Transformation of cultivated tomato by a binary vector in Agrobacterium rhizogenes: transgenic plants with normal phenotypes harbor binary vector T-DNA, but no Ri-plasmid T-DNA

Improved and Highly Efficient Agrobacterium rhizogenes-Mediated Genetic Transformation Protocol: Efficient Tools for Functional Analysis of Root-Specific Resistance Genes for Solanum lycopersicum cv. Micro-Tom

Gene function analysis, molecular breeding, and the introduction of new traits in crop plants all require the development of a high-performance genetic transformation system. In numerous crops, including tomatoes, Agrobacterium-mediated genetic transformation is the preferred method. As one of our ongoing research efforts, we are in the process of mapping a broad-spectrum nematode resistance gene (Me1) in pepper. We work to transform tomato plants with candidate genes to confer resistance to nematodes in Solanaceae members. The transformation technology development is designed to produce a reproducible, rapid, and highly effective Agrobacterium-mediated genetic transformation system of Micro-Tom. In our system, a transformation efficiency of over 90% was achieved. The entire procedure, starting from the germination of seeds to the establishment of transformed plants in soil, was completed in 53 days. We confirmed the presence of the NeoR/KanR and DsRed genes in the transformed roots by polymerase chain reaction. The hairy root plants were infected with nematodes, and after 3 months, the presence of DsRed and NeoR/KanR genes was detected in the transformant roots to confirm the long-term effectiveness of the method. The presented study may facilitate root-related research and exploration of root–pathogen interactions.

Engineering drought tolerant tomato plants over-expressing BcZAT12 gene encoding a C₂H₂ zinc finger transcription factor

Efficient genetic transformation of cotyledonary explants of tomato (Solanum lycopersicum, cv. H-86, Kashi vishesh) was obtained. Disarmed Agrobacterium tumifaciens strain GV 3101 was used in conjugation with binary vector pBinAR containing a construct consisting of the coding sequence of the BcZAT12 gene under the regulatory control of the stress inducible Bclea1a promoter. ZAT12 encodes a C₂H₂ zinc finger protein which confers multiple abiotic stress tolerance to plants. Integration of ZAT12 gene into nuclear genome of individual kanamycin resistant transformed T₀ tomato lines was confirmed by Southern blot hybridization with segregation analysis of T(1) plants showing Mendelian inheritance of the transgene. Expression of ZAT12 in drought-stressed transformed tomato lines was verified in T₂ generation plants using RT-PCR. Of the six transformed tomato lines (ZT1-ZT6) the transformants ZT1 and ZT5 showed maximum expression of BcZAT12 gene transcripts when exposed to 7 days drought stress. Analysis of relative water content (RWC), electrolyte leakage (EL), chlorophyll colour index (CCI), H₂O₂ level and catalase activity suggested that tomato BcZAT12 transformants ZT1 and ZT5 have significantly increased levels of drought tolerance. These results suggest that BcZAT12 transformed tomato cv. H-86 has real potential for molecular breeding programs aimed at augmenting yield of tomato in regions affected with drought stress.

Soybean transcription factor GmMYBZ2 represses catharanthine biosynthesis in hairy roots of Catharanthus roseus

Catharanthus roseus (L.) G. Don is a plant species known for its production of a variety of terpenoid indole alkaloids, many of which have pharmacological activities. Production of catharanthine in cell cultures or in hairy roots established by transformation with Agrobacterium rhizogenes is of interest because catharanthine can be chemically coupled to the abundant leaf alkaloid vindoline to form the valuable anticancer drug vinblastine. Here, we observed a high amount of catharanthine in hairy roots of C. roseus, established by infecting leaf explants with the A. rhizogenes >agropine-type A4 strain carrying plasmid pRi. T-DNA transfer from plasmid pRi into hairy roots was confirmed by PCR for the essential T-DNA genes rolA and rolB and the agropine synthesis gene ags. The results suggest that integration of T-DNA into the plant DNA plays a positive role on the catharanthine pathway in C. roseus hairy roots. Furthermore, co-transformation with the soybean transcription factor GmMYBZ2 indicated that GmMYBZ2 reduces the catharanthine production by alteration of expression of a number of genes linked to the pathway. Transcription levels of the zinc-finger transcription factor 1 gene ZCT1 were high, and the transcription levels of the anthranilate synthase gene ASα, the strictosidine synthase gene STR, and the key transcription factor gene octadecanoid-responsive Catharanthus APETALA2/ethylene response factor were low. In addition, GmMYBZ2 had a negative effect on the gene expression levels of A-type cyclin CYSA and B-type cyclin CYSB, which was correlated with a reduced growth rate of the hairy roots.

Miraculin, a taste-modifying protein is secreted into intercellular spaces in plant cells

A taste-modifying protein, miraculin, is highly accumulated in ripe fruit of miracle fruit (Richadella dulcifica) and the content can reach up to 10% of the total soluble protein in these fruits. Although speculated for decades that miraculin is secreted into intercellular spaces in miracle fruit, no evidence exists of its cellular localization. To study the cellular localization of miraculin in plant cells, using miracle fruit and transgenic tomato that constitutively express miraculin, immunoelectron microscopy, imaging GFP fusion proteins, and immunological detection of secreted proteins in culture medium of transgenic tomato were carried out. Immunoelectron microscopy showed the specific accumulation of miraculin in the intercellular layers of both miracle fruit and transgenic tomato. Imaging GFP fusion protein demonstrated that the miraculin-GFP fusion protein was accumulated in the intercellular spaces of tomato epidermal cells. Immunological detection of secreted proteins in culture medium of transgenic tomato indicated that miraculin was secreted from the roots of transgenic tomato expressing miraculin. This study firstly showed the evidences of the intercellular localization of miraculin, and provided a new insight of biological roles of miraculin in plants.

Efficient production of Agrobacterium rhizogenes-transformed roots and composite plants for studying gene expression in coffee roots

The possibility of rapid validation and functional analysis of nematode resistance genes is a common objective for numerous species and particularly for woody species. In this aim, we developed an Agrobacterium rhizogenes-mediated transformation protocol for Coffea arabica enabling efficient and rapid regeneration of transformed roots from the hypocotyls of germinated zygotic embryos, and the subsequent production of composite plants. The A. rhizogenes strain A4RS proved to be the most virulent. High transformation efficiencies (70%) were obtained using a 2-week co-cultivation period at a temperature of 15-18 degrees C. Using a p35S-gusA-int construct inserted in the pBIN19 binary plasmid, we could estimate that 35% of transformed roots were GUS positive (co-transformed). Using the GUS assay as visual marker, 40% composite plants bearing a branched co-transformed rootstock could be obtained after only 12 weeks without selection with herbicides or antibiotics. Transgenic coffee roots obtained with A. rhizogenes did not exhibit the 'hairy' disturbed phenotype and were morphologically similar to normal roots. PCR analyses demonstrated that all co-transformed roots were positive for the expected rolB and gusA genes. Transformed and non-transformed root systems from both susceptible and resistant varieties were inoculated with Meloidogyne exigua nematode individuals. Inoculation of composite plants from the Caturra susceptible variety resulted in the normal development of nematode larvae. Numbers of extracted nematodes demonstrated that transformed roots retain the resistance/sensibility phenotype of varieties from which they are derived. These results suggest that composite plants constitute a powerful tool for studying nematode resistance genes.

Stable transformation and direct regeneration in Coffea canephora P ex. Fr. by Agrobacterium rhizogenes mediated transformation without hairy-root phenotype

A system for genetic transformation of Coffea canephora by co-cultivation with Agrobacterium rhizogenes harbouring a binary vector has been developed. The objective of the present study was the genetic transformation and direct regeneration of transformants through secondary embryos bypassing an intervening hairy root stage. Transformants were obtained with a transformation efficiency up to 3% depending on the medium adjuvant used. A. rhizogenes strain A4 harbouring plasmid pCAMBIA 1301 with an intron uidA reporter and hygromycin phosphotransferase (hptII) marker gene was used for sonication-assisted transformation of Coffea canephora. The use of hygromycin in the secondary embryo induction medium allowed the selection of transgenic secondary embryos having Ri T-DNA along with the T-DNA from the pCAMBIA 1301 binary vector. In addition transgenic secondary embryos devoid of Ri-T-DNA but with stable integration of the T-DNA from the binary vector were obtained. The putative transformants were positive for the expression of the uidA gene. PCR and Southern blot analysis confirmed the independent, transgenic nature of the analysed plants and indicated single and multiple locus integrations. The study clearly demonstrates that A. rhizogenes can be used for delivering transgenes into tree species like Coffea using binary vectors with Agrobacterium tumefaciens T-DNA borders.

Characterization of the ethanol-inducible alc gene expression system in tomato

The efficacy of the ethanol-inducible alc transgene expression system, derived from the filamentous fungus Aspergillus nidulans, has been demonstrated in transgenic tomato. Two direct comparisons have been made. First, this study has utilized two transgenic lines carrying distinct reporter genes (chloramphenicol acetyltransferase and beta-glucuronidase) to distinguish aspects of induction determined by the nature of the gene/gene product rather than that of the plant. Second, comparisons have been made to data generated in other species in order to identify any species-specific effects. The induction profiles for different genes in different species have shown remarkable similarity indicating the broad applicability of this gene switch. While there are minor differences observed between species, these probably arise from diversity in their metabolism. A series of potential alternative inducers have also been tested, revealing that ethanol (through metabolism to acetaldehyde) is better than other alcohols and ketones included in this study. Expression driven by alc was demonstrated to vary spatially, the upper younger leaves having higher activity than the lower older leaves; this will be important for some applications, and for experimental design. The highest levels of activity from ethanol-inducible transgene expression were determined to be the equivalent of those from the constitutive Cauliflower Mosaic Virus 35S promoter. This suggests that the alc system could be an important tool for plant functional genomics.

An efficient mannose selection protocol for tomato that has no adverse effect on the ploidy level of transgenic plants

A protocol for Agrobacterium-mediated transformation with mannose selection was developed for cotyledon petiole, hypocotyl and leaf explants of tomato (Lycopersicon esculentum L. Mill). More than 400 transgenic plants from three tomato varieties were selected with 1% mannose in combination with 0.1-0.5% glucose. Average transformation frequencies ranged from 2.0 to 15.5% depending on the construct, genotype and type of tissue used for transformation. The highest transformation rate was obtained for hypocotyl explants from tomato variety SG048. The ploidy levels of 264 independent transgenic events and 233 non-transgenic plants regenerated from tissue culture were assessed by flow cytometry. The incidence of polyploids within the total population of transgenic plants varied from 10 to 78% and was not significantly different from the non-transgenic population. The greatest variation in the proportion of polyploids was observed in plants derived from different explant types, both in transgenic and non-transgenic regenerants, across three studied genotypes. Transgenic and non-transgenic plants regenerated from leaves included the highest number of normal diploid plants (82-100%), followed by cotyledon petiole-derived plants (63-78%). Transgenic plants produced from hypocotyls contained 22-58% diploids depending on the genotype used in transformation. Results described in this study demonstrate that, although transformation frequencies for leaf tissue are still lower under current protocols, the high percentage of diploids obtained make leaf tissue an attractive transformation target.

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.

An examination of factors affecting the efficiency ofAgrobacterium-mediated transformation of tomato

An improved protocol forAgrobacterium-mediated transformation of the tomato cultivar Moneymaker was developed by examining the effects of six different factors on the efficiency of transformation. Explant size, explant orientation, gelling agent and plate sealant were found to affect transformation efficiency. Two other factors, type of explant (hypocotyl or cotyledon) and frequency of transfer to fresh selective regeneration medium, did not have any effect on transformation efficiency. By combining the best treatments for each factor, an average transformation efficiency of 10.6% was obtained for Moneymaker.

Stable transfer and expression of chimeric genes in licorice (Glycyrrhiza uralensis) using an Ri plasmid binary vector

The pharmaceutically important plant, licorice (Glycyrrhiza uralenesis Fisher), was transformed with a binary vector system of an Ri plasmid, pRi15834, and a mini Ti vector, pGSGluc1, containing chimeric neo and gus genes. The transgenic state of transformed roots was confirmed by detection of agropine and mannopine and by Southern blot hybridization with T-DNA of pGSGluc1. One to three copies of T-DNA of pGSGluc1 was integrated into the genomic DNA of G. uralensis. The expression of chimeric neo and gus genes driven by TR 1' and 2' promoters, respectively, was demonstrated by enzymatic assays. Histochemical analysis showed that the chimeric TR2'-gus gene was expressed specifically in phloem and pericycle tissues of the transformed licorice roots.

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.

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.

Efficient transformation with regeneration of the tropical pasture legumeStylosanthes humilis usingAgrobacterium rhizogenes and a Ti plasmid-binary vector system

Agrobacterium rhizogenes carrying the binary Ti plasmid vector pGA492 was used to transform leaf and stem explants of the tropical pasture legumeStylosanthes humilis. Conditions which yielded kanamycin resistant roots at a frequency of up to 86% and subsequent plant regeneration at a frequency of 23% were defined. Transgenic plants were fertile and either grew normally or had stunted growth but otherwise showed only minor morphological abnormalities. Transgenic plants with normal phenotypes were obtained in the progeny of the primary regenerants. The presence of active neomycin phosphotransferase enzyme activity and binary vector DNA and TL-DNA was demonstrated in the regenerated plants. Evidence for the independent transfer of binary vector and TL-DNA was also obtained. This high frequency production of transgenic plants ofS. humilis is a major improvement over previous methods using disarmed strains ofA. tumefaciens as helper.

A selection method for the synthesis of triploid hybrids by fusion of microspore protoplasts (n) with somatic cell protoplasts (2n)

Microspore protoplasts (n) isolated at the tetrad stage from plants of Nicotiana tabacum Km(+) (2n=4x=48) were fused with somatic cell protoplasts (2n) of WT N. rustica (2n=4x=48) to produce triploid plants. A total of 21.2×10(6) microspore protoplasts were fused with 11.2×10(6) somatic cell protoplasts using the high pH/Ca(+ +) method. Microspore protoplasts did not divide and WT N. rustica protoplasts stopped dividing when the protoplast-derived colonies were transferred to a selection medium containing kanamycin. A total of 104 actively growing green colonies were recovered on the selection medium. Ninety-six of these colonies were tested for their hybrid nature by PAGE of peroxidases and were found to contain bands characteristic of both parents. Hybrid nature of the plants regenerated from some of the selected colonies was confirmed by IEF of leaf esterases, by NPT II activity assay and by hybridizing total DNAs restricted with EcoR I to a cloned 18s rDNA fragment. Root tip squashes of six of the hybrid plants revealed chromosome numbers ranging from 58-72. From chromosomal and biochemical analysis, it can be concluded that the procedure of fusing microspore protoplasts (n) of species A carrying a dominant selection marker with WT somatic cell protoplasts (2n) of species B can be a convenient selection method for the synthesis of triploid plants. The significance of triploids lies in their subsequent use for transferring alien chromosomes and genes of species A to species B.

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.

Establishment of new axenic hairy root lines by inoculation with Agrobacterium rhizogenes

Cultured hairy root lines resulting from infection by Agrobacterium rhizogenes are known for approximately thirty plant species. We extend this range by establishing forty original dicotyledonous hairy root lines with A. rhizogenes strain A4. Hairy roots have been cultured for at least 2-6 years on Murashige & Skoog medium. Some hairy root cultures such as Anagallis arvensis and Antirrhinum majus spontaneously regenerated whole plants.

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.

A new vector derived from Agrobacterium rhizogenes plasmids: a micro-Ri plasmid and its use to construct a mini-Ri plasmid

A new binary vector system has been constructed, based on agropine-type root-inducing plasmid (pRi) left transferred-region border sequences cloned in a plasmid containing the replication origin of another A. rhizogenes plasmid (pArA4a). This micro-pRi has been used to introduce a chimeric kanamycin resistance gene into tobacco plants, vir functions being provided by either octopine or nopaline tumor-inducing plasmids deleted of their own transferred regions. In addition, we show that cloning of pRi EcoRI fragment 15, which contains three open reading frames (which may correspond to loci rolA, B and C), in the micro-Ri vector generates a mini-pRi capable of inducing the proliferation of transformed roots.