Virulence of Agrobacterium tumefaciens requires lipid homeostasis mediated by the lysyl‐phosphatidylglycerol hydrolase AcvB

Agrobacterium tumefaciens transfers oncogenic T‐DNA via the type IV secretion system (T4SS) into plants causing tumor formation. The acvB gene encodes a virulence factor of unknown function required for plant transformation. Here we specify AcvB as a periplasmic lysyl‐phosphatidylglycerol (L‐PG) hydrolase, which modulates L‐PG homeostasis. Through functional characterization of recombinant AcvB variants, we showed that the C‐terminal domain of AcvB (residues 232–456) is sufficient for full enzymatic activity and defined key residues for catalysis. Absence of the hydrolase resulted in ~10‐fold increase in L‐PG in Agrobacterium membranes and abolished T‐DNA transfer and tumor formation. Overproduction of the L‐PG synthase gene (lpiA) in wild‐type A. tumefaciens resulted in a similar increase in the L‐PG content (~7‐fold) and a virulence defect even in the presence of intact AcvB. These results suggest that elevated L‐PG amounts (either by overproduction of the synthase or absence of the hydrolase) are responsible for the virulence phenotype. Gradually increasing the L‐PG content by complementation with different acvB variants revealed that cellular L‐PG levels above 3% of total phospholipids interfere with T‐DNA transfer. Cumulatively, this study identified AcvB as a novel virulence factor required for membrane lipid homeostasis and T‐DNA transfer.     

T-DNA from Agrobacterium tumefaciens as an efficient tool for gene targeting in Kluyveromyces lactis

The soil bacterium Agrobacterium tumefaciens can transfer a part of its tumour-inducing (Ti) plasmid, the T-DNA, to plant cells. The virulence (vir) genes, also located on the Ti plasmid, encode proteins involved in the transport of T-DNA into the plant cell. Once in the plant nucleus, T-DNA is able to integrate into the plant genome by an illegitimate recombination mechanism. The host range of A. tumefaciens is not restricted to plant species. A. tumefaciens is also able to transfer T-DNA to the yeast Saccharomyces cerevisiae. In this paper we demonstrate transfer of T-DNA from A. tumefaciens to the yeast Kluyveromyces lactis. Furthermore, we found that T-DNA serves as an ideal substrate for gene targeting in K. lactis. We have studied the efficiency of gene targeting at the K. lactis TRP1 locus using either direct DNA transfer (electroporation) or T-DNA transfer from Agrobacterium. We found that gene targeting using T-DNA was at least ten times more efficient than using linear double-stranded DNA introduced by electroporation. Therefore, the outcome of gene targeting experiments in some organisms may depend strongly upon the DNA substrate used. Received: 11 May 1998 / Accepted: 16 October 1998     

Induction of virulence response in Agrobacterium tumefaciens by tissue explants of various plant species

Forty-four plant species belonging to different taxa were tested for their ability to induce the expression of the virulence E gene in Agrobacterium tumefaciens containing virE:lacZ fusion constructs. With the exception of 6 algae, one fern and 2 monocots, tissue explants of all other plants (2 Algae, 3 Bryophytes, 2 Pteridophytes, 15 Gymnosperms, 8 Monocots and 5 Dicots) induced the expression of the virE gene as detected by the presence of -galactosidase activity in the bacteria.Abbreviations AS acetosyringone - vir virulence genes Scientific Contribution Number 1734 from the New Hampshire Agricultural Experiment Station     

Selection system and co-cultivation medium are important determinants of Agrobacterium-mediated transformation of sugarcane

A reproducible method for transformation of sugarcane using various strains of Agrobacterium tumefaciens (A. tumefaciens) (AGL0, AGL1, EHA105 and LBA4404) has been developed. The selection system and co-cultivation medium were the most important factors determining the success of transformation and transgenic plant regeneration. Plant regeneration at a frequency of 0.8–4.8% occurred only when callus was transformed with A. tumefaciens carrying a newly constructed superbinary plasmid containing neomycin phosphotransferase (nptII) and β-glucuronidase (gusA) genes, both driven by the maize ubiquitin (ubi-1) promoter. Regeneration was successful in plants carrying the nptII gene but not the hygromycin phosphotransferase (hph) gene. NptII gene selection was imposed at a concentration of 150 mg/l paromomycin sulphate and applied either immediately or 4 days after the co-cultivation period. Co-cultivation on Murashige and Skoog (MS)-based medium for a period of 4 days produced the highest number of transgenic plants. Over 200 independent transgenic lines were created using this protocol. Regenerated plants appeared phenotypically normal and contained both gusA and nptII genes. Southern blot analysis revealed 1–3 transgene insertion events that were randomly integrated in the majority of the plants produced.     

Transgenic sweet pepper plants fromAgrobacterium mediated transformation

Fertile transgenic sweet pepper (Capsicum annuum var. grossum) plants were regenerated at relatively high rate from various explants that were cocultivated withAgrobacterium tumefaciens strain GV3111-SE harbouring a plasmid that contains the cucumber mosaic virus coat protein (CMV-CP) gene. The rate of plant regeneration was found to depend on the types of explants cultured and the media used. Young leaves were most effective for bud induction and subsequent plant elongation while hypocotyls were the most inefficient. Southern analysis of DNA isolated from putative transgenic plants revealed that 3 out of 5 R₁ plant lines reacted positively with the CMV-CP gene. Western blot analysis of CMV-CP containing R₁ plants showed that two of them accumulated significant levels of the foreign gene product while the other two expressed it only, to low levels. Thus, like many other dicotyledonous plant species, sweet pepper can be transformed byA. tumefaciens and regenerated into healthy, fertile plants that express foreign genes.Abbreviations ABA abscisic acid - BA 6-benzyladenine - carb carbenicillin - CMV cucumber mosaic virus - CTAB cetyltriethylammonium bromide - GA gibberellic acid - GUS -glucuronidase - IgG immunoglobulin G - kan kanamycin - LB Luria-Bertani - MS Murashige and Skoog - NAA naphthalene acetic acid - OD optical density - SDS-PAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - spe spectinomycin     

<Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis>-mediated transformation of blackgram: An assessment of factors influencing the efficiency of <Emphasis Type="Italic">uidA</Emphasis> gene transfer

Agrobacterium tumefaciens strain EHA105 carrying a binary vector pCAMBIA2301, which contains a neomycin phosphotransferase gene (nptII) and a β-glucuronidase (GUS) gene (uidA) interrupted with an intron, was used for transformation of Vigna mungo cotyledonary node explants. Various factors such as preculture and wounding of explants, manipulations in inoculation and co-cultivation conditions were found to play a significant role in influencing tissue competence, Agrobacterium virulence and compatibility of both, for achieving the maximum transformation frequencies. The stable transformation with 4.31 % efficiency was achieved using the optimized conditions. The transformed green shoots that were selected and rooted on medium containing kanamycin and tested positive for nptII gene by polymerase chain reaction were established in soil to collect seeds. GUS activity was detected in leaves, roots, pollen grains and T₁ seedlings. Southern analysis of T₀ plants showed the integration of nptII into the plant genome.     

O-methyltransferase(s)-suppressed plants produce lower amounts of phenolic vir inducers and are less susceptible to Agrobacterium tumefaciens infection

The first step of Agrobacterium tumefaciens/plant interaction corresponds to the activation of a transduction pathway of the bacterium by plant exudate. Phenolic compounds rapidly secreted by wounded plant cells induce the expression of bacterial virulence (vir) genes; however, little is known about their biosynthesis in plant. Here we show that inoculation of an Agrobacterium tumefaciens virulent strain on orthodiphenol-O-methyltransferases-suppressed tobacco plants leads to significantly smaller tumors compared to control plants. These transgenic plants are inhibited for caffeic acid O-methyltransferase class I or II (OMT; EC 2.1.1.6) and/or caffeoyl-coenzyme A O-methyltransferase (CCoAOMT; EC 2.1.1.104) that are involved in monolignol biosynthesis. The significant decrease of tumor size could be suppressed by the pre-activation of bacterial virulence, before inoculation, using acetosyringone a known vir inducer. Total soluble phenolic amounts and cell wall composition analyzed by FT-IR analysis did not show significant differences between transgenic and control plants. The potential of phenolic extracts from control and OMT-suppressed plants to induce virulence was evaluated using an Agrobacterium tumefaciens reporter strain carrying a vir::LacZ gene fusion plasmid. Lower vir-inducing activities were recorded for plants that show inhibition to caffeic acid O-methyltransferase activity. HPLC analysis confirmed that the levels of several phenolic compounds were differently affected by wounding and/or by bacterial inoculation. Statistical correlations were established between tumor sizes, vir-inducing activities, O-methyltransferases proteins accumulations and the levels of various soluble phenolic compounds such as acetosyringone. These results demonstrate the role of the O-methyltransferases of the phenylpropanoid pathway in the early production of soluble Agrobacterium tumefaciens vir inducers.     

Improved <Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated genetic transformation of GNA transgenic sugarcane

Six plasmids carrying a snowdrop lectin (Galanthus nivalis agglutinin, GNA) and one of three selection markers were successfully transferred into two sugarcane cultivars (FN81–745 and Badila) via Agrobacterium-mediated transformation. Agrobacterium strains LBA4404, EHA105 and A281 that harboured a super-binary vector were used for sugarcane transformation. The use of the hygromycin (Hyg) resistance gene (hpt II), phosphinothrincin (PPT) resistance gene (bar) or G418 resistance gene (npt II) as a screenable marker facilitated the initial selection of GNA transgenic sugarcane callus with different efficiencies and helped the rapid segregation of individual transformation events. All the three selective marker genes were controlled by CaMV 35S promoter, while GNA gene was controlled by promoter of RSs-1 (rice sucrose synthase-1) or Ubi (maize ubiquitin). Factors important to successful transformation mediated by Agrobacterium tumefaciens were optimized, which included concentration of A. tumefaciens, medium composition, co-cultivated methods with plant tissue, strain virulence and different selective marker genes. An efficient protocol for sugarcane transformation mediated by A. tumefaciens was established. The GNA gene has been integrated into sugarcane genome as demonstrated by PCR and Southern dot blotting detections. The preliminary results from bioassay demonstrated a significant resistance of the transgenic sugarcane plants to woolly aphid (Ceratovacuna lanigera Zehnther) indicating thus the possibility for obtaining a transgenic sugarcane cultivar with resistance to woolly aphid.     

<Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis>-mediated transformation of the plant pathogenic fungus <Emphasis Type="Italic">Rosellinia necatrix</Emphasis>

Rosellinia necatrix is a soil-borne root pathogen affecting a wide range of commercially important plant species. The mycelium of R. necatrix was transformed to hygromycin B resistance by an Agrobacterium tumefaciens-mediated transformation system using a binary plasmid vector containing the hygromycin B phosphotransferase (hph) gene controlled by the heterologous fungal Aspergillus nidulans P-gpd (glyceraldehyde 3-phosphate dehydrogenase) promoter and the trpC terminator. Co-cultivation of R. necatrix strain W1015 and A. tumefaciens strain AGL-1 at 25°C using the binary vector pAN26-CB1300, which contained the hygromycin B resistance cassette based on pAN26 and pCAMBIA1300, resulted in high frequencies of transformation. The presence of the hph gene in the transformants was detected by PCR, and single-copy integration of the marker gene was demonstrated by Southern b lot analy s is. This report of an Agrobacterium-mediated transformation method should allow the development of T-DNA tagging as a system for insertional mutagenesis in R. necatrix and provide a simple and reliable method for genetic manipulation.     

Metabolic factors capable of inducing Agrobacterium vir gene expression are present in rice (Oryza sativa L.)

Summary The effects of exudates and extracts from suspension cultures or various parts of rice (Oryza sativa L.) plants on induction of vir (virulence) gene expression in Agrobacterium tumefaciens were examined. Only leaf extracts from panicle-differentiating plants to flowering plants were able to strongly induce activation and expression of vir genes. This induction was similar to that observed with 2 M acetosyringone (AS), yet there was no synergy between AS and rice extracts. Responses to vir-inducing metabolites and signal molecules were different among various vir loci. These results demonstrate that one or more inducing factors for vir gene expression are also present in rice, but only in specific parts and developmental stages.     

Co-expression of<Emphasis Type="Italic">flavonoid 3′ 5′-hydroxylase</Emphasis> and<Emphasis Type="Italic">flavonoid 3′-hydroxylase</Emphasis> Accelerates Decolorization in Transgenic Chrysanthemum Petals

Theflavonoid 3′,5′-hydroxylase (F3′,5′H) gene, derived from petunia, was introduced into chrysanthemum tissues by Agrobacterium-mediated genetic transformation. Cotyledon expiants were co-cultured withA. tumefaciens LBA 4404 harboring the vector pMBP that carriesF3′,5′H under the control of the CaMV 35S promoter andnptll as a selectable marker gene. After 72 h of co-cultivation, the expiants were placed on an MS medium supplemented with 4 mg L^-1 BA, 0.1 mg L^-1 NAA, 400 mg L^-1 carbenicillin, and 100 mg L^-1; kanamycin. After 4 weeks, kanamycin-resistant adventitious shoots had developed at a frequency of 6.3%. These shoots were then rooted and acclimatized in potting soil. Integration ofF3′,5′H into the plant genome was confirmed by Southern blot analysis. Flower buds that had red petals did not differ between the transgenic and the wild-type plants. However, petal color did change from red to bright orange to yellow when the buds developed into fully opened flowers on the transgenics. Spectrometric analysis revealed that the content of flavonoid compounds was more rapidly reduced in the transgenic petals as floral development proceeded. RT-PCR analysis showed thatF3′,5′H andflavonoid 3′hydroxylase (F3′H) were expressed simultaneously in the transgenic plants. Therefore, we suggest that this more rapid change in petal color results from 1) competition between levels of transgenicF3′,5′H and endogenousF3′H, each of which uses the same substrate in the flavonoid biosynthetic pathway and 2) the intrinsic substrate specificity of chrysanthemumDFR (dihydroflavonol 4-reductase).     

Transgenic tomato (Lycopersicon esculentum L.) transformed with a binary vector in Agrobacterium rhizogenes: Non-chimeric origin of callus clone and low copy numbers of integrated vector T-DNA

Summary We transformed tomato (Lycopersicon esculentum L.) by using Agrobacterium rhizogenes containing two independent plasmids: the wild-type Ri-plasmid, and the vector plasmid, pARC8. The T-DNA of the vector plasmid contained a marker gene (Nos/Kan) encoding neomycin phosphotransferase which conferred resistance to kanamycin in transformed plant cells. Transgenic plants (R ₀) with normal phenotype were regenerated from transformed organogenic calli by the punctured cotyledon transformation method. Southern blot analysis of the DNA from these transgenic plants showed that one or two copies of the vector plasmid T-DNA, but none of the Ri-plamid T-DNA, were integrated into the plant genome. Different transgenic plants derived from the same callus clone showed an identical DNA banding pattern, indicating the non-chimeric origin of these plants. We also transformed tomato by using A. tumefaciens strain LBA4404 containing a disarmed Ti-plasmid (pAL4404), and a vector plasmid (pARC8). Transgenic plants derived via A. tumefaciens transformation, like those via A. rhizogenes, contained one to two copies of the integrated vector T-DNA. The kanamycin resistance trait in the progeny (R ₁) of most transgenic plants segregated at a ratio of 3:1, suggesting that the vector T-DNAs were integrated at a single site on a tomato chromosome. In some cases, the expression of the marker gene (Nos/Kan) seemed to be suppressed or lost in the progeny.     

Agrobacterium‐produced and exogenous cytokinin‐modulated Agrobacterium‐mediated plant transformation

Agrobacterium tumefaciens is a plant pathogenic bacterium that causes neoplastic growths, called ‘crown gall’, via the transfer and integration of transferred DNA (T‐DNA) from the bacterium into the plant genome. We characterized an acetosyringone (AS)‐induced tumour‐inducing (Ti) plasmid gene, tzs (trans‐zeatin synthesizing), that is responsible for the synthesis of the plant hormone cytokinin in nopaline‐type A. tumefaciens strains. The loss of Tzs protein expression and trans‐zeatin secretions by the tzs frameshift (tzs‐fs) mutant is associated with reduced tumorigenesis efficiency on white radish stems and reduced transformation efficiencies on Arabidopsis roots. Complementation of the tzs‐fs mutant with a wild‐type tzs gene restored wild‐type levels of trans‐zeatin secretions and transformation efficiencies. Exogenous application of cytokinin during infection increased the transient transformation efficiency of Arabidopsis roots infected by strains lacking Tzs, which suggests that the lower transformation efficiency resulted from the lack of Agrobacterium‐produced cytokinin. Interestingly, although the tzs‐fs mutant displayed reduced tumorigenesis efficiency on several tested plants, the loss of Tzs enhanced tumorigenesis efficiencies on green pepper and cowpea. These data strongly suggest that Tzs, by synthesizing trans‐zeatin at early stage(s) of the infection process, modulates plant transformation efficiency by A. tumefaciens.     

Transformation of cultured cells of Chenopodium quinoa by binary vectors that carry a fragment of DNA from the virulence region of pTiBo542

Summary A 15.2-kb KpnI fragment from the virulence region of pTiBo542, the Ti plasmid harbored by Agrobacterium tumefaciens strain A281, was introduced into binary vectors. The fragment contained the virB, virC and virG genes, and it is known to have the ability to increase the virulence of strains of A. tumefaciens. The strains of A. tumefaciens that carried the resulting plasmids were able to transform cells in a suspension culture of Chenopodium quinoa Willd cells which were not transformable by common vectors. Although the sizes of the plasmids was very large, a foreign segment of DNA was introduced into one of the plasmids by homologous recombination in A. tumefaciens cells, and the segment was subsequently transferred to plant cells.Abbreviations NPT neomycin phosphotransferase - SPT streptomycin/spectinomycin phosphotransferase     

Genetic transformation and regeneration of rubber tree (Hevea brasiliensis Muell. Arg) transgenic plants with a constitutive version of an anti-oxidative stress superoxide dismutase gene

Agrobacterium tumefaciens-mediated genetic transformation and the regeneration of transgenic plants was achieved in Hevea brasiliensis. Immature anther-derived calli were used to develop transgenic plants. These calli were co-cultured with A. tumefaciens harboring a plasmid vector containing the H. brasiliensis superoxide dismutase gene (HbSOD) under the control of the CaMV 35S promoter. The -glucuronidase gene (uidA) was used for screening and the neomycin phosphotransferase gene (nptII) was used for selection of the transformed calli. Factors such as co-cultivation time, co-cultivation media and kanamycin concentration were assessed to establish optimal conditions for the selection of transformed callus lines. Transformed calli surviving on medium containing 300 mg l^-1 kanamycin showed a strong GUS-positive reaction. Somatic embryos were then regenerated from these transgenic calli on MS2 medium containing 2.0 mg l^-1 spermine and 0.1 mg l^-1 abscisic acid. Mature embryos were germinated and developed into plantlets on MS4 medium supplemented with 0.2 mg l^-1 gibberellic acid, 0.2 mg l^-1 kinetin (KIN) and 0.1 mg l^-1 indole-3-acetic acid. A transformation frequency of 4% was achieved. The morphology of the transgenic plants was similar to that of untransformed plants. Histochemical GUS assay revealed the expression of the uidA gene in embryos as well as leaves of transgenic plants. The presence of the uidA, nptII and HbSOD genes in the Hevea genome was confirmed by polymerase chain reaction amplification and genomic Southern blot hybridization analyses.Communicated by L. Peña     

<Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis>-mediated transformation of a medicinal plant <Emphasis Type="Italic">Taraxacum platycarpum</Emphasis>

Dandelion plants, the genus Taraxacum, are used in herbal medicine owing to their choleretic, diuretic and anti-carcinogenic activities and several medicinal compounds have been isolated from the roots of these plants. Metabolic manipulation of secondary metabolite biosynthesis is a potential strategy to improve the production of high-value secondary metabolites. The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) is known to control a key regulatory step in the isoprenoid pathway. We report an efficient transformation protocol for stable introduction of HMGR into dandelion plants (Taraxacum platycarpum H. Dablstaed), which is essential for the biotechnological approach. The Agrobacterium tumefaciens strain EHA105 containing the binary vector, pCAMBIA1301, with GUS and HMGR genes, showed high transformation efficiency after 3–5 week hygromycin selection. Southern blotting, GUS staining and RT-PCR analyses demonstrated stable integration of one copy of the HMGR gene into the dandelion genome. Expression of the integrated genes was particularly eminent in root tissues of primary transformant plants. The establishment of an efficient transformation method may facilitate the improvement of medicinal plant in terms of the accumulation levels of secondary metabolites.     

Restoration of attachment, virulence and nodulation of Agrobacterium tumefaciens chvB mutants by rhicadhesin

In contrast to wild-type Agrobacterium tumefaciens strains, β-1,2-glucan-deficient chvB mutants were found to be unable to attach to pea root hair tips. The mutants appeared to produce rhicadhesin, the protein that mediates the first step in attachment of Rhizobiaceae cells to plant root hairs, but the protein was inactive. Both attachment to root hairs and virulence of the ChvB mutants could be restored by treatment of the plants with active rhicadhesin, whereas treatment of plants with β-1,2-glucan had no effect on attachment or virulence. Moreover, nodulation ability of a chvB mutant carrying a Sym plasmid could be restored by pretreatment of the host plant with rhicadhesin. Apparently the attachment-minus and avirulence phenotype of chvB mutants is caused by lack of active rhicadhesin, rather than directly being caused by a deficiency in β-1,2-glucan synthesis. The results strongly suggest that rhicadhesin is essential for attachment and virulence of A. tumefaciens cells. They also indicate that the mechanisms of binding of Agrobacterium and Rhizobium bacteria to plant target cells are similar, despite differences between these target cells.     

The expression of analgesic-antitumor peptide (<Emphasis Type="Italic">AGAP</Emphasis>) from Chinese <Emphasis Type="Italic">Buthus</Emphasis><Emphasis Type="Italic">martensii</Emphasis> Karsch in transgenic tobacco and tomato

The present study aimed to obtain analgesic-antitumor peptide (AGAP) gene expression in plants. The analgesic-antitumor peptide (AGAP) gene was from the venom of Buthus martensii Karsch. Previous studies showed that AGAP has both analgesic and antitumor activities, suggesting that AGAP would be useful in clinical situations as an antitumor drug. Given that using a plant as an expression vector has more advantages than prokaryotic expression, we tried to obtain transgenic plants containing AGAP. In the present study, the AGAP gene was cloned into the plasmid pBI121 to obtain the plant expression vector pBI-AGAP. By tri-parental mating and freeze–thaw transformation, pBI-AGAP was transformed into Agrobacterium tumefaciens LBA4404. Tobacco (Nicotiana tabacum) and tomato (Lycopersicom esculentum) were transformed by the method of Agrobacterium-mediated leaf disc transformation. The transformants were then screened to grow and root on media containing kanamycin. Finally, transformations were confirmed by analysis of PCR, RT-PCR and western blotting. The results showed that the AGAP gene was integrated into the genomic DNA of tobacco and tomato and was successfully expressed. Therefore, the present study suggests a potential industrial application of AGAP expressed in plants.