Efficient transformation of potato (Solanum tuberosum L.) using a binary vector in Agrobacterium rhizogenes

Lignans of Sesame (Sesamum indicum L.): A Comprehensive Review

Major lignans of sesame sesamin and sesamolin are benzodioxol--substituted furofurans. Sesamol, sesaminol, its epimers, and episesamin are transformation products found in processed products. Synthetic routes to all lignans are known but only sesamol is synthesized industrially. Biosynthesis of furofuran lignans begins with the dimerization of coniferyl alcohol, followed by the formation of dioxoles, oxidation, and glycosylation. Most genes of the lignan pathway in sesame have been identified but the inheritance of lignan content is poorly understood. Health-promoting properties make lignans attractive components of functional food. Lignans enhance the efficiency of insecticides and possess antifeedant activity, but their biological function in plants remains hypothetical. In this work, extensive literature including historical texts is reviewed, controversial issues are critically examined, and errors perpetuated in literature are corrected. The following aspects are covered: chemical properties and transformations of lignans; analysis, purification, and total synthesis; occurrence in Seseamum indicum and related plants; biosynthesis and genetics; biological activities; health-promoting properties; and biological functions. Finally, the improvement of lignan content in sesame seeds by breeding and biotechnology and the potential of hairy roots for manufacturing lignans in vitro are outlined.

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.

Virus-induced gene silencing for comparative functional studies in Gladiolus hybridus

Functional analysis of genes in gladiolus has previously been impractical due to the lack of an efficient stable genetic transformation method. However, virus-induced gene silencing (VIGS) is effective in some plants which are difficult to transform through other methods. Although the Tobacco rattle virus (TRV)-based VIGS system has been developed and used for verifying gene functions in diverse plants, an appropriate TRV-VIGS approach for gladiolus has not been established yet. In this report we describe the first use of the TRV-VIGS system for gene silencing in gladiolus. Vacuum infiltration of cormels and young plants with the GhPDS-VIGS vector effectively down-regulated the PHYTOENE DESATURASE ortholog GhPDS gene and also resulted in various degrees of photobleaching in Gladiolus hybridus. The reduction in GhPDS expression was tested after TRV-based vector infection using real-time RT-PCR. In addition, the progress of TRV infection was detected by fluorescence visualization using a pTRV2: CP-GFP vector. In conclusion, the TRV-mediated VIGS described here will be an effective gene function analysis mechanism in gladiolus.

Agrobacterium tumefaciens and Agrobacterium rhizogenes transformed roots of the parasitic plant Triphysaria versicolor retain parasitic competence

Parasitic plants in the Orobanchaceae invade roots of neighboring plants to rob them of water and nutrients. Triphysaria is facultative parasite that parasitizes a broad range of plant species including maize and Arabidopsis. In this paper we describe transient and stable transformation systems for Triphysaria versicolor Fischer and C. Meyer. Agrobacterium tumefaciens and Agrobacterium rhizogenes were both able to transiently express a GUS reporter in Triphysaria seedlings following vacuum infiltration. There was a correlation between the length of time seedlings were conditioned in the dark prior to infiltration and the tissue type transformed. In optimized experiments, nearly all of the vacuum infiltrated seedlings transiently expressed GUS activity in some tissue. Calluses that developed from transformed tissues were selected using non-destructive GUS staining and after several rounds of in vivo GUS selection, we recovered uniformly staining GUS calluses from which roots were subsequently induced. The presence and expression of the transgene in Triphysaria was verified using genomic PCR, RT PCR and Southern hybridizations. Transgenic roots were also obtained by inoculating A. rhizogenes into wounded Triphysaria seedlings. Stable transformed roots were identified using GUS staining or fluorescent microscopy following transformation with vectors containing GFP, dsRED or EYFP. Transgenic roots derived from both A. tumefaciens and A. rhizogenes transformations were morphologically normal and developed haustoria that attached to and invaded lettuce roots. Transgenic roots also remained competent to form haustoria in response to purified inducing factors. These transformation systems will allow an in planta assessment of genes predicted to function in plant parasitism.

Green fluorescent protein as an efficient selection marker for Agrobacterium rhizogenes mediated carrot transformation

Agrobacterium rhizogenes mediated transformation combined with a visual selection for green fluorescent protein (GFP) has been applied effectively in carrot (Daucus carota L.) transformation. Carrot root discs were inoculated with A4, A4T, LBA1334 and LBA9402 strains, all bearing gfp gene in pBIN-m-gfp5-ER. The results indicate that transformed adventitious roots can be visually selected solely based on GFP fluorescence with a very high accuracy. The method requires no selection agents like antibiotics or herbicides and enables a reduction of labour and time necessary for tissue culture. Moreover, individual transformants can be easily excised from the host tissue and cultured separately. All of the 12 used carrot cultivars produced transformed adventitious roots and the frequency of discs producing GFP expressing adventitious roots varied from 13 to 85%. The highest transformation rate was found for A4T and LBA1334 strains possessing chromosomal background of A. tumefaciens C58. The results encourage that visual selection of transformed, fluorescing adventitious roots can be highly effective and applied routinely for the production of carrot transgenic plants.

An improved protocol for Agrobacterium-mediated transformation of Antirrhinum majus L

Efficient Agrobacterium -mediated transformation of Antirrhinum majus L. was achieved via indirect shoot organogenesis from hypocotyl explants of seedlings. Stable transformants were obtained by inoculating explants with A. tumefaciens strain GV2260 harboring the binary vector pBIGFP121, which contains the neomycin phosphotransferase gene ( NPT II) as a selectable marker and the gene for the Green Fluorescent Protein ( GFP) as a visual marker. Putative transformants were identified by selection for kanamycin resistance and by examining the shoots using fluorescence microscopy. PCR and Southern analyses confirmed integration of the GFP gene into the genomes of the transformants. The transformants had a morphologically normal phenotype. The transgene was shown to be inherited in a Mendelian manner. This improved method requires only a small number of seeds for explant preparation, and three changes of medium; the overall transformation efficiency achieved, based on the recovery of transformed plants after 4-5 months of culture, reached 8-9%. This success rate makes the protocol very useful for producing transgenic A. majus plants.

Transformation of Antirrhinum majus L. by a rol-type multi-auto-transformation (MAT) vector system

A total of 11 independent beta-glucuronidase (GUS) positive hairy roots were induced following co-cultivation of leaf explants of Antirrhinum majus L. with Agrobacterium tumefaciens strain GV2260 containing rol-type multi-auto-transformation (MAT) vector pNPI702. A total of 326 adventitious shoots were regenerated from the hairy root lines on 1/2 MS medium without plant growth regulators at 25 degrees C under a 16 h/day photoperiod condition 4 months after infection of the A. tumefaciens GV2260. The absence of the rol genes in five plants was verified by polymerase chain reaction (PCR) and Southern blot analysis. Acclimatized transformants exhibited normal phenotypes in height and in the morphology of leaves and flowers. Furthermore, the GUS gene was strongly expressed in the leaves, inflorescence of the transformed plant, and the progeny. This result demonstrates that the rol-type MAT vector can be used to study gene functions controlling the morphogenesis of Antirrhinum majus plants.

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.

Genetic localisation of transformation competence in diploid potato

In the course of improving diploid potato genotypes for transformation ability, selection for specific components affecting regeneration and transformation was carried out. From a segregating population between two good regenerating clones a selection was made to yield an optimal well-transforming and fertile genotype J92-6400-A16. This plant yielded predominantly diploid transformants and was heterozygous for the gene R1, conferring resistance to Phytophthora infestans. The speed of, and competence for, regeneration and transformation on both sides of the stem explant were improved. A competence factor for tranformation was found to be linked with the R1 locus and a molecular marker on chromosome 5. The male fertility of transformants was frequently decreased to a great extent, whereas female fertility was not so markedly affected.

Isolation and characterization of a cDNA encoding granule-bound starch synthase in cassava (Manihot esculenta Crantz) and its antisense expression in potato

A tuber-specific cDNA library of cassava (Manihot esculenta Crantz) was constructed and a full-length cDNA for granule-bound starch synthase (GBSS, also known as waxy protein), the enzyme responsible for the synthesis of amylose in reserve starch, was cloned. Sequencing of the cloned cDNA showed that it has 74% identity with potato GBSS and 60-72% identity with GBSS from other plant species. The cDNA encodes a 608 amino acid protein of which 78 amino acids form a chloroplast/amyloplast transit peptide of 8.37 kDa. The mature protein has a predicted molecular mass of 58.61 kDa (530 amino acids). Comparison of the GBSS proteins of various plant species and glycogen synthase of bacteria showed extensive identity among the mature form of plant GBSS proteins, in which the monocots and dicots form two separate branches in the evolutionary tree. From analysis of the genomic DNA of allotetraploid cassava, it is shown that GBSS is a low-copy-number gene. GBSS transcript is synthesized in a number of different organs, but most abundantly in tubers. Potato plants were transformed with the cassava GBSS cDNA in antisense orientation fused between the potato GBSS promoter and the nopaline synthase terminator. The expression of the endogenous GBSS gene in these transgenic potato plants was partially or completely inhibited. Complete inhibition of GBSS activity by the cassava antisense gene resulted in absence of GBSS protein and amylose giving rise to almost complete amylose-free potato starch. This shows that also heterologous genes can be used to achieve antisense effects in other plant species.

Conditional inhibition of beta-glucuronidase expression by antisense gene fragments in petunia protoplasts

Antisense RNA-mediated inhibition of gene expression is a valuable tool to induce mutant phenotypes. We are interested in the application of antisense gene fragments with the aim to improve the efficiency of inhibition and to be able to selectively suppress gene family members in plants. Protoplasts may provide a rapid system to screen the efficiency of antisense gene segments. As a first step, we set up a transient expression system for leaf protoplasts of Petunia hybrida and used as a model system the inhibition of beta-glucuronidase (uidA) expression by uidA antisense gene segments. Both GUS enzyme activities and uidA RNA levels were measured. Co-introducing equal amounts of a full-length uidA antisense gene and a uidA sense gene reduced GUS activity by 60-70%. Various uidA antisense fragments also inhibited expression although with different efficiencies and we show that strong antisense fragments can be retrieved from weak antisense gene fragments. A promoter-less antisense gene did not reduce uidA expression indicating that the inhibition is mediated by antisense transcripts. Using quantitative PCR on first-strand cDNA we show that expression of functional antisense genes lead to reduced levels of uidA mRNA. This suggests that the mechanism of antisense RNA inhibition in protoplasts is similar to that in transgenic plants and that the protoplast system in combination with PCR can be used to preselect antisense fragments of any gene.

Complementation of the amylose-free starch mutant of potato (Solanum tuberosum.) by the gene encoding granule-bound starch synthase

Agrobacterium rhizogenes-mediated introduction of the wild-type allele of the gene encoding granulebound starch synthase (GBSS) into the amylose-free starch mutantamf of potato leads to restoration of GBSS activity and amylose synthesis, which demonstrates thatAmf is the structural gene for GBSS. Amylose was found in columella cells of root tips, in stomatal guard cells, tubers, and pollen, while in the control experiments using only vector DNA, these tissues remained amylose free. This confirms the fact that, in potato, GBSS is the only enzyme responsible for the presence of amylose, accumulating in all starch-containing tissues. Amylose-containing transformants showed no positive correlation between GBSS activity and amylose content, which confirms that the former is not the sole regulating factor in amylose metabolism.

Cotransformation and differential expression of introduced genes into potato (Solanum tuberosum L.) cv Bintje

The Dutch potato cultivar Bintje has been transformed by Agrobacterium strain LBA1060KG, which contains two plasmids carrying three different DNAs (TL- and TR-DNA on the Agrobacterium rhizogenes plasmid and TKG-DNA on the pBI121 plasmid). Several transformed root clones were obtained after transformation of leaf, stem, and tuber segments, and plants were then regenerated from these root clones. The expression of the various marker genes [rol, opine, β-glucuronidase (GUS), and neomycin phosphotransferase (NPTII)] was determined in several root clones and in regenerated plants. The selection of vigorously growing root clones was as efficient as selection for kanamycin resistance. In spite of the location of NPTII and GUS genes on the same T-DNA, 17% of the root clones did not show GUS activity. Nevertheless, Southern blot analysis showed that these root clones contained at least three copies of the GUS gene. Sixty-four per cent of the root clones contained opines. The expression of these genes, however, was negatively correlated with plant regeneration capacity and normal plant development. The differential expression of the marker genes in the transgenic potato tissues is discussed.

Inhibition of the expression of the gene for granule-bound starch synthase in potato by antisense constructs

Granule-bound starch synthase [GBSS; EC 24.1.21] determines the presence of amylose in reserve starches. Potato plants were transformed to produce antisense RNA from a gene construct containing a full-length granule-bound starch synthase cDNA in reverse orientation, fused between the cauliflower mosaic virus 35S promoter and the nopaline synthase terminator. The construct was integrated into the potato genome by Agrobacterium rhizogenes-mediated transformation. Inhibition of GBSS activity in potato tuber starch was found to vary from 70% to 100%. In those cases where total suppression of GBSS activity was found both GBSS protein and amylose were absent, giving rise to tubers containing amylose-free starch. The variable response of the transformed plants indicates that position effects on the integrated sequences might be important. The results clearly demonstrate that in tubers of potato plants which constitutively synthesize antisense RNA the starch composition is altered.

Agrobacterium-mediated plant transformation by novel mini-T vectors in conjunction with a high-copy vir region helper plasmid

A new binary vector system for Agrobacterium-mediated plant transformation was developed. A set of four mini-T vectors comprised of T-DNA border sequences from nopaline-type Ti-plasmid pTiC58 flanking a chimaeric hygromycin-resistance gene for selection of transformants and up to eight unique restriction sites for cloning foreign DNA was constructed on a broad-host replicon containing the oriV of plasmid pSa. In two of the constructs these multiple cloning sites are flanked by a strong promoter to activate transcription of inserted DNA in planta. High-efficiency transformation was prompted by a high-copy, stable virulence helper plasmid pUCD2614, which contains a cloned virulence region of pTiC58 and tandem copies of the par locus of plasmid pTAR. Southern blot hybridization and genetic analyses of the progeny of transformed plants showed that the hygromycin resistance gene was stably inherited.

Expression and inheritance of inserted markers in binary vector carrying Agrobacterium rhizogenes-transformed potato (Solanum tuberosum L.)

Transgenic shoots were regenerated from eight diploid potato hairy root clones obtained by transformation with Agrobacterium rhizogenes harboring next to its wild-type Ri-plasmid a binary vector containing the neomycin phosphotransferase and the β-glucuronidase genes. The plants exhibited the typical hairy root phenotype. Of the plants isolated, 58% were tetraploid and 38% were diploid. Flowering and tuberization was much better in the diploid than in the tetraploid plants. Transgenic plants formed a significantly larger root system when grown on kanamycin-containing medium as compared to growth on kanamycin-free medium. Direct evidence for genetic transformation was obtained by opine, neomycin phosphotransferase and β-glucuronidase assays, and by molecular hybridization. Fourteen flowering diploid plants were reciprocally crossed with untransformed S. tuberosum plants, but only six were successful. Seedlings obtained from four crosses showed that all traits were transmitted to the offspring. Molecular analysis confirmed the presence of multiple integrations (copies) of both vector T-DNA and Ri-T-DNA. The genetic data, furthermore, suggest that the traits derived from Ri-T-DNA and binary vector T-DNA are linked, as no recombination between the different traits was observed.

Efficient transformation of potato (Solanum tuberosum L.) using a binary vector in Agrobacterium rhizogenes

We transformed three potato (Solanum tuberosum L.) genotypes by using A. rhizogenes or a mixture of A. rhizogenes and A. tumefaciens. Inoculations of potato stem segments were performed with Agrobacterium rhizogenes AM8703 containing two independent plasmids: the wild-type Ri-plasmid, pRI1855, and the binary vector plasmid, pBI121. In mixed inoculation experiments, Agrobacterium rhizogenes LBA1334 (pRI1855) and Agrobacterium tumefaciens AM8706 containing the disarmed Ti-plasmid (pAL4404) and the binary vector plasmid (pBI121) were mixed in a 1∶1 ratio. The T-DNA of the binary vector plasmid pBI121 contained two marker genes encoding neomycin phosphotransferase, which confers resistance to kanamycin, and β-glucuronidase. Both transformation procedures gave rise to hairy roots on potato stem segments within 2 weeks. With both procedures it was possible to obtain transformed hairy roots, able to grow on kanamycin and possessing β-glucuronidase activity, without selection pressure. The efficiency of the A. rhizogenes AM8703 transformation, however, was much higher than that of the "mixed" transformation. Up to 60% of the hairy roots resulting from the former transformation method were kanamycin resistant and possessed β-glucuronidase activity. There was no correlation between the height of the kanamycin resistance and that of the β-glucuronidase activity in a root clone. Hairy roots obtained from a diploid potato genotype turned out to be diploid in 80% of the cases. Transformed potato plants were recovered from Agrobacterium rhizogenes AM8703-induced hairy roots.