CRISPR/Cas9 genome editing through in planta transformation

Gene transformation can be done in direct and indirect (Agrobacterium-mediated) ways. The most efficient method of gene transformation to date is Agrobacterium-mediated method. The main problem of Agrobacterium-method is that some plant species and mutant lines are recalcitrant to regeneration. Requirements for sterile conditions for plant regeneration are another problem of Agrobacterium-mediated transformation. Development of genotype-independent gene transformation method is of great interest in many plants. Some tissue culture-independent Agrobacterium-mediated gene transformation methods are reported in individual plants and crops. Generally, these methods are called in planta gene transformation. In planta transformation methods are free from somaclonal variation and easier, quicker, and simpler than tissue culture-based transformation methods. Vacuum infiltration, injection of Agrobacterium culture to plant tissues, pollen-tube pathway, floral dip and floral spray are the main methods of in planta transformation. Each of these methods has its own advantages and disadvantages. Simplicity and reliability are the primary reasons for the popularity of the in planta methods. These methods are much quicker than regular tissue culture-based Agrobacterium-mediated gene transformation and success can be achieved by non-experts. In the present review, we highlight all methods of in planta transformation comparing them with regular tissue culture-based Agrobacterium-mediated transformation methods and then recently successful transformations using these methods are presented.

RNA-Guided Genome Editing in Plants Using a CRISPR–Cas System

Tissue culture technology through organogenesis by in vitro plant regeneration has been applied and successfully developed in several economically important legume and nonlegume plant species. This method has not provided robust regeneration protocol for application and limits the utilization of genetic manipulation to its full potential. Therefore, in planta methods are quite easy to perform and do not require much proficiency and labour and are also less cost-effective. Here, we have explained three economically important legumes such as horse gram, black gram and cowpea using in planta transformation methods. In the present study, methods of phytochemical screening of transgenic and non-transgenic leguminous plant species have been analysed, and observed results were reported. The results showed that metabolic changes were observed due to stress of Agrobacterium tumefaciens. Phytochemical analysis revealed that the total phenolic content was gradually increased in all the transgenic plants as compared to the control plant. The flavonoid and anthocyanin contents increased in black gram, whereas primary metabolic compounds such as protein, amino acids and chlorophyll contents were increased in the cowpea and decreased in the horse gram and black gram, respectively. But the amounts of carbohydrate and starch contents were decreased among all three transgenic plants. This simple procedure can be used for the genetic manipulation of genes to enhance the production of important leguminous species.

The CRISPR (clustered regularly interspaced short palindromic repeat)-Cas9 (CRISPR-associated nuclease 9) system is a versatile tool for genetic engineering that uses cas9 to target the sequence-specific region and introduce a double-stranded break in the target area. This simple RNA-guided genome editing technology has become a revolutionary tool in biology and has many innovative applications in different fields. This technique helps to make precise genome modification in many different tissues and organisms. Development of genetically edited crops will assist sustainable productive agriculture for better feeding of the rapidly growing population in a changing climate. The emerging areas of research for the genome editing in plants include rewiring the regulatory signaling networks, and interrogating gene functions and sgRNA library for high-throughput loss-of-function screening. This chapter deals with the strengths and weaknesses of Cas9 nuclease-mediated genome editing in plants for development of designer crops like sugarcane. With this powerful and innovative technique, the genetically engineered non-GM plants will support the sustainable agriculture and maximize yield by combating abiotic and biotic stresses.

Progresses of CRISPR/Cas9 genome editing in forage crops

The genetic transformation of plants has provided fundamental insights into plant biology. However, the genetic transformation systems for most horticultural plants remain incomplete. Genome editing has significantly contributed to the improvement of crop traits, but it heavily relies on effective genetic transformation. Currently, reducing costs and improving the efficiency of genetic transformation are crucial for promoting the widespread application of genome editing in plants. Here, we review the advances in plant genetic transformation research, performing analysis of three methods for plant gene function analysis that bypass tissue culture: Agrobacterium rhizogenes-mediated root genetic transformation, developmental regulators (DRs)-mediated genetic transformation, and virus-mediated genome editing. We analyzed transformation efficiency in strawberry and citrus using the A. rhizogenes infiltration method, employing GFP to label different subcellular locations to investigate the morphology of microfilaments, nuclei, and peroxisomes in strawberry cells. Sequence analysis revealed that a series of developmental regulators critical for enhancing genetic transformation efficiency in specific species are highly conserved across different plant species. Additionally, we successfully edited the endogenous Pds gene in Cas9-overexpressing transgenic tobacco using TRV and CLBV containing the gRNA module. These three methods offer the benefits of being cost-effective and time-efficient, providing valuable technical insights for the application of plant genome editing.

The ever-increasing food requirement with globally growing population demands advanced agricultural practices to improve grain yield, to gain crop resilience under unpredictable extreme weather, and to reduce production loss caused by insects and pathogens. To fulfill such requests, genome engineering technology has been applied to various plant species. To date, several generations of genome engineering methods have been developed. Among these methods, the new mainstream technology is clustered regularly interspaced short palindromic repeats (CRISPR) with nucleases. One of the most important processes in genome engineering is to deliver gene cassettes into plant cells. Conventionally used systems have several shortcomings, such as being labor- and time-consuming procedures, potential tissue damage, and low transformation efficiency. Taking advantage of nanotechnology, the nanoparticle-mediated gene delivery method presents technical superiority over conventional approaches due to its high efficiency and adaptability in different plant species. In this review, we summarize the evolution of plant biomolecular delivery methods and discussed their characteristics as well as limitations. We focused on the cutting-edge nanotechnology-based delivery system, and reviewed different types of nanoparticles, preparation of nanomaterials, mechanism of nanoparticle transport, and advanced application in plant genome engineering. On the basis of established methods, we concluded that the combination of genome editing, nanoparticle-mediated gene transformation and de novo regeneration technologies can accelerate crop improvement efficiently in the future.

Agrobacterium-mediated in planta transformation of horticultural plants: Current status and future prospects

Our review has described principles and functional importance of CRISPR-Cas9 with emphasis on the recent advancements, such as CRISPR-Cpf1, base editing (BE), prime editing (PE), epigenome editing, tissue-specific (CRISPR-TSKO), and inducible genome editing and their potential applications in generating stress-tolerant plants. Improved agricultural practices and enhanced food crop production using innovative crop breeding technology is essential for increasing access to nutritious foods across the planet. The crop plants play a pivotal role in energy and nutrient supply to humans. The abiotic stress factors, such as drought, heat, and salinity cause a substantial yield loss in crop plants and threaten food security. The most sustainable and eco-friendly way to overcome these challenges are the breeding of crop cultivars with improved tolerance against abiotic stress factors. The conventional plant breeding methods have been highly successful in developing abiotic stress-tolerant crop varieties, but usually cumbersome and time-consuming. Alternatively, the CRISPR/Cas genome editing has emerged as a revolutionary tool for making efficient and precise genetic manipulations in plant genomes. Here, we provide a comprehensive review of the CRISPR/Cas genome editing (GE) technology with an emphasis on recent advances in the plant genome editing, including base editing (BE), prime editing (PE), epigenome editing, tissue-specific (CRISPR-TSKO), and inducible genome editing (CRISPR-IGE), which can be used for obtaining cultivars with enhanced tolerance to various abiotic stress factors. We also describe tissue culture-free, DNA-free GE technology, and some of the CRISPR-based tools that can be modified for their use in crop plants.

Finding and explaining the functions of genes in plants have promising applications in crop improvement and bioprospecting and hence, it is important to compare various techniques available for gene function identification in plants. Today, the most popular technology among researchers to identify the functions of genes is the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR associated protein 9 (Cas9)-based genome editing method. But by no means can we say that CRISPR/Cas9 is the go-to method for all purposes. It comes with its own baggage. Researchers will agree and have lived through at least seven more technologies deployed to find the functions of genes, which come under three umbrellas: 1. genetic engineering, 2. transient expression, and 3. chemical/physical mutagenesis. Each of the methods evolved when the previous one ran into an insurmountable problem. In this review, we compare the eight technologies against one another on 14 parameters. This review lays bare the pros and cons, and similarities and dissimilarities of various methods. Every method comes with its advantages and disadvantages. For example, the CRISPR/Cas9-based genome editing is an excellent method for modifying gene sequences, creating allelic versions of genes, thereby aiding the understanding of gene function. But it comes with the baggage of unwanted or off-target mutations. Then, we have methods based on random or targeted knockout of the gene, knockdown, and overexpression of the gene. Targeted disruption of genes is required for complete knockout of gene function, which may not be accomplished by editing. We have also discussed the strategies to overcome the shortcomings of the targeted gene-knockout and the CRISPR/Cas9-based methods. This review serves as a comprehensive guide towards the understanding and comparison of various technologies available for gene function identification in plants and hence, it will find application for crop improvement and bioprospecting related research.

Lack of appropriate methods for delivery of genome-editing reagents is a major barrier to CRISPR/Cas-mediated genome editing in plants. Agrobacterium-mediated genetic transformation (AMGT) is the preferred method of CRISPR/Cas reagent delivery, and researchers have recently made great improvements to this process. In this article, we review the development of AMGT and AMGT-based delivery of CRISPR/Cas reagents. We give an overview of the development of AMGT vectors including binary vector, superbinary vector, dual binary vector, and ternary vector systems. We also review the progress in Agrobacterium genomics and Agrobacterium genetic engineering for optimal strains. We focus in particular on the ternary vector system and the resources we developed. In summary, it is our opinion that Agrobacterium-mediated CRISPR/Cas genome editing in plants is entering an era of ternary vector systems, which are often integrated with morphogenic regulators. The new vectors described in this article are available from Addgene and/or MolecularCloud for sharing with academic investigators for noncommercial research.

The development of new adaptations of CRISPR-based genome editing platforms, such as base editing and prime editing, made it possible to broaden the scope and applications of genome editing in plants. First base editing and, more recently, prime editing evade the creation of double-stranded breaks in deoxyribonucleic acid (DNA) and the requirement of donor template of DNA for repair while enhancing editing efficiency and product purity over CRISPR/Cas9. As base-pair changes in genomic DNA determine many significant agronomic traits, crop varieties can be developed by precisely converting specific single bases in plant genomes. While base editing can introduce specific nucleotide changes, such as transition and transversion mutations in the targeted region, prime editing can create precise insertions, deletions, and all 12 types of point mutations using the “search-and-replace” method.This chapter provides the basic principles of base editing and prime editing technologies and their practical applications in plants. The chapter also summarizes the recent breakthroughs in applying base and prime editors in diverse plant species, including their use in improving disease resistance, herbicide resistance, nutritional quality, crop yield, and quality. Finally, this chapter aims to clearly understand base editing and prime editing in plants by outlining potential developments.

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

Erratum

The clustered regularly interspaced short palindromic repeats (CRISPR) system has been adopted for a wide range of biological applications including genome editing. In some cases, dissection of genome functions requires allele-specific genome editing, but the use of CRISPR for this purpose has not been studied in detail. In this study, using the p16INK4a gene in HCT116 as a model locus, we investigated whether chromatin states, such as CpG methylation, or a single-nucleotide gap form in a target site can be exploited for allele-specific locus binding and genome editing by CRISPR in vivo. First, we showed that allele-specific locus binding and genome editing could be achieved by targeting allele-specific CpG-methylated regions, which was successful for one, but not all guide RNAs. In this regard, molecular basis underlying the success remains elusive at this stage. Next, we demonstrated that an allele-specific single-nucleotide gap form could be employed for allele-specific locus binding and genome editing by CRISPR, although it was important to avoid CRISPR tolerance of a single nucleotide mismatch brought about by mismatched base skipping. Our results provide information that might be useful for applications of CRISPR in studies of allele-specific functions in the genomes.