Advanced CRISPR Systems Transform Crop Breeding Through Precision Base and Prime Editing

A comprehensive scientific review published in Discover Plants by Kishor U. Tribhuvan and colleagues details how precision genome editing technologies have fundamentally transformed agricultural biotechnology. Driven by the pressing need to enhance crop resilience, productivity, and nutritional quality, molecular plant breeding has rapidly expanded beyond conventional genetic modification techniques. Early genomic engineering platforms relied on protein-guided nucleases such as meganucleases, zinc finger nucleases, and transcription activator-like effector nucleases, yet these initial systems were constrained by labor-intensive engineering and limited targeting flexibility. The discovery and adaptation of clustered regularly interspaced short palindromic repeats, known as CRISPR, revolutionized the field by introducing programmable RNA-guided nucleic acid cleavage. While conventional CRISPR-associated endonucleases have proven exceptionally powerful for introducing targeted mutations, their reliance on double-strand DNA breaks has presented inherent biological limitations, including random insertions or deletions and unintended off-target alterations. To surmount these bottlenecks, researchers have developed advanced precision editing platforms, specifically cytosine and adenine base editors along with prime editing systems, which enable single-nucleotide conversions, defined insertions, and deletions without cutting both strands of genomic DNA.

Understanding the molecular evolution of plant genome editing requires examining the natural biological defense mechanisms from which CRISPR tools originate. In bacteria and archaea, CRISPR-Cas configurations function as adaptive immune systems that protect microbial hosts against invading bacteriophages, viral pathogens, and foreign plasmids through three distinct operational phases: adaptation, expression, and interference. During adaptation, microbial Cas protein complexes capture short invading foreign DNA segments called protospacers and integrate them into the host CRISPR array between repetitive sequences as spacers, establishing a heritable genetic memory. In the subsequent expression phase, the host transcribes this genomic array into long precursor transcripts termed pre-crRNAs, which specialized Cas enzymes process into mature crRNAs. During interference, these crRNAs guide associated nuclease effectors to recognize complementary foreign nucleic acids and execute targeted cleavage. Modern classifications divide these diverse natural systems into Class 1 and Class 2 architectures based on effector complexity. Class 1 systems utilize multi-subunit protein complexes across Types I, III, IV, and VII, where Type I recruits the Cascade complex and Cas3 helicase-nuclease, Type III targets both DNA and RNA via Csm or Cmr complexes, Type IV provides plasmid defense, and Type VII employs Cas14 to cleave target RNA. Class 2 systems rely on streamlined single-protein effectors across Types II, V, and VI, which include the foundational Cas9 endonuclease, staggered-cutting Cas12a nucleases, and RNA-cleaving Cas13 enzymes.

Among Class 2 systems, the Type II CRISPR-Cas9 platform remains the most widely deployed endonuclease in plant functional genomics and crop trait enhancement. The Streptococcus pyogenes Cas9 enzyme operates as an RNA-guided endonuclease structured into six functional domains: the Recognition Lobe containing REC1 for guide RNA binding, an Arginine-Rich Bridge Helix providing structural stability, REC2 and REC3 domains coordinating protospacer adjacent motif recognition and RNA-DNA hybrid stabilization, an HNH catalytic domain cleaving the target strand, and a RuvC domain cutting the non-complementary strand. Cas9 identifies genomic targets through a 5′-NGG-3′ protospacer adjacent motif and introduces a blunt double-strand break three base pairs upstream of this motif. Host plant cells subsequently repair the double-strand break via two primary internal repair pathways: homology-directed repair, which requires a homologous donor template for precise sequence replacement, or non-homologous end joining, an inherently error-prone pathway that directly ligates broken DNA ends. Because non-homologous end joining frequently generates variable insertions, deletions, or frameshift mutations, CRISPR-Cas9 has served as an exceptionally efficient tool for targeted gene knockouts. However, generating predictable single-base changes or scarless sequence replacements via homology-directed repair in plants is severely hindered by low endogenous repair efficiency and reliance on donor DNA delivery.

Despite the stochastic nature of double-strand break repair, CRISPR-Cas9 has achieved broad agronomic utility across diverse crop species to enhance abiotic stress tolerance, disease resistance, and grain quality. In Oryza sativa, targeted knockouts of OsERA1 and OsPYL9 significantly boosted drought tolerance, with OsERA1 disruption simultaneously increasing grain yield, while editing OsRAV2 and OsPUB7 validated key regulatory mechanisms governing salinity and drought resilience. Furthermore, disrupting OsSWEET14 conferred resistance against devastating bacterial blight, and precise mutation of QT12 demonstrated improved thermotolerance with superior grain quality under high-temperature conditions. In hexaploid wheat, mutating homoeologous TaMLO alleles and TaEDR1 established durable resistance against powdery mildew and Blumeria graminis, whereas disrupting TaALS conferred resistance to sulfonylurea and imidazolinone herbicides, and mutating TaCIPK14 protected wheat against stripe rust. CRISPR-Cas9 disruption of the susceptibility allele TaLr34 provided durable leaf rust resistance, while editing TaPDIL5-1 and TaeIF4E protected wheat against yellow mosaic virus. In Zea mays, knockout of ZmLOX3 conferred resistance to smut disease, and editing ZmPL1 and the ZmDnaJ-ZmNCED6 module enhanced drought survival. Horticultural crops have realized comparable benefits, including elevated omega-3 fatty acids in tomato via SlFAD2-1 disruption, late blight resistance in potato via StDND1, StCHL1, and StDMR6-1 knockouts, and controlled chromosome fusions in Arabidopsis thaliana that created fertile eight-chromosome plants.

To overcome the unpredictability and off-target risks associated with double-strand breaks, researchers developed base editing technologies that install specific point mutations with nucleotide-level precision. Cytosine base editors convert cytosine-guanine base pairs into thymine-adenine pairs through targeted deamination coupled with endogenous base excision repair. First-generation cytosine base editors fused the rat cytidine deaminase rAPOBEC1 to catalytically dead Cas9, achieving modest editing efficiencies of approximately 7.7 percent because endogenous host uracil N-glycosylase recognized uracil as damaged DNA and excised it. Second-generation editors incorporated a uracil DNA glycosylase inhibitor at the C-terminus of the fusion construct, preventing uracil removal and raising base conversion efficiency to 20 percent, which climbed to 50 percent in high-fidelity variants capable of dual-strand conversion. Third-generation cytosine base editors replaced catalytically dead Cas9 with Cas9 nickase harboring a D10A mutation, introducing a single-strand nick that directs cellular repair machinery toward the non-edited strand without inducing double-strand breaks. Subsequent structural refinements incorporated nuclear localization signals to achieve efficiencies reaching 93 percent, engineered deaminase variants like YE1-BE3 and YEE-BE3 to constrict editing windows to two nucleotides, and introduced bacteriophage Mu Gam protein in fourth-generation systems to suppress unwanted indel formation. To curb guide-independent promiscuous deamination, researchers engineered split deaminase systems and advanced variants like A3Bctd-VHM-BE3 that eliminate detectable off-target modifications.

Parallel engineering efforts led to adenine base editors, which mediate targeted adenine-to-guanine conversions without inducing double-strand breaks or requiring exogenous donor repair templates. Developed by fusing engineered Escherichia coli TadA adenosine deaminases to Cas9 nickase, adenine base editors catalyze the deamination of adenine into inosine, which host DNA polymerases interpret as guanine during subsequent DNA replication cycles. Early adenine base editors exhibited baseline editing efficiencies ranging from 1.7 to 20 percent across a restricted four-to-seven nucleotide activity window, but subsequent structural evolutions dramatically expanded their performance. Engineered variants like xCas9-ABE7.10 relaxed protospacer adjacent motif constraints while increasing efficiency to 69 percent, and hyper-active ABE8e architectures incorporating dimeric TadA8e deaminases attained editing efficiencies between 18 and 86 percent. To address the functional constraint that canonical editors perform only single-base transition mutations, researchers constructed dual base editors such as PhieDBEs and STEMEs, which fuse both cytidine and adenosine deaminases to Cas9 nickase, thereby enabling concurrent cytosine-to-thymine and adenine-to-guanine conversions under single guide RNA control. Furthermore, glycosylase base editors and adenine-to-transversion editors were established using uracil-DNA glycosylase or alkyladenine DNA glycosylase modules, allowing transversion edits including cytosine-to-guanine, cytosine-to-adenine, and adenine-to-thymine changes directly in rice and tomato genomes.

Expanding the targetable space of plant genomes has further accelerated through the discovery and integration of alternative Cas nucleases and compact non-Cas effectors. While Streptococcus pyogenes Cas9 typically requires a canonical 5′-NGG-3′ motif, alternative nucleases recognize diverse PAM sequences while offering advantageous biochemical properties. Type V Cas12a endonucleases recognize thymine-rich motifs, process their own precursor crRNA arrays to simplify multiplex genome engineering, and generate staggered sticky ends that facilitate directional fragment integration. Researchers successfully harnessed Cas12a variants to build multiplex base editors, achieving editing efficiencies up to 68.75 percent across six distinct genomic loci simultaneously. Compact nucleases present distinct delivery advantages, illustrated by engineered Cas12i3 effectors fused with T5 exonuclease that achieved 60.71 to 90.00 percent editing efficiency in wheat, hypercompact Cas12f platforms, and miniature Cas12j-8 nucleases consisting of only 717 amino acids that recognize 5′-TTN-3′ motifs. Even smaller natural transposon-encoded nucleases, known as TnpB systems, have emerged as functional genome editors in plants, demonstrating robust editing of OsPDS, OsYSA, and OsCERK1 in rice. Integrating catalytically inactive ISDge10 TnpB effectors with engineered omegaRNAs, DNA-binding domains from Sulfolobus solfataricus, and adenosine deaminases has successfully unlocked both targeted adenine base editing and programmable transcriptional regulation within ultra-compact delivery constraints.

While base editors excel at targeted transition and transversion edits, they cannot install arbitrary sequence replacements, defined insertions, or precise deletions, limitations that catalyzed the creation of prime editing. Prime editing operates as an exceptionally versatile search-and-replace technology that writes programmed genetic information directly into specified genomic sites without double-strand breaks or donor repair templates. The prime editor complex couples a Cas9 nickase to an engineered reverse transcriptase, guided by an extended prime editing guide RNA consisting of a target-specifying spacer, a primer-binding site, and a reverse transcription template. Upon single-strand target nicking by Cas9, the liberated 3′ genomic DNA flap anneals to the primer-binding site, priming reverse transcription of the adjacent template sequence to synthesize a newly edited DNA strand. Iterative structural optimizations produced advanced prime editing variants: PE2 incorporated an engineered Moloney murine leukemia virus reverse transcriptase; PE3 introduced a second sgRNA to nick the non-edited strand; PE4 and PE5 deployed dominant-negative mismatch repair inhibitors; and PEmax refined nuclear localization and codon usage. Plant-tailored iterations, including ePPE with viral nucleocapsid fusion, PAiD-PE featuring artificial-intelligence-designed nickases, and PE6c utilizing an evolved Tf1 retrotransposon reverse transcriptase, have elevated editing efficiencies more than 3.5-fold over PEmax across multiple rice target loci.

The agricultural implementation of prime editing has validated its capability to introduce complex, agronomically beneficial alleles across monocot and dicot crops. Initial plant demonstrations in rice and wheat protoplasts achieved regenerated edited rice plants at frequencies up to 21.8 percent, establishing the feasibility of whole-plant regeneration following prime editing. Subsequent design optimizations, including melting-temperature-guided primer-binding site design and paired prime editing guide RNAs, boosted editing efficiencies by up to 17.4-fold while maintaining minimal genome-wide off-target frequencies between zero and 0.23 percent. Agronomic applications quickly followed: researchers installed the W548L point mutation in OsALS to confer herbicide resistance, engineered the complex TAP-IVS triple substitution into OsEPSPS to establish robust glyphosate resistance, and precisely edited TFIIAgamma5 and the xa23 promoter to confer resistance against Xanthomonas oryzae blight strains. In Zea mays, prime editing introduced the herbicide-tolerance alleles W542L and S621I into ZmALS1 and ZmALS2, while successful transversion editing was verified in tetraploid potato.

Translating precision genome editing from laboratory demonstrations into commercial agriculture depends upon navigating international regulatory frameworks, public acceptance, and technical delivery hurdles. Jurisdictions such as the United States and India have established product-oriented regulatory paths that exempt genome-edited crops categorized under Site Directed Nuclease 1 and Site Directed Nuclease 2 classifications from burdensome transgenic regulations, provided the final commercial varieties are entirely transgene-free and devoid of foreign recombinant DNA. Because point mutations generated through base editing mimic naturally occurring single-nucleotide polymorphisms or conventional mutagenesis, base-edited crops are positioned for favorable regulatory consideration and public acceptance. In contrast, complex prime editing outcomes involving substantial fragment insertions may encounter closer regulatory scrutiny under frameworks governing foreign insertions. Beyond regulatory harmonization, practical implementation must resolve persistent technical challenges, including low transformation efficiency, tissue-culture dependency in recalcitrant germplasm, and genetic redundancy in polyploid species such as wheat and cotton. Continued convergence of artificial-intelligence-assisted guide design, structure-guided deaminase engineering, compact TnpB platforms, and nanoparticle-mediated delivery systems will ultimately accelerate the translation of precision editing tools into climate-resilient, high-yielding crop varieties.

Subject of Research: Precision genome modification in crops using advanced CRISPR, base editing, and prime editing systems

Article Title: Precision genome modifications through CRISPR systems including base and prime editing for crop improvement

Article References: Tribhuvan, K. U., Singh, D. K., Joshi, P. A., Das, J., Kumar, R., & Karkute, S. G. (2026). Precision genome modifications through CRISPR systems including base and prime editing for crop improvement. Discover Plants, 3(1), Article 540. https://doi.org/10.1007/s44372-026-01016-x

Image Credits: AI Generated

DOI: 10.1007/s44372-026-01016-x

Keywords: CRISPR-Cas9, base editing, prime editing, crop improvement, cytosine base editor, adenine base editor, pegRNA, reverse transcriptase, Cas12a, TnpB, cereal genomics, herbicide resistance, disease resistance, plant biotechnology, climate resilience, SDN regulatory policy