Efficient Protoplast Regeneration Protocol and CRISPR/Cas9-Mediated Editing of Glucosinolate Transporter (GTR) Genes in Rapeseed (Brassica napus L.)

Phytosulfokine alpha enhances regeneration of transformed and untransformed protoplasts of Brassica oleracea

Phytosulfokine-α (PSK-α) is a disulfated pentapeptide (YIYTQ) acting as an intercellular signal peptide and growth factor. It was originally isolated from conditioned medium of asparagus mesophyll cell cultures in 1996 and later characterized as a hormone-like signal molecule with important roles in numerous processes of in vivo plant growth and development. It is currently becoming a valuable mitogenic factor in plant breeding and biotechnology due to its stimulatory effect on in vitro cell elongation, proliferation and differentiation. The focus of our work was to review current knowledge about the roles of PSK-α in plant biotechnology and to evaluate its influence on the regeneration of protoplasts of four Brassica oleracea cultivars (two cauliflower and two cabbage) cultured under two distinctive protocols and with different protoplast densities. Protoplast regeneration was studied due to its high value for plant genome editing, which is generally limited by the inefficient regeneration of treated protoplasts of numerous important plant genotypes. Our hypothesis was that the stress related to PEG-mediated protoplast transformation and the following decrease in viable protoplast density in culture could be alleviated by the addition of PSK-α to the culture medium. We therefore tested whether PSK-α could increase cell division at the early stages of culture (5 and 15 days after protoplast isolation) and stimulate the formation of microcallus colonies up to the 30st day of culture and to evaluate its influence on callus organogenesis leading to shoot regeneration. The PSK-α showed a strong stimulatory effect on untransformed protoplast regeneration already during the first days of culture, accelerating cell division up to 5.3-fold and the formation of multicellular microcallus colonies up to 37.0-fold. The beneficial influence was retained at later stages of regeneration, when PSK improved shoot organogenesis even if it was present only during the first 10 days of culture. The highest numbers of shoots, however, were regenerated when PSK was present during the first days of culture and later in solid shoot regeneration medium. Finally, the addition of PSK-α to PEG-transformed protoplasts significantly enhanced their division rate and the formation of microcallus colonies in selection media, up to 44.0-fold.

Editing Metabolism, Sex, and Microbiome: How Can We Help Poplar Resist Pathogens?

Poplar (Populus) is a genus of woody plants of great economic value. Due to the growing economic importance of poplar, there is a need to ensure its stable growth by increasing its resistance to pathogens. Genetic engineering can create organisms with improved traits faster than traditional methods, and with the development of CRISPR/Cas-based genome editing systems, scientists have a new highly effective tool for creating valuable genotypes. In this review, we summarize the latest research data on poplar diseases, the biology of their pathogens and how these plants resist pathogens. In the final section, we propose to plant male or mixed poplar populations; consider the genes of the MLO group, transcription factors of the WRKY and MYB families and defensive proteins BbChit1, LJAMP2, MsrA2 and PtDef as the most promising targets for genetic engineering; and also pay attention to the possibility of microbiome engineering.

Optimization of cabbage (Brassica oleracea var. capitata L.) protoplast transformation for genome editing using CRISPR/Cas9

Genome editing techniques, such as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated systems (CRISPR/Cas9) are undoubtedly becoming an indispensable tool for improving food crops and tackling agricultural challenges. In the present study, key factors affecting transformation efficiency, such as PEG4000 concentration, incubation time, and plasmid amount were evaluated to achieve efficient delivery of CRISPR/Cas9 vector into cabbage protoplasts. Using amplicon sequencing, we confirmed a significant effect of PEG4000 concentration and incubation time on the induced target mutations. By optimizing the transformation protocol, editing efficiency of 26.4% was achieved with 40 µg of plasmid and 15 minutes incubation with 50% PEG4000. While these factors strongly affected the mutation rate, the viability of the transformed protoplasts remained high. Our findings would be useful for successful genome editing in cabbage and other brassicas, as well as in research areas such as gene function analysis and subcellular localization that rely on transient transformation methods in protoplasts.

Improvements in Protoplast Isolation Protocol and Regeneration of Different Cabbage (Brassica oleracea var. capitata L.) Cultivars

Protoplasts are a versatile tool in plant biotechnology since they can be used for basic biological studies as well as for breeding strategies based on genome editing. An efficient protoplast isolation protocol is essential for conducting protoplast-based studies. To optimize the protoplast isolation protocol in cabbage (Brassica oleracea var. capitata L.), different enzyme solutions were tested for the isolation of leaf mesophyll protoplasts. In our experiments, the combination of 0.5% Cellulase Onozuka RS and 0.1% Macerozyme R-10 showed the best result. The optimized protocol proved suitable for the isolation of protoplasts from five different cabbage cultivars with yields ranging from 2.38 to 4.63 × 106 protoplasts/g fresh weight (fw) and a viability of 93% or more. After three weeks in culture, protoplasts from all of the tested cultivars formed micro-calli, but further callus growth and shoot regeneration depended strongly on the genotype and regeneration protocol used. For shoot formation, 1 mg/L BAP in combination with auxin 0.2 mg/L NAA showed the best results with a regeneration of 23.5%. The results obtained will contribute to the development of different applications of cabbage protoplasts and facilitate the breeding process of this important horticultural crop.

Targeted genome editing in polyploids: lessons from Brassica

CRISPR-mediated genome editing has emerged as a powerful tool for creating targeted mutations in the genome for various applications, including studying gene functions, engineering resilience against biotic and abiotic stresses, and increasing yield and quality. However, its utilization is limited to model crops for which well-annotated genome sequences are available. Many crops of dietary and economic importance, such as wheat, cotton, rapeseed-mustard, and potato, are polyploids with complex genomes. Therefore, progress in these crops has been hampered due to genome complexity. Excellent work has been conducted on some species of Brassica for its improvement through genome editing. Although excellent work has been conducted on some species of Brassica for genome improvement through editing, work on polyploid crops, including U's triangle species, holds numerous implications for improving other polyploid crops. In this review, we summarize key examples from genome editing work done on Brassica and discuss important considerations for deploying CRISPR-mediated genome editing more efficiently in other polyploid crops for improvement.

CRISPR-mediated technology for seed oil improvement in rapeseed: Challenges and future perspectives

Rapeseed not only provide considerable amount of edible oil with high nutritional properties but can also be used as a raw material for biofuel production in many industries. It is therefore in high demand to bring genetic changes in order to fulfill the need of human and of industries. Though traditional breeding techniques such as hybridization and mutagenesis remained the top methods for long time to create improved varieties in oilseed rape. Clustered regularly interspaced short palindromic repeats (CRISPR) is becoming one of the most valuable gene editing technologies that allow precise genome engineering, and open new ways for research in plant functional genomics. Though CRISPR has been used in many other crops for genetic improvement it is expected to be an effective tool for genome editing and molecular design in oilseed rape for seed oil improvement. This mini review will discuss and summarize the past and ongoing research and development in rapeseed in terms of seed oil improvement and fatty acid composition using CRISPR technology. In addition, the factors that hinder the efficiency of this tool and how to eliminate those factors will be briefly summarized. The improvement of CRISPR technology for getting better results in oilseed rape will also be considered here. This minireview will open new windows for researchers in Brassica napus oil improvement research and genetic improvement using CRISPR technology.

Developing multifunctional crops by engineering Brassicaceae glucosinolate pathways

Glucosinolates (GSLs), found mainly in species of the Brassicaceae family, are one of the most well-studied classes of secondary metabolites. Produced by the action of myrosinase on GSLs, GSL-derived hydrolysis products (GHPs) primarily defend against biotic stress in planta. They also significantly affect the quality of crop products, with a subset of GHPs contributing unique food flavors and multiple therapeutic benefits or causing disagreeable food odors and health risks. Here, we explore the potential of these bioactive functions, which could be exploited for future sustainable agriculture. We first summarize our accumulated understanding of GSL diversity and distribution across representative Brassicaceae species. We then systematically discuss and evaluate the potential of exploited and unutilized genes involved in GSL biosynthesis, transport, and hydrolysis as candidate GSL engineering targets. Benefiting from available information on GSL and GHP functions, we explore options for multifunctional Brassicaceae crop ideotypes to meet future demand for food diversification and sustainable crop production. An integrated roadmap is subsequently proposed to guide ideotype development, in which maximization of beneficial effects and minimization of detrimental effects of GHPs could be combined and associated with various end uses. Based on several use-case examples, we discuss advantages and limitations of available biotechnological approaches that may contribute to effective deployment and could provide novel insights for optimization of future GSL engineering.

Improved fatty acid composition of field cress (Lepidium campestre) by CRISPR/Cas9-mediated genome editing

The wild species field cress (Lepidium campestre) has the potential to become a novel cover and oilseed crop for the Nordic climate. Its seed oil is however currently unsuitable for most food, feed, and industrial applications, due to the high contents of polyunsaturated fatty acids (PUFAs) and erucic acid (C22:1). As the biosynthesis of these undesirable fatty acids is controlled by a few well-known major dominant genes, knockout of these genes using CRISPR/Cas9 would thus be more effective in improving the seed oil quality. In order to increase the level of the desirable oleic acid (C18:1), and reduce the contents of PUFAs and C22:1, we targeted three important genes FATTY ACID ELONGASE1 (FAE1), FATTY ACID DESATURASE2 (FAD2), and REDUCED OLEATE DESATURASE1 (ROD1) using a protoplast-based CRISPR/Cas9 gene knockout system. By knocking out FAE1, we obtained a mutated line with almost no C22:1, but an increase in C18:1 to 30% compared with 13% in the wild type. Knocking out ROD1 resulted in an increase of C18:1 to 23%, and a moderate, but significant, reduction of PUFAs. Knockout of FAD2, in combination with heterozygous FAE1fae1 genotype, resulted in mutated lines with up to 66% C18:1, very low contents of PUFAs, and a significant reduction of C22:1. Our results clearly show the potential of CRISPR/Cas9 for rapid trait improvement of field cress which would speed up its domestication process. The mutated lines produced in this study can be used for further breeding to develop field cress into a viable crop.

A high-efficiency PEG-Ca2+-mediated transient transformation system for broccoli protoplasts

Transient transformation of plant protoplasts is an important method for studying gene function, subcellular localization and plant morphological development. In this study, an efficient transient transformation system was established by optimizing the plasmid concentration, PEG4000 mass concentration and genotype selection, key factors that affect transformation efficiency. Meanwhile, an efficient and universal broccoli protoplast isolation system was established. Using 0.5% (w/v) cellulase R-10 and 0.1% (w/v) pectolyase Y-23 to hydrolyze broccoli cotyledons of three different genotypes for 3 h, the yield was more than 5×10⁶/mL/g, and the viability was more than 95%, sufficient to meet the high standards for protoplasts to be used in various experiments. The average transformation efficiency of the two plasmid vectors PHG-eGFP and CP507-YFP in broccoli B1 protoplasts were 61.4% and 41.7%, respectively. Using this system, we successfully performed subcellular localization of the products of three target genes (the clubroot resistance gene CRa and two key genes regulated by glucosinolates, Bol029100 and Bol031350).The results showed that the products of all three genes were localized in the nucleus. The high-efficiency transient transformation system for broccoli protoplasts constructed in this study makes it possible to reliably acquire high-viability protoplasts in high yield. This research provides important technical support for international frontier research fields such as single-cell sequencing, spatial transcriptomics, plant somatic hybridization, gene function analysis and subcellular localization.

Hairy root transformation system as a tool for CRISPR/Cas9-directed genome editing in oilseed rape (Brassica napus)

Our study examined the mutation efficiency of the CRISPR/Cas9 method for tryptophan aminotransferase BnaTAA1 genes involved in the auxin biosynthesis pathway. We made nine CRISPR/Cas9 constructs with various promoters driving the expression of a Cas9 from Staphylococcus aureus (SaCas9) or a plant-codon-optimized Streptococcus pyogenes Cas9 (pcoCas9). We developed a fast and efficient system for evaluating the variety and frequency of mutations caused by each construct using Brassica napus hairy roots. We showed that pcoCas9 is more efficient in mutating the targeted loci than SaCas9 and the presence of the NLS signal enhanced the chance of mutagenesis by 25%. The mutations were studied further in regenerated lines, and we determined the BnaTAA1 gene expression and heritability of the gene modifications in transgenic plants. Hairy root transformation combined with CRISPR/Cas9-mediated gene editing represents a fast and straightforward system for studying target gene function in the important oilseed crop B. napus.

Invited Mini-Review Research Topic: Utilization of Protoplasts to Facilitate Gene Editing in Plants: Schemes for In Vitro Shoot Regeneration From Tissues and Protoplasts of Potato and Rapeseed: Implications of Bioengineering Such as Gene Editing of Broad-Leaved Plants

Schemes for efficient regenerationand recovery of shoots from in vitro tissues or single cells, such as protoplasts, are only available for limited numbers of plant species and genotypes and are crucial for establishing gene editing tools on a broader scale in agriculture and plant biology. Growth conditions, including hormone and nutrient composition as well as light regimes in key steps of known regeneration protocols, display significant variations, even between the genotypes within the same species, e.g., potato (Solanum tuberosum). As fresh plant material is a prerequisite for successful shoot regeneration, the plant material often needs to be refreshed for optimizing the growth and physiological state prior to genetic transformation. Utilization of protoplasts has become a more important approach for obtaining transgene-free edited plants by genome editing, CRISPR/Cas9. In this approach, callus formation from protoplasts is induced by one set of hormones, followed by organogenesis, i.e., shoot formation, which is induced by a second set of hormones. The requirements on culture conditions at these key steps vary considerably between the species and genotypes, which often require quantitative adjustments of medium compositions. In this mini-review, we outline the protocols and notes for clonal regeneration and cultivation from single cells, particularly protoplasts in potato and rapeseed. We focus mainly on different hormone treatment schemes and highlight the importance of medium compositions, e.g., sugar, nutrient, and light regimes as well as culture durations at the key regeneration steps. We believe that this review would provide important information and hints for establishing efficient regeneration strategies from other closely related and broad-leaved plant species in general.

Advances in protoplast transfection promote efficient CRISPR/Cas9-mediated genome editing in tetraploid potato

An efficient method of DNA-free gene-editing in potato protoplasts was developed using linearized DNA fragments, UBIQUITIN10 promoters of several plant species, kanamycin selection, and transient overexpression of the BABYBOOM transcription factor. Plant protoplasts represent a reliable experimental system for the genetic manipulation of desired traits using gene editing. Nevertheless, the selection and regeneration of mutated protoplasts are challenging and subsequent recovery of successfully edited plants is a significant bottleneck in advanced plant breeding technologies. In an effort to alleviate the obstacles related to protoplasts' transgene expression and protoplasts' regeneration, a new method was developed. In so doing, it was shown that linearized DNA could efficiently transfect potato protoplasts and that UBIQUITIN10 promoters from various plants could direct transgene expression in an effective manner. Also, the inhibitory concentration of kanamycin was standardized for transfected protoplasts, and the NEOMYCIN PHOSPHOTRANSFERASE2 (NPT2) gene could be used as a potent selection marker for the enrichment of transfected protoplasts. Furthermore, transient expression of the BABYBOOM (BBM) transcription factor promoted the regeneration of protoplast-derived calli. Together, these methods significantly increased the selection for protoplasts that displayed high transgene expression, and thereby significantly increased the rate of gene editing events in protoplast-derived calli to 95%. The method developed in this study facilitated gene-editing in tetraploid potato plants and opened the way to sophisticated genetic manipulation in polyploid organisms.

Targeted Gene Knockouts by Protoplast Transformation in the Moss Physcomitrella patens

Targeted gene knockout is particularly useful for analyzing gene functions in plant growth, signaling, and development. By transforming knockout cassettes consisting of homologous sequences of the target gene into protoplasts, the classical gene targeting method aims to obtain targeted gene replacement, allowing for the characterization of gene functions in vivo. The moss Physcomitrella patens is a known model organism for a high frequency of homologous recombination and thus harbors a remarkable rate of gene targeting. Other moss features, including easy to culture, dominant haploidy phase, and sequenced genome, make gene targeting prevalent in Physcomitrella patens. However, even gene targeting was powerful to generate knockouts, researchers using this method still experienced technical challenges. For example, obtaining a good number of targeted knockouts after protoplast transformation and regeneration disturbed the users. Off-target mutations such as illegitimate random integration mediated by nonhomologous end joining and targeted insertion wherein one junction on-target but the other end off-target is commonly present in the knockouts. Protoplast fusion during transformation and regeneration was also a problem. This review will discuss the advantages and technical challenges of gene targeting. Recently, CRISPR-Cas9 is a revolutionary technology and becoming a hot topic in plant gene editing. In the second part of this review, CRISPR-Cas9 technology will be focused on and compared to gene targeting regarding the practical use in Physcomitrella patens. This review presents an updated perspective of the gene targeting and CRISPR-Cas9 techniques to plant biologists who may consider studying gene functions in the model organism Physcomitrella patens.

Establishment of an Efficient Protoplast Regeneration and Transfection Protocol for Field Cress (Lepidium campestre)

Field cress (Lepidium campestre) is a potential oilseed crop that has been under domestication in recent decades. CRISPR/Cas9 is a powerful tool for rapid trait improvement and gene characterization and for generating transgene-free mutants using protoplast transfection system. However, protoplast regeneration remains challenging for many plant species. Here we report an efficient protoplast regeneration and transfection protocol for field cress. Important factors such as type of basal media, type/combination of plant growth regulators, and culture duration on different media were optimized. Among the basal media tested, Nitsch was the best for protoplast growth in MI and MII media. For cell wall formation during the early stage of protoplast growth, relatively high auxin concentrations (0.5 mg L⁻¹ NAA and 2,4-D), without addition of cytokinin was preferred for maintaining protoplast viability. After cell wall formation, 1.1 mg L⁻¹ TDZ combined with either 0.05 mg L⁻¹ NAA or 2,4-D was found to efficiently promote protoplast growth. On solid shoot induction medium, 1.1 mg L⁻¹ TDZ without any auxin resulted in over 80% shoot generation frequency. A longer culture duration in MI medium would inhibit protoplast growth, while a longer culture duration in MII medium significantly delayed shoot formation. Using this optimized protoplast regeneration protocol, we have established an efficient PEG-mediated transfection protocol using a vector harboring the GFP gene, with transfection efficiencies of 50–80%. This efficient protoplast protocol would facilitate further genetic improvement of field cress via genome editing, and be beneficial to development of protoplast regeneration protocols for related plant species.