Enhancement of the CRISPR/Cas9-Based Genome Editing System in Lettuce (Lactuca sativa L.) Using the Endogenous U6 Promoter

A Truncated Endogenous U6 Promoter Enables High-Efficiency CRISPR Editing in Flax (Linum usitatissimum L.)

Functional U6 promoters are widely utilized in CRISPR gene editing systems for crops. The identification of endogenous U6 promoter activity and the establishment of CRISPR/Cas9 gene editing systems in various crops can enhance the efficiency and accuracy of gene editing in molecular breeding. In this study, four U6 snRNAs were identified in the genome of the oil flax (Linum usitatissimum L.) cultivar Longya 10, which exhibit high homology with the promoter regions of Arabidopsis thaliana U6 snRNA. We cloned and constructed fusion expression vectors with U6 promoter-driven dual-luciferase reporter genes. Transient transformation of flax and Nicotiana benthamiana was performed to measure the relative activity of dual luciferase. The U6-4 on chromosome 14 showed the highest transcriptional activity. Truncations of varying lengths from the 5' end of this promoter were tested, revealing that a 342 bp U6 promoter fragment possesses high transcriptional activity and an optimal length. Subsequently, we constructed a CRISPR/Cas9 gene editing vector with LuU6-5P/AtU6-P driving LusPDS sgRNA. Agrobacterium-mediated infection of flax hypocotyls yielded transgenic albino flax shoots. DNA from these shoots was used as a template to amplify LusPDS fragments, which were then sequenced. Sequencing analysis revealed that CRISPR/Cas9 vectors using Lu14U6-4-5P achieved higher editing frequencies at LusPDS compared to AtU6-P-driven systems.

Viral-Based Gene Editing System for Nutritional Improvement of Fructan Content in Lettuce

Lettuce is a globally cultivated and consumed leafy crop. Here we developed an efficient tobacco rattle virus (TRV)-based guide RNA (gRNA) delivery system for CRISPR/Cas editing in the commercial lettuce cultivar 'Noga'. Plants stably expressing Cas9 were inoculated with TRV vectors carrying gRNAs targeting five nutrient-associated genes. The system achieved an average editing efficiency of 48.7%, with up to 78.9% of regenerated plantlets showing independent mutations. This approach eliminates the need for antibiotic selection, simplifying tissue culture processes. The system supports diverse applications, including Cas12a editing and large-fragment deletions using dual gRNA sets. Targeting the fructan 1-exohydrolase 2 (1-FEH2) gene produced knockout lines with significant increases in prebiotic dietary fibre fructan content, up to 5.2-fold, and an average rise in the degree of polymerisation by 2.15 units compared with controls. Combining 1-FEH1 and 1-FEH2 knockouts did not further increase fructan levels, revealing 1-FEH2 as the predominant isozyme in lettuce. RT-qPCR analysis showed reduced expression of the upstream biosynthetic enzyme sucrose:sucrose 1-fructosyl transferase (1-SST), suggesting potential feedback inhibition in fructan metabolism. This TRV-based gene editing approach, utilised here to increase fructan content, could be applied to improve other valuable traits in lettuce, and may inspire similar systems to enhance nutritional content of crops.

CRISPR-Cas9-mediated deletions of FvMYB46 in Fragaria vesca reveal its role in regulation of fruit set and phenylpropanoid biosynthesis

The phenylpropanoid pathway, regulated by transcription factors of the MYB family, produces secondary metabolites that play important roles in fertilization and early phase of fruit development. The MYB46 transcription factor is a key regulator of secondary cell wall structure, lignin and flavonoid biosynthesis in many plants, but little is known about its activity in flowers and berries in F. vesca. For functional analysis of FvMYB46, we designed a CRISPR-Cas9 construct with an endogenous F. vesca-specific U6 promoter for efficient and specific expression of two gRNAs targeting the first exon of FvMYB46. This generated mutants with an in-frame 81-bp deletion of the first conserved MYB domain or an out-of-frame 82-bp deletion potentially knocking out gene function. In both types of mutant plants, pollen germination and fruit set were significantly reduced compared to wild type. Transcriptomic analysis of flowers revealed that FvMYB46 positively regulates the expression of genes involved in processes like xylan biosynthesis and metabolism, homeostasis of reactive oxygen species (ROS) and the phenylpropanoid pathway, including secondary cell wall biosynthesis and flavonoid biosynthesis. Genes regulating carbohydrate metabolism and signalling were also deregulated, suggesting that FvMYB46 might regulate the crosstalk between carbohydrate metabolism and phenylpropanoid biosynthesis. In the FvMYB46-mutant flowers, the flavanol and flavan-3-ol contents, especially epicatechin, quercetin-glucoside and kaempferol-3-coumaroylhexoside, were reduced, and we observed a local reduction in the lignin content in the anthers. Together, these results suggest that FvMYB46 controls fertility and efficient fruit set by regulating the cell wall structure, flavonoid biosynthesis, carbohydrate metabolism, and sugar and ROS signalling in flowers and early fruit development in F. vesca.

An Improved Method for Agrobacterium-Mediated Genetic Transformation of Three Types of Lettuce

Lettuce genetic transformation is genotype-dependent. In the present study, we have successfully developed an optimized Agrobacterium-mediated transformation protocol for elite lettuce cultivars, which belong to the romaine, leaf, and butterhead cultivar types. We optimized the type and concentration of plant growth regulators (PGRs) and selection antibiotics and found that the use of 1-naphthaleneacetic acid (NAA; 0.10 mg/L) and 6-benzyladenine (BA; 0.25 mg/L) as plant growth regulators, the use of hygromycin (15 mg/L) for transgenic plant selection, and the use of cotyledons and the first true leaf as explants efficiently yielded transformed plants for seven out of the eleven tested cultivars, achieving a 24.3-100% transformation efficiency. These seven cultivars include two romaine-type cultivars, three leaf-type cultivars, and two butterhead-type cultivars, and mark the first successful genetic transformation of the romaine cultivars 'Kahu' and 'Rosalita', the leaf cultivars 'Red Sails' and 'Royal Oak Leaf', and the butterhead cultivar 'Lollo Biondo'. We also observed that substituting hygromycin selection with kanamycin selection (40 mg/L) resulted in a 64.3% transformation efficiency in the butterhead-type 'Mariska', one of the remaining four cultivars. Our newly optimized protocols are applicable in elite lettuce cultivars for Agrobacterium-mediated genetic transformation and regeneration, enabling hygromycin or kanamycin selection.

Identification of U6 Promoter and Establishment of Gene-Editing System in Larix kaempferi (Lamb.) Carr

This study aimed to establish a CRISPR/Cas9 gene-editing system for Larix kaempferi (Lamb.) Carr. (Japanese larch). We screened L. kaempferi U6 promoters and used them to drive sgRNA expression in the CRISPR/Cas9 gene-editing system. The L. kaempferi embryogenic callus was used as the receptor material for genetic transformation, and the frequency and types of gene editing were then analyzed. The results showed various mutations in the transgenic materials, including base substitutions and deletions, and the editing frequency ranged from 5% to 14.29%. In summary, we established a CRISPR/Cas9 gene-editing system for L. kaempferi. Our results demonstrate that the CRISPR/Cas9 system can efficiently edit genes in L. kaempferi, with significantly higher editing frequencies observed when sgRNA expression is driven by endogenous LaU6 promoters compared to the exogenous promoter ProAtU6-26. This work provides technical support for the study of L. kaempferi gene functions and genetic improvement.

Silencing of RDR1 and RDR6 genes by a single RNAi enhances lettuce's capacity to express recombinant proteins in transient assays

KEY MESSAGE

Enhanced recombinant protein expression was achieved in Salinas lettuce and commercial lettuce by designing a unique RNAi that knockdown the gene-silencing mechanism in transient assays. Improved yields of recombinant proteins (RP) are necessary for protein-production efficiency and ease of purification. Achieving high yield in non-tobacco plants will enable diverse plants to be used as hosts in transient protein-expression systems. With improved protein yield, lettuce (Lactuca sativa) could take the lead as a plant host for RP production. Therefore, this study aimed to improve RP production in lettuce var. Salinas by designing a single RNA interference (RNAi) construct targeting LsRDR1 and LsRDR6 using the Tsukuba system vector. Two RNAi constructs, RNAi-1 and RNAi-2, targeting common regions of LsRDR1 and LsRDR6 with 75% and 76% similarity, respectively, were employed to evaluate simultaneous gene silencing. Quantitative transcription analysis demonstrated that both RNAi constructs effectively knocked down LsRDR6 and LsRDR1, but not LsRDR2, at both 3 and 5 days post-infiltration (dpi), with RNAi-1 exhibited slightly higher efficiency. Based on the protein yield, co-expression of RNAi-1 with enhanced green fluorescent protein (EGFP) increased EGFP expression by approximately 4.9-fold and 3.7-fold at 3 dpi and 5 dpi, respectively, compared to control. A similar but slightly lower increase (2.4-fold and 2.33-fold) was observed in commercial lettuce at 3 and 5 dpi, respectively. To confirm these results, co-infiltration with Bet v 1, a major allergen from birch pollen, resulted in a 2.5-fold increase in expression in Salinas lettuce at 5 dpi. This study marks a significant advancement in enhancing transient protein production in lettuce, elevating its potential as a host for recombinant protein production.

Establishing CRISPR-Cas9 in the sexually dimorphic moss, Ceratodon purpureus

The development of CRISPR technologies provides a powerful tool for understanding the evolution and functionality of essential biological processes. Here we demonstrate successful CRISPR-Cas9 genome editing in the dioecious moss species, Ceratodon purpureus. Using an existing selection system from the distantly related hermaphroditic moss, Physcomitrium patens, we generated knock-outs of the APT reporter gene by employing CRISPR-targeted mutagenesis under expression of native U6 snRNA promoters. Next, we used the native homology-directed repair (HDR) pathway, combined with CRISPR-Cas9, to knock in two reporter genes under expression of an endogenous RPS5A promoter in a newly developed landing site in C. purpureus. Our results show that the molecular tools developed in P. patens can be extended to other mosses across this ecologically important and developmentally variable group. These findings pave the way for precise and powerful experiments aimed at identifying the genetic basis of key functional variation within the bryophytes and between the bryophytes and other land plants.

Isolation and Functional Characterization of a Constitutive Promoter in Upland Cotton (Gossypium hirsutum L.)

Multiple cis-acting elements are present in promoter sequences that play critical regulatory roles in gene transcription and expression. In this study, we isolated the cotton FDH (Fiddlehead) gene promoter (pGhFDH) using a real-time reverse transcription-PCR (qRT-PCR) expression analysis and performed a cis-acting elements prediction analysis. The plant expression vector pGhFDH::GUS was constructed using the Gateway approach and was used for the genetic transformation of Arabidopsis and upland cotton plants to obtain transgenic lines. Histochemical staining and a β-glucuronidase (GUS) activity assay showed that the GUS protein was detected in the roots, stems, leaves, inflorescences, and pods of transgenic Arabidopsis thaliana lines. Notably, high GUS activity was observed in different tissues. In the transgenic lines, high GUS activity was detected in different tissues such as leaves, stalks, buds, petals, androecium, endosperm, and fibers, where the pGhFDH-driven GUS expression levels were 3-10-fold higher compared to those under the CaMV 35S promoter at 10-30 days post-anthesis (DPA) during fiber development. The results indicate that pGhFDH can be used as an endogenous constitutive promoter to drive the expression of target genes in various cotton tissues to facilitate functional genomic studies and accelerate cotton molecular breeding.

Cloning and Functional Verification of Endogenous U6 Promoters for the Establishment of Efficient CRISPR/Cas9-Based Genome Editing in Castor (Ricinus communis)

Castor (Ricinus communis) seeds are rich in a type of hydroxy fatty acid called ricinoleic acid, which is in high demand for the production of plant-based plastics, lubricants, and hydraulic oils. However, the high content of ricin, a toxic protein, in these seeds has restricted further expansion in the area of castor cultivation. Therefore, the development of ricin-free castor is needed. Genome editing technology, although successfully applied in several plant species, is still in the developing stages in castor and awaits the identification of an endogenous U6 promoter with robust function. Here, we searched for U6 small nuclear RNA (snRNA) genes in the castor genome. This led to the identification of six U6 snRNA genes. The promoters of these U6 snRNA genes were cloned, and their function was examined in castor cells using the particle delivery method. The results showed that a U6 promoter length of approximately 300 bp from the transcription start site was sufficient to activate gene expression. This study provides insights into the endogenous castor U6 promoter sequences and outlines a method for verifying the function of U6 promoters in plants using the particle delivery system.