CRISPR/Cas9-mediated targeted mutagenesis in Nicotiana tabacum

Harnessing promoter elements to enhance gene editing in plants: perspectives and advances

Genome-edited plants, endowed with climate-smart traits, have been promoted as tools for strengthening resilience against climate change. Successful plant gene editing (GE) requires precise regulation of the GE machinery, a process controlled by the promoters, which drives its transcription through interactions with transcription factors (TFs) and RNA polymerase. While constitutive promoters are extensively used in GE constructs, their limitations highlight the need for alternative approaches. This review emphasizes the promise of tissue/organ specific as well as inducible promoters, which enable targeted GE in a spatiotemporal manner with no effects on other tissues. Advances in synthetic biology have paved the way for the creation of synthetic promoters, offering refined control over gene expression and augmenting the potential of plant GE. The integration of these novel promoters with synthetic systems presents significant opportunities for precise and conditional genome editing. Moreover, the advent of bioinformatic tools and artificial intelligence is revolutionizing the characterization of regulatory elements, enhancing our understanding of their roles in plants. Thus, this review provides novel insights into the strategic use of promoters and promoter editing to enhance the precision, efficiency and specificity of plant GE, setting the stage for innovative crop improvement strategies.

Cutin formation in tomato is controlled by a multipartite module of synergistic and antagonistic transcription factors

Cuticles protect plants from water loss and pathogen attack. We address here the functional significance of SlGRAS9, SlZHD17, and SlMBP3 in regulating cutin formation in tomato fruit. The study unveils the role of the multipartite "SlGRAS9-SlZHD17-SlMBP3-SlMIXTA-like" transcription factor module in cutin biosynthesis. Plants deficient in SlGRAS9, SlZHD17, or SlMBP3 exhibit thickened cuticles and a higher accumulation of cutin monomers, conferring extended fruit shelf life and higher tolerance to postharvest fungal infection. SlGRAS9 regulation of cutin is mediated by SlZHD17, a negative regulator of SlCYP86A69. SlZHD17 acts synergistically with SlMBP3 to repress SlCYP86A69, and its interaction with SlMIXTA-like prevents the binding to the SlCYP86A69 promoter, thereby releasing the repression of cutin biosynthesis. SlZHD17 and SlMBP3 synergistically repress cutin biosynthesis, while SlMIXTA-like and SlCD2 act antagonistically to SlZHD17 and SlMBP3 on this metabolic pathway. The study defines targets for breeding strategies aimed at improving cuticle-associated traits in tomato and potentially other crops.

Knocking out artificially selected gene GmAOC4H8 improves germination in soybean

Seed germination is an essential stage in the life cycle of flowering plants, influencing the field emergence rates of seeds. Consequently, the role of GmAOC4 in soybean seed germination was investigated in the present study. Results suggested that the chloroplast-localized GmAOC4 exhibited high expression levels in the roots and young pods and during the seed germination stage in soybeans. It was found that GmAOC4 has been artificially selected during soybean domestication and improvement and that GmAOC4H8 showed repressed seed germination, of which the frequency in landraces and cultivars decreased when compared with wild soybean. Knocking out GmAOC4H8 via CRISPR/Cas9 led to enhanced germination in gmaoc4 mutants, suggesting its negative regulation on seed germination in soybeans. Additionally, decreased endogenous jasmonic acid (JA) and JA precursor, 12-oxo-phytodienoic acid, were found in gmaoc4 mutants. RNA-seq analyses revealed that 91 and 269 differentially expressed genes (DEGs) were up-regulated and down-regulated in gmaoc4 mutants, respectively. Among these DEGs, three genes were involved in JA biosynthetic and signaling pathways. Our results offer new insights into the mechanism of soybean seed germination regulation by GmAOC4.

Establishment of a CRISPR-Cas9-Mediated Genome Editing System in Flax

Flax is an important crop used for oil and fiber production. Although genetic engineering has been possible in flax, it is not commonly used to produce cultivars. However, the use of genome editing technology, which can produce site-specific mutations without introducing foreign genes, may be a valuable tool for creating elite cultivars that can be easily cultivated. The purpose of this study was to investigate the potential of genome editing in flax by establishing the clustered regularly interspaced short palindromic repeats (CR ISPR)-CRISPR-associated protein 9 (CRISPR-Cas9) genome editing system using the phytoene desaturase (PDS) gene, which produces albino mutants that are easily identifiable. Four sgRNAs were designed from two PDS genes of Flax (LuPDS1 and LuPDS2), and CRISPR-Cas9 genome editing vectors were constructed. After gene transformation, albino phenotypes were observed in transformed callus and regenerated plantlets on selection media. Polymerase chain reaction (PCR) amplification and sequencing of the PDS genes revealed deletions and insertions in the albino tissues, indicating successful editing of the PDS genes. Potential off-target sites were analyzed, but no off-target mutations were found, indicating the specificity of the CRISPR-Cas9 system. The establishment of a flax genome editing system using the CRISPR-Cas9 technology opens up new possibilities for the genetic engineering of flax. This study demonstrates the potential of genome editing in creating elite cultivars that can be easily cultivated, which can have significant implications for the flax industry.

Application and development of CRISPR technology in the secondary metabolic pathway of the active ingredients of phytopharmaceuticals

As an efficient gene editing tool, the CRISPR/Cas9 system has been widely employed to investigate and regulate the biosynthetic pathways of active ingredients in medicinal plants. CRISPR technology holds significant potential for enhancing both the yield and quality of active ingredients in medicinal plants. By precisely regulating the expression of key enzymes and transcription factors, CRISPR technology not only deepens our understanding of secondary metabolic pathways in medicinal plants but also opens new avenues for drug development and the modernization of traditional Chinese medicine. This article introduces the principles of CRISPR technology and its efficacy in gene editing, followed by a detailed discussion of its applications in the secondary metabolism of medicinal plants. This includes an examination of the composition of active ingredients and the implementation of CRISPR strategies within metabolic pathways, as well as the influence of Cas9 protein variants and advanced CRISPR systems in the field. In addition, this article examines the long-term impact of CRISPR technology on the progress of medicinal plant research and development. It also raises existing issues in research, including off-target effects, complexity of genome structure, low transformation efficiency, and insufficient understanding of metabolic pathways. At the same time, this article puts forward some insights in order to provide new ideas for the subsequent application of CRISPR in medicinal plants. In summary, CRISPR technology presents broad application prospects in the study of secondary metabolism in medicinal plants and is poised to facilitate further advancements in biomedicine and agricultural science. As technological advancements continue and challenges are progressively addressed, CRISPR technology is expected to play an increasingly vital role in the research of active ingredients in medicinal plants.

Integration of CRISPR/Cas9 with multi-omics technologies to engineer secondary metabolite productions in medicinal plant: Challenges and Prospects

Plants acts as living chemical factories that may create a large variety of secondary metabolites, most of which are used in pharmaceutical products. The production of these secondary metabolites is often much lower. Moreover, the primary constraint after discovering potential metabolites is the capacity to manufacture sufficiently for use in industrial and therapeutic contexts. The development of omics technology has brought revolutionary discoveries in various scientific fields, including transcriptomics, metabolomics, and genome sequencing. The metabolic pathways leading to the utilization of new secondary metabolites in the pharmaceutical industry can be identified with the use of these technologies. Genome editing (GEd) is a versatile technology primarily used for site-directed DNA insertions, deletions, replacements, base editing, and activation/repression at the targeted locus. Utilizing GEd techniques such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 (CRISPR-associated protein 9), metabolic pathways engineered to synthesize bioactive metabolites optimally. This article will briefly discuss omics and CRISPR/Cas9-based methods to improve secondary metabolite production in medicinal plants.

Stress-Induced Autophagy Is Essential for Microspore Cell Fate Transition to the Initial Cell of Androgenesis

The isolated microspores can be reprogrammed towards embryogenesis via stress treatment during in vitro culture, and produce (doubled) haploid plants as a breeding source of new genetic variability. However, the mechanism underlying the cell fate transition from gametogenesis to embryogenesis remains largely unknown. Here, we report that autophagy plays a key role in cell fate transition for microspore embryogenesis (referred to as androgenesis) in Nicotiana tabacum. Immunofluorescence and transmission electronic microscopy detection unveiled that autophagy was triggered in microspores following exposure to inductive stress, and a transient wave of the numerous autophagy-related genes (ATGs) expression occurred before the initiation of microspore embryogenesis. Suppression or promotion of the original autophagy levels could inhibit microspore embryogenesis, indicating that stress-induced autophagic homeostasis is essential for cell fate transition. Furthermore, quantitative proteomics analysis revealed that autophagy might be involved in lignin biosynthesis and chromatin decondensation for promoting reprogramming for androgenesis initiation. Altogether, we reveal an essential role of autophagy in the microspore cell fate transition and androgenesis initiation, providing novel insight for understanding this critical developmental process.

Advancements and Future Prospects of CRISPR-Cas-Based Population Replacement Strategies in Insect Pest Management

Population replacement refers to the process by which a wild-type population of insect pests is replaced by a population possessing modified traits or abilities. Effective population replacement necessitates a gene drive system capable of spreading desired genes within natural populations, operating under principles akin to super-Mendelian inheritance. Consequently, releasing a small number of genetically edited insects could potentially achieve population control objectives. Currently, several gene drive approaches are under exploration, including the newly adapted CRISPR-Cas genome editing system. Multiple studies are investigating methods to engineer pests that are incapable of causing crop damage or transmitting vector-borne diseases, with several notable successful examples documented. This review summarizes the recent advancements of the CRISPR-Cas system in the realm of population replacement and provides insights into research methodologies, testing protocols, and implementation strategies for gene drive techniques. The review also discusses emerging trends and prospects for establishing genetic tools in pest management.

Advancing crop disease resistance through genome editing: a promising approach for enhancing agricultural production

Modern agriculture has encountered several challenges in achieving constant yield stability especially due to disease outbreaks and lack of long-term disease-resistant crop cultivars. In the past, disease outbreaks in economically important crops had a major impact on food security and the economy. On the other hand climate-driven emergence of new pathovars or changes in their host specificity further poses a serious threat to sustainable agriculture. At present, chemical-based control strategies are frequently used to control microbial pathogens and pests, but they have detrimental impact on the environment and also resulted in the development of resistant phyto-pathogens. As a replacement, cultivating engineered disease-resistant crops can help to minimize the negative impact of regular pesticides on agriculture and the environment. Although traditional breeding and genetic engineering have been instrumental in crop disease improvement but they have certain limitations such as labour intensity, time consumption, and low efficiency. In this regard, genome editing has emerged as one of the potential tools for improving disease resistance in crops by targeting multiple traits with more accuracy and efficiency. For instance, genome editing techniques, such as CRISPR/Cas9, CRISPR/Cas13, base editing, TALENs, ZFNs, and meganucleases, have proved successful in improving disease resistance in crops through targeted mutagenesis, gene knockouts, knockdowns, modifications, and activation of target genes. CRISPR/Cas9 is unique among these techniques because of its remarkable efficacy, low risk of off-target repercussions, and ease of use. Some primary targets for developing CRISPR-mediated disease-resistant crops are host-susceptibility genes (the S gene method), resistance genes (R genes) and pathogen genetic material that prevents their development, broad-spectrum disease resistance. The use of genome editing methods has the potential to notably ameliorate crop disease resistance and transform agricultural practices in the future. This review highlights the impact of phyto-pathogens on agricultural productivity. Next, we discussed the tools for improving disease resistance while focusing on genome editing. We provided an update on the accomplishments of genome editing, and its potential to improve crop disease resistance against bacterial, fungal and viral pathogens in different crop systems. Finally, we highlighted the future challenges of genome editing in different crop systems for enhancing disease resistance.

The SlGRAS9-SlZHD17 transcriptional cascade regulates chlorophyll and carbohydrate metabolism contributing to fruit quality traits in tomato

Some essential components of fleshy fruits are dependent on photosynthetic activity and carbohydrate metabolism. Nevertheless, the regulatory mechanisms linking chlorophyll and carbohydrate metabolism remain partially understood. Here, we uncovered the role of SlGRAS9 and SlZHD17 transcription factors in controlling chlorophyll and carbohydrate accumulation in tomato fruit. Knockout or knockdown of SlGRAS9 or SlZHD17 resulted in marked increase in chlorophyll content, reprogrammed chloroplast biogenesis and enhanced accumulation of starch and soluble sugars. Combined genome-wide transcriptomic profiling and promoter-binding experiments unveiled a complex mechanism in which the SlGRAS9/SlZHD17 regulatory module modulates the expression of chloroplast and sugar metabolism either via a sequential transcriptional cascade or through binding of both TFs to the same gene promoters, or, alternatively, via parallel pathways where each of the TFs act on different target genes. For instance, the regulation of SlAGPaseS1 and SlSUS1 is mediated by SlZHD17 whereas that of SlVI and SlGLK1 occurs only through SlGRAS9 without the intervention of SlZHD17. Both SlGRAS9 and SlZHD17 can also directly bind the promoter of SlPOR-B to regulate its expression. Taken together, our findings uncover two important regulators acting synergistically to manipulate chlorophyll and carbohydrate accumulation and provide new potential breeding targets for improving fruit quality in fleshy fruits.

CRISPR/Cas9-gene editing approaches in plant breeding

CRISPR/Cas9 gene editing system is recently developed robust genome editing technology for accelerating plant breeding. Various modifications of this editing system have been established for adaptability in plant varieties as well as for its improved efficiency and portability. This review provides an in-depth look at the various strategies for synthesizing gRNAs for efficient delivery in plant cells, including chemical synthesis and in vitro transcription. It also covers traditional analytical tools and emerging developments in detection methods to analyze CRISPR/Cas9 mediated mutation in plant breeding. Additionally, the review outlines the various analytical tools which are used to detect and analyze CRISPR/Cas9 mediated mutations, such as next-generation sequencing, restriction enzyme analysis, and southern blotting. Finally, the review discusses emerging detection methods, including digital PCR and qPCR. Hence, CRISPR/Cas9 has great potential for transforming agriculture and opening avenues for new advancements in the system for gene editing in plants.

CRISPR/Cas9 Editing Sites Identification and Multi-Elements Association Analysis in Camellia sinensis

CRISPR/Cas9 is an efficient genome-editing tool, and the identification of editing sites and potential influences in the Camellia sinensis genome have not been investigated. In this study, bioinformatics methods were used to characterise the Camellia sinensis genome including editing sites, simple sequence repeats (SSRs), G-quadruplexes (GQ), gene density, and their relationships. A total of 248,134,838 potential editing sites were identified in the genome, and five PAM types, AGG, TGG, CGG, GGG, and NGG, were observed, of which 66,665,912 were found to be specific, and they were present in all structural elements of the genes. The characteristic region of high GC content, GQ density, and PAM density in contrast to low gene density and SSR density was identified in the chromosomes in the joint analysis, and it was associated with secondary metabolites and amino acid biosynthesis pathways. CRISPR/Cas9, as a technology to drive crop improvement, with the identified editing sites and effector elements, provides valuable tools for functional studies and molecular breeding in Camellia sinensis.

Knockout of CAFFEOYL-COA 3-O-METHYLTRANSFERASE 6/6L enhances the S/G ratio of lignin monomers and disease resistance in Nicotiana tabacum

Background

Nicotiana tabacum is an important economic crop, which is widely planted in the world. Lignin is very important for maintaining the physiological and stress-resistant functions of tobacco. However, higher lignin content will produce lignin gas, which is not conducive to the formation of tobacco quality. To date, how to precisely fine-tune lignin content or composition remains unclear.

Results

Here, we annotated and screened 14 CCoAOMTs in Nicotiana tabacum and obtained homozygous double mutants of CCoAOMT6 and CCoAOMT6L through CRSIPR/Cas9 technology. The phenotype showed that the double mutants have better growth than the wild type whereas the S/G ratio increased and the total sugar decreased. Resistance against the pathogen test and the extract inhibition test showed that the transgenic tobacco has stronger resistance to tobacco bacterial wilt and brown spot disease, which are infected by Ralstonia solanacearum and Alternaria alternata, respectively. The combined analysis of metabolome and transcriptome in the leaves and roots suggested that the changes of phenylpropane and terpene metabolism are mainly responsible for these phenotypes. Furthermore, the molecular docking indicated that the upregulated metabolites, such as soyasaponin Bb, improve the disease resistance due to highly stable binding with tyrosyl-tRNA synthetase targets in Ralstonia solanacearum and Alternaria alternata.

Conclusions

CAFFEOYL-COA 3-O-METHYLTRANSFERASE 6/6L can regulate the S/G ratio of lignin monomers and may affect tobacco bacterial wilt and brown spot disease resistance by disturbing phenylpropane and terpene metabolism in leaves and roots of Nicotiana tabacum, such as soyasaponin Bb.

Development of an Agrobacterium-delivered codon-optimized CRISPR/Cas9 system for chickpea genome editing

Chickpea is considered recalcitrant to in vitro tissue culture amongst all edible legumes. The clustered, regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9)-based genome editing in chickpea can remove the bottleneck of limited genetic variation in this cash crop, which is rich in nutrients and protein. However, generating stable mutant lines using CRISPR/Cas9 requires efficient and highly reproducible transformation protocols. As an attempt to solve this problem, we developed a modified and optimized protocol for chickpea transformation. This study transformed the single cotyledon half-embryo explants using CaMV35S promoter to drive two marker genes (β-glucuronidase gene; GUS and green fluorescent protein; GFP) through binary vectors pBI101.2 and modified pGWB2, respectively. These vectors were delivered in the explants through three different strains of Agrobacterium tumefaciens, viz., GV3101, EHA105, and LBA4404. We found better efficiency with the strain GV3101 (17.56%) compared with two other strains, i.e., 8.54 and 5.43%, respectively. We recorded better regeneration frequencies in plant tissue culture for the constructs GUS and GFP, i.e., 20.54% and 18.09%, respectively. The GV3101 was further used for the transformation of the genome editing construct. For the development of genome-edited plants, we used this modified protocol. We also used a modified binary vector pPZP200 by introducing a CaMV35S-driven chickpea codon-optimized SpCas9 gene. The promoter of the Medicago truncatula U6.1 snRNA gene was used to drive the guide RNA cassettes. This cassette targeted and edited the chickpea phytoene desaturase (CaPDS) gene. A single gRNA was found sufficient to achieve high efficiency (42%) editing with the generation of PDS mutants with albino phenotypes. A simple, rapid, highly reproducible, stable transformation and CRISPR/Cas9-based genome editing system for chickpea was established. This study aimed to demonstrate this system's applicability by performing a gene knockout of the chickpea PDS gene using an improved chickpea transformation protocol for the first time.

Male sterility in plants: an overview of advancements from natural CMS to genetically manipulated systems for hybrid seed production

KEY MESSAGE

The male sterility system in plants has traditionally been utilized for hybrid seed production. In last three decades, genetic manipulation for male sterility has revolutionized this area of research related to hybrid seed production technology. Here, we have surveyed some of the natural cytoplasmic male sterility (CMS) systems that existed/ were developed in different crop plants for developing male sterility-fertility restoration systems used in hybrid seed production and highlighted some of the recent biotechnological advancements in the development of genetically engineered systems that occurred in this area. We have indicated the possible future directions toward the development of engineered male sterility systems. Cytoplasmic male sterility (CMS) is an important trait that is naturally prevalent in many plant species, which has been used in the development of hybrid varieties. This is associated with the use of appropriate genes for fertility restoration provided by the restorer line that restores fertility on the corresponding CMS line. The development of hybrids based on a CMS system has been demonstrated in several different crops. However, there are examples of species, which do not have usable cytoplasmic male sterility and fertility restoration systems (Cytoplasmic Genetic Male Sterility Systems-CGMS) for hybrid variety development. In such plants, it is necessary to develop usable male sterile lines through genetic engineering with the use of heterologous expression of suitable genes that control the development of male gametophyte and fertile male gamete formation. They can also be developed through gene editing using the recently developed CRISPR-Cas technology to knock out suitable genes that are responsible for the development of male gametes. The present review aims at providing an insight into the development of various technologies for successful production of hybrid varieties and is intended to provide only essential information on male sterility systems starting from naturally occurring ones to the genetically engineered systems obtained through different means.

Role of BraRGL1 in regulation of Brassica rapa bolting and flowering

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

Breakthrough in CRISPR/Cas system: Current and future directions and challenges

Targeted genome editing (GE) technology has brought a significant revolution in fictional genomic research and given hope to plant scientists to develop desirable varieties. This technology involves inducing site-specific DNA perturbations that can be repaired through DNA repair pathways. GE products currently include CRISPR-associated nuclease DNA breaks, prime editors generated DNA flaps, single nucleotide-modifications, transposases, and recombinases. The discovery of double-strand breaks, site-specific nucleases (SSNs), and repair mechanisms paved the way for targeted GE, and the first-generation GE tools, ZFNs and TALENs, were successfully utilized in plant GE. However, CRISPR-Cas has now become the preferred tool for GE due to its speed, reliability, and cost-effectiveness. Plant functional genomics has benefited significantly from the widespread use of CRISPR technology for advancements and developments. This review highlights the progress made in CRISPR technology, including multiplex editing, base editing (BE), and prime editing (PE), as well as the challenges and potential delivery mechanisms.

Using A Protoplast Transformation System to Enable Functional Studies in Mangifera indica L

Mangoes (Mangifera indica L.) are an important kind of perennial fruit tree, but their biochemical testing method and transformation technology were insufficient and had not been rigorously explored. The protoplast technology is an excellent method for creating a rapid and effective tool for transient expression and transformation assays, particularly in plants that lack an Agrobacterium-mediated plant transformation system. This study optimized the conditions of the protoplast isolation and transformation system, which can provide a lot of help in the gene expression regulation study of mango. The most beneficial protoplast isolation conditions were 150 mg/mL of cellulase R-10 and 180 mg/mL of macerozyme R-10 in the digestion solution at pH 5.6 and 12 h of digestion time. The 0.16 M and 0.08 M mannitol in wash solution (WI) and suspension for counting (MMG), respectively, were optimal for the protoplast isolation yield. The isolated leaf protoplasts (~5.4 × 105 cells/10 mL) were transfected for 30 min mediated by 40% calcium-chloride-based polyethylene glycol (PEG)-4000-CaCl2, from which 84.38% of the protoplasts were transformed. About 0.08 M and 0.12 M of mannitol concentration in MMG and transfection solutions, respectively, were optimal for protoplast viability. Under the florescence signal, GFP was seen in the transformed protoplasts. This showed that the target gene was successfully induced into the protoplast and that it can be transcribed and translated. Experimental results in this paper show that our high-efficiency protoplast isolation and PEG-mediated transformation protocols can provide excellent new methods for creating a rapid and effective tool for the molecular mechanism study of mangoes.

Effects of editing DFR genes on flowers, leaves, and roots of tobacco

BACKGROUND

DFR is a crucial structural gene in plant flavonoid and polyphenol metabolism, and DFR knockout (DFR-KO) plants may have increased biomass accumulation. It is uncertain whether DFR-KO has comparable effects in tobacco and what the molecular mechanism is. We employed the CRISPR/Cas9 method to generate a knockout homozygous construct and collected samples from various developmental phases for transcriptome and metabolome detection and analysis.

RESULTS

DFR-KO turned tobacco blossoms white on homozygous tobacco (Nicotiana tabacum) plants with both NtDFR1 and NtDFR2 knockout. RNA-seq investigation of anthesis leaf (LF), anthesis flower (FF), mature leaf (LM), and mature root (RM) variations in wild-type (CK) and DFR-KO lines revealed 2898, 276, 311, and 101 differentially expressed genes (DEGs), respectively. DFR-KO primarily affected leaves during anthesis. According to KEGG and GSEA studies, DFR-KO lines upregulated photosynthetic pathway carbon fixation and downregulated photosystem I and II genes. DFR-KO may diminish tobacco anthesis leaf photosynthetic light reaction but boost dark reaction carbon fixation. DFR-KO lowered the expression of pathway-related genes in LF, such as oxidative phosphorylation and proteasome, while boosting those in the plant-pathogen interaction and MAPK signaling pathways, indicating that it may increase biological stress resistance. DFR-KO greatly boosted the expression of other structural genes involved in phenylpropanoid production in FF, which may account for metabolite accumulation. The metabolome showed that LF overexpressed 8 flavonoid metabolites and FF downregulated 24 flavone metabolites. In DFR-KO LF, proteasome-related genes downregulated 16 amino acid metabolites and reduced free amino acids. Furthermore, the DEG analysis on LM revealed that the impact of DFR-KO on tobacco growth may progressively diminish with time.

CONCLUSION

The broad impact of DFR-KO on different phases and organs of tobacco development was thoroughly and methodically investigated in this research. DFR-KO decreased catabolism and photosynthetic light reactions in leaves during the flowering stage while increasing carbon fixation and disease resistance pathways. However, the impact of DFR-KO on tobacco growth steadily declined as it grew and matured, and transcriptional and metabolic modifications were consistent. This work offers a fresh insight and theoretical foundation for tobacco breeding and the development of gene-edited strains.

CRISPR/Cas9-Mediated Mutation in XSP10 and SlSAMT Genes Impart Genetic Tolerance to Fusarium Wilt Disease of Tomato (Solanum lycopersicum L.)

Fusarium wilt is a major devastating fungal disease of tomato (Solanum lycopersicum L.) caused by Fusarium oxysporum f. sp. lycopersici (Fol) which reduces the yield and production. Xylem sap protein 10 (XSP10) and Salicylic acid methyl transferase (SlSAMT) are two putative negative regulatory genes associated with Fusarium wilt of tomato. Fusarium wilt tolerance in tomato can be developed by targeting these susceptible (S) genes. Due to its efficiency, high target specificity, and versatility, CRISPR/Cas9 has emerged as one of the most promising techniques for knocking out disease susceptibility genes in a variety of model and agricultural plants to increase tolerance/resistance to various plant diseases in recent years. Though alternative methods, like RNAi, have been attempted to knock down these two S genes in order to confer resistance in tomato against Fusarium wilt, there has been no report of employing the CRISPR/Cas9 system for this specific intent. In this study, we provide a comprehensive downstream analysis of the two S genes via CRISPR/Cas9-mediated editing of single (XSP10 and SlSAMT individually) and dual-gene (XSP10 and SlSAMT simultaneously). Prior to directly advancing on to the generation of stable lines, the editing efficacy of the sgRNA-Cas9 complex was first validated using single cell (protoplast) transformation. In the transient leaf disc assay, the dual-gene editing showed strong phenotypic tolerance to Fusarium wilt disease with INDEL mutations than single-gene editing. In stable genetic transformation of tomato at the GE1 generation, dual-gene CRISPR transformants of XSP10 and SlSAMT primarily exhibited INDEL mutations than single-gene-edited lines. The dual-gene CRISPR-edited lines (CRELs) of XSP10 and SlSAMT at GE1 generation conferred a strong phenotypic tolerance to Fusarium wilt disease compared to single-gene-edited lines. Taken together, the reverse genetic studies in transient and stable lines of tomato revealed that, XSP10 and SlSAMT function together as negative regulators in conferring genetic tolerance to Fusarium wilt disease.

Genome Editing by CRISPR/Cas9 in Polyploids

CRISPR/Cas system has been widely used for genome editing in the past few years. Even though it has been performed in many polyploid species to date, its efficient accomplishment in these organisms is still a challenge. The presence of multiple homoeologous genes as targets for their editing requires more rigorous work and specific needs to assess successful genome editing. Here, we describe a general stepwise protocol to select target sites, design sgRNAs, indicate vector requirements, and screen CRISPR/Cas9-mediated genome editing in polyploid species.

Improvement of Soybean; A Way Forward Transition from Genetic Engineering to New Plant Breeding Technologies

Soybean is considered one of the important crops among legumes. Due to high nutritional contents in seed (proteins, sugars, oil, fatty acids, and amino acids), soybean is used globally for food, feed, and fuel. The primary consumption of soybean is vegetable oil and feed for chickens and livestock. Apart from this, soybean benefits soil fertility by fixing atmospheric nitrogen through root nodular bacteria. While conventional breeding is practiced for soybean improvement, with the advent of new biotechnological methods scientists have also engineered soybean to improve different traits (herbicide, insect, and disease resistance) to fulfill consumer requirements and to meet the global food deficiency. Genetic engineering (GE) techniques such as transgenesis and gene silencing help to minimize the risks and increase the adaptability of soybean. Recently, new plant breeding technologies (NPBTs) emerged such as zinc-finger nucleases, transcription activator-like effector nucleases, and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR/Cas9), which paved the way for enhanced genetic modification of soybean. These NPBTs have the potential to improve soybean via gene functional characterization precision genome engineering for trait improvement. Importantly, these NPBTs address the ethical and public acceptance issues related to genetic modifications and transgenesis in soybean. In the present review, we summarized the improvement of soybean through GE and NPBTs. The valuable traits that have been improved through GE for different constraints have been discussed. Moreover, the traits that have been improved through NPBTs and potential targets for soybean improvements via NPBTs and solutions for ethical and public acceptance are also presented.

Machine learning in the estimation of CRISPR-Cas9 cleavage sites for plant system

CRISPR-Cas9 system is one of the recent most used genome editing techniques. Despite having a high capacity to alter the precise target genes and genomic regions that the planned guide RNA (or sgRNA) complements, the off-target effect still exists. But there are already machine learning algorithms for people, animals, and a few plant species. In this paper, an effort has been made to create models based on three machine learning-based techniques [namely, artificial neural networks (ANN), support vector machines (SVM), and random forests (RF)] for the prediction of the CRISPR-Cas9 cleavage sites that will be cleaved by a particular sgRNA. The plant dataset was the sole source of inspiration for all of these machine learning-based algorithms. 70% of the on-target and off-target dataset of various plant species that was gathered was used to train the models. The remaining 30% of the data set was used to evaluate the model's performance using a variety of evaluation metrics, including specificity, sensitivity, accuracy, precision, F1 score, F2 score, and AUC. Based on the aforementioned machine learning techniques, eleven models in all were developed. Comparative analysis of these produced models suggests that the model based on the random forest technique performs better. The accuracy of the Random Forest model is 96.27%, while the AUC value was found to be 99.21%. The SVM-Linear, SVM-Polynomial, SVM-Gaussian, and SVM-Sigmoid models were trained, making a total of six ANN-based models (ANN1-Logistic, ANN1-Tanh, ANN1-ReLU, ANN2-Logistic, ANN2-Tanh, and ANN-ReLU) and Support Vector Machine models (SVM-Linear, SVM-Polynomial, SVM-Gaussian However, the overall performance of Random Forest is better among all other ML techniques. ANN1-ReLU and SVM-Linear model performance were shown to be better among Artificial Neural Network and Support Vector Machine-based models, respectively.

RNA Pol III promoters-key players in precisely targeted plant genome editing

The clustered regularly interspaced short palindrome repeat (CRISPR)/CRISPR-associated protein Cas) system is a powerful and highly precise gene-editing tool in basic and applied research for crop improvement programs. CRISPR/Cas tool is being extensively used in plants to improve crop yield, quality, and nutritional value and make them tolerant to environmental stresses. CRISPR/Cas system consists of a Cas protein with DNA endonuclease activity and one CRISPR RNA transcript that is processed to form one or several short guide RNAs that direct Cas9 to the target DNA sequence. The expression levels of Cas proteins and gRNAs significantly influence the editing efficiency of CRISPR/Cas-mediated genome editing. This review focuses on insights into RNA Pol III promoters and their types that govern the expression levels of sgRNA in the CRISPR/Cas system. We discussed Pol III promoters structural and functional characteristics and their comparison with Pol II promoters. Further, the use of synthetic promoters to increase the targeting efficiency and overcome the structural, functional, and expressional limitations of RNA Pol III promoters has been discussed. Our review reports various studies that illustrate the use of endogenous U6/U3 promoters for improving editing efficiency in plants and the applicative approach of species-specific RNA pol III promoters for genome editing in model crops like Arabidopsis and tobacco, cereals, legumes, oilseed, and horticultural crops. We further highlight the significance of optimizing these species-specific promoters' systematic identification and validation for crop improvement and biotic and abiotic stress tolerance through CRISPR/Cas mediated genome editing.

Highly efficient transgene-free genome editing in tobacco using an optimized CRISPR/Cas9 system, pOREU3TR

CRISPR/Cas9 genome-editing technology has revolutionized plant science and holds enormous promise for crop improvement. The exploration of this system received much attention regarding plant genome editing. Here, by editing the NtPDS gene in tobacco, we first verified that incorporating an OsU3-tRNA promoter combination into the CRISPR/Cas9 system contributed to the highest editing efficiency, as the sgRNA expression level was greater than that resulting from the AtU6-tRNA and AtU6 promoters. Then, we optimized the existing tobacco CRISPR/Cas9 system, pORE-Cas9, by using the OsU3-tRNA promoter combination instead of AtU6 and by fusing an AtUb10-Ros1 expression cassette to the T-DNA to monitor the transgene events. The new system was named pOREU3TR. As expected, 49 transgene-free and homozygous gene-edited green plants were effectively screened in the T₁ generation as a result of editing the NtLHT1 gene in tobacco, and the plant height and the contents of most free amino acids in the leaves of the T₂ mutant plants were significantly different from those in the leaves of WT plants, demonstrating the high efficiency of the new editing system. This OsU3-tRNA-sgRNA/AtUb10-Ros1 system provides essential improvements for increasing the efficiency of plant genome editing.

Genome editing technologies, mechanisms and improved production of therapeutic phytochemicals: Opportunities and prospects

Plants produce a large number of secondary metabolites, known as phytometabolites that may be employed as medicines, dyes, poisons, and insecticides in the field of medicine, agriculture, and industrial use, respectively. The rise of genome management approaches has promised a factual revolution in genetic engineering. Targeted genome editing in living entities permits the understanding of the biological systems very clearly, and also sanctions to address a wide-ranging objective in the direction of improving features of plant and their yields. The last few years have introduced a number of unique genome editing systems, including transcription activator-like effector nucleases, zinc finger nucleases, and miRNA-regulated clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9). Genome editing systems have helped in the transformation of metabolic engineering, allowing researchers to modify biosynthetic pathways of different secondary metabolites. Given the growing relevance of editing genomes in plant research, the exciting novel methods are briefly reviewed in this chapter. Also, this chapter highlights recent discoveries on the CRISPR-based modification of natural products in different medicinal plants.

Comprehensive analysis of the carboxylesterase gene reveals that NtCXE22 regulates axillary bud growth through strigolactone metabolism in tobacco

Carboxylesterases (CXE) are a class of hydrolytic enzymes with α/β-folding domains that play a vital role in plant growth, development, stress response, and activation of herbicide-active substances. In this study, 49 Nicotiana tabacum L. CXE genes (NtCXEs) were identified using a sequence homology search. The basic characteristics, phylogenetic evolution, gene structure, subcellular location, promoter cis-elements, and gene expression patterns of the CXE family were systematically analyzed. RNA-seq data and quantitative real-time PCR showed that the expression level of CXEs was associated with various stressors and hormones; gene expression levels were significantly different among the eight tissues examined and at different developmental periods. As a new class of hormones, strigolactones (SLs) are released from the roots of plants and can control the germination of axillary buds.NtCXE7, NtCXE9, NtCXE22, and NtCXE24 were homologous to Arabidopsis SLs hydrolase AtCXE15, and changes in their expression levels were induced by topping and by GR24 (a synthetic analogue of strigolactone). Further examination revealed that NtCXE22-mutant (ntcxe22) plants generated by CRISPR-Cas9 technology had shorter bud outgrowth with lower SLs content. Validation of NtCXE22 was also performed in NtCCD8-OE plants (with fewer axillary buds) and in ntccd8 mutant plants (with more axillary buds). The results suggest that NtCXE22 may act as an efficient SLs hydrolase and affects axillary bud development, thereby providing a feasible method for manipulating endogenous SLs in crops and ornamental plants.

CRISPR/Cas Genome Editing Technologies for Plant Improvement against Biotic and Abiotic Stresses: Advances, Limitations, and Future Perspectives

Crossbreeding, mutation breeding, and traditional transgenic breeding take much time to improve desirable characters/traits. CRISPR/Cas-mediated genome editing (GE) is a game-changing tool that can create variation in desired traits, such as biotic and abiotic resistance, increase quality and yield in less time with easy applications, high efficiency, and low cost in producing the targeted edits for rapid improvement of crop plants. Plant pathogens and the severe environment cause considerable crop losses worldwide. GE approaches have emerged and opened new doors for breeding multiple-resistance crop varieties. Here, we have summarized recent advances in CRISPR/Cas-mediated GE for resistance against biotic and abiotic stresses in a crop molecular breeding program that includes the modification and improvement of genes response to biotic stresses induced by fungus, virus, and bacterial pathogens. We also discussed in depth the application of CRISPR/Cas for abiotic stresses (herbicide, drought, heat, and cold) in plants. In addition, we discussed the limitations and future challenges faced by breeders using GE tools for crop improvement and suggested directions for future improvements in GE for agricultural applications, providing novel ideas to create super cultivars with broad resistance to biotic and abiotic stress.

Simultaneously induced mutations in eIF4E genes by CRISPR/Cas9 enhance PVY resistance in tobacco

Tobacco is an important commercial crop and a rich source of alkaloids for pharmaceutical and agricultural applications. However, its yield can be reduced by up to 70% due to virus infections, especially by a potyvirus Potato virus Y (PVY). The replication of PVY relies on host factors, and eukaryotic translation initiation factor 4Es (eIF4Es) have already been identified as recessive resistance genes against potyviruses in many plant species. To investigate the molecular basis of PVY resistance in the widely cultivated allotetraploid tobacco variety K326, we developed a dual guide RNA CRISPR/Cas9 system for combinatorial gene editing of two clades, eIF4E1 (eIF4E1-S and eIF4E1-T) and eIF4E2 (eIF4E2-S and eIF4E2-T) in the eIF4E gene family comprising six members in tobacco. We screened for CRISPR/Cas9-induced mutations by heteroduplex analysis and Sanger sequencing, and monitored PVY^O accumulation in virus challenged regenerated plants by DAS-ELISA both in T0 and T1 generations. We found that all T0 lines carrying targeted mutations in the eIF4E1-S gene displayed enhanced resistance to PVY^O confirming previous reports. More importantly, our combinatorial approach revealed that eIF4E1-S is necessary but not sufficient for complete PVY resistance. Only the quadruple mutants harboring loss-of-function mutations in eIF4E1-S, eIF4E1-T, eIF4E2-S and eIF4E2-T showed heritable high-level resistance to PVY^O in tobacco. Our work highlights the importance of understanding host factor redundancy in virus replication and provides a roadmap to generate virus resistance by combinatorial CRISPR/Cas9-mediated editing in non-model crop plants with complex genomes.

Characterization of NAC transcription factor NtNAC028 as a regulator of leaf senescence and stress responses

NAC proteins constitute one of the largest transcription factor families and are involved in regulation of plant development and stress responses. Our previous transcriptome analyses of tobacco revealed a significant increase in the expression of NtNAC028 during leaf yellowing. In this study, we found that NtNAC028 was rapidly upregulated in response to high salinity, dehydration, and abscisic acid (ABA) stresses, suggesting a vital role of this gene in abiotic stress response. NtNAC028 loss-of-function tobacco plants generated via CRISPR-Cas9 showed delayed leaf senescence and increased tolerance to drought and salt stresses. Meanwhile NtNAC028 overexpression led to precocious leaf senescence and hypersensitivity to abiotic stresses in Arabidopsis, indicating that NtNAC028 functions as a positive regulator of natural leaf senescence and a negative regulator of stress tolerance. Furthermore, NtNAC028-overexpressing Arabidopsis plants showed lower antioxidant enzyme activities, higher reactive oxygen species (ROS), and H2O2 accumulation under high salinity, resulted in more severe oxidative damage after salt stress treatments. On the other hand, NtNAC028 mutation in tobacco resulted in upregulated expression of ROS-scavenging and abiotic stress-related genes, higher antioxidant enzyme activities, and enhanced tolerance against abiotic stresses, suggesting that NtNAC028 might act as a vital regulator for plant stress response likely by mediating ROS scavenging ability. Collectively, our results indicated that the NtNAC028 plays a key regulatory role in leaf senescence and response to multiple abiotic stresses.

Transcriptome-based gene regulatory network analyses of differential cold tolerance of two tobacco cultivars

BACKGROUND

Cold is one of the main abiotic stresses that severely affect plant growth and development, and crop productivity as well. Transcriptional changes during cold stress have already been intensively studied in various plant species. However, the gene networks involved in the regulation of differential cold tolerance between tobacco varieties with contrasting cold resistance are quite limited.

RESULTS

Here, we conducted multiple time-point transcriptomic analyses using Tai tobacco (TT, cold susceptibility) and Yan tobacco (YT, cold resistance) with contrasting cold responses. We identified similar DEGs in both cultivars after comparing with the corresponding control (without cold treatment), which were mainly involved in response to abiotic stimuli, metabolic processes, kinase activities. Through comparison of the two cultivars at each time point, in contrast to TT, YT had higher expression levels of the genes responsible for environmental stresses. By applying Weighted Gene Co-Expression Network Analysis (WGCNA), we identified two main modules: the pink module was similar while the brown module was distinct between the two cultivars. Moreover, we obtained 100 hub genes, including 11 important transcription factors (TFs) potentially involved in cold stress, 3 key TFs in the brown module and 8 key TFs in the pink module. More importantly, according to the genetic regulatory networks (GRNs) between TFs and other genes or TFs by using GENIE3, we identified 3 TFs (ABI3/VP1, ARR-B and WRKY) mainly functioning in differential cold responses between two cultivars, and 3 key TFs (GRAS, AP2-EREBP and C2H2) primarily involved in cold responses.

CONCLUSION

Collectively, our study provides valuable resources for transcriptome- based gene network studies of cold responses in tobacco. It helps to reveal how key cold responsive TFs or other genes are regulated through network. It also helps to identify the potential key cold responsive genes for the genetic manipulation of tobacco cultivars with enhanced cold tolerance in the future.

Application of CRISPR/Cas9 System for Efficient Gene Editing in Peanut

Peanuts are an economically important crop cultivated worldwide. However, several limitations restrained its productivity, including biotic/abiotic stresses. CRISPR/Cas9-based gene-editing technology holds a promising approach to developing new crops with improved agronomic and nutritional traits. Its application has been successful in many important crops. However, the application of this technology in peanut research is limited, probably due to the lack of suitable constructs and protocols. In this study, two different constructs were generated to induce insertion/deletion mutations in the targeted gene for a loss of function study. The first construct harbors the regular gRNA scaffold, while the second construct has the extended scaffold plus terminator. The designed gRNA targeting the coding sequence of the FAD2 genes was cloned into both constructs, and their functionality and efficiency were validated using the hairy root transformation system. Both constructs displayed insertions and deletions as the types of edits. The construct harboring the extended plus gRNA terminator showed a higher editing efficiency than the regular scaffold for monoallelic and biallelic mutations. These two constructs can be used for gene editing in peanuts and could provide tools for improving peanut lines for the benefit of peanut breeders, farmers, and industry.

CRISPR/Cas9 and Nanotechnology Pertinence in Agricultural Crop Refinement

The CRISPR/Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated protein 9) method is a versatile technique that can be applied in crop refinement. Currently, the main reasons for declining agricultural yield are global warming, low rainfall, biotic and abiotic stresses, in addition to soil fertility issues caused by the use of harmful chemicals as fertilizers/additives. The declining yields can lead to inadequate supply of nutritional food as per global demand. Grains and horticultural crops including fruits, vegetables, and ornamental plants are crucial in sustaining human life. Genomic editing using CRISPR/Cas9 and nanotechnology has numerous advantages in crop development. Improving crop production using transgenic-free CRISPR/Cas9 technology and produced fertilizers, pesticides, and boosters for plants by adopting nanotechnology-based protocols can essentially overcome the universal food scarcity. This review briefly gives an overview on the potential applications of CRISPR/Cas9 and nanotechnology-based methods in developing the cultivation of major agricultural crops. In addition, the limitations and major challenges of genome editing in grains, vegetables, and fruits have been discussed in detail by emphasizing its applications in crop refinement strategy.

CRISPR-Cas9-mediated editing of starch branching enzymes results in altered starch structure in Brassica napus

Starch branching enzymes (SBEs) are one of the major classes of enzymes that catalyze starch biosynthesis in plants. Here, we utilized the clustered regularly interspaced short palindromic repeats-CRISPR associated protein 9 (CRISPR-Cas9)-mediated gene editing system to investigate the effects of SBE mutation on starch structure and turnover in the oilseed crop Brassica napus. Multiple single-guide RNA (sgRNA) expression cassettes were assembled into a binary vector and two rounds of transformation were employed to edit all six BnaSBE genes. All mutations were heterozygous monoallelic or biallelic, and no chimeric mutations were detected from a total of 216 editing events. Previously unannotated gene duplication events associated with two BnaSBE genes were characterized through analysis of DNA sequencing chromatograms, reflecting the complexity of genetic information in B. napus. Five Cas9-free homozygous mutant lines carrying two to six mutations of BnaSBE were obtained, allowing us to compare the effect of editing different BnaSBE isoforms. We also found that in the sextuple sbe mutant, although indels were introduced at the genomic DNA level, an alternate transcript of one BnaSBE2.1 gene bypassed the indel-induced frame shift and was translated to a modified full-length protein. Subsequent analyses showed that the sextuple mutant possesses much lower SBE enzyme activity and starch branching frequency, higher starch-bound phosphate content, and altered pattern of amylopectin chain length distribution relative to wild-type (WT) plants. In the sextuple mutant, irregular starch granules and a slower rate of starch degradation during darkness were observed in rosette leaves. At the pod-filling stage, the sextuple mutant was distinguishable from WT plants by its thick main stem. This work demonstrates the applicability of the CRISPR-Cas9 system for the study of multi-gene families and for investigation of gene-dosage effects in the oil crop B. napus. It also highlights the need for rigorous analysis of CRISPR-Cas9-mutated plants, particularly with higher levels of ploidy, to ensure detection of gene duplications.

Genetic Dissection of CRISPR-Cas9 Mediated Inheritance of Independently Targeted Alleles in Tobacco α-1,3-Fucosyltransferase 1 and β-1,2-Xylosyltransferase 1 Loci

We determined the specificity of mutations induced by the CRISPR-Cas9 gene-editing system in tobacco (Nicotiana benthamiana) alleles and subsequent genetic stability. For this, we prepared 248 mutant plants using an Agrobacterium-delivered CRISPR-Cas9 system targeting α-1,3-fucosyltransferase 1 (FucT1) and β-1,2-xylosyltransferase1 (XylT1) genes, for which the mutation rates were 22.5% and 25%, respectively, with 20.5% for both loci. Individuals with wild-type (WT) alleles at the NbFucT1 locus in T0 were further segregated into chimeric progeny (37-54%) in the next generation, whereas homozygous T0 mutants tended to produce more (~70%) homozygotes than other bi-allelic and chimeric progenies in the T1 generation. Approximately 81.8% and 77.4% of the homozygous and bi-allelic mutations in T0 generation, respectively, were stably inherited in the next generation, and approximately 50% of the Cas9-free mutants were segregated in T2 generation. One homozygous mutant (Ta 161-1) with a +1 bp insertion in NbFucT1 and a -4 bp deletion in NbXylT1 was found to produce T2 progenies with the same alleles, indicating no activity of the integrated Cas9 irrespective of the insertion or deletion type. Our results provide empirical evidence regarding the genetic inheritance of alleles at CRISPR-targeted loci in tobacco transformants and indicate the potential factors contributing to further mutagenesis.

Using CRISPR-Cas9 Technology to Eliminate Xyloglucan in Tobacco Cell Walls and Change the Uptake and Translocation of Inorganic Arsenic

Xyloglucan is a quantitatively major polysaccharide in the primary cell walls of flowering plants and has been reported to affect plants' ability to tolerate toxic elements. However, it is not known if altering the amounts of xyloglucan in the wall influences the uptake and translocation of inorganic arsenic (As). Here, we identified two Nicotiana tabacum genes that encode xyloglucan-specific xylosyltransferases (XXT), which we named NtXXT1 and NtXXT2. We used CRISPR-Cas9 technology to generate ntxxt1, ntxxt2, and ntxxt1/2 mutant tobacco plants to determine if preventing xyloglucan synthesis affects plant growth and their ability to accumulate As. We show that NtXXT1 and NtXXT2 are required for xyloglucan biosynthesis because no discernible amounts of xyloglucan were present in the cell walls of the ntxxt1/2 double mutant. The tobacco double mutant (ntxxt1/2) and the corresponding Arabidopsis mutant (atxxt1/2) do not have severe growth defects but do have a short root hair phenotype and a slow growth rate. This phenotype is rescued by overexpressing NtXXT1 or NtXXT2 in atxxt1/2. Growing ntxxt mutants in the presence of AsIII or AsV showed that the absence of cell wall xyloglucan affects the accumulation and translocation of As. Most notably, root retention of As increased substantially and the amounts of As translocated to the shoots decreased in ntxxt1/2. Our results suggest that xyloglucan-deficient plants provide a strategy for the phytoremediation of As contaminated soils.

The Genetic Components of a Natural Color Palette: A Comprehensive List of Carotenoid Pathway Mutations in Plants

Carotenoids comprise the most widely distributed natural pigments. In plants, they play indispensable roles in photosynthesis, furnish colors to flowers and fruit and serve as precursor molecules for the synthesis of apocarotenoids, including aroma and scent, phytohormones and other signaling molecules. Dietary carotenoids are vital to human health as a source of provitamin A and antioxidants. Hence, the enormous interest in carotenoids of crop plants. Over the past three decades, the carotenoid biosynthesis pathway has been mainly deciphered due to the characterization of natural and induced mutations that impair this process. Over the year, numerous mutations have been studied in dozens of plant species. Their phenotypes have significantly expanded our understanding of the biochemical and molecular processes underlying carotenoid accumulation in crops. Several of them were employed in the breeding of crops with higher nutritional value. This compendium of all known random and targeted mutants available in the carotenoid metabolic pathway in plants provides a valuable resource for future research on carotenoid biosynthesis in plant species.

NtCycB2 negatively regulates tobacco glandular trichome formation, exudate accumulation, and aphid resistance

KEY MESSAGE

NtCycB2 negatively regulates the initiation of tobacco long stalk glandular trichomes and influences the expression of diterpenoid biosynthesis- and environmental stress resistance-related genes. Many asterid plants possess multicellular trichomes on their surface, both glandular and non-glandular. The CycB2 gene plays a key role in multicellular trichome initiation, but has distinct effects on different types of trichomes; its mechanisms remain unknown. In tomato (Solanum lycopersicum), SlCycB2 negatively regulates non-glandular trichome formation, but its effects on glandular trichomes are ambiguous. In this study, we cloned the SlCycB2 homolog of Nicotiana tabacum, NtCycB2, and analyzed its effect on three types of trichomes, long stalk glandular trichomes (LGT), short stalk glandular trichomes (SGT), and non-glandular trichomes (NGT). Knocking out NtCycB2 (NtCycB2-KO) promoted LGT formation, while overexpression of NtCycB2 (NtCycB2-OE) decreased LGT density. SGT and NGT were not significantly influenced in either NtCycB2-KO or NtCycB2-OE plants, indicating that NtCycB2 regulated only LGT formation in tobacco. In addition, compared with NtCycB2-OE and control plants, NtCycB2-KO plants produced more trichome exudates, including diterpenoids and sugar esters, and exhibited stronger aphid resistance. To further elucidate the function of NtCycB2, RNA-Seq analysis of the NtCycB2-KO, NtCycB2-OE, and control plants was conducted. 2,552 and 1,933 differentially expressed genes (DEGs) were found in NtCycB2-KO and NtCycB2-OE plants, respectively. Gene Ontology analysis of the common DEGs revealed that ion transport, carbohydrate and amino acid metabolism, photosynthesis, and transcription regulation processes were significantly enriched. Among these DEGs, diterpenoid biosynthesis genes were upregulated in NtCycB2-KO plants and downregulated in NtCycB2-OE plants. Two MYB transcription factors and several stress resistance-related genes were also identified, suggesting they may participate in regulating LGT formation and aphid resistance.

CRISPER/Cas in Plant Natural Product Research: Therapeutics as Anticancer and other Drug Candidates and Recent Patents

BACKGROUND

Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR- associated9 (Cas9) endonuclease system is a facile, highly efficient and selective site-directed mutagenesis tool for RNA-guided genome-editing. CRISPR/Cas9 genome-editing strategy uses designed guide-RNAs that recognizes a 3 base-pair protospacer adjacent motif (PAM) sequence in the target-DNA. CRISPR/Cas-editing tools have mainly been employed in crop plants in relation to yield and stress tolerance. However, the immense potential of this technology has not yet been fully utilized in medicinal plants in deciphering or modulating secondary metabolic pathways producing therapeutically active phytochemicals against cancer and other diseases.

OBJECTIVE

The present review elucidates the use of CRISPR-Cas9 as a promising genome-editing tool in plants and plant-derived natural products with anticancer and other therapeutic applications. It also includes recent patents on the therapeutic applications of CRISPR-CAS systems implicated to cancer and other human medical conditions.

METHODS

Popular search engines, such as PubMed, Scopus, Google Scholar, Google Patents, Medline, ScienceDirect, SpringerLink, EMBASE, Mendeley, etc., were searched in order to retrieve literature using relevant keywords viz. CRISPER/Cas, plant natural product research, anticancer, therapeutics, etc., either singly or in various combinations.

RESULTS

Retrieved citations and further cross-referencing among the literature have resulted in a total number of 71 publications and 3 patents are being cited in this work. Information presented in this review aims to support further biotechnological and clinical strategies to be carried using CRISPER/ Cas mediated optimization of plant natural products against cancer and an array of other human medical conditions.

CONCLUSION

Off late, knock-in and knock-out, point mutation, controlled tuning of gene-expression and targeted mutagenesis have enabled the versatile CRISPR/Cas-editing device to engineer medicinal plants' genomes. In addition, by combining CRISPR/Cas-editing tool with next-generation sequencing (NGS) and various tools of system biology, many medicinal plants have been engineered genetically to optimize the production of valuable bioactive compounds of industrial significance.

Unravelling the promise and limitations of CRISPR/Cas system in natural product research: Approaches and challenges

An incredible array of natural products are produced by plants that serve several ecological functions, including protecting them from herbivores and microbes, attracting pollinators, and dispersing seeds. In addition to their obvious medical applications, natural products serve as flavouring agents, fragrances and many other uses by humans. With the increasing demand for natural products and the development of various gene engineering systems, researchers are trying to modify the plant genome to increase the biosynthetic pathway of the compound of interest or blocking the pathway of unwanted compound synthesis. The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 has had widespread success in genome editing due to the system's high efficiency, ease of use, and accuracy which revolutionized the genome editing system in living organisms. This article highlights the method of the CRISPR/Cas system, its application in different organisms including microbes, algae, fungi and also higher plants in natural product research, its shortcomings and future prospects. This article is protected by copyright. All rights reserved.

Efficient knockout of the phytoene desaturase gene in a hybrid poplar (Populus alba × Populus glandulosa) using the CRISPR/Cas9 system with a single gRNA

The CRISPR/Cas9 system has been used for genome editing in several plant species; however, there are few reports on its use in trees. Here, CRISPR/Cas9 was used to mutate a target gene in Populus alba × Populus glandulosa hybrid poplars. The hybrid poplar is routinely used in molecular biological studies due to the well-established Agrobacterium-mediated transformation method. A single guide RNA (sgRNA) with reported high mutation efficiency in other popular species was designed with a protospacer adjacent motif sequence for the phytoene desaturase 1 (PagPDS1) gene. The pHSE/Cas9-PagPDS1 sgRNA vector was delivered into hybrid poplar cells using Agrobacterium-mediated transformation. The transgenic plants were propagated and classified them into three groups according to their phenotypes. Among a total of 110 lines of transgenic hybrid poplars, 82 lines showed either an albino or a pale green phenotype, indicating around 74.5% phenotypic mutation efficiency of the PagPDS1 gene. The albino phenotypes were observed when the CRISPR/Cas9-mediated mutations in both PagPDS1 alleles in the transgenic plants. There was no off-target modification of the PagPDS2 gene, which has a potential sgRNA target sequence with two mismatches. The results confirmed that the sgRNA can specifically edit PagPDS1 rather than PagPDS2, indicating that CRISPR/Cas9-mediated genome editing can effectively induce target mutations in the hybrid poplar. This technique will be useful to improve tree quality in hybrid poplars (P. alba × P. glandulosa); for example, by enhancing biomass or stress tolerance.

A Critical Review: Recent Advancements in the Use of CRISPR/Cas9 Technology to Enhance Crops and Alleviate Global Food Crises

Genome editing (GE) has revolutionized the biological sciences by creating a novel approach for manipulating the genomes of living organisms. Many tools have been developed in recent years to enable the editing of complex genomes. Therefore, a reliable and rapid approach for increasing yield and tolerance to various environmental stresses is necessary to sustain agricultural crop production for global food security. This critical review elaborates the GE tools used for crop improvement. These tools include mega-nucleases (MNs), such as zinc-finger nucleases (ZFNs), and transcriptional activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR). Specifically, this review addresses the latest advancements in the role of CRISPR/Cas9 for genome manipulation for major crop improvement, including yield and quality development of biotic stress- and abiotic stress-tolerant crops. Implementation of this technique will lead to the production of non-transgene crops with preferred characteristics that can result in enhanced yield capacity under various environmental stresses. The CRISPR/Cas9 technique can be combined with current and potential breeding methods (e.g., speed breeding and omics-assisted breeding) to enhance agricultural productivity to ensure food security. We have also discussed the challenges and limitations of CRISPR/Cas9. This information will be useful to plant breeders and researchers in the thorough investigation of the use of CRISPR/Cas9 to boost crops by targeting the gene of interest.

Multiplex Genome-Editing Technologies for Revolutionizing Plant Biology and Crop Improvement

Multiplex genome-editing (MGE) technologies are recently developed versatile bioengineering tools for modifying two or more specific DNA loci in a genome with high precision. These genome-editing tools have greatly increased the feasibility of introducing desired changes at multiple nucleotide levels into a target genome. In particular, clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) [CRISPR/Cas] system-based MGE tools allow the simultaneous generation of direct mutations precisely at multiple loci in a gene or multiple genes. MGE is enhancing the field of plant molecular biology and providing capabilities for revolutionizing modern crop-breeding methods as it was virtually impossible to edit genomes so precisely at the single base-pair level with prior genome-editing tools, such as zinc-finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). Recently, researchers have not only started using MGE tools to advance genome-editing applications in certain plant science fields but also have attempted to decipher and answer basic questions related to plant biology. In this review, we discuss the current progress that has been made toward the development and utilization of MGE tools with an emphasis on the improvements in plant biology after the discovery of CRISPR/Cas9. Furthermore, the most recent advancements involving CRISPR/Cas applications for editing multiple loci or genes are described. Finally, insights into the strengths and importance of MGE technology in advancing crop-improvement programs are presented.

Frontiers in the Standardization of the Plant Platform for High Scale Production of Vaccines

The recent COVID-19 pandemic has highlighted the value of technologies that allow a fast setup and production of biopharmaceuticals in emergency situations. The plant factory system can provide a fast response to epidemics/pandemics. Thanks to their scalability and genome plasticity, plants represent advantageous platforms to produce vaccines. Plant systems imply less complicated production processes and quality controls with respect to mammalian and bacterial cells. The expression of vaccines in plants is based on transient or stable transformation systems and the recent progresses in genome editing techniques, based on the CRISPR/Cas method, allow the manipulation of DNA in an efficient, fast, and easy way by introducing specific modifications in specific sites of a genome. Nonetheless, CRISPR/Cas is far away from being fully exploited for vaccine expression in plants. In this review, an overview of the potential conjugation of the renewed vaccine technologies (i.e., virus-like particles-VLPs, and industrialization of the production process) with genome editing to produce vaccines in plants is reported, illustrating the potential advantages in the standardization of the plant platforms, with the overtaking of constancy of large-scale production challenges, facilitating regulatory requirements and expediting the release and commercialization of the vaccine products of genome edited plants.

Data Comparison and Software Design for Easy Selection and Application of CRISPR-based Genome Editing Systems in Plants

CRISPR-based genome editing systems have been successfully and effectively used in many organisms. However, only a few studies have reported the comparison between CRISPR/Cas9 and CRISPR/Cpf1 systems in the whole-genome applications. Although many web-based toolkits are available, there is still a shortage of comprehensive, user-friendly, and plant-specific CRISPR databases and desktop software. In this study, we identified and analyzed the similarities and differences between CRISPR/Cas9 and CRISPR/Cpf1 systems by considering the abundance of proto-spacer adjacent motif (PAM) sites, the effects of GC content, optimal proto-spacer length, potential universality within the plant kingdom, PAM-rich region (PARR) inhibiting ratio, and the effects of G-quadruplex (G-Q) structures. Using this information, we built a comprehensive CRISPR database (including 138 plant genome data sources, www.grapeworld.cn/pc/index.html), which provides search tools for the identification of CRISPR editing sites in both CRISPR/Cas9 and CRISPR/Cpf1 systems. We also developed a desktop software on the basis of the Perl/Tk tool, which facilitates and improves the detection and analysis of CRISPR editing sites at the whole-genome level on Linux and/or Windows platform. Therefore, this study provides helpful data and software for easy selection and application of CRISPR-based genome editing systems in plants.

Genome-wide characterization and expression profiling of the PDR gene family in tobacco (Nicotiana tabacum)

The pleiotropic drug resistance (PDR) proteins of the ATP-binding cassette (ABC) family play essential roles in physiological processes and have been characterized in many plant species. However, no comprehensive investigation of tobacco (Nicotiana tabacum), an important economic crop and a useful model plant for scientific research, has been presented. We identified 32 PDR genes in the tobacco genome and explored their domain organization, chromosomal distribution and evolution, promoter cis-elements, and expression profiles. A phylogenetic analysis revealed that tobacco has a significantly expanded number of PDR genes involved in plant defense. It also revealed that two tobacco PDR proteins may function as strigolactone transporters to regulate shoot branching, and several NtPDR genes may be involved in cadmium transport. Moreover, tissue expression profiles of NtPDR genes and their responses to several hormones and abiotic stresses were assessed using quantitative real-time PCR. Most of the NtPDR genes were regulated by jasmonate or salicylic acid, suggesting the important regulatory roles of NtPDRs in plant defense and secondary metabolism. They were also responsive to abiotic stresses, like drought and cold, and there was a strong correlation between the presence of promoter cis-elements and abiotic/biotic stress responses. These results provide useful clues for further in-depth studies on the functions of the tobacco PDR genes.

Efficient genome editing and gene knockout in Setaria viridis with CRISPR/Cas9 directed gene editing by the non-homologous end-joining pathway

The CRISPR/Cas9 system has been used for genome editing in several organisms, including higher plants. This system induces site-specific mutations in the genome based on the nucleotide sequence of engineered guide RNAs. The complex genomes of C4 grasses makes genome editing a challenge in key grass crops like maize (Zea mays), sorghum (Sorghum bicolor), Brachiaria spp., switchgrass (Panicum virgatum), and sugarcane (Saccharum spp.). Setaria viridis is a diploid C4 grass widely used as a model for these C4 crop plants. Here, an optimized CRISPR/Cas9 binary vector that exploits the non-homologous end joining (NHEJ) system was used to knockout a green fluorescent protein (gfp) transgene in S. viridis accession A10.1. Transformation of embryogenic callus by A. tumefaciens generated ten glufosinate-ammonium resistant transgenic events. In the T0 generation, 60% of the events were biallelic mutants in the gfp transgene with no detectable accumulation of GFP protein and without insertions or deletions in predicted off-target sites. The gfp mutations generated by CRISPR/Cas9 were stable and displayed Mendelian segregation in the T1 generation. Altogether, the system described here is a highly efficient genome editing system for S. viridis, an important model plant for functional genomics studies in C4 grasses. Also, this system is a potential tool for improvement of agronomic traits in C4 crop plants with complex genomes.

CRISPR-Mediated Endogenous Activation of Fibroin Heavy Chain Gene Triggers Cellular Stress Responses in Bombyx mori Embryonic Cells

Simple Summary

Based on a CRISPRa approach, activating endogenous fibroin heavy chain (FibH) gene expression in Bombyx mori embryonic (BmE) cells, which was driven by a combination of the dCas9-VPR (a tripartite activator, composed of VP64, p65, and Rta) and the sgRNA targeting to the promoter of FibH gene, was performed for investigating the biological roles of FibH in the development of silk gland cells. The activation of the endogenous FibH gene lead to up-regulation of cellular stress responses-related genes, which suggested a significant positive correlation between activated FibH gene expression and cellular stress responses. Moreover, the present findings might provide a potential model for studying the cellular stress responses caused by silk secretion disorder and lay a foundation for the understanding of silk gland development in silk-spinning insects.

Abstract

The silkworm Bombyx mori is an economically important insect, as it is the main producer of silk. Fibroin heavy chain (FibH) gene, encoding the core component of silk protein, is specifically and highly expressed in silk gland cells but not in the other cells. Although the silkworm FibH gene has been well studied in transcriptional regulation, its biological functions in the development of silk gland cells remain elusive. In this study, we constructed a CRISPRa system to activate the endogenous transcription of FibH in Bombyx mori embryonic (BmE) cells, and the mRNA expression of FibH was successfully activated. In addition, we found that FibH expression was increased to a maximum at 60 h after transient transfection of sgRNA/dCas9-VPR at a molar ratio of 9:1. The qRT-PCR analysis showed that the expression levels of cellular stress response-related genes were significantly up-regulated along with activated FibH gene. Moreover, the lyso-tracker red and monodansylcadaverine (MDC) staining assays revealed an apparent appearance of autophagy in FibH-activated BmE cells. Therefore, we conclude that the activation of FibH gene leads to up-regulation of cellular stress responses-related genes in BmE cells, which is essential for understanding silk gland development and the fibroin secretion process in B. mori.