CRISPR-Cas9-mediated induction of heritable chromosomal translocations in Arabidopsis

CRISPR-Cas applications in agriculture and plant research

Growing world population and deteriorating climate conditions necessitate the development of new crops with high yields and resilience. CRISPR-Cas-mediated genome engineering presents unparalleled opportunities to engineer crop varieties cheaper, easier and faster than ever. In this Review, we discuss how the CRISPR-Cas toolbox has rapidly expanded from Cas9 and Cas12 to include different Cas orthologues and engineered variants. We present various CRISPR-Cas-based methods, including base editing and prime editing, which are used for precise genome, epigenome and transcriptome engineering, and methods used to deliver the genome editors into plants, such as bacterial-mediated and viral-mediated transformation. We then discuss how promoter editing and chromosome engineering are used in crop breeding for trait engineering and fixation, and important applications of CRISPR-Cas in crop improvement, such as de novo domestication and enhancing tolerance to abiotic stresses. We conclude with discussing future prospects of plant genome engineering.

Chromosomal structural variation loci HSS1 and HSS6 lead to hybrid sterility in rice

KEY MESSAGE

Chromosomal structural variation leads to rice hybrid sterility, and this study has established a new inheritance model and offers a novel insight into hybrid sterility in rice. The utilization of heterosis among different species significantly enhances rice yield. However, the direct application of this advantage is hindered by hybrid sterility (HS). Here we identify a stable semi-sterile inbred line (SSIL) in the hybridization between rice species O. sativa indica and wild rice O. longistaminata. Both pollen and spikelet fertility in heterozygous SSIL plants are semi-sterile, and the homozygous plants show normal fertility. Interestingly, unlike previously reported hybrid sterility loci, SSIL does not induce segregation distortion in progeny. The genotypes of chromosome 1 (Chr.1) exhibit tight pseudo-linkage on Chr.6 in SSIL population. HSS1 and HSS6 are named as the two loci of Chr.1 and Chr.6. Cytological observations revealed abnormalities in male and female gametes during meiosis, ultimately resulting in semi-sterile pollen and spikelet. HSS1 was narrowed down to a 190.6-kb interval and HSS6 to a 1391.6-kb region using an SSIL population of 10,393 plants. Through high-throughput sequencing and observation of chromosomal behavior during meiosis, a reciprocal translocation between the short arm of Chr.1 and the end of Chr.6 in the translocated RD23-type (RT-type) gamete in SSIL was discovered. The presence of RT-type gametes in SSIL is the direct causative factor for semi-sterility in both male and female gametes. This translocation led to abnormal SSIL-RT/L synapses and the formation of tetravalent ring structures in chromosomes during pachynema. Our findings have uncovered a pair of reciprocal chromosome translocations that control reproductive isolation in rice. These insights offer valuable guidance for optimizing hybrid breeding applications, ultimately enhancing the benefits of heterosis.

The future of genome editing in plants

The future of genome editing in plants differs from how it is used today. For both research and product development, we need to think beyond the creation of simple single-nucleotide polymorphisms and short deletions in genes. We believe that the future of genome editing in plants involves mimicking the natural evolutionary processes that have shaped plant genomes and been the target of artificial selection during crop domestication and improvement. This includes programming large structural variations (insertions, duplications, deletions, inversions and translocations) and controlling plant recombination and endogenous transposable elements that naturally reshape plant genomes. The key is that genome editing will be used to reshape plant genomes in a manner that could have happened naturally, but now these changes can be directed rapidly in the laboratory.

Molecular breeding of tomato: Advances and challenges

The modern cultivated tomato (Solanum lycopersicum) was domesticated from Solanum pimpinellifolium native to the Andes Mountains of South America through a "two-step domestication" process. It was introduced to Europe in the 16th century and later widely cultivated worldwide. Since the late 19th century, breeders, guided by modern genetics, breeding science, and statistical theory, have improved tomatoes into an important fruit and vegetable crop that serves both fresh consumption and processing needs, satisfying diverse consumer demands. Over the past three decades, advancements in modern crop molecular breeding technologies, represented by molecular marker technology, genome sequencing, and genome editing, have significantly transformed tomato breeding paradigms. This article reviews the research progress in the field of tomato molecular breeding, encompassing genome sequencing of germplasm resources, the identification of functional genes for agronomic traits, and the development of key molecular breeding technologies. Based on these advancements, we also discuss the major challenges and perspectives in this field.

Past innovations and future possibilities in plant chromosome engineering

Plant chromosome engineering has emerged as a pivotal tool in modern plant breeding, facilitating the transfer of desirable traits through the incorporation of alien chromosome fragments into plants. Here, we provide a comprehensive overview of the past achievements, current methodologies and future prospects of plant chromosome engineering. We begin by examining the successful integration of specific examples such as the incorporation of rye chromosome segments (e.g. the 1BL/1RS translocation), Dasypyrum villosum segments (e.g. the 6VS segment for powdery mildew resistance), Thinopyrum intermedium segments (e.g. rust resistance genes) and Thinopyrum elongatum segments (e.g. Fusarium head blight resistance genes). In addition to trait transfer, advancements in plant centromere engineering have opened new possibilities for chromosomal manipulation. This includes the development of plant minichromosomes via centromere-mediated techniques, the generation of haploids through CENH3 gene editing, and the induction of aneuploidy using KaryoCreate. The advent of CRISPR/Cas technology has further revolutionized chromosome engineering, enabling large-scale chromosomal rearrangements, such as inversions and translocations, as well as enabling targeted insertion of large DNA fragments and increasing genetic recombination frequency. These advancements have significantly expanded the toolkit for genetic improvement in plants, opening new horizons for the future of plant breeding.

Epigenetic state and gene expression remain stable after CRISPR/Cas-mediated chromosomal inversions

The epigenetic state of chromatin, gene activity and chromosomal positions are interrelated in plants. In Arabidopsis thaliana, chromosome arms are DNA-hypomethylated and enriched with the euchromatin-specific histone mark H3K4me3, while pericentromeric regions are DNA-hypermethylated and enriched with the heterochromatin-specific mark H3K9me2. We aimed to investigate how the chromosomal location affects epigenetic stability and gene expression by chromosome engineering. Two chromosomal inversions of different sizes were induced using CRISPR/Cas9 to move heterochromatic, pericentric sequences into euchromatic regions. The epigenetic status of these lines was investigated using whole-genome bisulfite sequencing and chromatin immunoprecipitation. Gene expression changes following the induction of the chromosomal inversions were studied via transcriptome analysis. Both inversions had a minimal impact on the global distribution of histone marks and DNA methylation patterns, although minor epigenetic changes were observed across the genome. Notably, the inverted chromosomal regions and their borders retained their original epigenetic profiles. Gene expression analysis showed that only 0.5-1% of genes were differentially expressed genome-wide following the induction of the inversions. CRISPR/Cas-induced chromosomal inversions minimally affect epigenetic landscape and gene expression, preserving their profiles in subsequent generations.

Chromosome engineering points to the cis-acting mechanism of chromosome arm-specific telomere length setting and robustness of plant phenotype, chromatin structure and gene expression

The study investigates the impact of targeted chromosome engineering on telomere dynamics, chromatin structure, gene expression, and phenotypic stability in Arabidopsis thaliana. Using precise CRISPR/Cas-based engineering, reciprocal translocations of chromosome arms were introduced between non-homologous chromosomes. The subsequent homozygous generations of plants were assessed for phenotype, transcriptomic changes and chromatin modifications near translocation breakpoints, and telomere length maintenance. Phenotypically, translocated lines were indistinguishable from wild-type plants, as confirmed through morphological assessments and principal component analysis. Gene expression profiling detected minimal differential expression, with affected genes dispersed across the genome, indicating negligible transcriptional impact. Similarly, ChIPseq analysis showed no substantial alterations in the enrichment of key histone marks (H3K27me3, H3K4me1, H3K56ac) near junction sites or across the genome. Finally, bulk and arm-specific telomere lengths remained stable across multiple generations, except for minor variations in one translocation line. These findings highlight the remarkable genomic and phenotypic robustness of A. thaliana despite large-scale chromosomal rearrangements. The study offers insights into the cis-acting mechanisms underlying chromosome arm-specific telomere length setting and establishes the feasibility of chromosome engineering for studies of plant genome evolution and crop improvement strategies.

Precise deletion, replacement and inversion of large DNA fragments in plants using dual prime editing

Precise manipulation of genome structural variations holds great potential for plant trait improvement and biological research. Here we present a genome-editing approach, dual prime editing (DualPE), that efficiently facilitates precise deletion, replacement and inversion of large DNA fragments in plants. In our experiments, DualPE enabled the production of specific genomic deletions ranging from ~500 bp to 2 Mb in wheat protoplasts and plants. DualPE was effective in directly replacing wheat genomic fragments of up to 258 kb with desired sequences in the absence of donor DNA. Additionally, DualPE allowed precise DNA inversions of up to 205.4 kb in wheat plants with efficiencies of up to 51.5%. DualPE also successfully edited large DNA fragments in the dicots Nicotiana benthamiana and tomato, with editing efficiencies of up to 72.7%. DualPE thus provides a precise and efficient approach for large DNA sequence and chromosomal engineering, expanding the availability of precision genome-editing tools for crop improvement.

Advances in lignocellulosic feedstocks for bioenergy and bioproducts

Lignocellulose, an abundant renewable resource, presents a promising alternative for sustainable energy and industrial applications. However, large-scale adoption of lignocellulosic feedstocks faces considerable obstacles, including scalability, bioprocessing efficiency, and resilience to climate change. This Review examines current efforts and future opportunities for leveraging lignocellulosic feedstocks in bio-based energy and products, with a focus on enhancing conversion efficiency and scalability. It also explores emerging biotechnologies such as CRISPR-based genome editing informed by machine learning, aimed at improving feedstock traits and reducing the environmental impact of fossil fuel dependence.

Enhancing CRISPR-Cas-based gene targeting in tomato using a dominant-negative ku80

The CRISPR-Cas-based gene targeting (GT) method has enabled precise modifications of genomic DNA ranging from single base to several kilobase scales through homologous recombination (HR). In plant somatic cells, canonical non-homologous end-joining (cNHEJ) is the predominant mechanism for repairing double-stranded breaks (DSBs), thus limiting the HR-mediated GT. In this study, we implemented an approach to shift the repair pathway preference toward HR by using a dominant-negative ku80 mutant protein (KUDN) to disrupt the initiation of cNHEJ. The employment of KUDN conferred a 1.71- to 3.55-fold improvement in GT efficiency at the callus stage. When we screened transformants, there was a more remarkable increase in GT efficiency, ranging from 1.62- to 9.84-fold, at two specific tomato loci, SlHKT1;2 and SlEPSPS1. With practical levels of efficiency, this enhanced KUDN-based GT tool successfully facilitated a 9-bp addition at an additional locus, SlCAB13. These findings provide another promising method for more efficient and precise plant breeding.

Synthetic minichromosomes in plants: past, present, and promise

The status of engineered mini-chromosomes/artificial chromosomes/synthetic chromosomes in plants is summarized. Their promise is that they provide a means to accumulate foreign genes on an independent entity other than the normal chromosomes, which would facilitate stacking of novel traits in a way that would not be linked to endogenous genes and that would facilitate transfer between lines. Centromeres in plants are epigenetic, and therefore the isolation of DNA underlying centromeres and reintroduction into plant cells will not establish a functional kinetochore, which obviates this approach for in vitro assembly of plant artificial chromosomes. This issue was bypassed by using telomere-mediated chromosomal truncation to produce mini-chromosomes with little more than an endogenous centromere that could in turn be used as a foundation to build synthetic chromosomes. Site-specific recombinases and various iterations of CRISPR-Cas9 editing provide many tools for the development and re-engineering of synthetic chromosomes.

Targeted genome-modification tools and their advanced applications in crop breeding

Crop improvement by genome editing involves the targeted alteration of genes to improve plant traits, such as stress tolerance, disease resistance or nutritional content. Techniques for the targeted modification of genomes have evolved from generating random mutations to precise base substitutions, followed by insertions, substitutions and deletions of small DNA fragments, and are finally starting to achieve precision manipulation of large DNA segments. Recent developments in base editing, prime editing and other CRISPR-associated systems have laid a solid technological foundation to enable plant basic research and precise molecular breeding. In this Review, we systematically outline the technological principles underlying precise and targeted genome-modification methods. We also review methods for the delivery of genome-editing reagents in plants and outline emerging crop-breeding strategies based on targeted genome modification. Finally, we consider potential future developments in precise genome-editing technologies, delivery methods and crop-breeding approaches, as well as regulatory policies for genome-editing products.

Uncovering the dynamics of precise repair at CRISPR/Cas9-induced double-strand breaks

CRISPR/Cas9 is widely used for precise mutagenesis through targeted DNA double-strand breaks (DSBs) induction followed by error-prone repair. A better understanding of this process requires measuring the rates of cutting, error-prone, and precise repair, which have remained elusive so far. Here, we present a molecular and computational toolkit for multiplexed quantification of DSB intermediates and repair products by single-molecule sequencing. Using this approach, we characterize the dynamics of DSB induction, processing and repair at endogenous loci along a 72 h time-course in tomato protoplasts. Combining this data with kinetic modeling reveals that indel accumulation is determined by the combined effect of the rates of DSB induction processing of broken ends, and precise versus error repair. In this study, 64-88% of the molecules were cleaved in the three targets analyzed, while indels ranged between 15-41%. Precise repair accounts for most of the gap between cleavage and error repair, representing up to 70% of all repair events. Altogether, this system exposes flux in the DSB repair process, decoupling induction and repair dynamics, and suggesting an essential role of high-fidelity repair in limiting the efficiency of CRISPR-mediated mutagenesis.

A word of caution: T-DNA-associated mutagenesis in plant reproduction research

T-DNA transformation is prevalent in Arabidopsis research and has expanded to a broad range of crops and model plants. While major progress has been made in optimizing the Agrobacterium-mediated transformation process for various species, a variety of pitfalls associated with the T-DNA insertion may lead to the misinterpretation of T-DNA mutant analysis. Indeed, secondary mutagenesis either on the integration site or elsewhere in the genome, together with epigenetic interactions between T-DNA inserts or frequent genomic rearrangements, can be tricky to differentiate from the effect of the knockout of the gene of interest. These are mainly the case for genomic rearrangements that become balanced in filial generations without consequential phenotypical defects, which may be confusing particularly for studies that aim to investigate fertility and gametogenesis. As a cautionary note to the plant research community studying gametogenesis, we here report an overview of the consequences of T-DNA-induced secondary mutagenesis with emphasis on the genomic imbalance on gametogenesis. Additionally, we present a simple guideline to evaluate the T-DNA-mutagenized transgenic lines to decrease the risk of faulty analysis with minimal experimental effort.

Regulation of gene-edited plants in Europe: from the valley of tears into the shining sun?

Some 20 years ago, the EU introduced complex regulatory rules for the growth of transgenic crops, which resulted in a de facto ban to grow these plants in fields within most European countries. With the rise of novel genome editing technologies, it has become possible to improve crops genetically in a directed way without the need for incorporation of foreign genes. Unfortunately, in 2018, the European Court of Justice ruled that such gene-edited plants are to be regulated like transgenic plants. Since then, European scientists and breeders have challenged this decision and requested a revision of this outdated law. Finally, after 5 years, the European Commission has now published a proposal on how, in the future, to regulate crops produced by new breeding technologies. The proposal tries to find a balance between the different interest groups in Europe. On one side, genetically modified plants, which cannot be discerned from their natural counterparts, will exclusively be used for food and feed and are-besides a registration step-not to be regulated at all. On the other side, plants expressing herbicide resistance are to be excluded from this regulation, a concession to the strong environmental associations and NGOs in Europe. Moreover, edited crops are to be excluded from organic farming to protect the business interests of the strong organic sector in Europe. Nevertheless, if this law passes European parliament and council, unchanged, it will present a big step forward toward establishing a more sustainable European agricultural system. Thus, it might soon be possible to develop and grow crops that are more adapted to global warming and whose cultivation will require lower amounts of pesticides. However, there is still a long way to go until the law is passed. Too often, the storm of arguments raised by the opponents, based on irrational fears of mutations and a naive understanding of nature, has fallen on fruitful ground in Europe.

Identification of allelic relationship and translocation region among chromosomal translocation lines that leads to less-seed watermelon

Less-seed and seedless traits are desirable characteristics in watermelon (Citrullus lanatus). Hybridization between watermelon chromosomal translocated lines and wild lines significantly reduced seed counts in the hybrid fruits, approaching even seedless. However, the allelic relationships and the chromosomal translocation breakpoints from different sources are unclear, which limits their utility in breeding practices. This study focused on three groups of chromosomal translocation materials from different sources and conducted inheritance and allelic relationship analysis of translocation points. The results from third-generation genome sequencing and fluorescence in situ hybridization (FISH) revealed that the specific translocations in the naturally mutated material MT-a involved reciprocal translocations between Chr6 and Chr10. The Co60γ radiation-induced mutant material MT-b involved reciprocal translocations between Chr1 and Chr5, Chr4 and Chr8. The Co60γ radiation-induced mutant material MT-c involved complex translocations among Chr1, Chr5, and Chr11. Cytological observation showed that heterozygous translocation hybrids showed chromosomal synapsis abnormalities during meiotic diakinesis. Further, dominant and codominant molecular markers were developed on both sides of the translocation breakpoints, which could facilitate rapid and efficient identification of chromosome translocation lines. This study provides technical guidance for utilizing chromosomal translocation materials in the development of less-seed watermelon varieties.

Modulation of the microhomology-mediated end joining pathway suppresses large deletions and enhances homology-directed repair following CRISPR-Cas9-induced DNA breaks

BACKGROUND

CRISPR-Cas9 genome editing often induces unintended, large genomic rearrangements, posing potential safety risks. However, there are no methods for mitigating these risks.

RESULTS

Using long-read individual-molecule sequencing (IDMseq), we found the microhomology-mediated end joining (MMEJ) DNA repair pathway plays a predominant role in Cas9-induced large deletions (LDs). We targeted MMEJ-associated genes genetically and/or pharmacologically and analyzed Cas9-induced LDs at multiple gene loci using flow cytometry and long-read sequencing. Reducing POLQ levels or activity significantly decreases LDs, while depleting or overexpressing RPA increases or reduces LD frequency, respectively. Interestingly, small-molecule inhibition of POLQ and delivery of recombinant RPA proteins also dramatically promote homology-directed repair (HDR) at multiple disease-relevant gene loci in human pluripotent stem cells and hematopoietic progenitor cells.

CONCLUSIONS

Our findings reveal the contrasting roles of RPA and POLQ in Cas9-induced LD and HDR, suggesting new strategies for safer and more precise genome editing.

Genomic view of heavy-ion-induced deletions associated with distribution of essential genes in Arabidopsis thaliana

Heavy-ion beam, a type of ionizing radiation, has been applied to plant breeding as a powerful mutagen and is a promising tool to induce large deletions and chromosomal rearrangements. The effectiveness of heavy-ion irradiation can be explained by linear energy transfer (LET; keV µm-1). Heavy-ion beams with different LET values induce different types and sizes of mutations. It has been suggested that deletion size increases with increasing LET value, and complex chromosomal rearrangements are induced in higher LET radiations. In this study, we mapped heavy-ion beam-induced deletions detected in Arabidopsis mutants to its genome. We revealed that deletion sizes were similar between different LETs (100 to 290 keV μm-1), that their upper limit was affected by the distribution of essential genes, and that the detected chromosomal rearrangements avoid disrupting the essential genes. We also focused on tandemly arrayed genes (TAGs), where two or more homologous genes are adjacent to one another in the genome. Our results suggested that 100 keV µm-1 of LET is enough to disrupt TAGs and that the distribution of essential genes strongly affects the heritability of mutations overlapping them. Our results provide a genomic view of large deletion inductions in the Arabidopsis genome.

Deficiency of both classical and alternative end-joining pathways leads to a synergistic defect in double-strand break repair but not to an increase in homology-dependent gene targeting in Arabidopsis

In eukaryotes, double-strand breaks (DSBs) are either repaired by homologous recombination (HR) or non-homologous end-joining (NHEJ). In somatic plant cells, HR is very inefficient. Therefore, the vast majority of DSBs are repaired by two different pathways of NHEJ. The classical (cNHEJ) pathway depends on the heterodimer KU70/KU80, while polymerase theta (POLQ) is central to the alternative (aNHEJ) pathway. Surprisingly, Arabidopsis plants are viable, even when both pathways are impaired. However, they exhibit severe growth retardation and reduced fertility. Analysis of mitotic anaphases indicates that the double mutant is characterized by a dramatic increase in chromosome fragmentation due to defective DSB repair. In contrast to the single mutants, the double mutant was found to be highly sensitive to the DSB-inducing genotoxin bleomycin. Thus, both pathways can complement for each other efficiently in DSB repair. We speculated that in the absence of both NHEJ pathways, HR might be enhanced. This would be especially attractive for gene targeting (GT) in which predefined changes are introduced using a homologous template. Unexpectedly, the polq single mutant as well as the double mutant showed significantly lower GT frequencies in comparison to wildtype plants. Accordingly, we were able to show that elimination of both NHEJ pathways does not pose an attractive approach for Agrobacterium-mediated GT. However, our results clearly indicate that a loss of cNHEJ leads to an increase in GT frequency, which is especially drastic and attractive for practical applications, in which the in planta GT strategy is used.

Crop bioengineering via gene editing: reshaping the future of agriculture

Genome-editing technologies have revolutionized research in plant biology, with major implications for agriculture and worldwide food security, particularly in the face of challenges such as climate change and increasing human populations. Among these technologies, clustered regularly interspaced short palindromic repeats [CRISPR]-CRISPR-associated protein [Cas] systems are now widely used for editing crop plant genomes. In this review, we provide an overview of CRISPR-Cas technology and its most significant applications for improving crop sustainability. We also review current and potential technological advances that will aid in the future breeding of crops to enhance food security worldwide. Finally, we discuss the obstacles and challenges that must be overcome to realize the maximum potential of genome-editing technologies for future crop and food production.

Plant Artificial Chromosomes: Construction and Transformation

With the decline of cultivated land and increase of the population in recent years, an agricultural revolution is urgently needed to produce more food to improve the living standards of humans. As one of the foundations of synthetic biology, artificial chromosomes hold great potential for advancing crop improvement. They offer opportunities to increase crop yield and quality, while enhancing crop resistance to disease. The progress made in plant artificial chromosome technology enables selective modification of existing chromosomes or the synthesis of new ones to improve crops and study gene function. However, current artificial chromosome technologies still face limitations, particularly in the synthesis of repeat sequences and the transformation of large DNA fragments. In this review, we will introduce the structure of plant centromeres, the construction of plant artificial chromosomes, and possible methods for transforming large fragments into plant cells.

Plant chromosome engineering - past, present and future

Spontaneous chromosomal rearrangements (CRs) play an essential role in speciation, genome evolution and crop domestication. To be able to use the potential of CRs for breeding, plant chromosome engineering was initiated by fragmenting chromosomes by X-ray irradiation. With the rise of the CRISPR/Cas system, it became possible to induce double-strand breaks (DSBs) in a highly efficient manner at will at any chromosomal position. This has enabled a completely new level of predesigned chromosome engineering. The genetic linkage between specific genes can be broken by inducing chromosomal translocations. Natural inversions, which suppress genetic exchange, can be reverted for breeding. In addition, various approaches for constructing minichromosomes by downsizing regular standard A or supernumerary B chromosomes, which could serve as future vectors in plant biotechnology, have been developed. Recently, a functional synthetic centromere could be constructed. Also, different ways of genome haploidization have been set up, some based on centromere manipulations. In the future, we expect to see even more complex rearrangements, which can be combined with previously developed engineering technologies such as recombinases. Chromosome engineering might help to redefine genetic linkage groups, change the number of chromosomes, stack beneficial genes on mini cargo chromosomes, or set up genetic isolation to avoid outcrossing.

Genomic consequences associated with Agrobacterium-mediated transformation of plants

Attenuated strains of the naturally occurring plant pathogen Agrobacterium tumefaciens can transfer virtually any DNA sequence of interest to model plants and crops. This has made Agrobacterium-mediated transformation (AMT) one of the most commonly used tools in agricultural biotechnology. Understanding AMT, and its functional consequences, is of fundamental importance given that it sits at the intersection of many fundamental fields of study, including plant-microbe interactions, DNA repair/genome stability, and epigenetic regulation of gene expression. Despite extensive research and use of AMT over the last 40 years, the extent of genomic disruption associated with integrating exogenous DNA into plant genomes using this method remains underappreciated. However, new technologies like long-read sequencing make this disruption more apparent, complementing previous findings from multiple research groups that have tackled this question in the past. In this review, we cover progress on the molecular mechanisms involved in Agrobacterium-mediated DNA integration into plant genomes. We also discuss localized mutations at the site of insertion and describe the structure of these DNA insertions, which can range from single copy insertions to large concatemers, consisting of complex DNA originating from different sources. Finally, we discuss the prevalence of large-scale genomic rearrangements associated with the integration of DNA during AMT with examples. Understanding the intended and unintended effects of AMT on genome stability is critical to all plant researchers who use this methodology to generate new genetic variants.

Prime editing: Mechanism insight and recent applications in plants

Prime editing (PE) technology utilizes an extended prime editing guide RNA (pegRNA) to direct a fusion peptide consisting of nCas9 (H840) and reverse transcriptase (RT) to a specific location in the genome. This enables the installation of base changes at the targeted site using the extended portion of the pegRNA through RT activity. The resulting product of the RT reaction forms a 3' flap, which can be incorporated into the genomic site through a series of biochemical steps involving DNA repair and synthesis pathways. PE has demonstrated its effectiveness in achieving almost all forms of precise gene editing, such as base conversions (all types), DNA sequence insertions and deletions, chromosomal translocation and inversion and long DNA sequence insertion at safe harbour sites within the genome. In plant science, PE could serve as a groundbreaking tool for precise gene editing, allowing the creation of desired alleles to improve crop varieties. Nevertheless, its application has encountered limitations due to efficiency constraints, particularly in dicotyledonous plants. In this review, we discuss the step-by-step mechanism of PE, shedding light on the critical aspects of each step while suggesting possible solutions to enhance its efficiency. Additionally, we present an overview of recent advancements and future perspectives in PE research specifically focused on plants, examining the key technical considerations of its applications.

Genome editing for plant synthetic metabolic engineering and developmental regulation

Plant metabolism and development are a reflection of the orderly expression of genetic information intertwined with the environment interactions. Genome editing is the cornerstone for scientists to modify endogenous genes or introduce exogenous functional genes and metabolic pathways, holding immense potential applications in molecular breeding and biosynthesis. Over the course of nearly a decade of development, genome editing has advanced significantly beyond the simple cutting of double-stranded DNA, now enabling precise base and fragment replacements, regulation of gene expression and translation, as well as epigenetic modifications. However, the utilization of genome editing in plant synthetic metabolic engineering and developmental regulation remains exploratory. Here, we provide an introduction and a comprehensive overview of the editing attributes associated with various CRISPR/Cas tools, along with diverse strategies for the meticulous control of plant metabolic pathways and developments. Furthermore, we discuss the limitations of current approaches and future prospects for genome editing-driven plant breeding.

CRISPR/Cas9-induced DNA breaks trigger crossover, chromosomal loss, and chromothripsis-like rearrangements

DNA double-stranded breaks (DSBs) generated by the Cas9 nuclease are commonly repaired via nonhomologous end-joining (NHEJ) or homologous recombination (HR). However, little is known about unrepaired DSBs and the type of damage they trigger in plants. We designed an assay that detects loss of heterozygosity (LOH) in somatic cells, enabling the study of a broad range of DSB-induced genomic events. The system relies on a mapped phenotypic marker which produces a light purple color (betalain pigment) in all plant tissues. Plants with sectors lacking the Betalain marker upon DSB induction between the marker and the centromere were tested for LOH events. Using this assay, we detected a tomato (Solanum lycopersicum) flower with a twin yellow and dark purple sector, corresponding to a germinally transmitted somatic crossover event. We also identified instances of small deletions of genomic regions spanning the T-DNA and whole chromosome loss. In addition, we show that major chromosomal rearrangements including loss of large fragments, inversions, and translocations were clearly associated with the CRISPR-induced DSB. Detailed characterization of complex rearrangements by whole-genome sequencing and molecular and cytological analyses supports a model in which a breakage-fusion-bridge cycle followed by chromothripsis-like rearrangements had been induced. Our LOH assay provides a tool for precise breeding via targeted crossover detection. It also uncovers CRISPR-mediated chromothripsis-like events in plants.

Hidden prevalence of deletion-inversion bi-alleles in CRISPR-mediated deletions of tandemly arrayed genes in plants

Tandemly arrayed genes (TAGs) with functional redundancy and chromosomal linkage constitute 14 ~ 35% in sequenced plant genomes. The multiplex CRISPR system is the tool of choice for creating targeted TAG deletions. Here, we show that up to ~80% of CRISPR-mediated TAG knockout alleles in Arabidopsis and rice are deletion-inversion (delinver) bi-alleles, which are easily misidentified as homozygous deletion alleles by routine PCR-based genotyping. This can lead to misinterpretation of experimental data and production of progenies with genetic heterogeneity in an unnoticed manner. In ~2,650 transgenic events, delinver mutation frequencies are predominantly correlated with deletion frequencies but unrelated to chromosomal locations or deletion sizes. Delinver mutations also occur frequently at genomic non-TAG loci during multiplexed CRISPR editing. Our work raises the alarm about delinver mutations as common unwanted products of targeted TAG deletions in plants and helps prevent false interpretation of plant TAG functions due to this hidden genotype issue.

Towards Plant Synthetic Genomics

Rapid advances in DNA synthesis techniques have allowed the assembly and engineering of viral and microbial genomes. Multicellular eukaryotic organisms, with their larger genomes, abundant transposons, and prevalent epigenetic regulation, present a new frontier to synthetic genomics. Plant synthetic genomics have long been proposed, and exciting progress has been made using the top-down approach. In this perspective, we propose applying bottom-up genome synthesis in multicellular plants, starting from the model moss Physcomitrium patens, in which homologous recombination, DNA delivery, and regeneration are possible, although further optimizations are necessary. We then discuss technical barriers, including genome assembly and plant transformation, associated with synthetic genomics in seed plants.

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

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

Engineering traits through CRISPR/cas genome editing in woody species to improve forest diversity and yield

Dangers confronting forest ecosystems are many and the strength of these biological systems is deteriorating, thus substantially affecting tree physiology, phenology, and growth. The establishment of genetically engineered trees into degraded woodlands, which would be adaptive to changing climate, could help in subsiding ecological threats and bring new prospects. This should not be resisted due to the apprehension of transgene dispersal in forests. Consequently, it is important to have a deep insight into the genetic structure and phenotypic limits of the reproductive capability of tree stands/population(s) to endure tolerance and survival. Importantly, for a better understanding of genes and their functional mechanisms, gene editing (GeEd) technology is an excellent molecular tool to unravel adaptation progressions. Therefore, GeEd could be harnessed for resolving the allelic interactions for the creation of gene diversity, and transgene dispersal may be alleviated among the population or species in different bioclimatic zones around the globe. This review highlights the potential of the CRISPR/Cas tools in genomic, transcriptomic, and epigenomic-based assorted and programmable alterations of genes in trees that might be able to fix the trait-specific gene function. Also, we have discussed the application of diverse forms of GeEd to genetically improve several traits, such as wood density, phytochemical constituents, biotic and abiotic stress tolerance, and photosynthetic efficiency in trees. We believe that the technology encourages fundamental research in the forestry sector besides addressing key aspects, which might fasten tree breeding and germplasm improvement programs worldwide.

Multiplex Editing of the Nucleoredoxin1 Tandem Array in Poplar: From Small Indels to Translocations and Complex Inversions

The CRISPR-Cas9 system has been deployed for precision mutagenesis in an ever-growing number of species, including agricultural crops and forest trees. Its application to closely linked genes with extremely high sequence similarities has been less explored. In this study, we used CRISPR-Cas9 to mutagenize a tandem array of seven Nucleoredoxin1 (NRX1) genes spanning ∼100 kb in Populus tremula × Populus alba. We demonstrated efficient multiplex editing with one single guide RNA in 42 transgenic lines. The mutation profiles ranged from small insertions and deletions and local deletions in individual genes to large genomic dropouts and rearrangements spanning tandem genes. We also detected complex rearrangements including translocations and inversions resulting from multiple cleavage and repair events. Target capture sequencing was instrumental for unbiased assessments of repair outcomes to reconstruct unusual mutant alleles. The work highlights the power of CRISPR-Cas9 for multiplex editing of tandemly duplicated genes to generate diverse mutants with structural and copy number variations to aid future functional characterization.

Enhancing Maize Transformation and Targeted Mutagenesis through the Assistance of Non-Integrating Wus2 Vector

Efficient genetic transformation is a prerequisite for rapid gene functional analyses and crop trait improvements. We recently demonstrated that new T-DNA binary vectors with NptII/G418 selection and a compatible helper plasmid can efficiently transform maize inbred B104 using our rapid Agrobacterium-mediated transformation method. In this work, we implemented the non-integrating Wuschel2 (Wus2) T-DNA vector method for Agrobacterium-mediated B104 transformation and tested its potential for recalcitrant inbred B73 transformation and gene editing. The non-integrating Wus2 (NIW) T-DNA vector-assisted transformation method uses two Agrobacterium strains: one carrying a gene-of-interest (GOI) construct and the other providing an NIW construct. To monitor Wus2 co-integration into the maize genome, we combined the maize Wus2 expression cassette driven by a strong constitutive promoter with a new visible marker RUBY, which produces the purple pigment betalain. As a GOI construct, we used a previously tested CRISPR-Cas9 construct pKL2359 for Glossy2 gene mutagenesis. When both GOI and NIW constructs were delivered by LBA4404Thy- strain, B104 transformation frequency was significantly enhanced by about two-fold (10% vs. 18.8%). Importantly, we were able to transform a recalcitrant inbred B73 using the NIW-assisted transformation method and obtained three transgene-free edited plants by omitting the selection agent G418. These results suggest that NIW-assisted transformation can improve maize B104 transformation frequency and provide a novel option for CRISPR technology for transgene-free genome editing.

CRISPR/Cas9 and Agrobacterium tumefaciens virulence proteins synergistically increase efficiency of precise genome editing via homology directed repair in plants

CRISPR/Cas9 genome editing and Agrobacterium tumefaciens-mediated genetic transformation are widely-used plant biotechnology tools derived from bacterial immunity-related systems, each involving DNA modification. The Cas9 endonuclease introduces DNA double-strand breaks (DSBs), and the A. tumefaciens T-DNA is released by the VirD2 endonuclease assisted by VirDl and attached by VirE2, transferred to the plant nucleus and integrated into the genome. Here, we explored the potential for synergy between the two systems and found that Cas9 and three virulence (Vir) proteins achieve precise genome editing via the homology directed repair (HDR) pathway in tobacco and rice plants. Compared with Cas9T (Cas9, VirD1, VirE2) and CvD (Cas9-VirD2) systems, the HDR frequencies of a foreign GFPm gene in the CvDT system (Cas9-VirD2, VirD1, VirE2) increased 52-fold and 22-fold, respectively. Further optimization of the CvDT process with a donor linker (CvDTL) achieved a remarkable increase in the efficiency of HDR-mediated genome editing. Additionally, the HDR efficiency of the three rice endogenous genes ACETOLACTATE SYNTHASE (ALS), PHYTOENE DESATURASE (PDS), and NITROGEN TRANSPORTER 1.1 B (NRT1.1B) increased 24-, 32- and 16-fold, respectively, in the CvDTL system, compared with corresponding Cas9TL (Cas9T process with a donor linker). Our results suggest that collaboration between CRISPR/Cas9 and Agrobacterium-mediated genetic transformation can make great progress towards highly efficient and precise genome editing via the HDR pathway.

Molecular cytogenetic map visualizes the heterozygotic genome and identifies translocation chromosomes in Citrus sinensis

Citrus sinensis is the most cultivated and economically valuable Citrus species in the world, whose genome has been assembled by three generation sequencings. However, chromosome recognition remains a problem due to the small size of chromosomes, and difficulty in differentiating between pseudo and real chromosomes because of a highly heterozygous genome. Here, we employ fluorescence in situ hybridization (FISH) with 9 chromosome painting probes, 30 oligo pools, and 8 repetitive sequences to visualize 18 chromosomes. Then, we develop an approach to identify each chromosome in one cell through single experiment of oligo-FISH and Chromoycin A3 (CMA) staining. By this approach, we construct a high-resolution molecular cytogenetic map containing the physical positions of CMA banding and 38 sequences of FISH including centromere regions, which enables us to visualize significant differences between homologous chromosomes. Based on the map, we locate several highly repetitive sequences on chromosomes and estimate sizes and copy numbers of each site. In particular, we discover the translocation regions of chromosomes 4 and 9 in C. sinensis "Valencia." The high-resolution molecular cytogenetic map will help improve understanding of sweet orange genome assembly and also provide a fundamental reference for investigating chromosome evolution and chromosome engineering for genetic improvement in Citrus.

An overview of genome engineering in plants, including its scope, technologies, progress and grand challenges

Genome editing is a useful, adaptable, and favored technique for both functional genomics and crop enhancement. Over the years, rapidly evolving genome editing technologies, including clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas), transcription activator-like effector nucleases (TALENs), and zinc finger nucleases (ZFNs), have shown broad application prospects in gene function research and improvement of critical agronomic traits in many crops. These technologies have also opened up opportunities for plant breeding. These techniques provide excellent chances for the quick modification of crops and the advancement of plant science in the future. The current review describes various genome editing techniques and how they function, particularly CRISPR/Cas9 systems, which can contribute significantly to the most accurate characterization of genomic rearrangement and plant gene functions as well as the enhancement of critical traits in field crops. To accelerate the use of gene-editing technologies for crop enhancement, the speed editing strategy of gene-family members was designed. As it permits genome editing in numerous biological systems, the CRISPR technology provides a valuable edge in this regard that particularly captures the attention of scientists.

Enhancing the quality of staple food crops through CRISPR/Cas-mediated site-directed mutagenesis

The enhancement of CRISPR-Cas gene editing with robust nuclease activity promotes genetic modification of desirable agronomic traits, such as resistance to pathogens, drought tolerance, nutritional value, and yield-related traits in crops. The genetic diversity of food crops has reduced tremendously over the past twelve millennia due to plant domestication. This reduction presents significant challenges for the future especially considering the risks posed by global climate change to food production. While crops with improved phenotypes have been generated through crossbreeding, mutation breeding, and transgenic breeding over the years, improving phenotypic traits through precise genetic diversification has been challenging. The challenges are broadly associated with the randomness of genetic recombination and conventional mutagenesis. This review highlights how emerging gene-editing technologies reduce the burden and time necessary for developing desired traits in plants. Our focus is to provide readers with an overview of the advances in CRISPR-Cas-based genome editing for crop improvement. The use of CRISPR-Cas systems in generating genetic diversity to enhance the quality and nutritional value of staple food crops is discussed. We also outlined recent applications of CRISPR-Cas in developing pest-resistant crops and removing unwanted traits, such as allergenicity from crops. Genome editing tools continue to evolve and present unprecedented opportunities to enhance crop germplasm via precise mutations at the desired loci of the plant genome.

Getting better all the time - recent progress in the development of CRISPR/Cas-based tools for plant genome engineering

Since their first adaptation for plant genome editing, clustered regularly interspaced short palindromic repeats/CRISPR-associated system nucleases and tools have revolutionized the field. While early approaches focused on targeted mutagenesis that relies on mutagenic repair of induced double-strand breaks, newly developed tools now enable the precise induction of predefined modifications. Constant efforts to optimize these tools have led to the generation of more efficient base editors with enlarged editing windows and have enabled previously unachievable C-G transversions. Prime editors were also optimized for the application in plants and now allow to accurately induce substitutions, insertions, and deletions. Recently, great progress was made through precise restructuring of chromosomes, which enables not only the breakage or formation of genetic linkages but also the swapping of promoters.

Applications of CRISPR/Cas genome editing in economically important fruit crops: recent advances and future directions

Fruit crops, consist of climacteric and non-climacteric fruits, are the major sources of nutrients and fiber for human diet. Since 2013, CRISPR/Cas (Clustered Regularly Interspersed Short Palindromic Repeats and CRISPR-Associated Protein) genome editing system has been widely employed in different plants, leading to unprecedented progress in the genetic improvement of many agronomically important fruit crops. Here, we summarize latest advancements in CRISPR/Cas genome editing of fruit crops, including efforts to decipher the mechanisms behind plant development and plant immunity, We also highlight the potential challenges and improvements in the application of genome editing tools to fruit crops, including optimizing the expression of CRISPR/Cas cassette, improving the delivery efficiency of CRISPR/Cas reagents, increasing the specificity of genome editing, and optimizing the transformation and regeneration system. In addition, we propose the perspectives on the application of genome editing in crop breeding especially in fruit crops and highlight the potential challenges. It is worth noting that efforts to manipulate fruit crops with genome editing systems are urgently needed for fruit crops breeding and demonstration.

Characterization and acceleration of genome shuffling and ploidy reduction in synthetic allopolyploids by genome sequencing and editing

Polyploidy and the subsequent ploidy reduction and genome shuffling are the major driving forces of genome evolution. Here, we revealed short-term allopolyploid genome evolution by sequencing a synthetic intergeneric hybrid (Raphanobrassica, RRCC). In this allotetraploid, the genome deletion was quick, while rearrangement was slow. The core and high-frequency genes tended to be retained while the specific and low-frequency genes tended to be deleted in the hybrid. The large-fragment deletions were enriched in the heterochromatin region and probably derived from chromosome breaks. The intergeneric translocations were primarily of short fragments dependent on homoeology, indicating a gene conversion origin. To accelerate genome shuffling, we developed an efficient genome editing platform for Raphanobrassica. By editing Fanconi Anemia Complementation Group M (FANCM) genes, homoeologous recombination, chromosome deletion and secondary meiosis with additional ploidy reduction were accelerated. FANCM was shown to be a checkpoint of meiosis and controller of ploidy stability. By simultaneously editing FLIP genes, gene conversion was precisely introduced, and mosaic genes were produced around the target site. This intergeneric hybrid and genome editing platform not only provides models that facilitate experimental evolution research by speeding up genome shuffling and conversion but also accelerates plant breeding by enhancing intergeneric genetic exchange and creating new genes.

Centromeres: From chromosome biology to biotechnology applications and synthetic genomes in plants

Centromeres are the genomic regions that organize and regulate chromosome behaviours during cell cycle, and their variations are associated with genome instability, karyotype evolution and speciation in eukaryotes. The highly repetitive and epigenetic nature of centromeres were documented during the past half century. With the aid of rapid expansion in genomic biotechnology tools, the complete sequence and structural organization of several plant and human centromeres were revealed recently. Here, we systematically summarize the current knowledge of centromere biology with regard to the DNA compositions and the histone H3 variant (CENH3)-dependent centromere establishment and identity. We discuss the roles of centromere to ensure cell division and to maintain the three-dimensional (3D) genomic architecture in different species. We further highlight the potential applications of manipulating centromeres to generate haploids or to induce polyploids offspring in plant for breeding programs, and of targeting centromeres with CRISPR/Cas for chromosome engineering and speciation. Finally, we also assess the challenges and strategies for de novo design and synthesis of centromeres in plant artificial chromosomes. The biotechnology applications of plant centromeres will be of great potential for the genetic improvement of crops and precise synthetic breeding in the future.

A detailed landscape of CRISPR-Cas-mediated plant disease and pest management

Genome editing technology has rapidly evolved to knock-out genes, create targeted genetic variation, install precise insertion/deletion and single nucleotide changes, and perform large-scale alteration. The flexible and multipurpose editing technologies have started playing a substantial role in the field of plant disease management. CRISPR-Cas has reduced many limitations of earlier technologies and emerged as a versatile toolbox for genome manipulation. This review summarizes the phenomenal progress of the use of the CRISPR toolkit in the field of plant pathology. CRISPR-Cas toolbox aids in the basic studies on host-pathogen interaction, in identifying virulence genes in pathogens, deciphering resistance and susceptibility factors in host plants, and engineering host genome for developing resistance. We extensively reviewed the successful genome editing applications for host plant resistance against a wide range of biotic factors, including viruses, fungi, oomycetes, bacteria, nematodes, insect pests, and parasitic plants. Recent use of CRISPR-Cas gene drive to suppress the population of pathogens and pests has also been discussed. Furthermore, we highlight exciting new uses of the CRISPR-Cas system as diagnostic tools, which rapidly detect pathogenic microorganism. This comprehensive yet concise review discusses innumerable strategies to reduce the burden of crop protection.

Massive crossover suppression by CRISPR-Cas-mediated plant chromosome engineering

Recent studies have demonstrated that not only genes but also entire chromosomes can be engineered using clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPER-associated protein 9 (Cas9)^1-5. A major objective of applying chromosome restructuring in plant breeding is the manipulation of genetic exchange⁶. Here we show that meiotic recombination can be suppressed in nearly the entire chromosome using chromosome restructuring. We were able to induce a heritable inversion of a >17 Mb-long chromosome fragment that contained the centromere and covered most of chromosome 2 of the Arabidopsis ecotype Col-0. Only the 2 and 0.5 Mb-long telomeric ends remained in their original orientation. In single-nucleotide polymorphism marker analysis of the offspring of crosses with the ecotype Ler-1, we detected a massive reduction of crossovers within the inverted chromosome region, coupled with a shift of crossovers to the telomeric ends. The few genetic exchanges detected within the inversion all originated from double crossovers. This not only indicates that heritable genetic exchange can occur by interstitial chromosome pairing, but also that it is restricted to the production of viable progeny.

The integrated genomics of crop domestication and breeding

As a major event in human civilization, wild plants were successfully domesticated to be crops, largely owing to continuing artificial selection. Here, we summarize new discoveries made during the past decade in crop domestication and breeding. The construction of crop genome maps and the functional characterization of numerous trait genes provide foundational information. Approaches to read, interpret, and write complex genetic information are being leveraged in many plants for highly efficient de novo or re-domestication. Understanding the underlying mechanisms of crop microevolution and applying the knowledge to agricultural productions will give possible solutions for future challenges in food security.

Genome editing and beyond: what does it mean for the future of plant breeding?

MAIN CONCLUSION

Genome editing offers revolutionized solutions for plant breeding to sustain food production to feed the world by 2050. Therefore, genome-edited products are increasingly recognized via more relaxed legislation and community adoption. The world population and food production are disproportionally growing in a manner that would have never matched each other under the current agricultural practices. The emerging crisis is more evident with the subtle changes in climate and the running-off of natural genetic resources that could be easily used in breeding in conventional ways. Under these circumstances, affordable CRISPR-Cas-based gene-editing technologies have brought hope and charged the old plant breeding machine with the most energetic and powerful fuel to address the challenges involved in feeding the world. What makes CRISPR-Cas the most powerful gene-editing technology? What are the differences between it and the other genetic engineering/breeding techniques? Would its products be labeled as "conventional" or "GMO"? There are so many questions to be answered, or that cannot be answered within the limitations of our current understanding. Therefore, we would like to discuss and answer some of the mentioned questions regarding recent progress in technology development. We hope this review will offer another view on the role of CRISPR-Cas technology in future of plant breeding for food production and beyond.

CRISPR-Cas9-mediated chromosome engineering in Arabidopsis thaliana

The rise of the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein (Cas) system has made it possible to induce double-strand breaks at almost any desired target site in the genome. In plant somatic cells, double-strand breaks are predominantly repaired by the error-prone nonhomologous end-joining pathway, which can lead to mutations at the break site upon repair. So far, it had only been possible to induce genomic changes of up to a few hundred kilobases in plants utilizing this mechanism. However, by combining the highly efficient Staphylococcus aureus Cas9 (SaCas9) with an egg-cell-specific promoter to facilitate heritable mutations, chromosomal rearrangements in the Mb range, such as inversion and translocations, were obtained in Arabidopsis thaliana recently. Here we describe the chromosome-engineering protocol used to generate these heritable chromosomal rearrangements in A. thaliana. The protocol is based on Agrobacterium-mediated transformation of A. thaliana with transfer DNA constructs containing SaCas9, which is driven by an egg-cell-specific promoter, and two guide RNAs that have been preselected based on their cutting efficiency. In the T1 generation, primary transformants are selected and, if required, analyzed by Droplet Digital PCR and propagated. In the following generations, junction-specific PCR screenings are carried out until plants that carry the rearrangement homozygously are identified. Using this protocol, overall rearrangement frequencies range between 0.03% and 0.5%, depending on the type of rearrangement. In total, it takes about 1 year to establish homozygous lines.

Recent advancements in CRISPR/Cas technology for accelerated crop improvement

Precise genome engineering approaches could be perceived as a second paradigm for targeted trait improvement in crop plants, with the potential to overcome the constraints imposed by conventional CRISPR/Cas technology. The likelihood of reduced agricultural production due to highly turbulent climatic conditions increases as the global population expands. The second paradigm of stress-resilient crops with enhanced tolerance and increased productivity against various stresses is paramount to support global production and consumption equilibrium. Although traditional breeding approaches have substantially increased crop production and yield, effective strategies are anticipated to restore crop productivity even further in meeting the world's increasing food demands. CRISPR/Cas, which originated in prokaryotes, has surfaced as a coveted genome editing tool in recent decades, reshaping plant molecular biology in unprecedented ways and paving the way for engineering stress-tolerant crops. CRISPR/Cas is distinguished by its efficiency, high target specificity, and modularity, enables precise genetic modification of crop plants, allowing for the creation of allelic variations in the germplasm and the development of novel and more productive agricultural practices. Additionally, a slew of advanced biotechnologies premised on the CRISPR/Cas methodologies have augmented fundamental research and plant synthetic biology toolkits. Here, we describe gene editing tools, including CRISPR/Cas and its imitative tools, such as base and prime editing, multiplex genome editing, chromosome engineering followed by their implications in crop genetic improvement. Further, we comprehensively discuss the latest developments of CRISPR/Cas technology including CRISPR-mediated gene drive, tissue-specific genome editing, dCas9 mediated epigenetic modification and programmed self-elimination of transgenes in plants. Finally, we highlight the applicability and scope of advanced CRISPR-based techniques in crop genetic improvement.

Nonhomologous end joining as key to CRISPR/Cas-mediated plant chromosome engineering

Although clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas)-mediated gene editing has revolutionized biology and plant breeding, large-scale, heritable restructuring of plant chromosomes is still in its infancy. Duplications and inversions within a chromosome, and also translocations between chromosomes, can now be achieved. Subsequently, genetic linkages can be broken or can be newly created. Also, the order of genes on a chromosome can be changed. While natural chromosomal recombination occurs by homologous recombination during meiosis, CRISPR/Cas-mediated chromosomal rearrangements can be obtained best by harnessing nonhomologous end joining (NHEJ) pathways in somatic cells. NHEJ can be subdivided into the classical (cNHEJ) and alternative NHEJ (aNHEJ) pathways, which partially operate antagonistically. The cNHEJ pathway not only protects broken DNA ends from degradation but also suppresses the joining of previously unlinked broken ends. Hence, in the absence of cNHEJ, more inversions or translocations can be obtained which can be ascribed to the unrestricted use of the aNHEJ pathway for double-strand break (DSB) repair. In contrast to inversions or translocations, short tandem duplications can be produced by paired single-strand breaks via a Cas9 nickase. Interestingly, the cNHEJ pathway is essential for these kinds of duplications, whereas aNHEJ is required for patch insertions that can also be formed during DSB repair. As chromosome engineering has not only been accomplished in the model plant Arabidopsis (Arabidopsis thaliana) but also in the crop maize (Zea mays), we expect that this technology will soon transform the breeding process.