of H3 and H2A variants in vitro reflects preferential deposition in vivo . DDM1 co-localizes with H3.1 and H3.3 during the cell cycle, and with the DNA methyltransferase MET1 Dnmt1 . DDM1 localization to the chromosome is blocked by H4K16 acetylation, which accumulates at DDM1 targets in ddm1 mutants, as does the sperm cell specific H3.3 variant MGH3 in pollen, which acts as a placeholder nucleosome in the germline and contributes to epigenetic inheritance.

Salt and osmotic stress can improve the editing efficiency of CRISPR/Cas9-mediated genome editing system in potato

CRISPR/Cas9-mediated genome editing technology has been widely used for the study of gene function in crops, but the differences between species have led to widely varying genome editing efficiencies. The present study utilized a potato hairy root genetic transformation system and incorporated a rapid assay with GFP as a screening marker. The results clearly demonstrated that salt and osmotic stress induced by NaCl (10 to 50 mM) and mannitol (50 to 200 mM) treatments significantly increased the positive rates of genetic transformation mediated by A. rhizogenes and the editing efficiency of the CRISPR/Cas9-mediated genome editing system in potato. However, it was observed that the regeneration of potato roots was partially inhibited as a result. The analysis of CRISPR/Cas9-mediated mutation types revealed that chimeras accounted for the largest proportion, ranging from 62.50% to 100%. Moreover, the application of salt and osmotic stress resulted in an increased probability of null mutations in potato. Notably, the highest rate of null mutations, reaching 37.5%, was observed at a NaCl concentration of 10 mM. Three potential off-target sites were sequenced and no off-targeting was found. In conclusion, the application of appropriate salt and osmotic stress significantly improved the editing efficiency of the CRISPR/Cas9-mediated genome editing system in potato, with no observed off-target effects. However, there was a trade-off as the regeneration of potato roots was partially inhibited. Overall, these findings present a new and convenient approach to enhance the genome editing efficiency of the CRISPR/Cas9-mediated gene editing system in potato.

Basis for high-affinity ethylene binding by the ethylene receptor ETR1 of Arabidopsis

The gaseous hormone ethylene is perceived in plants by membrane-bound receptors, the best studied of these being ETR1 from Arabidopsis. Ethylene receptors can mediate a response to ethylene concentrations at less than one part per billion; however, the mechanistic basis for such high-affinity ligand binding has remained elusive. Here we identify an Asp residue within the ETR1 transmembrane domain that plays a critical role in ethylene binding. Site-directed mutation of the Asp to Asn results in a functional receptor that has a reduced affinity for ethylene, but still mediates ethylene responses in planta. The Asp residue is highly conserved among ethylene receptor-like proteins in plants and bacteria, but Asn variants exist, pointing to the physiological relevance of modulating ethylene-binding kinetics. Our results also support a bifunctional role for the Asp residue in forming a polar bridge to a conserved Lys residue in the receptor to mediate changes in signaling output. We propose a new structural model for the mechanism of ethylene binding and signal transduction, one with similarities to that found in a mammalian olfactory receptor.

Multi-faceted CRISPR/Cas technological innovation aspects in the framework of 3P medicine

Since 2009, the European Association for Predictive, Preventive and Personalised Medicine (EPMA, Brussels) promotes the paradigm change from reactive approach to predictive, preventive, and personalized medicine (PPPM/3PM) to protect individuals in sub-optimal health conditions from the health-to-disease transition, to increase life-quality of the affected patient cohorts improving, therefore, ethical standards and cost-efficacy of healthcare to great benefits of the society at large. The gene-editing technology utilizing CRISPR/Cas gene-editing approach has demonstrated its enormous value as a powerful tool in a broad spectrum of bio/medical research areas. Further, CRISPR/Cas gene-editing system is considered applicable to primary and secondary healthcare, in order to prevent disease spread and to treat clinically manifested disorders, involving diagnostics of SARS-Cov-2 infection and experimental treatment of COVID-19. Although the principle of the proposed gene editing is simple and elegant, there are a lot of technological challenges and ethical considerations to be solved prior to its broadly scaled clinical implementation. This article highlights technological innovation beyond the state of the art, exemplifies current achievements, discusses unsolved technological and ethical problems, and provides clinically relevant outlook in the framework of 3PM.

A method for generating genome edited plant lines from CRISPR-transformed Shanxin poplar plants

Due to the reason of low efficiency of mutation in CRISPR-editing, a high frequency of CRISPR transformed plant lines failing in mutation had been generated and had to be discarded. In the present study, we built a method to increase the efficiency of CRISPR-editing. We used Shanxin poplar (Populus davidiana×P. bolleana) as the study material, and CRISPR-editing system was first built to generate the CRISPR-transformed lines. The line that failed in CRISPR-editing was used for improving the efficiency of mutation, which was treated with heat (37 °C) to improve the cleaving activity of Cas9, leading to increased frequency of the cleaved DNA. Our results indicated that 87-100% of cells in CRISPR-transformed plants whose DNA had been cleaved by heat treatment, and the heat treatment plants were then cut into explants to differentiate adventitious buds. Each differentiated bud can be considered as an independent line. Twenty independent lines were randomly selected for analysis, and all of them had been mutated by CRISPR editing, displaying 4 types of mutation. Our results indicated that heat treatment combined with re-differentiation can generate CRISPR-edited plants efficiently. This method could conquer the problem of low mutation efficiency of CRISPR-editing in Shanxin poplar, and will have a wide application in plant CRISPR-editing. DATA AVAILABILITY: The genome sequence of Populus davidiana × P. bolleana had been submitted to GenBank with the BioProject Accession number of PRJNA867039 (https://www.ncbi.nlm.nih.gov/bioproject/?term=PRJNA867039).

FLInt: single shot safe harbor transgene integration via Fluorescent Landmark Interference

The stable incorporation of transgenes and recombinant DNA material into the host genome is a bottleneck in many bioengineering applications. Due to the low efficiency, identifying the transgenic animals is often a needle in the haystack. Thus, optimal conditions require efficient screening procedures, but also known and safe landing sites that do not interfere with host expression, low input material and strong expression from the new locus. Here, we leverage an existing library of ≈300 different loci coding for fluorescent markers that are distributed over all 6 chromosomes in Caenorhabditis elegans as safe harbors for versatile transgene integration sites using CRISPR/Cas9. We demonstrated that a single crRNA was sufficient for cleavage of the target region and integration of the transgene of interest, which can be easily followed by loss of the fluorescent marker. The same loci can also be used for extrachromosomal landing sites and as co-CRISPR markers without affecting body morphology or animal behavior. Thus, our method overcomes the uncertainty of transgene location during random mutagenesis, facilitates easy screening through fluorescence interference and can be used as co-CRISPR markers without further influence in phenotypes.

Multi-Knock-a multi-targeted genome-scale CRISPR toolbox to overcome functional redundancy in plants

Plant genomes are characterized by large and complex gene families that often result in similar and partially overlapping functions. This genetic redundancy severely hampers current efforts to uncover novel phenotypes, delaying basic genetic research and breeding programmes. Here we describe the development and validation of Multi-Knock, a genome-scale clustered regularly interspaced short palindromic repeat toolbox that overcomes functional redundancy in Arabidopsis by simultaneously targeting multiple gene-family members, thus identifying genetically hidden components. We computationally designed 59,129 optimal single-guide RNAs that each target two to ten genes within a family at once. Furthermore, partitioning the library into ten sublibraries directed towards a different functional group allows flexible and targeted genetic screens. From the 5,635 single-guide RNAs targeting the plant transportome, we generated over 3,500 independent Arabidopsis lines that allowed us to identify and characterize the first known cytokinin tonoplast-localized transporters in plants. With the ability to overcome functional redundancy in plants at the genome-scale level, the developed strategy can be readily deployed by scientists and breeders for basic research and to expedite breeding efforts.

Nutraceutical and flavor profiles in underutilized desert legumes of India: gene editing strategies towards sustainable food development

Climate change has posed a challenge for food security all over the world in the form of fluctuating crop yields and novel disease outbreaks in plants. Human society's overdependence on a few food crops does not seem a wise precedence. There are numerous underutilized/orphan/neglected legumes growing in the Indian desert regions that can come to the rescue and act as balanced and sustainable sources of nutrients and health-benefitting nutraceuticals. However, challenges such as low plant yield, unidentified metabolic pathways and off-flavor in the food products derived from them prevent the realization of their full potential. Conventional breeding techniques are too slow to achieve the desired modifications and cater to the sharply rising demand for functional foods. The novel gene editing tools like CRISPR-Cas provide more precise tool to manipulate the target genes with or without introduction of foreign DNA and therefore, have better chances to be accepted by governments and societies. The current article reports some of the relevant 'gene editing' success stories with respect to nutraceutical and flavor profiles in the popular legumes. It highlights gaps and future potential, along with areas requiring caution, in underutilized edible legumes of the Indian (semi) arid regions like Prosopis cineraria, Acacia senegal and Cyamopsis tetragonoloba.

Editing efficiencies with Cas9 orthologs, Cas12a endonucleases, and temperature in rice

The advent of CRISPR-Cas technology has made it the genome editing tool of choice in all kingdoms of life, including plants, which can have large, highly duplicated genomes. As a result, finding adequate target sequences that meet the specificities of a given Cas nuclease on any gene of interest remains challenging in many cases. To assess target site flexibility, we tested five different Cas9/Cas12a endonucleases (SpCas9, SaCas9, St1Cas9, Mb3Cas12a, and AsCas12a) in embryogenic rice calli from Taipei 309 at 37°C (optimal temperature for most Cas9/Cas12a proteins) and 27°C (optimal temperature for tissue culture) and measured their editing rates under regular tissue culture conditions using Illumina sequencing. StCas9 and AsCas12 were not functional as tested, regardless of the temperature used. SpCas9 was the most efficient endonuclease at either temperature, regardless of whether monoallelic or biallelic edits were considered. Mb3Cas12a at 37°C was the next most efficient endonuclease. Monoallelic edits prevailed for both SaCas9 and Mb3Cas12a at 27°C, but biallelic edits prevailed at 37°C. Overall, the use of other Cas9 orthologs, the use of Cas12a endonucleases, and the optimal temperature can expand the range of targetable sequences.

CRISPR/Cas-mediated inplanta gene targeting: current advances and challenges

We can use gene targeting (GT) to make modifications at a specific region in a plant's genome and create high-precision tools for plant biotechnology and breeding. However, its low efficiency is a major barrier to its use in plants. The discovery of CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats)-Cas-based site-specific nucleases capable of inducing double-strand breaks in desired loci resulted in the development of novel approaches for plant GT. Several studies have recently demonstrated improvements in GT efficiency through cell-type-specific expression of Cas nucleases, the use of self-amplified GT-vector DNA, or manipulation of RNA silencing and DNA repair pathways. In this review, we summarize recent advances in CRISPR/Cas-mediated GT in plants and discuss potential efficiency improvements. Increasing the efficiency of GT technology will help us pave the way for increased crop yields and food safety in environmentally friendly agriculture.

Effects of Different Gene Editing Modes of CRISPR/Cas9 on Soybean Fatty Acid Anabolic Metabolism Based on GmFAD2 Family

Δ¹²-fatty acid dehydrogenase (FAD2) is the essential enzyme responsible for catalyzing the formation of linoleic acid from oleic acid. CRISPR/Cas9 gene editing technology has been an essential tool for molecular breeding in soybeans. To evaluate the most suitable type of gene editing in soybean fatty acid synthesis metabolism, this study selected five crucial enzyme genes of the soybean FAD2 gene family-GmFAD2-1A, GmFAD2-1B, GmFAD2-2A, GmFAD2-2B, and GmFAD2-2C-and created a CRISPR/Cas9-mediated single gene editing vector system. The results of Sanger sequencing showed that 72 transformed plants positive for T₁ generation were obtained using Agrobacterium-mediated transformation, of which 43 were correctly edited plants, with the highest editing efficiency of 88% for GmFAD2-2A. The phenotypic analysis revealed that the oleic acid content of the progeny of GmFAD2-1A gene-edited plants had a higher increase of 91.49% when compared to the control JN18, and the rest of the gene-edited plants in order were GmFAD2-2A, GmFAD2-1B, GmFAD2-2C, and GmFAD2-2B. The analysis of gene editing type has indicated that base deletions greater than 2bp were the predominant editing type in all editing events. This study provides ideas for the optimization of CRISPR/Cas9 gene editing technology and the development of new tools for precise base editing in the future.

Efficient gene activation in plants by the MoonTag programmable transcriptional activator

CRISPR/Cas-based transcriptional activators have been developed to induce gene expression in eukaryotic and prokaryotic organisms. The main advantages of CRISPR-Cas based systems is that they can achieve high levels of transcriptional activation and are very easy to program via pairing between the guide RNA and the DNA target strand. SunTag is a second-generation system that activates transcription by recruiting multiple copies of an activation domain (AD) to its target promoters. SunTag is a strong activator; however, in some species it is difficult to stably express. To overcome this problem, we designed MoonTag, a new activator that worked on the same basic principle as SunTag, but whose components are better tolerated when stably expressed in transgenic plants. We demonstrate that MoonTag is capable of inducing high levels of transcription in all plants tested. In Setaria, MoonTag is capable of inducing high levels of transcription of reporter genes as well as of endogenous genes. More important, MoonTag components are expressed in transgenic plants to high levels without any deleterious effects. MoonTag is also able to efficiently activate genes in eudicotyledonous species such as Arabidopsis and tomato. Finally, we show that MoonTag activation is functional across a range of temperatures, which is promising for potential field applications.

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.

Canalization of genome-wide transcriptional activity in Arabidopsis thaliana accessions by MET1-dependent CG methylation

BACKGROUND

Despite its conserved role on gene expression and transposable element (TE) silencing, genome-wide CG methylation differs substantially between wild Arabidopsis thaliana accessions.

RESULTS

To test our hypothesis that global reduction of CG methylation would reduce epigenomic, transcriptomic, and phenotypic diversity in A. thaliana accessions, we knock out MET1, which is required for CG methylation, in 18 early-flowering accessions. Homozygous met1 mutants in all accessions suffer from common developmental defects such as dwarfism and delayed flowering, in addition to accession-specific abnormalities in rosette leaf architecture, silique morphology, and fertility. Integrated analysis of genome-wide methylation, chromatin accessibility, and transcriptomes confirms that MET1 inactivation greatly reduces CG methylation and alters chromatin accessibility at thousands of loci. While the effects on TE activation are similarly drastic in all accessions, the quantitative effects on non-TE genes vary greatly. The global expression profiles of accessions become considerably more divergent from each other after genome-wide removal of CG methylation, although a few genes with diverse expression profiles across wild-type accessions tend to become more similar in mutants. Most differentially expressed genes do not exhibit altered chromatin accessibility or CG methylation in cis, suggesting that absence of MET1 can have profound indirect effects on gene expression and that these effects vary substantially between accessions.

CONCLUSIONS

Systematic analysis of MET1 requirement in different A. thaliana accessions reveals a dual role for CG methylation: for many genes, CG methylation appears to canalize expression levels, with methylation masking regulatory divergence. However, for a smaller subset of genes, CG methylation increases expression diversity beyond genetically encoded differences.

A dual sgRNA-directed CRISPR/Cas9 construct for editing the fruit-specific β-cyclase 2 gene in pigmented citrus fruits

CRISPR/Cas9 genome editing is a modern biotechnological approach used to improve plant varieties, modifying only one or a few traits of a specific variety. However, this technology cannot be easily used to improve fruit quality traits in citrus, due to the lack of knowledge of key genes, long juvenile stage, and the difficulty regenerating whole plants of specific varieties. Here, we introduce a genome editing approach with the aim of producing citrus plantlets whose fruits contain both lycopene and anthocyanins. Our method employs a dual single guide RNA (sgRNA)-directed genome editing approach to knockout the fruit-specific β-cyclase 2 gene, responsible for the conversion of lycopene to beta-carotene. The gene is targeted by two sgRNAs simultaneously to create a large deletion, as well as to induce point mutations in both sgRNA targets. The EHA105 strain of Agrobacterium tumefaciens was used to transform five different anthocyanin-pigmented sweet oranges, belonging to the Tarocco and Sanguigno varietal groups, and 'Carrizo' citrange, a citrus rootstock as a model for citrus transformation. Among 58 plantlets sequenced in the target region, 86% of them were successfully edited. The most frequent mutations were deletions (from -1 to -74 nucleotides) and insertions (+1 nucleotide). Moreover, a novel event was identified in six plantlets, consisting of the inversion of the region between the two sgRNAs. For 20 plantlets in which a single mutation occurred, we excluded chimeric events. Plantlets did not show an altered phenotype in vegetative tissues. To the best of our knowledge, this work represents the first example of the use of a genome editing approach to potentially improve qualitative traits of citrus fruit.

A Simple and Low-Tech Heat-Shock Method to Increase Genome Editing Efficiency in Plants

CRISPR/Cas is now the standard technique to generate novel plant genotypes. However, optimizing the efficiency of the system continues to be an aspect of research and development. One of the improvements for increasing mutagenesis efficiency in different species is the application of heat stress. However, many experimental setups are limited by the requirement of using dedicated climate chambers to impose heat stress and by difficulties in the phenotyping of soil-grown plants. Here, we describe a simplified heat stress assay for in vitro-grown plants that can be completed in 6 days using commonly available laboratory equipment. We show that three 24-hr heat shocks (3×HS) at 37°C alternated with 24 hr of recovery at 21°C efficiently increases indel rates of LbCas12a and Cas9. We illustrate how visual mutant phenotypes (pds3 and gl1) can assist in quantifying genome editing efficiency, and describe how to quantify genome editing efficiency using genotyping by Sanger sequencing. We also provide a support protocol to efficiently clone a CRISPR expression vector in a single step. Together, our methods allow researchers to increase CRISPR-induced mutations using a low-tech setup in plants. © 2022 Wiley Periodicals LLC. Basic Protocol 1: 3×HS protocol Basic Protocol 2: Genotyping by Sanger sequencing Support Protocol: One-step cloning of a CRISPR expression vector.

CRISPR/Cas mediated genome editing in potato: Past achievements and future directions

Genome engineering has been re-shaping plant biotechnology and agriculture. Crop improvement using the recently developed gene editing techniques is now easier, faster, and more precise than ever. Although considered to be a global food security crop, potato has not benefitted enough from diverse collection of these techniques. Unique genetic features of cultivated potatoes such as tetrasomic inheritance, high genomic heterozygosity, and inbreeding depression hamper conventional breeding of this important crop. Therefore, genome editing provides an excellent arsenal of tools for trait improvement in potato. Moreover, using specific transformation protocols, it is possible to engineer transgene free commercial varieties. In this review we first describe the past achievements in potato genome editing and highlight some of the missing aspects of these efforts. Then, we discuss about technical challenges of genome editing in potato and present approaches to overcome these difficulties. Finally, we talk about genome editing applications that have not been explored in potato and point out some of the missing venues in literature.

Systematic histone H4 replacement in Arabidopsis thaliana reveals a role for H4R17 in regulating flowering time

Despite the broad array of roles for epigenetic mechanisms on regulating diverse processes in eukaryotes, no experimental system is currently available in plants for the direct assessment of histone function. In this work, we present the development of a genetic strategy in Arabidopsis (Arabidopsis thaliana) whereby modified histone H4 transgenes can completely replace the expression of endogenous histone H4 genes. Accordingly, we established a collection of plants expressing different H4 point mutants targeting residues that may be post-translationally modified in vivo. To demonstrate its utility, we screened this new H4 mutant collection to uncover substitutions in H4 that alter flowering time. We identified different mutations in the H4 tail (H4R17A) and the H4 globular domain (H4R36A, H4R39K, H4R39A, and H4K44A) that strongly accelerate the floral transition. Furthermore, we identified a conserved regulatory relationship between H4R17 and the ISWI chromatin remodeling complex in plants: As with other biological systems, H4R17 regulates nucleosome spacing via ISWI. Overall, this work provides a large set of H4 mutants to the plant epigenetics community that can be used to systematically assess histone H4 function in Arabidopsis and a roadmap to replicate this strategy for studying other histone proteins in plants.

Temperature dependent in vitro binding and release of target DNA by Cas9 enzyme

The CRISPR-associated protein 9 (Cas9) system has proven to be a powerful technology for genome editing in a wide variety of in vivo and in vitro applications. CRISPR-Cas9, when loaded with the guide RNA, cleaves the DNA at the target position as recognized by the guide RNA sequence. For successful application of this technology, it is important to study the biophysical parameters affecting its function. Temperature dependence of the Cas9 binding as well as energetics of product release after cleavage has not been well reported in the literature. In this work, we study the binding properties of Cas9 enzyme to the sequence specific target DNA at a range of temperatures and, surprisingly, find that the Cas9 enzyme, in our study, can find and bind its target DNA with 90 ± 20% efficiency at temperatures as low as 4 °C. Further, we show that the cleaved DNA products remain bound to the Cas9 enzyme strongly and is released from the enzyme only at higher temperatures. Using the gel shift assays, we quantify the rate of Cas9 binding to target DNA to be 0.8 ± 0.2 min-1 at 37 °C. We also tested denaturant (SDS) dependent release of cleaved product which showed a similar release pattern with a dissociation constant of 0.23 ± 0.04 mM. Our results of heat and denaturant dependence on Cas9-DNA binding and release mechanics will provide valuable insights for developing temperature dependent applications of the CRISPR-Cas9 technology.

DNA methylation can alter CRISPR/Cas9 editing frequency and DNA repair outcome in a target-specific manner

The impact of epigenetic modifications on the efficacy of CRISPR/Cas9-mediated double-stranded DNA breaks and subsequent DNA repair is poorly understood, especially in plants. In this study, we investigated the effect of the level of cytosine methylation on the outcome of CRISPR/Cas9-induced mutations at multiple Cas9 target sites in Nicotiana benthamiana leaf cells using next-generation sequencing. We found that high levels of promoter methylation, but not gene-body methylation, decreased the frequency of Cas9-mediated mutations. DNA methylation also influenced the ratio of insertions and deletions and potentially the type of Cas9 cleavage in a target-specific manner. In addition, we detected an over-representation of deletion events governed by a single 5'-terminal nucleotide at Cas9-induced DNA breaks. Our findings suggest that DNA methylation can indirectly impair Cas9 activity and subsequent DNA repair, probably through changes in the local chromatin structure. In addition to the well described Cas9-induced blunt-end double-stranded DNA breaks, we provide evidence for Cas9-mediated staggered DNA cuts in plant cells. Both types of cut may direct microhomology-mediated DNA repair by a novel, as yet undescribed, mechanism.

Improvement of Gene Delivery and Mutation Efficiency in the CRISPR-Cas9 Wheat (Triticum aestivum L.) Genomics System via Biolistics

Discovery of the CRISPR-Cas9 gene editing system revolutionized the field of plant genomics. Despite advantages in the ease of designing gRNA and the low cost of the CRISPR-Cas9 system, there are still hurdles to overcome in low mutation efficiencies, specifically in hexaploid wheat. In conjunction with gene delivery and transformation frequency, the mutation efficiency bottleneck has the potential to slow down advancements in genomic editing of wheat. In this study, nine bombardment parameter combinations using three gold particle sizes and three rupture disk pressures were tested to establish optimal stable transformation frequencies in wheat. Utilizing the best transformation protocol and a knockout cassette of the phytoene desaturase gene, we subjected transformed embryos to four temperature treatments and compared mutation efficiencies. The use of 0.6 μm gold particles for bombardment increased transformation frequencies across all delivery pressures. A heat treatment of 34 °C for 24 h resulted in the highest mutation efficiency with no or minimal reduction in transformation frequency. The 34 °C treatment produced two M₀ mutant events with albino phenotypes, requiring biallelic mutations in all three genomes of hexaploid wheat. Utilizing optimal transformation and heat treatment parameters greatly increases mutation efficiency and can help advance research efforts in wheat genomics.

CRISPR/Cas9 Technique for Temperature, Drought, and Salinity Stress Responses

Global warming and climate change have severely affected plant growth and food production. Therefore, minimizing these effects is required for sustainable crop yields. Understanding the molecular mechanisms in response to abiotic stresses and improving agricultural traits to make crops tolerant to abiotic stresses have been going on unceasingly. To generate desirable varieties of crops, traditional and molecular breeding techniques have been tried, but both approaches are time-consuming. Clustered regularly interspaced short palindromic repeat/Cas9 (CRISPR/Cas9) and transcription activator-like effector nucleases (TALENs) are genome-editing technologies that have recently attracted the attention of plant breeders for genetic modification. These technologies are powerful tools in the basic and applied sciences for understanding gene function, as well as in the field of crop breeding. In this review, we focus on the application of genome-editing systems in plants to understand gene function in response to abiotic stresses and to improve tolerance to abiotic stresses, such as temperature, drought, and salinity stresses.

What Did We Learn From Current Progress in Heat Stress Tolerance in Plants? Can Microbes Be a Solution?

Temperature is a significant parameter in agriculture since it controls seed germination and plant growth. Global warming has resulted in an irregular rise in temperature posing a serious threat to the agricultural production around the world. A slight increase in temperature acts as stress and exert an overall negative impact on different developmental stages including plant phenology, development, cellular activities, gene expression, anatomical features, the functional and structural orientation of leaves, twigs, roots, and shoots. These impacts ultimately decrease the biomass, affect reproductive process, decrease flowering and fruiting and significant yield losses. Plants have inherent mechanisms to cope with different stressors including heat which may vary depending upon the type of plant species, duration and degree of the heat stress. Plants initially adapt avoidance and then tolerance strategies to combat heat stress. The tolerance pathway involves ion transporter, osmoprotectants, antioxidants, heat shock protein which help the plants to survive under heat stress. To develop heat-tolerant plants using above-mentioned strategies requires a lot of time, expertise, and resources. On contrary, plant growth-promoting rhizobacteria (PGPRs) is a cost-effective, time-saving, and user-friendly approach to support and enhance agricultural production under a range of environmental conditions including stresses. PGPR produce and regulate various phytohormones, enzymes, and metabolites that help plant to maintain growth under heat stress. They form biofilm, decrease abscisic acid, stimulate root development, enhance heat shock proteins, deamination of ACC enzyme, and nutrient availability especially nitrogen and phosphorous. Despite extensive work done on plant heat stress tolerance in general, very few comprehensive reviews are available on the subject especially the role of microbes for plant heat tolerance. This article reviews the current studies on the retaliation, adaptation, and tolerance to heat stress at the cellular, organellar, and whole plant levels, explains different approaches, and sheds light on how microbes can help to induce heat stress tolerance in plants.

A CRISPR/Cas9-Based System with Controllable Auto-Excision Feature Serving Cisgenic Plant Breeding and Beyond

Transgenic or genetically modified crops have great potential in modern agriculture but still suffer from heavy regulations worldwide due to biosafety concerns. As a promising alternative route, cisgenic crops have received higher public acceptance and better reviews by governing authorities. To serve the purpose of cisgenic plant breeding, we have developed a CRISPR/Cas9-based vector system, which is capable of delivering target gene-of-interest (GOI) into recipient plants while removing undesired genetic traces in the plants. The new system features a controllable auto-excision feature, which is realized by a core design of embedded multi-clonal sequence and the use of inducible promoters controlling the expression of Cas9 nuclease. In the current proof-of-concept study in Arabidopsis thaliana (L.) Heynh., we have successfully incorporated a GOI into the plant and removed the selection marker and CRISPR/Cas9 components from the final product. Following the designed workflow, we have demonstrated that novel cisgenic plant germplasms with desired traits could be developed within one to two generations. Further characterizations of the vector system have shown that heat treatment at 37 °C could significantly improve the editing efficiency (up to 100%), and no off-target mutations were identified in the Arabidopsis background. This novel vector system is the first CRISPR/Cas9-based genome editing tool for cisgenic plant breeding and should prove powerful for other similar applications in the bright future of precision molecular breeding.

Genome Editing: A Promising Approach for Achieving Abiotic Stress Tolerance in Plants

The susceptibility of crop plants towards abiotic stresses is highly threatening to assure global food security as it results in almost 50% annual yield loss. To address this issue, several strategies like plant breeding and genetic engineering have been used by researchers from time to time. However, these approaches are not sufficient to ensure stress resilience due to the complexity associated with the inheritance of abiotic stress adaptive traits. Thus, researchers were prompted to develop novel techniques with high precision that can address the challenges connected to the previous strategies. Genome editing is the latest approach that is in the limelight for improving the stress tolerance of plants. It has revolutionized crop research due to its versatility and precision. The present review is an update on the different genome editing tools used for crop improvement so far and the various challenges associated with them. It also highlights the emerging potential of genome editing for developing abiotic stress-resilient crops.

Expanding the targeting scope of FokI-dCas nuclease systems with SpRY and Mb2Cas12a

CRISPR-Cas9 and Cas12a are widely used sequence specific nucleases (SSNs) for genome editing. The nuclease domains of Cas proteins can induce DNA double strand breaks upon RNA guided DNA targeting. Zinc finger nucleases (ZFNs) and Transcription Activator-Like Effector Nucleases (TALENs) have been popular SSNs prior to CRISPR. Both ZFNs and TALENs are based on reconstitution of two monomers with each consisting of a DNA binding domain and a FokI nuclease domain. Inspired by the configuration of ZFNs and TALENs, dimeric FokI-dCas9 systems were previously demonstrated in human cells. Such configuration, based on a pair of guide RNAs (gRNAs), offers great improvement on targeting specificity. To expand the targeting scope of dimeric FokI-dCas systems, we explored the PAM (protospacer adjacent motif)-less SpRY Cas9 variant and the PAM-relaxed Mb2Cas12a system. We showed in rice cells that FokI-dSpRY had more robust editing efficiency than a paired SpRY nickase system. Furthermore, we developed a dimeric FokI-dMb2Cas12a system that displayed comparable editing activity to Mb2Cas12a nuclease in rice cells. Finally, we developed a single-chain FokI-FokI-dMb2Cas12a system that cuts DNA outside its targeting sequence, which could be useful for many versatile applications. Together, this work greatly expanded the FokI based CRISPR-Cas systems for genome editing. This article is protected by copyright. All rights reserved.