Transformation of cucumber (Cucumis sativus L.) plants with Agrobacterium rhizogenes

Heterologous mogrosides biosynthesis in cucumber and tomato by genetic manipulation

Mogrosides are widely used as high-value natural zero-calorie sweeteners that exhibit an array of biological activities and allow for vegetable flavour breeding by modern molecular biotechnology. In this study, we developed an In-fusion based gene stacking strategy for transgene stacking and a multi-gene vector harbouring 6 mogrosides biosynthesis genes and transformed it into Cucumis sativus and Lycopersicon esculentum. Here we show that transgenic cucumber can produce mogroside V and siamenoside I at 587 ng/g FW and 113 ng/g FW, respectively, and cultivated transgenic tomato with mogroside III. This study provides a strategy for vegetable flavour improvement, paving the way for heterologous biosynthesis of mogrosides.

An Efficient Hairy Root System for Validation of Plant Transformation Vector and CRISPR/Cas Construct Activities in Cucumber (Cucumis sativus L.)

Hairy root induction system has been applied in various plant species as an effective method to study gene expression and function due to its fast-growing and high genetic stability. Recently, these systems have shown to be an effective tool to evaluate activities of CRISPR/Cas9 systems for genome editing. In this study, Rhizobium rhizogenes mediated hairy root induction was optimized to provide an effective tool for validation of plant transformation vector, CRISPR/Cas9 construct activities as well as selection of targeted gRNAs for gene editing in cucumber (Cucumis sativus L.). Under the optimized conditions including OD₆₅₀ at 0.4 for infection and 5 days of co-cultivation, the highest hairy root induction frequency reached 100% for the cucumber variety Choka F1. This procedure was successfully utilized to overexpress a reporter gene (gus) and induce mutations in two Lotus japonicus ROOTHAIRLESS1 homolog genes CsbHLH66 and CsbHLH82 using CRISPR/Cas9 system. For induced mutation, about 78% of transgenic hairy roots exhibited mutant phenotypes including sparse root hair and root hair-less. The targeted mutations were obtained in individual CsbHLH66, CsbHLH82, or both CsbHLH66 and CsbHLH82 genes by heteroduplex analysis and sequencing. The hairy root transformation system established in this study is sufficient and potential for further research in genome editing of cucumber as well as other cucumis plants.

Hairy root culture technology: applications, constraints and prospect

Hairy root (HR) culture, a successful biotechnology combining in vitro tissue culture with recombinant DNA machinery, is intended for the genetic improvement of plants. This technology has been put to use since the last three decades for genetic advancement of medicinal and aromatic plants and also to harvest the economical products in the form of secondary metabolites that are significantly important for their ethnobotanical and pharmacological properties. It also provides an efficient way out for the quicker extraction and quantification of the valuable phytochemicals. The current review provides an account of the in vitro HR culture technology and its wide-scale applications in the field of research as well as in pharmaceutical industries. Different facets of HR with respect to the culture establishment, phytochemical production as well as research investigations concerning the areas of gene manipulation, biotransformation of the secondary metabolites, phytoremediation, their industrial utilisations and different problems encountered during the application of this technology have been covered in this appraisal. Eventually, an idea has been provided on HR about the recent trends on the progress of this technology that may open up newer prospects in near future and calls for further research and explorations in this field. KEY POINTS: • Genetic engineering-based HR culture aims towards enhanced secondary metabolite production. • This review explores an insight in the HR technology and its multi-faceted approaches. • Up-to-date ground-breaking research applications and constraints of HR culture are discussed.

A method of transformation and current progress in transgenic research on cucumbers and Cucurbita species

Cucumber (Cucumis sativus L.) and Cucurbita species (squashes, pumpkins, and gourds), belonging to the Cucurbitaceae family, are among the major vegetable crops in the world. Transgenic approaches could contribute to the accumulation of new knowledge of these species and to the development of elite cultivars. Despite this, research reports using transformants of these species are very limited so far. One of the reasons for this may be that although there are effective transformation methods, these methods are not well known among researchers. In the present review, we describe efficient protocols for the transformation of cucumber and squash plants and mention possible pitfalls in and advice for following these protocols. In addition, we discuss the current progress of genetic transformation research using cucumbers and squash, including genome editing.

Secondary Metabolite Production in Transgenic Hairy Root Cultures of Cucurbits

Cucurbits are important group of vegetables due to their nutritional significance and are also used for valuable traditional medicine. The infection of plants by Agrobacterium rhizogenes results in a hairy root (HR) phenotype characterized by rapid growth in hormone-free medium, an unusual ageotropism and extensive lateral branching. These genetically transformed root cultures (hairy roots) can produce levels of secondary metabolites comparable to that of intact plants. Hairy root cultures offer promise for high production and productivity of valuable secondary metabolites in many plants. High stability and productivity features allow the exploitation of HRs as valuable biotechnological tool for the production of plant secondary metabolites. While these chemical compounds are employed by plants for interactions with their environment, humans have long since explored and exploited plant secondary metabolites for medicinal and practical uses. The main constraint for commercial exploitation of hairy root cultivations is the development and scaling up of appropriate reactor vessels (bioreactors) that permit the growth of interconnected tissues normally unevenly distributed throughout the vessel. Emphasis has focused on designing appropriate bioreactors suitable to culture the delicate and sensitive plant hairy roots. To this end, hairy root culture presents an excellent platform for producing valuable secondary metabolites. For these reasons, this chapter describes the establishment of hairy roots and production of secondary metabolites from hairy roots of cucurbits and also phytochemicals uses for biological activity.

The Fate of Integrated Ri T-DNA rol Genes during Regeneration via Somatic Embryogenesis in Tylophora indica

The fate of integrated Ri T-DNA rol genes during regeneration via indirect somatic embryogenesis and stability of its effect on morphology and tylophorine content of Ri-transformed plants have been studied in Tylophora indica. Integration and expression of Ri T-DNA genes in transformed embryogenic callus lines derived from transformed root lines, 300 Ri-transformed somatic embryos, and 23 Ri-transformed plant lines were analysed. Fifty root lines studied showed integration and expression of four rol genes of TL-DNA. Spontaneous regeneration via indirect somatic embryogenesis was obtained from root lines that were TL+/TR−. Stable integration and expression of rol genes were observed in root lines, embryogenic callus lines, and the spontaneously induced somatic embryos. Nineteen out of the 23 Ri-transformed plant lines and their clones showed phenotypic and genetic stability over the period of 3 years. Four Ri-transformed plants were morphologically similar to nontransformed plants but showed variation with the integration and expression of the rolA gene and absence of other rol genes. Variant Ri-transformed plant line A428#1-V showed highest tylophorine content (2.93±0.03 mg gDW−1) among plant lines studied. The effects of T-DNA genes on growth, morphology, and tylophorine content of the Ri-transformed plants were stable in the long term culture.

Improvement of Agrobacterium-mediated transformation of cucumber (Cucumis sativus L.) by combination of vacuum infiltration and co-cultivation on filter paper wicks

An improved method for genetic transformation of cucumber (Cucumis sativus L. cv. Shinhokusei No. 1) was developed. Vacuum infiltration of cotyledonary explants with Agrobacterium suspension enhanced the efficiency of Agrobacterium infection in the proximal regions of explants. Co-cultivation on filter paper wicks suppressed necrosis of explants, leading to increased regeneration efficiency. Putative transgenic plants were screened by kanamycin resistance and green fluorescent protein (GFP) fluorescence, and integration of the transgene into the cucumber genome was confirmed by genomic polymerase chain reaction (PCR) and Southern blotting. These transgenic plants grew normally and T₁ seeds were obtained from 7 lines. Finally, stable integration and transmission of the transgene in T₁ generations were confirmed by GFP fluorescence and genomic PCR. The average transgenic efficiency for producing cucumbers with our method was 11.9 ± 3.5 %, which is among the highest values reported until date using kanamycin as a selective agent.

Genetically engineering plants for crop improvement

Dramatic progress has been made in the development of gene transfer systems for higher plants. The ability to introduce foreign genes into plant cells and tissues and to regenerate viable, fertile plants has allowed for explosive expansion of our understanding of plant biology and has provided an unparalleled opportunity to modify and improve crop plants. Genetic engineering of plants offers significant potential for seed, agrichemical, food processing, specialty chemical, and pharmaceutical industries to develop new products and manufacturing processes. The extent to which genetically engineered plants will have an impact on key industries will be determined both by continued technical progress and by issues such as regulatory approval, proprietary protection, and public perception.

Vegetables

The conscious promotion of health by an appropriate, balanced diet has become an important social request. Vegetable thereby possesses a special importance due to its high vitamin, mineral and dietary fibre content. Major progress has been made over the past few years in the transformation of vegetables. The expression of several genes has been inhibited by sense gene suppression, and new traits caused by new gene constructs are stably inherited. This chapter reviews advances in various traits such as disease resistance, abiotic stress tolerance, quality improvement, pharmaceutical and industrial application. Results are presented from most important vegetable families, like Solanaceae, Brassicaceae, Fabaceae, Cucurbitaceae, Asteraceae, Apiaceae, Chenopodiaceae and Liliaceae. Although many research trends in this report are positive, only a few transgenic vegetables have been released from confined into precommercial testing or into use.

The DefH9-iaaM-containing construct efficiently induces parthenocarpy in cucumber

Parthenocarpy (seedless fruits) is a desirable trait that has been achieved in many plant cultivars. We generated parthenocarpic cucumber fruits by introducing the chimeric DefH9-iaaM construct into the cucumber genome using an Agrobacterium tumefaciens-mediated protocol. The construct consists of the DefH9 promoter from Antirrhinum majus and the iaaM coding sequence from Pseudomonas syringae. Transgenic plants were obtained from nine independent transformation events: half of these were tetraploid and did not produce seeds following self-pollination, while the remaining half were capable of displaying parthenocarpy in the subsequent reproductive generation. Of the fruits produced by the transgenic lines, 70-90% were parthenocarpic. The segregation of the marker gene in the transgenic T(1) progeny indicated single gene inheritance. The seed set in the transgenic lines and their F(1) hybrids was lower than in the non-transgenic control plants. Some of the methodological details and the practical significance of the results are discussed.

The expression of the Saccharomyces cerevisiae HAL1 gene increases salt tolerance in transgenic watermelon [Citrullus lanatus (Thunb.) Matsun. & Nakai.]

An optimised Agrobacterium-mediated gene transfer protocol was developed in order to obtain watermelon transgenic plants [Citrullus lanatus (Thunb.) Matsun. & Nakai.]. Transformation efficiencies ranged from 2.8% to 5.3%, depending on the cultivar. The method was applied to obtain genetically engineered watermelon plants expressing the Saccharomyces cerevisiae HAL1 gene related to salt tolerance. In order to enhance its constitutive expression in plants, the HAL1 gene was cloned in a pBiN19 plasmid under control of the 35S promoter with a double enhancer sequence from the cauliflower mosaic virus and the RNA4 leader sequence of the alfalfa mosaic virus. This vector was introduced into Agrobacterium tumefaciens strain LBA4404 for further inoculation of watermelon half-cotyledon explants. The introduction of both the neomycin phosphotransferase II and HAL1 genes was assessed in primary transformants (TG1) by polymerase chain reaction analysis and Southern hybridisation. The expression of the HAL1 gene was determined by Northern analysis, and the diploid level of transgenic plants was confirmed by flow cytometry. The presence of the selectable marker gene in the expected Mendelian ratios was demonstrated in TG2 progenies. The TG2 kanamycin-resistant plantlets elongated better and produced new roots and leaves in culture media supplemented with NaCl compared with the control. Salt tolerance was confirmed in a semi-hydroponic system (EC=6 dS m(-1)) on the basis of the higher growth performance of homozygous TG3 lines with respect to their respective azygous control lines without the transgene. The halotolerance observed confirmed the inheritance of the trait and supports the potential usefulness of the HAL1 gene of S. cerevisiae as a molecular tool for genetic engineering of salt-stress protection in other crop species.

Transgenic hairy roots

Agrobacterium rhizogenes causes hairy root disease in plants. The neoplastic roots produced by A. rhizogenes infection is characterized by high growth rate and genetic stability. These genetically transformed root cultures can produce higher levels of secondary metabolites or amounts comparable to that of intact plants. Hairy root cultures offer promise for production of valuable secondary metabolites in many plants. The main constraint for commercial exploitation of hairy root cultures is their scaling up, as there is a need for developing a specially designed bioreactor that permits the growth of interconnected tissues unevenly distributed throughout the vessel. Rheological characteristics of heterogeneous system should also be taken into consideration during mass scale culturing of hairy roots. Development of bioreactor models for hairy root cultures is still a recent phenomenon. It is also necessary to develop computer-aided models for different parameters such as oxygen consumption and excretion of product to the medium. Further, transformed roots are able to regenerate genetically stable plants as transgenics or clones. This property of rapid growth and high plantlet regeneration frequency allows clonal propagation of elite plants. In addition, the altered phenotype of hairy root regenerants (hairy root syndrome) is useful in plant breeding programs with plants of ornamental interest. In vitro transformation and regeneration from hairy roots facilitates application of biotechnology to tree species. The ability to manipulate trees at a cellular and molecular level shows great potential for clonal propagation and genetic improvement. Transgenic root system offers tremendous potential for introducing additional genes along with the Ri T-DNA genes for alteration of metabolic pathways and production of useful metabolites or compounds of interest. This article discusses various applications and perspectives of hairy root cultures and the recent progress achieved with respect to transformation of plants using A. rhizogenes.

Characterization of transgenic plants derived from hairy roots ofHyoscyamus muticus

Mature plants were regenerated via protoplasts fromAgrobacterium rhizogenes-transformed root cultures ofHyoscyamus muticus L., and chemical analyses were performed on 34 individual plants. The regenerated plants showed strong phenotypic differences from clone to clone as well as from the control plants. Polymerase chain reaction studies revealed that the plants exhibiting the strongest phenotypic alterations contained therol (A, B and C) genes, whereas the plants with fewer alterations had lost them. The plants produced hyoscyamine, scopolamine and a range of different calystegins, and considerable somaclonal variation was observed. Alkaloid production in the plants transgenic for therol genes was clearly reduced. The pattern of calystegins was similar within all the regenerated plants lackingrol genes. Among the plants withrol genes, the calystegin B₁ was not detectable. It seems clear that the presence ofrol genes is detrimental to the alkaloid accumulation in the transgenic plants in contrast to hairy root cultures.

Use of roots transformed by Agrobacterium rhizogenes in rhizosphere research: applications in studies of cadmium assimilation from sewage sludges

The use of roots transformed by Agrobacterium rhizogenes in models for the rhizosphere is discussed. A list of species for which transformed root cultures have been obtained is provided and the example of studies of cadmium assimilation from sewage sludges is given to illustrate how transformed root cultures can be used in physiological tests under non-sterile conditions.

Fate of introduced genetic markers in transformed root clones and regenerated plants of monohaploid and diploid potato genotypes

Agrobacterium transformation of stem internodes of four monohaploid (839-79, 849-7, 851-23, 855-1) and two diploid (M9 and HH260) potato genotypes using hairy root-inducing single (LBA 1020, LBA 9365, LBA 9402) and binary (LBA 1060KG) vectors is reported. Various media and successive culture steps were tested for plant regeneration from different transformed root clones. The fate of introduced genetic markers in root clones and regenerated plants (hairy root phenotype, hormone autotrophy, opine production, kanamycin resistance, β-glucuronidase activity), the ploidy stability and protoplast yield were analysed. The transformation efficiency of stem internodes (hairy root production) and the regeneration capacity of the transformed root clones greatly differed within and between the various potato genotypes. The regenerated plants obtained after transformation with both types of vectors often showed the absence of one or more genetic markers. However, transformation with the binary Agrobacterium vector generally resulted in the stable presence of the opines in all transformed root clones and most regenerated plants. In HH260, transformation efficiency, plant regeneration of transformed root clones, protoplast yield and ploidy stability were the highest as compared to the other genotypes. The application of these transformed plants as marker lines in gene mapping and gene expression studies is indicated.

rolA locus of the Ri plasmid directs developmental abnormalities in transgenic tobacco plants

Plants containing the left T-DNA (TL) of Agrobacterium rhizogenes show a variety of developmental abnormalities that include severely wrinkled leaves, loss of apical dominance, reduced geotropism of roots, reduced internode distances, and floral hyperstyly. The TL-DNA also affects the morphology of tumor tissue at the site of inoculation on Kalanchoe diagremontiana leaves. Single mutations at four loci of the TL-DNA (rolA, rolB, rolC, and rolD) are known to affect tumor morphology on K. diagremontiana leaves. We regenerated plants from tissues transformed with TL-DNA containing mutations in each of the rol loci in order to determine which of the rol loci, if any, control the abnormal plant phenotype. Only plants regenerated after infection with bacteria containing a mutation in rolA locus showed loss of the wrinkled leaf phenotype. The rolA locus was cloned into the plant transformation vector pGA472 and introduced alone into plants. Transgenic plants containing rolA displayed the abnormal phenotype. These results indicate that rolA is the primary determinant of the severely wrinkled phenotype of Ri plasmid transgenic plants. Other rol loci may influence the degree of developmental abnormalities.

Somatic embryogenesis and plant regeneration from cell suspension cultures of Cucumis sativus L

A procedure for the regeneration of whole cucumber plants (Cucumis sativus L. cv. Poinsett 76) by embryogenesis from cell suspension cultures is described. Embryogenic callus was initiated from the primary leaves of 14-17 day old plants. Suspension cultures of embryogenic cells were grown in liquid Murashige and Skoog basal medium containing 5 uM 2,4,5-trichlorophenoxyacetic acid and 4 uM 6-benzylaminopurine. Suspension cultures were composed of a population of cells that were densely cytoplasmic and potentially embryogenic. Differentiation of embryos was enhanced by washing the suspension culture cells with MS basal medium containing 0.5% activated charcoal and twice with MS basal medium followed by liquid shake cultures in MS basal medium. Sixty to 70 percent of the embryos prewashed with activated charcoal germinated into plantlets with normal morphology. Embryos obtained from suspension cultured cells without prewashing with activated charcoal organized into plantlets with abnormal primary leaves. Morphologically normal plantlets were obtained by excising the shoot tips and transferring them to fresh medium.

Assessment of the efficiency of cotransformation of the T-DNA of disarmed binary vectors derived from Agrobacterium tumefaciens and the T-DNA of A. rhizogenes

Co-transfer of Agrobacterium rhizogenes T-DNA and T-DNA from the A. tumefaciens binary vector pBin19 (Bevan, 1984) was studied in detail using Nicotiana rustica. High frequencies of co-transfer of T-DNA's were observed, even when no selection pressure was exerted. Increased levels of pBin19 T-DNA were found in hairy root cultures with selection at higher levels of kanamycin sulphate (50-200 μg ml(-1)). Several other species were also transformed by A. rhizogenes carrying pBin19 and A. rhizogenes harbouring a different binary factor, pAGS125 (Van den Elzen et al., 1985), was used to transform N. rustica hairy roots to confer hygromycin B resistance.