Live-cell CRISPR imaging in plants reveals dynamic telomere movements

PRIMER cells: immune hotspots in plants

Advances in single-cell and spatial biology have transformed the study of plant immunity, revealing distinct immune cell states such as primary immune responder (PRIMER) cells and offering a deeper understanding of defense mechanisms. These insights offer opportunities for the development of advanced strategies for crop protection and disease resistance.

CRISPR-CISH: an in situ chromogenic DNA repeat detection system for research and life science education

In situ hybridization is a technique to visualize specific DNA sequences within nuclei and chromosomes. Various DNA in situ fluorescent labeling methods have been developed, which typically involve global DNA denaturation prior to the probe hybridization and often require fluorescence microscopes for visualization. Here, we report the development of a CRISPR/dCas9-mediated chromogenic in situ DNA detection (CRISPR-CISH) method that combines chromogenic signal detection with CRISPR imaging. This non-fluorescent approach uses 3' biotin-labeled tracrRNA and target-specific crRNA to form mature gRNA, which activates dCas9 to bind to target sequences. The subsequent application of streptavidin alkaline phosphatase or horseradish peroxidase generates chromogenic, target-specific signals that can be analyzed using conventional bright-field microscopes. Additionally, chromatin counterstains were identified to aid in the interpretation of CRISPR-CISH-generated target signals. This advancement makes in situ DNA detection techniques more accessible to researchers, diagnostic applications, and educational institutions in resource-limited settings.

CRISPR/Cas: a toolkit for plant disease diagnostics

Genetic factors and infectious pathogens that cause plant diseases have a major impact on agricultural production. In recent years, the potential of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) system in nucleic acid analysis and plant disease diagnostics has been demonstrated. We highlight progress of CRISPR/Cas technology that is significant for monitoring plant growth and preventing diseases.

Emerging trends in nucleic acid and peptide aptamers in plant science research

MAIN CONCLUSION

Aptamer technology has significantly advanced the field of plant research, emerging as a tool for enhancing agricultural productivity, plant growth, and environmental monitoring. Aptamers are short nucleotide or amino acid sequences that can bind to a range of target molecules with high affinity and selectivity. In recent years, these affinity molecules have piqued the interest of researchers across various scientific fields, including pharmaceuticals, analytical chemistry, and plant science. Advancements in aptamer technology have significantly broadened the horizons of plant science, particularly in the areas of plant analyte detection, pathogen targeting, and protein function analysis. Despite the use of various other bioassays and molecular techniques for plant analyte detection, the small size, chemical stability, and cost-effective synthesis of aptamers make them invaluable tools for unravelling the complexities of plant cells. Here, we discuss the progress in the development of nucleic acid and peptide aptamers and summarize their applications in plant biotechnology. The principles and signalling methods of various aptamer-based biosensors and their prospects as biotechnological tools for functional genomic studies, pathogen resistance, and bioimaging are discussed. Finally, the present challenges and future perspectives of aptamer-based technology in plant research are also summarized.

The histone chaperones ASF1 and HIRA are required for telomere length and 45S rDNA copy number homeostasis

Genome stability is significantly influenced by the precise coordination of chromatin complexes that facilitate the loading and eviction of histones from chromatin during replication, transcription, and DNA repair processes. In this study, we investigate the role of the Arabidopsis H3 histone chaperones ANTI-SILENCING FUNCTION 1 (ASF1) and HISTONE REGULATOR A (HIRA) in the maintenance of telomeres and 45S rDNA loci, genomic sites that are particularly susceptible to changes in the chromatin structure. We find that both ASF1 and HIRA are essential for telomere length regulation, as telomeres are significantly shorter in asf1a1b and hira mutants. However, these shorter telomeres remain localized around the nucleolus and exhibit a comparable relative H3 occupancy to the wild type. In addition to regulating telomere length, ASF1 and HIRA contribute to silencing 45S rRNA genes and affect their copy number. Besides, ASF1 supports global heterochromatin maintenance. Our findings also indicate that ASF1 transiently binds to the TELOMERE REPEAT BINDING 1 protein and the N terminus of telomerase in vivo, suggesting a physical link between the ASF1 histone chaperone and the telomere maintenance machinery.

The potential of ALFA-tag and tyramide-based fluorescence signal amplification to expand the CRISPR-based DNA imaging toolkit

Understanding the spatial organization of genomes within chromatin is crucial for deciphering gene regulation. A recently developed CRISPR-dCas9-based genome labeling tool, known as CRISPR-FISH, allows efficient labeling of repetitive sequences. Unlike standard fluorescence in situ hybridization (FISH), CRISPR-FISH eliminates the need for global DNA denaturation, allowing for superior preservation of chromatin structure. Here, we report on further development of the CRISPR-FISH method, which has been enhanced for increased efficiency through the engineering of a recombinant dCas9 protein containing an ALFA-tag. Using an ALFA-tagged dCas9 protein assembled with an Arabidopsis centromere-specific guide RNA, we demonstrate target-specific labeling with a fluorescence-labeled NbALFA nanobody. The dCas9 protein possessing multiple copies of the ALFA-tag, in combination with a minibody and fluorescence-labeled anti-rabbit secondary antibody, resulted in enhanced target-specific signals. The dCas9-ALFA-tag system was also instrumental in live cell imaging of telomeres in Nicotiana benthamiana. This method will further expand the CRISPR imaging toolkit, facilitating a better understanding of genome organization. Furthermore, we report the successful integration of the highly sensitive tyramide signal amplification method with CRISPR-FISH, demonstrating effective labeling of Arabidopsis centromeres.

Regulatory trends in engineering bioactive-phytocompounds

The secondary plant metabolites are of enormous importance because of their extensive medicinal, nutraceutical, and industrial applications. In plants, these secondary metabolites are often found in extremely small amounts, therefore, following the discovery of any prospective metabolite, the main constraining element is the ability to generate enough material for use in both industrial and therapeutic settings. In order to satisfy the rising demand for value-added metabolites, researchers prefer to use different molecular approaches for scalable and sustainable production of these phytocompounds. Here, we discuss the emerging regulatory trends in engineering these bioactive-phytocompounds and provide recommendation on successful employment of these state-of-the-art technologies for translation of these academic researches into novel process and products.

Human RNA Polymerase II Segregates from Genes and Nascent RNA and Transcribes in the Presence of DNA-Bound dCas9

RNA polymerase II (Pol II) dysfunction is frequently implied in human disease. Understanding its functional mechanism is essential for designing innovative therapeutic strategies. To visualize its supra-molecular interactions with genes and nascent RNA, we generated a human cell line carrying ~335 consecutive copies of a recombinant β-globin gene. Confocal microscopy showed that Pol II was not homogeneously concentrated around these identical gene copies. Moreover, Pol II signals partially overlapped with the genes and their nascent RNA, revealing extensive compartmentalization. Using a cell line carrying a single copy of the β-globin gene, we also tested if the binding of catalytically dead CRISPR-associated system 9 (dCas9) to different gene regions affected Pol II transcriptional activity. We assessed Pol II localization and nascent RNA levels using chromatin immunoprecipitation and droplet digital reverse transcription PCR, respectively. Some enrichment of transcriptionally paused Pol II accumulated in the promoter region was detected in a strand-specific way of gRNA binding, and there was no decrease in nascent RNA levels. Pol II preserved its transcriptional activity in the presence of DNA-bound dCas9. Our findings contribute further insight into the complex mechanism of mRNA transcription in human cells.

CRISPR-based gene editing in plants: Focus on reagents and their delivery tools

Introduction

CRISPR-Cas9 technology has revolutionized plant genome editing, providing precise and efficient methods for genetic modification. This study focuses on the advancements and delivery of CRISPR-Cas9 in plant gene editing.

Methods

A comprehensive search in scientific databases, including PubMed, ScienceDirect, and Google Scholar, was conducted to gather information on CRISPR-Cas9 gene editing and its delivery in precise gene modification in plants.

Results

The evolving landscape of CRISPR nucleases has led to the development of innovative technologies, enhancing plant research. However, successful editing is contingent on efficient delivery of genome engineering reagents. CRISPR-based gene editing in plants utilizes diverse delivery methods: Agrobacterium-mediated transformation for bacterial transfer, biolistic transformation for physical gene insertion, electroporation for direct gene entry, expression of developmental regulators for gene expression modulation, and tobacco rattle virus as a viral vector, each offering distinct advantages for precise and efficient genetic modification in plants.

Conclusion

CRISPR-Cas9 gene editing stands as a pivotal advancement in plant genetics, offering precise gene manipulation with applications in agriculture and biotechnology. The continuous refinement of reagent delivery tools reinforces CRISPR-Cas9's transformative role in plant genome editing, with significant implications for broader scientific applications.

Plant chromatin on the move: an overview of chromatin mobility during transcription and DNA repair

It has become increasingly clear in recent years that chromosomes are highly dynamic entities. Chromatin mobility and re-arrangement are involved in many biological processes, including gene regulation and the maintenance of genome stability. Despite extensive studies on chromatin mobility in yeast and animal systems, up until recently, not much had been investigated at this level in plants. For plants to achieve proper growth and development, they need to respond rapidly and appropriately to environmental stimuli. Therefore, understanding how chromatin mobility can support plant responses may offer profound insights into the functioning of plant genomes. In this review, we discuss the state of the art related to chromatin mobility in plants, including the available technologies for their role in various cellular processes.

Revolutionizing in vivo therapy with CRISPR/Cas genome editing: breakthroughs, opportunities and challenges

Genome editing using the CRISPR/Cas system has revolutionized the field of genetic engineering, offering unprecedented opportunities for therapeutic applications in vivo. Despite the numerous ongoing clinical trials focusing on ex vivo genome editing, recent studies emphasize the therapeutic promise of in vivo gene editing using CRISPR/Cas technology. However, it is worth noting that the complete attainment of the inherent capabilities of in vivo therapy in humans is yet to be accomplished. Before the full realization of in vivo therapeutic potential, it is crucial to achieve enhanced specificity in selectively targeting defective cells while minimizing harm to healthy cells. This review examines emerging studies, focusing on CRISPR/Cas-based pre-clinical and clinical trials for innovative therapeutic approaches for a wide range of diseases. Furthermore, we emphasize targeting cancer-specific sequences target in genes associated with tumors, shedding light on the diverse strategies employed in cancer treatment. We highlight the various challenges associated with in vivo CRISPR/Cas-based cancer therapy and explore their prospective clinical translatability and the strategies employed to overcome these obstacles.

CRISPR-Based Split Luciferase as a Biosensor for Unique DNA Sequences In Situ

To date, CRISPR-based DNA targeting approaches have typically used fusion proteins between full fluorescent reporters and catalytically inactive Cas9 (dCas9) for imaging rather than detection of endogenous genomic DNA sequences. A promising alternative strategy for DNA targeting is the direct biosensing of user-defined sequences at single copy with single-cell resolution. Our recently described DNA biosensing approach using a dual fusion protein biosensor comprised of two independently optimized fragments of NanoLuc luciferase (NLuc) directionally fused to dCas9 paired with user-defined single-guide RNAs (sgRNAs) could allow users to sensitively detect unique copies of a target sequence in individual living cells using common laboratory equipment such as a microscope or a luminescence-equipped microplate reader. Here we describe a protocol for using such a DNA biosensor noninvasively in situ.

Live cell imaging of DNA and RNA with fluorescent signal amplification and background reduction techniques

Illuminating DNA and RNA dynamics in live cell can elucidate their life cycle and related biochemical activities. Various protocols have been developed for labeling the regions of interest in DNA and RNA molecules with different types of fluorescent probes. For example, CRISPR-based techniques have been extensively used for imaging genomic loci. However, some DNA and RNA molecules can still be difficult to tag and observe dynamically, such as genomic loci in non-repetitive regions. In this review, we will discuss the toolbox of techniques and methodologies that have been developed for imaging DNA and RNA. We will also introduce optimized systems that provide enhanced signal intensity or low background fluorescence for those difficult-to-tag molecules. These strategies can provide new insights for researchers when designing and using techniques to visualize DNA or RNA molecules.

CRISPR-Based Genome Editing Tools: An Accelerator in Crop Breeding for a Changing Future

Genome editing is an important strategy to maintain global food security and achieve sustainable agricultural development. Among all genome editing tools, CRISPR-Cas is currently the most prevalent and offers the most promise. In this review, we summarize the development of CRISPR-Cas systems, outline their classification and distinctive features, delineate their natural mechanisms in plant genome editing and exemplify the applications in plant research. Both classical and recently discovered CRISPR-Cas systems are included, detailing the class, type, structures and functions of each. We conclude by highlighting the challenges that come with CRISPR-Cas and offer suggestions on how to tackle them. We believe the gene editing toolbox will be greatly enriched, providing new avenues for a more efficient and precise breeding of climate-resilient crops.

Completing the TRB family: newly characterized members show ancient evolutionary origins and distinct localization, yet similar interactions

Telomere repeat binding proteins (TRBs) belong to a family of proteins possessing a Myb-like domain which binds to telomeric repeats. Three members of this family (TRB1, TRB2, TRB3) from Arabidopsis thaliana have already been described as associated with terminal telomeric repeats (telomeres) or short interstitial telomeric repeats in gene promoters (telo-boxes). They are also known to interact with several protein complexes: telomerase, Polycomb repressive complex 2 (PRC2) E(z) subunits and the PEAT complex (PWOs-EPCRs-ARIDs-TRBs). Here we characterize two novel members of the TRB family (TRB4 and TRB5). Our wide phylogenetic analyses have shown that TRB proteins evolved in the plant kingdom after the transition to a terrestrial habitat in Streptophyta, and consequently TRBs diversified in seed plants. TRB4-5 share common TRB motifs while differing in several others and seem to have an earlier phylogenetic origin than TRB1-3. Their common Myb-like domains bind long arrays of telomeric repeats in vitro, and we have determined the minimal recognition motif of all TRBs as one telo-box. Our data indicate that despite the distinct localization patterns of TRB1-3 and TRB4-5 in situ, all members of TRB family mutually interact and also bind to telomerase/PRC2/PEAT complexes. Additionally, we have detected novel interactions between TRB4-5 and EMF2 and VRN2, which are Su(z)12 subunits of PRC2.

Advances and challenges in CRISPR-based real-time imaging of dynamic genome organization

Nuclear chromosome compaction is non-random and dynamic. The spatial distance among genomic elements instantly modulates transcription. Visualization of the genome organization in the cell nucleus is essential to understand nuclear function. In addition to cell type-dependent organization, high-resolution 3D imaging shows heterogeneous compaction of chromatin organization among the same cell type. Questions remain to be answered if these structural variations were the snapshots of dynamic organization at different time points and if they are functionally different. Live-cell imaging has provided unique insights into dynamic genome organization at short (milliseconds) and long (hours) time scales. The recent development of CRISPR-based imaging opened windows for studying dynamic chromatin organization in single cells in real time. Here we highlight these CRISPR-based imaging techniques and discuss their advances and challenges as a powerful live-cell imaging method that poses high potential to generate paradigm-shifting discoveries and reveal functional implications of dynamic chromatin organization.

CRISPR-FISH: A CRISPR/Cas9-Based In Situ Labeling Method

Fluorescence in situ hybridization (FISH) has been widely used to visualize target DNA sequences in fixed chromosome samples by denaturing the dsDNA to allow complementary probe hybridization, thus damaging the chromatin structure by harsh treatments. To overcome this limitation, a CRISPR/Cas9-based in situ labeling method was developed, termed CRISPR-FISH. This method is also known as RNA-guided endonuclease-in situ labeling (RGEN-ISL). Here we present different protocols for the application of CRISPR-FISH on acetic acid: ethanol or formaldehyde-fixed nuclei and chromosomes as well as tissue sections for labeling repetitive sequences in a range of plant species. In addition, methods on how immunostaining can be combined with CRISPR-FISH are provided.

Visualizing the Genome: Experimental Approaches for Live-Cell Chromatin Imaging

Over the years, our vision of the genome has changed from a linear molecule to that of a complex 3D structure that follows specific patterns and possesses a hierarchical organization. Currently, genomics is becoming "four-dimensional": our attention is increasingly focused on the study of chromatin dynamics over time, in the fourth dimension. Recent methods for visualizing the movements of chromatin loci in living cells by targeting fluorescent proteins can be divided into two groups. The first group requires the insertion of a special sequence into the locus of interest, to which proteins that recognize the sequence are recruited (e.g., FROS and ParB-INT methods). In the methods of the second approach, "programmed" proteins are targeted to the locus of interest (i.e., systems based on CRISPR/Cas, TALE, and zinc finger proteins). In the present review, we discuss these approaches, examine their strengths and weaknesses, and identify the key scientific problems that can be studied using these methods.

Brachypodium: 20 years as a grass biology model system; the way forward?

It has been 20 years since Brachypodium distachyon was suggested as a model grass species, but ongoing research now encompasses the entire genus. Extensive Brachypodium genome sequencing programmes have provided resources to explore the determinants and drivers of population diversity. This has been accompanied by cytomolecular studies to make Brachypodium a platform to investigate speciation, polyploidisation, perenniality, and various aspects of chromosome and interphase nucleus organisation. The value of Brachypodium as a functional genomic platform has been underscored by the identification of key genes for development, biotic and abiotic stress, and cell wall structure and function. While Brachypodium is relevant to the biofuel industry, its impact goes far beyond that as an intriguing model to study climate change and combinatorial stress.

Telomeres and Their Neighbors

Telomeres are essential structures formed from satellite DNA repeats at the ends of chromosomes in most eukaryotes. Satellite DNA repeat sequences are useful markers for karyotyping, but have a more enigmatic role in the eukaryotic cell. Much work has been done to investigate the structure and arrangement of repetitive DNA elements in classical models with implications for species evolution. Still more is needed until there is a complete picture of the biological function of DNA satellite sequences, particularly when considering non-model organisms. Celebrating Gregor Mendel’s anniversary by going to the roots, this review is designed to inspire and aid new research into telomeres and satellites with a particular focus on non-model organisms and accessible experimental and in silico methods that do not require specialized equipment or expensive materials. We describe how to identify telomere (and satellite) repeats giving many examples of published (and some unpublished) data from these techniques to illustrate the principles behind the experiments. We also present advice on how to perform and analyse such experiments, including details of common pitfalls. Our examples are a selection of recent developments and underexplored areas of research from the past. As a nod to Mendel’s early work, we use many examples from plants and insects, especially as much recent work has expanded beyond the human and yeast models traditional in telomere research. We give a general introduction to the accepted knowledge of telomere and satellite systems and include references to specialized reviews for the interested reader.

Recent advances of the biological and biomedical applications of CRISPR/Cas systems

The clustered, regularly interspaced, short palindromic repeats (CRISPR)-associated endonuclease (Cas) system, referred to as CRISPR/Cas system, has attracted significant interest in scientific community due to its great potential in translating into versatile therapeutic tools in biomedical field. For instance, a myriad of studies has demonstrated that the CRISPR/Cas system is capable of detecting various types of viruses, killing antibiotic-resistant bacteria, treating inherited genetic diseases, and providing new strategies for cancer therapy. Furthermore, CRISPR/Cas systems are also exploited as research tools such as genome engineering tool that allows researchers to interrogate the biological roles of unexplored genes or uncover novel functions of known genes. Additionally, the CRISPR/Cas system has been employed to edit the genome of a wide range of eukaryotic, prokaryotic organisms and experimental models, including but not limited to mammalian cells, mice, zebrafish, plants, yeast, and Escherichia coli. The present review mainly focuses on summarizing recent discoveries regarding the type II CRISPR/Cas9 and type VI CRISPR/Cas13a systems to give researchers a glimpse of their potential applications in the biological and biomedical field.

Dissecting Plant Gene Functions Using CRISPR Toolsets for Crop Improvement

The CRISPR-based gene editing technology has become more and more powerful in genome manipulation for agricultural breeding, with numerous improved toolsets springing up. In recent years, many CRISPR toolsets for gene editing, such as base editors (BEs), CRISPR interference (CRISPRi), CRISPR activation (CRISPRa), and plant epigenetic editors (PEEs), have been developed to clarify gene function and full-level gene regulation. Here, we comprehensively summarize the application and capacity of the different CRISPR toolsets in the study of plant gene expression regulation, highlighting their potential application in gene regulatory networks' analysis. The general problems in CRISPR application and the optimal solutions in the existing schemes for high-throughput gene function analysis are also discussed. The CRISPR toolsets targeting gene manipulation discussed here provide new solutions for further genetic improvement and molecular breeding of crops.

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.

Genome Editing and Designer Crops for the Future

Domestication spanning over thousands of years led to the evolution of crops that are being cultivated in recent times. Later, selective breeding methods were practiced by human to produce improved cultivars/germplasm. Classical breeding was further transformed into molecular- and genomics-assisted breeding strategies, however, these approaches are labor-intensive and time-consuming. The advent of omics technologies has facilitated the identification of genes and genetic determinants that regulate particular traits allowing the direct manipulation of target genes and genomic regions to achieve desirable phenotype. Recently, genome editing technologies such as meganucleases (MN), zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and CRISPR (clustered regularly interspaced short palindromic repeats)/CRISPR-Associated protein 9 (Cas9) have gained popularity for precise editing of genes to develop crop varieties with superior agronomic, physiological, climate-resilient, and nutritional traits. Owing to the efficiency and precision, genome editing approaches have been widely used to design the crops that can survive the challenges posed by changing climate, and also cater the food and nutritional requirements for ever-growing population. Here, we briefly review different genome editing technologies deployed for crop improvement, and the fundamental differences between GE technology and transgene-based approach. We also summarize the recent advances in genome editing and how this radical expansion can complement the previously established technologies along with breeding for creating designer crops.

Imaging Unique DNA Sequences in Individual Cells Using a CRISPR-Cas9-Based, Split Luciferase Biosensor

An extensive arsenal of biosensing tools has been developed based on the clustered regularly interspaced short palindromic repeat (CRISPR) platform, including those that detect specific DNA sequences both in vitro and in live cells. To date, DNA imaging approaches have traditionally used full fluorescent reporter-based fusion probes. Such “always-on” probes differentiate poorly between bound and unbound probe and are unable to sensitively detect unique copies of a target sequence in individual cells. Herein we describe a DNA biosensor that provides a sensitive readout for such low-copy DNA sequences through proximity-mediated reassembly of two independently optimized fragments of NanoLuc luciferase (NLuc), a small, bright luminescent reporter. Applying this “turn-on” probe in live cells, we demonstrate an application not easily achieved by fluorescent reporter-based probes, detection of individual endogenous genomic loci using standard epifluorescence microscopy. This approach could enable detection of gene edits during ex vivo editing procedures and should be a useful platform for many other live cell DNA biosensing applications.

Tracing the Evolution of the Angiosperm Genome from the Cytogenetic Point of View

Cytogenetics constitutes a branch of genetics that is focused on the cellular components, especially chromosomes, in relation to heredity and genome structure, function and evolution. The use of modern cytogenetic approaches and the latest microscopes with image acquisition and processing systems enables the simultaneous two- or three-dimensional, multicolour visualisation of both single-copy and highly-repetitive sequences in the plant genome. The data that is gathered using the cytogenetic methods in the phylogenetic background enable tracing the evolution of the plant genome that involve changes in: (i) genome sizes; (ii) chromosome numbers and morphology; (iii) the content of repetitive sequences and (iv) ploidy level. Modern cytogenetic approaches such as FISH using chromosome- and genome-specific probes have been widely used in studies of the evolution of diploids and the consequences of polyploidy. Nowadays, modern cytogenetics complements analyses in other fields of cell biology and constitutes the linkage between genetics, molecular biology and genomics.

Engineered tracrRNA for enabling versatile CRISPR-dCas9-based biosensing concepts

In recent years, CRISPR-Cas (stands for: clustered regularly interspaced short palindromic repeats - CRISPR associated protein) based technologies have gained increasing attention in the biosensing field. Thanks to excellent sequence specificity, their use is of particular interest for detecting nucleic acid (NA) targets. In this context, signal generation and amplification can be realized by employing the cis-cleavage activity of the Cas9 protein, although other options involving the catalytically inactive dead Cas9 (dCas9) are increasingly explored. The latter are however mostly based on complex protein engineering processes and often lack efficient signal amplification. Here we showed for the first time that flexible signal generation and amplification properties can be integrated into the CRISPR-dCas9 complex based on a straightforward incorporation of a DNA sequence into the trans-activating CRISPR RNA (tracrRNA). The intrinsic nuclease activity of the engineered complex remained conserved, while the incorporated DNA stretch enabled two modes of amplified fluorescent signal generation: (1) as an RNA-cleaving DNA-based enzyme (DNAzyme) or (2) as hybridization site for biotinylated DNA probes, allowing subsequent enzyme labeling. Both signal generation strategies were demonstrated in solution as well as while coupled to a solid surface. Finally, in a proof of concept bioassay, we demonstrated the successful detection of single stranded DNA on magnetic microbeads using the engineered CRISPR-dCas9 complex. Thanks to the flexibility of incorporating different NA-based signal generation and amplification strategies, this novel NA engineering approach holds enormous promise for many new CRISPR-based biosensing applications.

Gene Editing Technologies for Sugarcane Improvement: Opportunities and Limitations

Plant-based biofuels present a promising alternative to depleting non-renewable fuel resources. One of the benefits of biofuel is reduced environmental impact, including reduction in greenhouse gas emission which causes climate change. Sugarcane is one of the most important bioenergy crops. Sugarcane juice is used to produce table sugar and first-generation biofuel (e.g., bioethanol). Sugarcane bagasse is also a potential material for second-generation cellulosic biofuel production. Researchers worldwide are striving to improve sugarcane biomass yield and quality by a variety of means including biotechnological tools. This paper reviews the use of sugarcane as a feedstock for biofuel production, and gene manipulation tools and approaches, including RNAi and genome-editing tools, such as TALENs and CRISPR-Cas9, for improving its quality. The specific focus here is on CRISPR system because it is low cost, simple in design and versatile compared to other genome-editing tools. The advance of CRISPR-Cas9 technology has transformed plant research with its ability to precisely delete, insert or replace genes in recent years. Lignin is the primary material responsible for biomass recalcitrance in biofuel production. The use of genome editing technology to modify lignin composition and distribution in sugarcane cell wall has been realized. The current and potential applications of genome editing technology for sugarcane improvement are discussed. The advantages and limitations of utilizing RNAi and TALEN techniques in sugarcane improvement are discussed as well.

Understanding 3D Genome Organization and Its Effect on Transcriptional Gene Regulation Under Environmental Stress in Plant: A Chromatin Perspective

The genome of a eukaryotic organism is comprised of a supra-molecular complex of chromatin fibers and intricately folded three-dimensional (3D) structures. Chromosomal interactions and topological changes in response to the developmental and/or environmental stimuli affect gene expression. Chromatin architecture plays important roles in DNA replication, gene expression, and genome integrity. Higher-order chromatin organizations like chromosome territories (CTs), A/B compartments, topologically associating domains (TADs), and chromatin loops vary among cells, tissues, and species depending on the developmental stage and/or environmental conditions (4D genomics). Every chromosome occupies a separate territory in the interphase nucleus and forms the top layer of hierarchical structure (CTs) in most of the eukaryotes. While the A and B compartments are associated with active (euchromatic) and inactive (heterochromatic) chromatin, respectively, having well-defined genomic/epigenomic features, TADs are the structural units of chromatin. Chromatin architecture like TADs as well as the local interactions between promoter and regulatory elements correlates with the chromatin activity, which alters during environmental stresses due to relocalization of the architectural proteins. Moreover, chromatin looping brings the gene and regulatory elements in close proximity for interactions. The intricate relationship between nucleotide sequence and chromatin architecture requires a more comprehensive understanding to unravel the genome organization and genetic plasticity. During the last decade, advances in chromatin conformation capture techniques for unravelling 3D genome organizations have improved our understanding of genome biology. However, the recent advances, such as Hi-C and ChIA-PET, have substantially increased the resolution, throughput as well our interest in analysing genome organizations. The present review provides an overview of the historical and contemporary perspectives of chromosome conformation capture technologies, their applications in functional genomics, and the constraints in predicting 3D genome organization. We also discuss the future perspectives of understanding high-order chromatin organizations in deciphering transcriptional regulation of gene expression under environmental stress (4D genomics). These might help design the climate-smart crop to meet the ever-growing demands of food, feed, and fodder.

Hairy CRISPR: Genome Editing in Plants Using Hairy Root Transformation

CRISPR/Cas-mediated genome editing is a powerful tool of plant functional genomics. Hairy root transformation is a rapid and convenient approach for obtaining transgenic roots. When combined, these techniques represent a fast and effective means of studying gene function. In this review, we outline the current state of the art reached by the combination of these approaches over seven years. Additionally, we discuss the origins of different Agrobacterium rhizogenes strains that are widely used for hairy root transformation; the components of CRISPR/Cas vectors, such as the promoters that drive Cas or gRNA expression, the types of Cas nuclease, and selectable and screenable markers; and the application of CRISPR/Cas genome editing in hairy roots. The modification of the already known vector pKSE401 with the addition of the rice translational enhancer OsMac3 and the gene encoding the fluorescent protein DsRed1 is also described.

Caught in the Act: Live-Cell Imaging of Plant Meiosis

Live-cell imaging is a powerful method to obtain insights into cellular processes, particularly with respect to their dynamics. This is especially true for meiosis, where chromosomes and other cellular components such as the cytoskeleton follow an elaborate choreography over a relatively short period of time. Making these dynamics visible expands understanding of the regulation of meiosis and its underlying molecular forces. However, the analysis of meiosis by live-cell imaging is challenging; specifically in plants, a temporally resolved understanding of chromosome segregation and recombination events is lacking. Recent advances in live-cell imaging now allow the analysis of meiotic events in plants in real time. These new microscopy methods rely on the generation of reporter lines for meiotic regulators and on the establishment of ex vivo culture and imaging conditions, which stabilize the specimen and keep it alive for several hours or even days. In this review, we combine an overview of the technical aspects of live-cell imaging in plants with a summary of outstanding questions that can now be addressed to promote live-cell imaging in Arabidopsis and other plant species and stimulate ideas on the topics that can be addressed in the context of plant meiotic recombination.

CRISPR/dCas9-Based Systems: Mechanisms and Applications in Plant Sciences

RNA-guided genomic transcriptional regulation tools, namely clustered regularly interspaced short palindromic repeats interference (CRISPRi) and CRISPR-mediated gene activation (CRISPRa), are a powerful technology for gene functional studies. Deriving from the CRISPR/Cas9 system, both systems consist of a catalytically dead Cas9 (dCas9), a transcriptional effector and a single guide RNA (sgRNA). This type of dCas9 is incapable to cleave DNA but retains its ability to specifically bind to DNA. The binding of the dCas9/sgRNA complex to a target gene results in transcriptional interference. The CRISPR/dCas9 system has been explored as a tool for transcriptional modulation and genome imaging. Despite its potential applications and benefits, the challenges and limitations faced by the CRISPR/dCas9 system include the off-target effects, protospacer adjacent motif (PAM) sequence requirements, efficient delivery methods and the CRISPR/dCas9-interfered crops being labeled as genetically modified organisms in several countries. This review highlights the progression of CRISPR/dCas9 technology as well as its applications and potential challenges in crop improvement.

Recent advancements in CRISPR-Cas toolbox for imaging applications

The imaging of chromatin, genomic loci, RNAs, and proteins is very important to study their localization, interaction, and coordinated regulation. Recently, several clustered regularly interspaced short palindromic repeats (CRISPR) based imaging methods have been established. The refurbished tool kits utilizing deactivated Cas9 (dCas9) and dCas13 have been established to develop applications of CRISPR-Cas technology beyond genome editing. Here, we review recent advancements in CRISPR-based methods that enable efficient imaging and visualization of chromatin, genomic loci, RNAs, and proteins. RNA aptamers, Pumilio, SuperNova tagging system, molecular beacons, halotag, bimolecular fluorescence complementation, RNA-guided endonuclease in situ labeling, and oligonucleotide-based imaging methods utilizing fluorescent proteins, organic dyes, or quantum dots have been developed to achieve improved fluorescence and signal-to-noise ratio for the imaging of chromatin or genomic loci. RNA-guided RNA targeting CRISPR systems (CRISPR/dCas13) and gene knock-in strategies based on CRISPR/Cas9 mediated site-specific cleavage and DNA repair mechanisms have been employed for efficient RNA and protein imaging, respectively. A few CRISPR-Cas-based methods to investigate the coordinated regulation of DNA-protein, DNA-RNA, or RNA-protein interactions for understanding chromatin dynamics, transcription, and protein function are also available. Overall, the CRISPR-based methods offer a significant improvement in elucidating chromatin organization and dynamics, RNA visualization, and protein imaging. The current and future advancements in CRISPR-based imaging techniques can revolutionize genome biology research for various applications.

Plant 3D genomics: the exploration and application of chromatin organization

Eukaryotic genomes are highly folded for packing into higher-order chromatin structures in the nucleus. With the emergence of state-of-the-art chromosome conformation capture methods and microscopic imaging techniques, the spatial organization of chromatin and its functional implications have been interrogated. Our knowledge of 3D chromatin organization in plants has improved dramatically in the past few years, building on the early advances in animal systems. Here, we review recent advances in 3D genome mapping approaches, our understanding of the sophisticated organization of spatial structures, and the application of 3D genomic principles in plants. We also discuss directions for future developments in 3D genomics in plants.

CRISPR-Based Genome Editing Tools: Insights into Technological Breakthroughs and Future Challenges

Genome-editing (GE) is having a tremendous influence around the globe in the life science community. Among its versatile uses, the desired modifications of genes, and more importantly the transgene (DNA)-free approach to develop genetically modified organism (GMO), are of special interest. The recent and rapid developments in genome-editing technology have given rise to hopes to achieve global food security in a sustainable manner. We here discuss recent developments in CRISPR-based genome-editing tools for crop improvement concerning adaptation, opportunities, and challenges. Some of the notable advances highlighted here include the development of transgene (DNA)-free genome plants, the availability of compatible nucleases, and the development of safe and effective CRISPR delivery vehicles for plant genome editing, multi-gene targeting and complex genome editing, base editing and prime editing to achieve more complex genetic engineering. Additionally, new avenues that facilitate fine-tuning plant gene regulation have also been addressed. In spite of the tremendous potential of CRISPR and other gene editing tools, major challenges remain. Some of the challenges are related to the practical advances required for the efficient delivery of CRISPR reagents and for precision genome editing, while others come from government policies and public acceptance. This review will therefore be helpful to gain insights into technological advances, its applications, and future challenges for crop improvement.

Visualizing and Measuring Single Locus Dynamics in Arabidopsis thaliana

In eukaryotes, DNA is packed into an incredibly complex structure called chromatin. Although chromatin was often considered as a static entity, it is now clear that chromatin proteins and the chromatin fiber itself are in fact very dynamic. For instance, the packaging of the DNA into the nucleus requires an extraordinary degree of compaction but this should be achieved without compromising the accessibility to the transcription machinery and other nuclear processes. Approaches such as gene tagging have been established for living cells in order to detect, track, and analyze the mobility of single loci. In this chapter, we provide an experimental protocol for performing locus tracking in Arabidopsis thaliana roots and for characterizing locus mobility behavior via a Mean Square Displacement analysis.

Telomeres and Subtelomeres Dynamics in the Context of Early Chromosome Interactions During Meiosis and Their Implications in Plant Breeding

Genomic architecture facilitates chromosome recognition, pairing, and recombination. Telomeres and subtelomeres play an important role at the beginning of meiosis in specific chromosome recognition and pairing, which are critical processes that allow chromosome recombination between homologs (equivalent chromosomes in the same genome) in later stages. In plant polyploids, these terminal regions are even more important in terms of homologous chromosome recognition, due to the presence of homoeologs (equivalent chromosomes from related genomes). Although telomeres interaction seems to assist homologous pairing and consequently, the progression of meiosis, other chromosome regions, such as subtelomeres, need to be considered, because the DNA sequence of telomeres is not chromosome-specific. In addition, recombination operates at subtelomeres and, as it happens in rye and wheat, homologous recognition and pairing is more often correlated with recombining regions than with crossover-poor regions. In a plant breeding context, the knowledge of how homologous chromosomes initiate pairing at the beginning of meiosis can contribute to chromosome manipulation in hybrids or interspecific genetic crosses. Thus, recombination in interspecific chromosome associations could be promoted with the aim of transferring desirable agronomic traits from related genetic donor species into crops. In this review, we summarize the importance of telomeres and subtelomeres on chromatin dynamics during early meiosis stages and their implications in recombination in a plant breeding framework.

Application of Aptamers Improves CRISPR-Based Live Imaging of Plant Telomeres

Development of live imaging techniques for providing information how chromatin is organized in living cells is pivotal to decipher the regulation of biological processes. Here, we demonstrate the improvement of a live imaging technique based on CRISPR/Cas9. In this approach, the sgRNA scaffold is fused to RNA aptamers including MS2 and PP7. When the dead Cas9 (dCas9) is co-expressed with chimeric sgRNA, the fluorescent coat protein-tagged for MS2 and PP7 aptamers (tdMCP-FP and tdPCP-FP) are recruited to the targeted sequence. Compared to previous work with dCas9:GFP, we show that the quality of telomere labeling was improved in transiently transformed Nicotiana benthamiana using aptamer-based CRISPR-imaging constructs. Labeling is influenced by the copy number of aptamers and less by the promoter types. The same constructs were not applicable for labeling of repeats in stably transformed plants and roots. The constant interaction of the RNP complex with its target DNA might interfere with cellular processes.

Directed Evolution of CRISPR/Cas Systems for Precise Gene Editing

CRISPR technology is a universal tool for genome engineering that has revolutionized biotechnology. Recently identified unique CRISPR/Cas systems, as well as re-engineered Cas proteins, have rapidly expanded the functions and applications of CRISPR/Cas systems. The structures of Cas proteins are complex, containing multiple functional domains. These protein domains are evolutionarily conserved polypeptide units that generally show independent structural or functional properties. In this review, we propose using protein domains as a new way to classify protein engineering strategies for these proteins and discuss common ways to engineer key domains to modify the functions of CRISPR/Cas systems.

Tidying-up the plant nuclear space: domains, functions, and dynamics

Understanding how the packaging of chromatin in the nucleus is regulated and organized to guide complex cellular and developmental programmes, as well as responses to environmental cues is a major question in biology. Technological advances have allowed remarkable progress within this field over the last years. However, we still know very little about how the 3D genome organization within the cell nucleus contributes to the regulation of gene expression. The nuclear space is compartmentalized in several domains such as the nucleolus, chromocentres, telomeres, protein bodies, and the nuclear periphery without the presence of a membrane around these domains. The role of these domains and their possible impact on nuclear activities is currently under intense investigation. In this review, we discuss new data from research in plants that clarify functional links between the organization of different nuclear domains and plant genome function with an emphasis on the potential of this organization for gene regulation.

Two combinatorial patterns of telomere histone marks in plants with canonical and non-canonical telomere repeats

Telomeres, nucleoprotein structures at the ends of linear eukaryotic chromosomes, are crucial for the maintenance of genome integrity. In most plants, telomeres consist of conserved tandem repeat units comprising the TTTAGGG motif. Recently, non-canonical telomeres were described in several plants and plant taxons, including the carnivorous plant Genlisea hispidula (TTCAGG/TTTCAGG), the genus Cestrum (Solanaceae; TTTTTTAGGG), and plants from the Asparagales order with either a vertebrate-type telomere repeat TTAGGG or Allium genus-specific CTCGGTTATGGG repeat. We analyzed epigenetic modifications of telomeric histones in plants with canonical and non-canonical telomeres, and further in telomeric chromatin captured from leaves of Nicotiana benthamiana transiently transformed by telomere CRISPR-dCas9-eGFP, and of Arabidopsis thaliana stably transformed with TALE_telo C-3×GFP. Two combinatorial patterns of telomeric histone modifications were identified: (i) an Arabidopsis-like pattern (A. thaliana, G. hispidula, Genlisea nigrocaulis, Allium cepa, Narcissus pseudonarcissus, Petunia hybrida, Solanum tuberosum, Solanum lycopersicum) with telomeric histones decorated predominantly by H3K9me2; (ii) a tobacco-like pattern (Nicotiana tabacum, N. benthamiana, C. elegans) with a strong H3K27me3 signal. Our data suggest that epigenetic modifications of plant telomere-associated histones are related neither to the sequence of the telomere motif nor to the lengths of the telomeres. Nor the phylogenetic position of the species plays the role; representatives of the Solanaceae family are included in both groups. As both patterns of histone marks are compatible with fully functional telomeres in respective plants, we conclude that the described specific differences in histone marks are not critical for telomere functions.

Counting on Crossovers: Controlled Recombination for Plant Breeding

Crossovers (COs), that drive genetic exchange between homologous chromosomes, are strongly biased toward subtelomeric regions in plant species. Manipulating the rate and positions of COs to increase the genetic variation accessible to breeders is a longstanding goal. Use of genome editing reagents that induce double-stranded breaks (DSBs) or modify the epigenome at desired sites of recombination, and manipulation of CO factors, are increasingly applicable approaches for achieving this goal. These strategies for 'controlled recombination' have potential to reduce the time and expense associated with traditional breeding, reveal currently inaccessible genetic diversity, and increase control over the inheritance of preferred haplotypes. Considerable challenges to address include translating knowledge from models to crop species and determining the best stages of the breeding cycle at which to control recombination.

Decrosslinking enables visualization of RNA-guided endonuclease-in situ labeling signals for DNA sequences in plant tissues

Information about the positioning of individual loci in the nucleus and the status of epigenetic modifications at these loci in each cell contained in plant tissue increases our understanding of how cells in a tissue coordinate gene expression. To obtain such information, a less damaging method of visualizing DNA in tissue that can be used with immunohistochemistry is required. Recently, a less damaging DNA visualization method using the CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/associated caspase 9) system, named RNA-guided endonuclease-in situ labeling (RGEN-ISL), was reported. This system made it possible to visualize a target DNA locus in a nucleus fixed on a glass slide with a set of simple operations, but it could not be applied to cells in plant tissues. In this work, we have developed a modified RGEN-ISL method with decrosslinking that made it possible to simultaneously detect the DNA loci and immunohistochemistry signals, including histone modification, in various types of plant tissues and species.

Live-Cell CRISPR Imaging in Plant Cells with a Telomere-Specific Guide RNA

Chromatin organization is highly dynamic in living cells. Therefore, it might have a regulatory role over biological mechanisms like transcription, replication, and DNA repair. To elucidate how these mechanisms are regulated, it is required to establish imaging methods to visualize the chromatin dynamic in living cells. Here, we provide a protocol for a live plant cell imaging technique based on application of two orthologs of the bacterial clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9) from Streptococcus pyogenes and Staphylococcus aureus. This technique uses the inactive variants of Cas9 combined with different fluorescent proteins (GFP and mRuby) and telomere-specific guide RNA to target telomeric repeats in Nicotiana benthamiana. Our immuno-FISH data revealed that signals arising from the CRISPR/dCas9 method are specifically belonging to telomeric regions.

CRISPR/dCas9 platforms in plants: strategies and applications beyond genome editing

Clustered regularly interspaced short palindromic repeat (CRISPR) and Cas9-associated protein systems provide a powerful genetic manipulation tool that can drive plant research forward. Nuclease-dead Cas9 (dCas9) is an enzymatically inactive mutant of Cas9 in which its endonuclease activity is non-functional. The applications of CRISPR/dCas9 have expanded and diversified in recent years. Originally, dCas9 was used as a CRISPR/Cas9 re-engineering tool that enables targeted expression of any gene or multiple genes through recruitment of transcriptional effector domains without introducing irreversible DNA-damaging mutations. Subsequent applications have made use of its ability to recruit modifying enzymes and reporter proteins to DNA target sites. In this paper, the most recent progress in the applications of CRISPR/dCas9 in plants, which include gene activation and repression, epigenome editing, modulation of chromatin topology, live-cell chromatin imaging and DNA-free genetic modification, will be reviewed. The associated strategies for exploiting the CRISPR/dCas9 system for crop improvement with a dimer of the future of the CRISPR/dCas9 system in the functional genomics of crops and the development of traits will be briefly discussed.

Genome editing in grass plants

Cereal crops including maize, rice, wheat, sorghum, barley, millet, oats and rye are the major calorie sources in our daily life and also important bioenergy sources of the world. The rapidly advancing and state-of-the-art genome-editing tools such as zinc finger nucleases, TAL effector nucleases, and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated systems (CRISPR-Cas9-, CRISPR-Cas12a- and CRISPR/Cas-derived base editors) have accelerated the functional genomics and have promising potential for precision breeding of grass crops. With the availability of annotated genomes of the major cereal crops, application of these established genome-editing toolkits to grass plants holds promise to increase the nutritional value and productivity. Furthermore, these easy-to-use and robust genome-editing toolkits have advanced the reverse genetics for discovery of novel gene functions in crop plants. In this review, we document some of important progress in development and utilization of genome-editing tool sets in grass plants. We also highlight present and future uses of genome-editing toolkits that can sustain and improve the quality of cereal grain for food consumption.

Application and prospects of CRISPR/Cas9-based methods to trace defined genomic sequences in living and fixed plant cells

The 3D organization of chromatin plays an important role in genome stability and many other pivotal biological programs. Therefore, the establishment of imaging methods, which enable us to study the dynamics of chromatin in living cells, is necessary. Although primary live cell imaging methods were a breakthrough, there is a need to develop more specific labeling techniques. With the discovery of programmable DNA binding proteins, such zinc finger proteins (ZFP), transcription activator-like effectors (TALE), and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), a major leap forward was made. Here, we review the applications and potential of fluorescent repressor-operator systems, programmable DNA binding proteins with an emphasis on CRISPR-based chromatin imaging in living and fixed cells, and their potential application in plant science.

Methods and applications of CRISPR/Cas system for genome editing in stem cells

Genome editing technology holds great promise for genome manipulation and gene therapy. While widespread utilization, genome editing has been used to unravel the roles of specific genes in differentiation and pluripotency of stem cells, and reinforce the stem cell-based applications. In this review, we summarize the advances of genome editing technology, as well as the derivative technologies from CRISPR/Cas system, which show tremendous potential in various fields. We also highlight the key findings in the studies of stem cells and regeneration by genome editing technology.