Optimized prime editing in monocot plants using PlantPegDesigner and engineered plant prime editors (ePPEs)

Small RNA Toxin-Assisted Evolution of GC-Preferred ErCas12a for Enhanced Genome Targeting Range

CRISPR/Cas12a, a promising gene editing technology, faces limitations due to its requirement for a thymine (T)-rich protospacer adjacent motif (PAM). Despite the development of Cas12a variants with expanded PAM profiles, many genomic loci, especially those with guanine-cytosine (GC)-rich PAMs, have remained inaccessible. This study develops a small RNA toxin-aided strategy to evolve ErCas12a for targeting GC-rich PAMs, resulting in the creation of enhanced ErCas12a (enErCas12a). EnErCas12a demonstrates the ability to recognize GC-rich PAMs and target five times more PAM sequences than the wild-type ErCas12a. Furthermore, enErCas12a achieves efficient gene editing in both bacterial and mammalian cells at various sites with non-canonical PAMs, including GC-rich PAMs such as GCCC, CGCC, and GGCC, which are inaccessible to previous Cas12a variants. Moreover, enErCas12a effectively targets PAM sequences with a GC content exceeding 75% in mammalian cells, providing a valuable alternative to the existing Cas12a toolkit. Importantly, enErCas12a maintains high specificity at targets with canonical PAMs, while also demonstrating enhanced specificity at targets with non-canonical PAMs. Collectively, this work establishes enErCas12a as a promising tool for gene editing in both eukaryotes and prokaryotes.

QBEmax is a sequence-permuted and internally protected base editor

Cytosine base editors (CBEs) show promise for multiplex gene knockout applications, but impure edits, indels and off-targets still frequently occur. We describe here QBEmax, which exhibits high efficiency, low indel and off-targets and high product purity with up to 99.8% of edits comprised of C-to-T. Through molecular dynamic modeling, QBEmax presents as a compact and stable base editor that shields protected bases from undesired repair processes.

Design of CoQ10 crops based on evolutionary history

Coenzyme Q (CoQ) is essential for energy production by mitochondrial respiration, and it is a supplement most often used to promote cardiovascular health. Humans make CoQ10, but cereals and some vegetable/fruit crops synthesize CoQ9 with a side chain of nine isoprene units. Engineering CoQ10 production in crops would benefit human health, but this is hindered by the fact that the specific residues of the enzyme Coq1 that control chain length are unknown. Based on an extensive investigation of the distribution of CoQ9 and CoQ10 in land plants and the associated Coq1 sequence variation, we identified key amino acid changes at the base of the Coq1 catalytic pocket that occurred independently in multiple angiosperm lineages and repeatedly drove CoQ9 formation. Guided by this knowledge, we used gene editing to modify the native Coq1 genes of rice and wheat to produce CoQ10, paving the way for developing additional dietary sources of CoQ10.

Prime editing enables precise genome modification of a Populus hybrid

CRISPR/Cas-based genome editing has been extensively employed in the breeding and genetic improvement of trees, yet precise editing remains challenging in these species. Prime editing (PE), a revolutionary technology for precise editing, allows for arbitrary base substitutions and the insertion/deletion of small fragments. In this study, we focused on the model tree poplar 84K (Populus alba × P. glandulosa). We used the 2 × 35S promoter to express a fusion protein of spCas9 nickase (nCas9) and engineered Moloney murine leukemia virus (MMLV), and the Arabidopsis thaliana AtU6 promoter to express an engineered PE guide RNA (epegRNA) and Nick gRNA, pioneering the establishment of the Prime Editor 3 (PE3) system in dicot poplar. Single-base substitutions, multiple-base substitutions, and small-fragment insertions/deletions were edited into three endogenous target genes. The desired edits were identified in hygromycin-resistant (transformed) calli at seven out of nine target sites, with an average editing efficiency ranging from 0.1 to 3.6%. Furthermore, stable T0 plants contained the desired edits at four out of nine targets, with editing efficiencies ranging from 3.6 to 22.2%. Establishment of the PE3 system provides a powerful tool for the precise modification of the poplar genome.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-024-00177-1.

Prime editing: A gene precision editing tool from inception to present

Genetic mutations significantly contribute to the onset of diseases, with over half of the cases caused by single-nucleotide mutations. Advances in gene editing technologies have enabled precise editing and correction of mutated genes, offering effective treatment methods for genetic disorders. CRISPR/Cas9, despite its power, poses risks of inducing gene mutations due to DNA double-strand breaks (DSB). The advent of base editing (BE) and prime editing (PE) has mitigated these risks by eliminating the hazards associated with DNA DSBs, allowing for more precise gene editing. This breakthrough lays a solid foundation for the clinical application of gene editing technologies. This review discusses the principles, development, and applications of PE gene editing technology in various genetic mutation-induced diseases.

Optimized dicot prime editing enables heritable desired edits in tomato and Arabidopsis

Prime editing (PE) enables almost all types of precise genome editing in animals and plants. It has been successfully adapted to edit several plants with variable efficiency and versatility. However, this technique is inefficient for dicots for unknown reasons. Here, using new combinations of PE components, including an RNA chaperone and altered engineered prime editing guide RNAs driven by a PolII-PolIII composite promoter and a viral replicon system, we obtained up to 9.7% of the desired PE efficiency at the callus stage as assessed by targeted deep sequencing. Subsequently, we identified that up to 38.2% of transformants contained desired PE alleles in tomatoes and Arabidopsis, marking successful heritable PE transmission. Our PE tools also showed high accuracy, specificity and multiplexing capability, which unlocked the potential for practical PE applications in dicots, paving the way for transformative advancements in plant sciences.

Prime editing in plants: prospects and challenges

Prime editors are reverse transcriptase (RT)-based genome-editing tools that utilize double-strand break (DSB)-free mechanisms to decrease off-target editing in genomes and enhance the efficiency of targeted insertions. The multiple prime editors that have been developed within a short span of time are a testament to the potential of this technique for targeted insertions. This is mainly because of the possibility of generation of all types of mutations including deletions, insertions, transitions, and transversions. Prime editing reverses several bottlenecks of gene editing technologies that limit the biotechnological applicability to produce designer crops. This review evaluates the status and evolution of the prime editing technique in terms of the types of editors available up to prime editor 5 and twin prime editors, and considers the developments in plants in a systematic manner. The various factors affecting prime editing efficiency in plants are discussed in detail, including the effects of temperature, the prime editing guide (peg)RNA, and RT template amongst others. We discuss the current obstructions, key challenges, and available resolutions associated with the technique, and consider future directions and further improvements that are feasible to elevate the efficiency in plants.

Strand-preferred base editing of organellar and nuclear genomes using CyDENT

Transcription-activator-like effector (TALE)-based tools for base editing of nuclear and organellar DNA rely on double-stranded DNA deaminases, which edit substrate bases on both strands of DNA, reducing editing precision. Here, we present CyDENT base editing, a CRISPR-free, strand-selective, modular base editor. CyDENT comprises a pair of TALEs fused with a FokI nickase, a single-strand-specific cytidine deaminase and an exonuclease to generate a single-stranded DNA substrate for deamination. We demonstrate effective base editing in nuclear, mitochondrial and chloroplast genomes. At certain mitochondrial sites, we show editing efficiencies of 14% and strand specificity of 95%. Furthermore, by exchanging the CyDENT deaminase with one that prefers editing GC motifs, we demonstrate up to 20% mitochondrial base editing at sites that are otherwise inaccessible to editing by other methods. The modular nature of CyDENT enables a suite of bespoke base editors for various applications.

Recent Advances in Studies of Genomic DNA Methylation and Its Involvement in Regulating Drought Stress Response in Crops

As global arid conditions worsen and groundwater resources diminish, drought stress has emerged as a critical impediment to plant growth and development globally, notably causing declines in crop yields and even the extinction of certain cultivated species. Numerous studies on drought resistance have demonstrated that DNA methylation dynamically interacts with plant responses to drought stress by modulating gene expression and developmental processes. However, the precise mechanisms underlying these interactions remain elusive. This article consolidates the latest research on the role of DNA methylation in plant responses to drought stress across various species, focusing on methods of methylation detection, mechanisms of methylation pattern alteration (including DNA de novo methylation, DNA maintenance methylation, and DNA demethylation), and overall responses to drought conditions. While many studies have observed significant shifts in genome-wide or gene promoter methylation levels in drought-stressed plants, the identification of specific genes and pathways involved remains limited. This review aims to furnish a reference for detailed research into plant responses to drought stress through epigenetic approaches, striving to identify drought resistance genes regulated by DNA methylation, specific signaling pathways, and their molecular mechanisms of action.

Recent advances in prime editing technologies and their promises for therapeutic applications

Prime editing (PE) is a groundbreaking genome editing technology offering unparalleled precision in targeted genome modifications and has great potential for therapeutic applications. This review delves into the core principles of PE and emphasizes its advancements, applications, and prospects. We begin with a brief introduction to PE principles, followed by a detailed examination of recent improvements in efficiency, precision, and the scale of feasible edits. These improvements have been made to the PE systems through guide RNA engineering, protein engineering, DNA repair pathway screening, chromosomal or epigenomic modification, and in silico design and optimization tools. Furthermore, we highlight in vivo studies showcasing the therapeutic potential of PE to model and treat genetic diseases. Moreover, we discuss PE's versatile applications in saturation genome editing and its applicability to nonhuman organisms. In conclusion, we address the challenges and opportunities linked with PE, emphasizing its profound impact on biological research and therapeutics.

Future-Proofing Agriculture: De Novo Domestication for Sustainable and Resilient Crops

The worldwide agricultural system confronts a significant challenge represented by the increasing demand for food in the face of a growing global population. This challenge is exacerbated by a reduction in cultivable land and the adverse effects of climate change on crop yield quantity and quality. Breeders actively embrace cutting-edge omics technologies to pursue resilient genotypes in response to these pressing issues. In this global context, new breeding techniques (NBTs) are emerging as the future of agriculture, offering a solution to introduce resilient crops that can ensure food security, particularly against challenging climate events. Indeed, the search for domestication genes as well as the genetic modification of these loci in wild species using genome editing tools are crucial steps in carrying out de novo domestication of wild plants without compromising their genetic background. Current knowledge allows us to take different paths from those taken by early Neolithic farmers, where crop domestication has opposed natural selection. In this process traits and alleles negatively correlated with high resource environment performance are probably eradicated through artificial selection, while others may have been lost randomly due to domestication and genetic bottlenecks. Thus, domestication led to highly productive plants with little genetic diversity, owing to the loss of valuable alleles that had evolved to tolerate biotic and abiotic stresses. Recent technological advances have increased the feasibility of de novo domestication of wild plants as a promising approach for crafting optimal crops while ensuring food security and using a more sustainable, low-input agriculture. Here, we explore what crucial domestication genes are, coupled with the advancement of technologies enabling the precise manipulation of target sequences, pointing out de novo domestication as a promising application for future crop development.

Modularly assembled multiplex prime editors for simultaneous editing of agronomically important genes in rice

Prime editing (PE) technology enables precise alterations in the genetic code of a genome of interest. PE offers great potential for identifying major agronomically important genes in plants and editing them into superior variants, ideally targeting multiple loci simultaneously to realize the collective effects of the edits. Here, we report the development of a modular assembly-based multiplex PE system in rice and demonstrate its efficacy in editing up to four genes in a single transformation experiment. The duplex PE (DPE) system achieved a co-editing efficiency of 46.1% in the T0 generation, converting TFIIAγ5 to xa5 and xa23 to Xa23SW11. The resulting double-mutant lines exhibited robust broad-spectrum resistance against multiple Xanthomonas oryzae pathovar oryzae (Xoo) strains in the T1 generation. In addition, we successfully edited OsEPSPS1 to an herbicide-tolerant variant and OsSWEET11a to a Xoo-resistant allele, achieving a co-editing rate of 57.14%. Furthermore, with the quadruple PE (QPE) system, we edited four genes-two for herbicide tolerance (OsEPSPS1 and OsALS1) and two for Xoo resistance (TFIIAγ5 and OsSWEET11a)-using one construct, with a co-editing efficiency of 43.5% for all four genes in the T0 generation. We performed multiplex PE using five more constructs, including two for triplex PE (TPE) and three for QPE, each targeting a different set of genes. The editing rates were dependent on the activity of pegRNA and/or ngRNA. For instance, optimization of ngRNA increased the PE rates for one of the targets (OsSPL13) from 0% to 30% but did not improve editing at another target (OsGS2). Overall, our modular assembly-based system yielded high PE rates and streamlined the cloning of PE reagents, making it feasible for more labs to utilize PE for their editing experiments. These findings have significant implications for advancing gene editing techniques in plants and may pave the way for future agricultural applications.

Application of genome editing techniques to regulate gene expression in crops

BACKGROUND

Enhanced agricultural production is urgently required to meet the food demands of the increasing global population. Abundant genetic diversity is expected to accelerate crop development. In particular, the development of the CRISPR/Cas genome editing technology has greatly enhanced our ability to improve crop's genetic diversity through direct artificial gene modification. However, recent studies have shown that most crop improvement efforts using CRISPR/Cas techniques have mainly focused on the coding regions, and there is a relatively lack of studies on the regulatory regions of gene expression.

RESULTS

This review briefly summarizes the development of CRISPR/Cas system in the beginning. Subsequently, the importance of gene regulatory regions in plants is discussed. The review focuses on recent developments and applications of mutations in regulatory regions via CRISPR/Cas techniques in crop breeding.

CONCLUSION

Finally, an outline of perspectives for future crop breeding using genome editing technologies is provided. This review provides new research insights for crop improvement using genome editing techniques.

Mapping nucleosome-resolution chromatin organization and enhancer-promoter loops in plants using Micro-C-XL

Although chromatin organizations in plants have been dissected at the scales of compartments and topologically associating domain (TAD)-like domains, there remains a gap in resolving fine-scale structures. Here, we use Micro-C-XL, a high-throughput chromosome conformation capture (Hi-C)-based technology that involves micrococcal nuclease (instead of restriction enzymes) and long cross-linkers, to dissect single nucleosome-resolution chromatin organization in Arabidopsis. Insulation analysis reveals more than 14,000 boundaries, which mostly include chromatin accessibility, epigenetic modifications, and transcription factors. Micro-C-XL reveals associations between RNA Pols and local chromatin organizations, suggesting that gene transcription substantially contributes to the establishment of local chromatin domains. By perturbing Pol II both genetically and chemically at the gene level, we confirm its function in regulating chromatin organization. Visible loops and stripes are assigned to super-enhancers and their targeted genes, thus providing direct insights for the identification and mechanistic analysis of distal CREs and their working modes in plants. We further investigate possible factors regulating these chromatin loops. Subsequently, we expand Micro-C-XL to soybean and rice. In summary, we use Micro-C-XL for analyses of plants, which reveal fine-scale chromatin organization and enhancer-promoter loops and provide insights regarding three-dimensional genomes in plants.

Efficient and heritable A-to-K base editing in rice and tomato

Cytosine and adenosine base editors (CBE and ABE) have been widely used in plants, greatly accelerating gene function research and crop breeding. Current base editors can achieve efficient A-to-G and C-to-T/G/A editing. However, efficient and heritable A-to-Y (A-to-T/C) editing remains to be developed in plants. In this study, a series of A-to-K base editor (AKBE) systems were constructed for monocot and dicot plants. Furthermore, nSpCas9 was replaced with the PAM-less Cas9 variant (nSpRY) to expand the target range of the AKBEs. Analysis of 228 T0 rice plants and 121 T0 tomato plants edited using AKBEs at 18 endogenous loci revealed that, in addition to highly efficient A-to-G substitution (41.0% on average), the plant AKBEs can achieve A-to-T conversion with efficiencies of up to 25.9 and 10.5% in rice and tomato, respectively. Moreover, the rice-optimized AKBE generates A-to-C conversion in rice, with an average efficiency of 1.8%, revealing the significant value of plant-optimized AKBE in creating genetic diversity. Although most of the A-to-T and A-to-C edits were chimeric, desired editing types could be transmitted to the T1 offspring, similar to the edits generated by the traditional ABE8e. Besides, using AKBEs to target tyrosine (Y, TAT) or cysteine (C, TGT) achieved the introduction of an early stop codon (TAG/TAA/TGA) of target genes, demonstrating its potential use in gene disruption.

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.

The Development, Optimization and Future of Prime Editing

Prime editing is a rapidly developing method of CRISPR/Cas-based genome editing. The increasing number of novel PE applications and improved versions demands constant analysis and evaluation. The present review covers the mechanism of prime editing, the optimization of the method and the possible next step in the evolution of CRISPR/Cas9-associated genome editing. The basic components of a prime editing system are a prime editor fusion protein, consisting of nickase and reverse transcriptase, and prime editing guide RNA, consisting of a protospacer, scaffold, primer binding site and reverse transcription template. Some prime editing systems include other parts, such as additional RNA molecules. All of these components were optimized to achieve better efficiency for different target organisms and/or compactization for viral delivery. Insights into prime editing mechanisms allowed us to increase the efficiency by recruiting mismatch repair inhibitors. However, the next step in prime editing evolution requires the incorporation of new mechanisms. Prime editors combined with integrases allow us to combine the precision of prime editing with the target insertion of large, several-kilobase-long DNA fragments.

CRISPR/Cas9-mediated genome editing techniques and new breeding strategies in cereals - current status, improvements, and perspectives

Cereal crops, including triticeae species (barley, wheat, rye), as well as edible cereals (wheat, corn, rice, oat, rye, sorghum), are significant suppliers for human consumption, livestock feed, and breweries. Over the past half-century, modern varieties of cereal crops with increased yields have contributed to global food security. However, presently cultivated elite crop varieties were developed mainly for optimal environmental conditions. Thus, it has become evident that taking into account the ongoing climate changes, currently a priority should be given to developing new stress-tolerant cereal cultivars. It is necessary to enhance the accuracy of methods and time required to generate new cereal cultivars with the desired features to adapt to climate change and keep up with the world population expansion. The CRISPR/Cas9 system has been developed as a powerful and versatile genome editing tool to achieve desirable traits, such as developing high-yielding, stress-tolerant, and disease-resistant transgene-free lines in major cereals. Despite recent advances, the CRISPR/Cas9 application in cereals faces several challenges, including a significant amount of time required to develop transgene-free lines, laboriousness, and a limited number of genotypes that may be used for the transformation and in vitro regeneration. Additionally, developing elite lines through genome editing has been restricted in many countries, especially Europe and New Zealand, due to a lack of flexibility in GMO regulations. This review provides a comprehensive update to researchers interested in improving cereals using gene-editing technologies, such as CRISPR/Cas9. We will review some critical and recent studies on crop improvements and their contributing factors to superior cereals through gene-editing technologies.

Tuning plant phenotypes by precise, graded downregulation of gene expression

The ability to control gene expression and generate quantitative phenotypic changes is essential for breeding new and desired traits into crops. Here we report an efficient, facile method for downregulating gene expression to predictable, desired levels by engineering upstream open reading frames (uORFs). We used base editing or prime editing to generate de novo uORFs or to extend existing uORFs by mutating their stop codons. By combining these approaches, we generated a suite of uORFs that incrementally downregulate the translation of primary open reading frames (pORFs) to 2.5-84.9% of the wild-type level. By editing the 5' untranslated region of OsDLT, which encodes a member of the GRAS family and is involved in the brassinosteroid transduction pathway, we obtained, as predicted, a series of rice plants with varied plant heights and tiller numbers. These methods offer an efficient way to obtain genome-edited plants with graded expression of traits.

Discovery of deaminase functions by structure-based protein clustering

The elucidation of protein function and its exploitation in bioengineering have greatly advanced the life sciences. Protein mining efforts generally rely on amino acid sequences rather than protein structures. We describe here the use of AlphaFold2 to predict and subsequently cluster an entire protein family based on predicted structure similarities. We selected deaminase proteins to analyze and identified many previously unknown properties. We were surprised to find that most proteins in the DddA-like clade were not double-stranded DNA deaminases. We engineered the smallest single-strand-specific cytidine deaminase, enabling efficient cytosine base editor (CBE) to be packaged into a single adeno-associated virus (AAV). Importantly, we profiled a deaminase from this clade that edits robustly in soybean plants, which previously was inaccessible to CBEs. These discovered deaminases, based on AI-assisted structural predictions, greatly expand the utility of base editors for therapeutic and agricultural applications.

Inoculation and Screening Methods for Major Sorghum Diseases Caused by Fungal Pathogens: Claviceps africana, Colletotrichum sublineola, Sporisorium reilianum, Peronosclerospora sorghi and Macrophomina phaseolina

Sorghum is the fifth most important crop globally. Researching interactions between sorghum and fungal pathogens is essential to further elucidate plant defense mechanisms to biotic stress, which allows breeders to employ genetic resistance to disease. A variety of creative and useful inoculation and screening methods have been developed by sorghum pathologists to study major fungal diseases. As inoculation and screening methods can be keys for successfully conducting experiments, it is necessary to summarize the techniques developed by this research community. Among many fungal pathogens of sorghum, here we summarize inoculation and screening methods for five important fungal pathogens of sorghum: Claviceps africana, Colletotrichum sublineola, Sporisorium reilianum, Peronosclerospora sorghi and Macrophomina phaseolina. The methods described within will be useful for researchers who are interested in exploring sorghum-fungal pathogen interactions. Finally, we discuss the latest biotechnologies and methods for studying plant-fungal pathogen interactions and their applicability to sorghum pathology.