Agrobacterium tumefaciens 6bgenes are strain-specific and affect the activity of auxin as well as cytokinin genes

The Ti Plasmid, Driver of Agrobacterium Pathogenesis

The phytopathogenic bacterium Agrobacterium tumefaciens causes crown gall disease in plants, characterized by the formation of tumor-like galls where wounds were present. Nowadays, however, the bacterium and its Ti (tumor-inducing) plasmid is better known as an effective vector for the genetic manipulation of plants and fungi. In this review, I will briefly summarize some of the major discoveries that have led to this bacterium now playing such a prominent role worldwide in plant and fungal research at universities and research institutes and in agricultural biotechnology for the production of genetically modified crops. I will then delve a little deeper into some aspects of Agrobacterium biology and discuss the diversity among agrobacteria and the taxonomic position of these bacteria, the diversity in Ti plasmids, the molecular mechanism used by the bacteria to transform plants, and the discovery of protein translocation from the bacteria to host cells as an essential feature of Agrobacterium-mediated transformation.

Organization of the TC and TE cellular T-DNA regions in Nicotiana otophora and functional analysis of three diverged TE-6b genes

Nicotiana otophora contains Agrobacterium-derived T-DNA sequences introduced by horizontal gene transfer (Chen et al., 2014). Sixty-nine contigs were assembled into four different cellular T-DNAs (cT-DNAs) totalling 83 kb. TC and TE result from two successive transformation events, each followed by duplication, yielding two TC and two TE inserts. TC is also found in other Nicotiana species, whereas TE is unique to N. otophora. Both cT-DNA regions are partially duplicated inverted repeats. Analysis of the cT-DNA divergence patterns allowed reconstruction of the evolution of the TC and TE regions. TC and TE carry 10 intact open reading frames. Three of these are TE-6b genes, derived from a single 6b gene carried by the Agrobacterium strain which inserted TE in the N. otophora ancestor. 6b genes have so far only been found in Agrobacterium tumefaciens or Agrobacterium vitis T-DNAs and strongly modify plant growth (Chen and Otten, 2016). The TE-6b genes were expressed in Nicotiana tabacum under the constitutive 2 × 35S promoter. TE-1-6b-R and TE-2-6b led to shorter plants, dark-green leaves, a strong increase in leaf vein development and modified petiole wings. TE-1-6b-L expression led to a similar phenotype, but in addition leaves show outgrowths at the margins, flowers were modified and plants became viviparous, i.e. embryos germinated in the capsules at an early stage of their development. Embryos could be rescued by culture in vitro. The TE-6b phenotypes are very different from the earlier described 6b phenotypes and could provide new insight into the mode of action of the 6b genes.

The Agrobacterium Phenotypic Plasticity (Plast) Genes

The transfer of T-DNA sequences from Agrobacterium to plant cells is a well-understood process of natural genetic engineering. The expression of T-DNA genes in plants leads to tumors, hairy roots, or transgenic plants. The transformed cells multiply and synthesize small molecules, called opines, used by Agrobacteria for their growth. Several T-DNA genes stimulate or influence plant growth. Among these, iaaH and iaaM encode proteins involved in auxin synthesis, whereas ipt encodes a protein involved in cytokinin synthesis. Growth can also be induced or modified by other T-DNA genes, collectively called plast genes (for phenotypic plasticity). The plast genes are defined by their common ancestry and are mostly found on T-DNAs. They can influence plant growth in different ways, but the molecular basis of their morphogenetic activity remains largely unclear. Only some plast genes, such as 6b, rolB, rolC, and orf13, have been studied in detail. Plast genes have a significant potential for applied research and may be used to modify the growth of crop plants. In this review, I summarize the most important findings and models from 30 years of plast gene research and propose some outlooks for the future.

T-6b allocates more assimilation product for oil synthesis and less for polysaccharide synthesis during the seed development of Arabidopsis thaliana

BACKGROUND

As an Agrobacterium tumefaciens T-DNA oncogene, T-6b induces the development of tumors and the enation syndrome in vegetative tissues of transgenic plants. Most of these effects are related to increases in soluble sugar contents. To verify the potential roles of T-6b in the distribution of carbon in developing seeds, not in vegetative tissues, we fused an endosperm-specific promoter to the T-6b gene for expression in transgenic Arabidopsis thaliana plants.

RESULTS

The expression of T-6b in reproductive organs did not induce the development of the enation syndrome, and moreover, promoted endosperm expansion, which increased the total seed biomass by more than 10%. Additionally, T-6b also increased oil content in mature seeds by more than 10% accompanied with the decrease of starch and mucilage content at the same time.

CONCLUSIONS

T-6b enhances seed biomass and helps oil biosynthesis but not polysaccharides in reproductive organs without disturbing vegetative growth and development. Our findings suggest T-6b may be very useful for increasing oil production in biodiesel plants.

Morphological analysis of the 6b oncogene-induced enation syndrome

MAIN CONCLUSION

The T-DNA 6b oncogene induces complex and partly unprecedented phenotypic changes in tobacco stems and leaves, which result from hypertrophy and hyperplasia with ectopic spot-like, ridge-like and sheet-like meristems. The Agrobacterium T-DNA oncogene 6b causes complex growth changes in tobacco including enations; this unusual phenotype has been called "6b enation syndrome". A detailed morphological and anatomical analysis of the aerial part of Nicotiana tabacum plants transformed with a dexamethasone-inducible dex-T-6b gene revealed several striking growth phenomena. Among these were: uniform growth of ectopic photosynthetic cells on the abaxial leaf side, gutter-like petioles with multiple parallel secondary veins, ectopic leaf primordia emerging behind large glandular trichomes, corniculate structures emerging from distal ends of secondary veins, pin-like structures with remarkable branching patterns, ectopic vascular strands in midveins and petioles extending down along the stem, epiascidia and hypoascidia, double enations and complete inhibition of leaf outgrowth. Ectopic stipule-like leaves and inverted leaves were found at the base of the petioles. Epinastic and hyponastic growth of petioles and midveins yielded complex but predictable leaf folding patterns. Detailed anatomical analysis of over sixty different 6b-induced morphological changes showed that the different modifications are derived from hypertrophy and abaxial hyperplasia, with ectopic photosynthetic cells forming spot-like, ridge-like and sheet-like meristems and ectopic vascular strands forming regular patterns in midveins, petioles and stems. Part of the enation syndrome is due to an unknown phloem-mobile enation factor. Graft experiments showed that the 6b mRNA is mobile and could be the enation factor. Our work provides a better insight in the basic effects of the 6b oncogene.

Reprogramming of plant cells induced by 6b oncoproteins from the plant pathogen Agrobacterium

Reprogramming of plant cells is an event characterized by dedifferentiation, reacquisition of totipotency, and enhanced cell proliferation, and is typically observed during formation of the callus, which is dependent on plant hormones. The callus-like cell mass, called a crown gall tumor, is induced at the sites of infection by Agrobacterium species through the expression of hormone-synthesizing genes encoded in the T-DNA region, which probably involves a similar reprogramming process. One of the T-DNA genes, 6b, can also by itself induce reprogramming of differentiated cells to generate tumors and is therefore recognized as an oncogene acting in plant cells. The 6b genes belong to a group of Agrobacterium T-DNA genes, which include rolB, rolC, and orf13. These genes encode proteins with weakly conserved sequences and may be derived from a common evolutionary origin. Most of these members can modify plant growth and morphogenesis in various ways, in most cases without affecting the levels of plant hormones. Recent studies have suggested that the molecular function of 6b might be to modify the patterns of transcription in the host nuclei, particularly by directly targeting the host transcription factors or by changing the epigenetic status of the host chromatin through intrinsic histone chaperone activity. In light of the recent findings on zygotic resetting of nucleosomal histone variants in Arabidopsis thaliana, one attractive idea is that acquisition of totipotency might be facilitated by global changes of epigenetic status, which might be induced by replacement of histone variants in the zygote after fertilization and in differentiated cells upon stimulation by plant hormones as well as by expression of the 6b gene.

Protein encoded by oncogene 6b from Agrobacterium tumefaciens has a reprogramming potential and histone chaperone-like activity

Crown gall tumors are formed mainly by actions of a group of genes in the T-DNA that is transferred from Agrobacterium tumefaciens and integrated into the nuclear DNA of host plants. These genes encode enzymes for biosynthesis of auxin and cytokinin in plant cells. Gene 6b in the T-DNA affects tumor morphology and this gene alone is able to induce small tumors on certain plant species. In addition, unorganized calli are induced from leaf disks of tobacco that are incubated on phytohormone-free media; shooty teratomas, and morphologically abnormal plants, which might be due to enhanced competence of cell division and meristematic states, are regenerated from the calli. Thus, the 6b gene appears to stimulate a reprogramming process in plants. To uncover mechanisms behind this process, various approaches including the yeast-two-hybrid system have been exploited and histone H3 was identified as one of the proteins that interact with 6b. It has been also demonstrated that 6b acts as a histone H3 chaperon in vitro and affects the expression of various genes related to cell division competence and the maintenance of meristematic states. We discuss current views on a role of 6b protein in tumorigenesis and reprogramming in plants.

Molecular insights into plant cell proliferation disturbance by Agrobacterium protein 6b

The Agrobacterium Ti plasmid (T-DNA) 6b proteins interact with many different host proteins implicated in plant cell proliferation. Here, we show that Arabidopsis plants overexpressing 6b display microRNA (miRNA) deficiency by directly targeting SERRATE and AGO1 via a specific loop fragment (residues 40-55). In addition, we report the crystal structures of Agrobacterium tumefaciens AK6b at 2.1 Å, Agrobacterium vitis AB6b at 1.65 Å, and Arabidopsis ADP ribosylation factor (ARF) at 1.8 Å. The 6b structure adopts an ADP-ribosylating toxin fold closely related to cholera toxin. In vitro ADP ribosylation analysis demonstrates that 6b represents a new toxin family, with Tyr 66, Thr 93, and Tyr 153 as the ADP ribosylation catalytic residues in the presence of Arabidopsis ARF and GTP. Our work provides molecular insights, suggesting that 6b regulates plant cell growth by the disturbance of the miRNA pathway through its ADP ribosylation activity.

Modulation of the venation pattern of cotyledons of transgenic tobacco for the tumorigenic 6b gene of Agrobacterium tumefaciens AKE10

Neoplastic plant-tissue formation, termed crown gall disease, is induced on infection with Agrobacterium tumefaciens. The tumorous tissues develop an extensive vascular system, with a venation pattern distinct from that of native host plants. We report here that the plant-tumorigenic 6b gene of the A. tumefaciens strain AKE10 is capable of inducing extensive vein formation in transgenic tobacco seedlings with distinct pattern formation. Unlike the wild-type cotyledons, transgenic cotyledons had wavy and striate veins depending on the extent of severity of leaf morphology. Graph analysis of the transgenic cotyledonous vein patterns revealed an increase in the number of branch points of veins, end-points of veins, and areas surrounded by the veins. Histological analysis showed abnormal tissue growth on the abaxial side of the cotyledon blades and continual formation of adventitious veins. These adventitiously formed veins included inverted dorso-ventrality and formation of a radial axis.

Abnormal accumulation of sugars and phenolics in tobacco roots expressing the Agrobacterium T-6b oncogene and the role of these compounds in 6b-induced growth

The Agrobacterium T-DNA oncogene 6b induces tumors and modifies the growth of transgenic plants by an unknown mechanism. We have investigated changes in roots of tobacco seedlings that express a dexamethasone-inducible T-6b (dex-T-6b) gene. On induction medium with sucrose, intact or isolated dex-T-6b roots accumulated sucrose, glucose, and fructose and changed their growth, contrary to noninduced roots. Root fragments bridging agar blocks with or without sucrose accumulated sugars at the site of sucrose uptake, resulting in local growth. Induced root fragments showed enhanced uptake of 14C-labeled sucrose, glucose, and fructose. When seedlings were placed on sucrose-free induction medium, sugar levels strongly decreased in roots and increased in cotyledons. Collectively, these results demonstrate that 6b stimulates sugar uptake and retention with drastic effects on growth. Apart from sugars, phenolic compounds also have been found to accumulate in 6b tissues and have been proposed earlier to play a role in 6b-induced growth. Induced dex-T-6b roots accumulated high levels of 5-caffeoylquinic acid (or chlorogenic acid [CGA]), but only under conditions where endogenous sugars increased. Inhibition of phenylalanine ammonia-lyase with the competitive inhibitor 2-aminoindan-2-phosphonic acid (AIP) abolished CGA accumulation without modifying sugar accumulation or affecting the 6b phenotype. We conclude that the absorption, retention, and abnormal accumulation of sugars are essential factors in 6b-induced growth changes, whereas phenylpropanoids only marginally contribute to the 6b seedling phenotype.

Oncogene 6b from Agrobacterium tumefaciens induces abaxial cell division at late stages of leaf development and modifies vascular development in petioles

The 6b gene in the T-DNA region of the Ti plasmids of Agrobacterium tumefaciens and A. vitis is able to generate shooty calli in phytohormone-free culture of leaf sections of tobacco transformed with 6b. In the present study, we report characteristic morphological abnormalities of the leaves of transgenic tobacco and Arabidopsis that express 6b from pTiAKE10 (AK-6b), and altered expression of genes related to cell division and meristem formation in the transgenic plants. Cotyledons and leaves of both transgenic tobacco and Arabidopsis exhibited various abnormalities including upward curling of leaf blades, and transgenic tobacco leaves produced leaf-like outgrowths from the abaxial side. Transcripts of some class 1 KNOX homeobox genes, which are thought to be related to meristem functions, and cell cycle regulating genes were ectopically accumulated in mature leaves. M phase-specific genes were also ectopically expressed at the abaxial sides of mature leaves. These results suggest that the AK-6b gene stimulates the cellular potential for division and meristematic functions preferentially in the abaxial side of leaves and that the leaf phenotypes generated by AK-6b are at least in part due to such biased cell division during polar development of leaves. The results of the present experiments with a fusion gene between the AK-6b gene and the glucocorticoid receptor gene showed that nuclear import of the AK-6b protein was essential for upward curling of leaves and hormone-free callus formation, suggesting a role for AK-6b in nuclear events.

The Agrobacterium vitis T-6b oncoprotein induces auxin-independent cell expansion in tobacco

Among the Agrobacterium T-DNA genes, rolB, rolC, orf13, orf8, lso, 6b and several other genes encode weakly homologous proteins with remarkable effects on plant growth. The 6b oncogene induces tumors and enations. In order to study its properties we have used transgenic tobacco plants that carry a dexamethasone-inducible 6b gene, dex-T-6b. Upon induction, dex-T-6b plants develop a large array of morphological modifications, some of which involve abnormal cell expansion. In the present investigation, dex-T-6b-induced expansion was studied in intact leaves and an in vitro leaf disc system. Although T-6b and indole-3-acetic acid (IAA) both induced expansion and were non-additive, T-6b expression did not increase IAA levels, nor did it induce an IAA-responsive gene. Fusicoccin (FC) is known to stimulate expansion by increasing cell wall plasticity. T-6b- and FC-induced expansion were additive at saturating FC concentrations, indicating that T-6b does not act by a similar mechanism to FC. T-6b expression led to higher leaf osmolality values, in contrast to FC, suggesting that the T-6b gene induces expansion by increasing osmolyte concentrations. Metabolite profiling showed that glucose and fructose played a major role in this increase. We infer that T-6b disrupts the osmoregulatory controls that govern cell expansion during development and wound healing.

Reduction of polar auxin transport in tobacco by the tumorigenic Agrobacterium tumefaciens AK-6b gene

The plant-tumorigenic 6b (AK-6b) gene of Agrobacterium tumefaciens strain AKE10 induces morphological alterations to tobacco plants, Nicotiana tabacum. To investigate the molecular mechanisms underlying these processes, we generated transgenic tobacco harboring the AK-6b gene under the control of a dexamethazone-inducible promoter. Upon induction, transgenic tobacco seedlings exhibited distinct classes of aberrant morphologies, most notably adventitious outgrowths and stunted epicotyls. Histological analysis revealed massive proliferation and altered venation in the newly established outgrowths. Prominent vascular development suggested that auxin metabolism or signaling had been altered. Indeed, basipetal auxin transport in the hypocotyls of the transgenic seedlings was reduced by 50-80%, whereas intracellular auxin contents were only slightly reduced. Analysis of cell extracts by HPLC revealed a large accumulation of phenolic compounds, including the flavonoid kaempferol-3-rutinoside, in transgenic plants compared with wild-type seedlings. As some naturally occurring flavonoids have been shown to affect auxin transport, we suggest that the AK-6b gene expression impairs auxin transport via modulation of phenylpropanoid metabolism, and ultimately results in the observed morphological alterations.

Nuclear localization and interaction of RolB with plant 14-3-3 proteins correlates with induction of adventitious roots by the oncogene rolB

The rooting-locus gene B (rolB) on the T-DNA of the root-inducing (Ri) plasmid in Agrobacterium rhizogenes is responsible for the induction of transformed adventitious roots, although the root induction mechanism is unknown. We report here that the RolB protein of pRi1724 (1724RolB) is associated with Nicotianatabacum14-3-3-like protein omegaII (Nt14-3-3 omegaII) in tobacco bright yellow (BY)-2 cells. Nt14-3-3 omegaII directly interacts with 1724RolB protein. Green fluorescent protein (GFP)-fused 1724RolB is localized to the nucleus. GFP-fused mutant 1724RolB proteins having a deletion or amino acid substitution are unable to interact with Nt14-3-3 omegaII and also show impaired nuclear localization. Moreover, these 1724RolB mutants show decreased capacity for adventitious root induction. These results suggest that adventitious root induction by 1724RolB protein correlates with its interaction with Nt14-3-3 omegaII and the nuclear localization of 1724RolB protein.

New plant growth-modifying properties of the Agrobacterium T-6b oncogene revealed by the use of a dexamethasone-inducible promoter

Agrobacterium 6b oncogenes induce tumours on Nicotiana glauca and enations and associated modifications in transgenic N. tabacum plants. 2x35S-AB-6b tobacco rootstocks produced a graft-transmissible factor that induced enations in wild-type scions; the nature of this enation factor remains to be identified. Here, we report on the properties of tobacco plants carrying a dexamethasone-inducible T-6b gene (dex-T-6b). Induction with dex led to complex growth modifications, many of which have not been reported previously. Modifications were only found in growing tissues; mature tissues remained unaffected. Growth could be either stimulated or inhibited. Dex induction of young plants led to morphogenetic gradients that included enations, tubular leaves and fragmented leaf primordia. Root elongation was increased or slowed down, while radial root growth was strongly enhanced. Additional cell divisions were found in the root pericycle and vasculature. Enation factor import from mature tissues did not have the same effects on growing tissues as local T-6b synthesis: normal scions grafted on induced dex-T-6b rootstocks formed enations, whereas local dex-T-6b induction at the shoot apex led to numerous dark-green spots on the abaxial side of the leaves. In leaf patch assays, the 23-kDa T-6b protein was found to move through leaves and to enter the vascular system. This and the fact that rootstocks of spontaneous tobacco enation mutants did not modify wild-type scions contrary to 6b plants indicate that the 6b protein might be the enation factor.

The rolB-like part of the Agrobacterium rhizogenes orf8 gene inhibits sucrose export in tobacco

Many Agrobacterium T-DNA genes belong to the highly diverse rolB family. The mode of action of most of these genes is still unknown. rolB-like sequences also are present at the 5' ends of the T-DNA-located iaaM genes and the iaaM homolog orf8, whereas iaaM genes from Pseudomonas and Erwinia spp. lack such sequences. iaaM genes encode tryptophan monooxygenases; these enzymes convert tryptophan into indole-3-acetamide, a precursor of indole-3-acetic acid. Tobacco plants expressing the rolB-like part of the A4 orf8 gene (2x35S-A4-Norf8 plants) accumulate glucose, fructose, sucrose, and starch and resemble sucrose transporter (NtSUT1) antisense plants. Different lines of evidence indicate that 2x35S-A4-Norf8 plants export less sucrose from source leaves. Glucose, fructose, sucrose, and starch accumulate in source leaves during sink-source transition, whereas sink tissues like petioles and midveins contain lower levels than normal. Petiole exudation experiments demonstrate a significant decrease in export of label after 14C-sucrose infiltration and after 14CO2 labeling. Grafting of stunted homozygous 2x35S-A4-Norf8 plants onto wild-type rootstocks restores growth, indicating that unloading is not affected. Growth of 2x35S-A4-Norf8 seedlings is inhibited on naphthalene acetic acid-containing media, suggesting a link between sucrose transport and auxin sensitivity.

Resistance of transgenic tobacco seedlings expressing the Agrobacterium tumefaciens C58-6b gene, to growth-inhibitory levels of cytokinin is associated with elevated IAA levels and activation of phenylpropanoid metabolism

We previously reported that the Agrobacterium tumefaciens C58-6b gene confers resistance to growth-inhibitory levels of exogenously applied N(6)-benzyladenine (BA, cytokinin) in transgenic tobacco (Nicotiana tabacum) seedlings. Here, we found that intracellular levels of indoleacetic acid (IAA, auxin) increased in transgenics but declined in wild-type seedlings upon BA treatment. Since exogenously supplied 1-naphthalene acetic acid (NAA), a stable synthetic auxin, counteracted the growth inhibition of wild-type seedlings by BA, we suggest that BA-induced growth inhibition in wild-type seedlings occurs, at least in part, as a result of intracellular IAA deficiency. Further HPLC analysis of cell extracts from BA-treated seedlings revealed that a fluorescent compound, later identified as the phenylpropanoid, scopolin, and the major phenolic compound, chlorogenic acid, accumulated earlier in transgenics than in wild-type seedlings. Gene transcripts encoding phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and 4-coumarate:CoA ligase, which are responsible for the early steps of phenylpropanoid biosynthesis, accumulated earlier and to higher levels in transgenics than in wild-type seedlings as determined by Northern hybridization analysis, thus accounting for the early accumulation of scopolin and chlorogenic acid in transgenics. As some phenolic compounds, including chlorogenic acid and scopoletin (aglycon of scopolin) are suggested to inhibit IAA catabolism, we further propose that C58-6b gene expression protects IAA from degradation by inducing the early phenylpropanoid pathway.

Biological activity of the rolB-like 5' end of the A4-orf8 gene from the Agrobacterium rhizogenes TL-DNA

The iaaM gene from different plant-associated bacteria encodes a tryptophan monooxygenase (IaaM) that catalyzes the synthesis of indole-3-acetamide (IAM), a precursor of indole-3-acetic acid (IAA). Unlike the IaaM proteins from other bacteria, Agrobacterium spp. T-DNA-encoded IaaM proteins carry a 200 amino acid N-terminal extension with low homology to various members of the RolB protein family. This family is composed of 18 highly divergent T-DNA-encoded proteins, the basic functions of which are still largely undetermined. Deletion of the 5' rolB-like extension of the iaaM gene from Agrobacterium tumefaciens strain Ach5 did not lead to a reduction in IAM synthesis in plants. When expressed in tobacco, the rolB-like fragment did not affect growth or morphology. An iaaM homolog (A4-orf8) from the TL-DNA of Agrobacterium rhizogenes strain A4 also was investigated. Neither the full-size A4-orf8 gene nor the 5'-truncated form induced detectable IAM synthesis. Plants expressing the rolB-like part of the A4-orf8 gene, however, were dwarfed and mottled to various extents and synthesized abnormally high amounts of glucose, fructose, sucrose, and starch.

CROWN GALL OF GRAPE: Biology and Disease Management

Not until 1973 was it reported that strains of Agrobacterium that cause crown gall disease of grape form a specific group (later characterized as Agrobacterium vitis). Tumorigenic and nontumorigenic A. vitis have since been isolated from infected and symptomless grapes worldwide. Research on the genetic makeup of A. vitis has led to an improved understanding of pathogen biology and bacterial evolution. In addition, the identification of significant gene sequences has facilitated the development of PCR and RFLP-based identification procedures that continue to improve the detection of A. vitis in plants and soil. Current control practices rely on the use of disease-resistant cultivars, cultural practices that minimize plant injury, and the production of pathogen-free vines. Promising future controls include employment of biological control agents and development of crown gall-resistant transgenic grapevines.

A T-DNA from the Agrobacterium tumefaciens limited-host-range strain AB2/73 contains a single oncogene

Agrobacterium tumefaciens strain AB2/73 isolated from Lippia canescens has been described as a limited-host-range strain. Its tumor-inducing (Ti) plasmid has been found to lack DNA homology to known T-DNAs (L. Unger, S. F. Ziegler, G. A. Huffman, V. C. Knauf, R. Peet, L. W. Moore, M. P. Gordon, and E. W. Nester. J. Bacteriol. 164:723-730, 1985). We have isolated a T-DNA from AB2/73 by using a heterologous border sequence as a probe. The AB2/73 T-DNA sequence (3,504 bp) is flanked by canonical border sequences, has no detectable DNA homology with other T-DNAs, and contains only two genes: lsn (Lippia strain nopaline synthaselike gene) and lso (Lippia strain oncogene). The lso gene induces nondifferentiating tumors on a limited number of hosts when transferred by a Ti plasmid from a wide-host-range strain. Part of the predicted Lso protein is weakly homologous to other Agrobacterium oncoproteins encoded by rolB, rolB, orf13, gene e, gene 5, and gene 3'. A 28-kb fragment corresponding to the virA to virE region was cloned by using a heterologous vir fragment as probe. The AB2/73 vir region is homologous to most of the C58 virulence region; however, the virA gene is most related to the virA gene of the Agrobacterium vitis limited-host-range strain Ag162.

Tobacco plants carrying a tms locus of Ti-plasmid origin and the Hl-1 allele are tumor prone

The autonomous growth of plant tumor cells is believed to result from their persistent loss of the requirement for growth hormones such as auxin and cytokinin. The partially dominant gene Habituated leaf-1 (Hl-1) regulates the requirement of cultures tissues of Havana 425 tobacco (Nicotiana tabacum L.) for cytokinins. The Hl-1 allele can partially restore the tumor phenotype in tobacco cells transformed with a Agrobacterium tumefaciens Ti plasmid defective in the isopentenyl transferase locus, which encodes a key enzyme in cytokinin biosynthesis and is required for neoplastic growth. To investigate the oncogenic function of Hl-1, we transformed wild-type (hl-1/hl-1) and Hl-1/Hl-1 tobacco plants with the tms locus derived from the limited-host-range Ti plasmid pTiAg162. This locus encodes enzymes for biosynthesis of the auxin indole-3-acetic acid. Grafting tests and measurements of the hormone requirement of cultured explants show that wound-induced overgrowths arising in tms transformed Hl-1 plants are tumorous. While some wound-induced overgrowths also formed in hl-1/hl-1 transformants, these showed slight hormone-autotrophic growth and weak tumorigenicity in grafting tests. In addition, Hl-1/Hl-1 tms/tms plants, but not hl-1/hl-1 tms/tms plants, spontaneously developed rooty teratomatous overgrowths, showed flowering abnormalities, and formed calli at the base of the stem in young seedlings. Thus, Hl-1 tms plants exhibit a tumor-prone phenotype, and in this regard closely resemble tumor-prone hybrids that arise in certain interspecific crosses of Nicotiana species. Our results show that the interaction of just two genetic elements-the mutant Hl-1 allele of the tobacco host with tms genes of Ti plasmid origin-are sufficient for a tumor-prone phenotype.

5'-regulatory region of Agrobacterium tumefaciens T-DNA gene 6b directs organ-specific, wound-inducible and auxin-inducible expression in transgenic tobacco

The regulatory activity of a 826 bp DNA fragment located upstream of the pTiBo542 TL-DNA gene 6b coding region was analyzed in transgenic tobacco, using beta-glucuronidase (gus) as a reporter gene. The region was shown to drive organ-specific, wound- and auxin-inducible expression of the reporter, the effect being dependent on the type and concentration of auxin.

Identification of the Agrobacterium tumefaciens C58 T-DNA genes e and f and their impact on crown gall tumour formation

DNA sequence analysis of the 4.4 kilobases (kb) Eco RI fragment 14 from T-DNA of Agrobacterium tumefaciens C58 revealed three open reading frames. One of them (945 bp) was supposed to encode the transcript e, the function of which has not been identified to date. Furthermore, a so far undescribed open reading frame (1035 bp) was identified, located in the centre of the Eco RI fragment 14 and termed gene f. The third open reading frame encoded the carboxy-terminal part of the agrocinopine synthase (Acs). The gene e-encoded protein showed significant homologies to the gene products of the Agrobacterium rhizogenes rolB gene and the Agrobacterium tumefaciens gene 5. Both gene products are supposed to regulate the plant's reaction on auxin. Depending on the plant species tested, Agrobacterium strains carrying mutations in gene e induced only small or almost no detectable crown gall tumours. According to these mutational studies and the protein homologies observed, the gene e product is suggested to be involved in tumour formation. Infection of several plant species with Agrobacterium carrying a mutated gene f, as well as expression of the gene f in transgenic tobacco plants did not lead to visible morphological changes. Therefore, in contrast to gene e, the gene f seems not to be essential for tumour formation. In order to study whether gene f is an active gene, its expression in agrobacteria and plants was monitored by translational lacZ fusion. In planta, the putative gene f-promoter mediates a tissue-specific expression pattern. Although gene f was expressed in free-living agrobacteria as well as in transgenic plants, the function of the f locus remained unclear. DNA homology studies with the f gene region revealed a mosaic-like DNA structure, indicating that this locus might be the result of genetic exchanges between different Agrobacterium strains during evolution.

Agrobacterium vitis nopaline Ti plasmid pTiAB4: relationship to other Ti plasmids and T-DNA structure

The Ti plasmid of the Agrobacterium vitis nopaline-type strain AB4 was subcloned and mapped. Several regions of the 157 kb Ti plasmid are similar or identical to parts of the A. vitis octopine/cucumopine (o/c)-type Ti plasmids, and other regions are homologous to the nopaline-type Ti plasmid pTiC58. The T-DNA of pTiAB4 is a chimaeric structure of recent origin: the left part is 99.2% homologous to the left part of the TA-DNA of the o/c-type Ti plasmids, while the right part is 97.1% homologous to the right part of an unusual nopaline T-DNA recently identified in strain 82.139, a biotype II strain from wild cherry. The 3' noncoding regions of the ipt genes from pTiAB4 and pTi82.139 are different from those of other ipt genes and contain a 62 bp fragment derived from the coding sequence of an ipt gene of unknown origin. A comparison of different ipt gene sequences indicates that the corresponding 62 bp sequence within the coding region of the AB4 ipt gene has been modified during the course of its evolution, apparently by sequence transfer from the 62 bp sequence in the 3' non-coding region. In pTi82.139 the original coding region of the ipt gene has remained largely unmodified. The pTiAB4 6b gene differs from its pTi82.139 counterpart by the lack of a 12 bp repeat in the 3' part of the coding sequence. This leads to the loss of four glutamic acid residues from a series of ten. In spite of these differences, the ipt and 6b genes of pTiAB4 are functional. Our results provide new insight into the evolution of Agrobacterium Ti plasmids and confirm the remarkable plasticity of these genetic elements. Possible implications for the study of bacterial phylogeny are discussed.

Oncogene arrangement in a shooty strain of Agrobacterium tumefaciens

The Agrobacterium tumefaciens nopaline strain 82.139 induces non-teratogenic shooty tumours on several plant species. We have determined the position of the T-region oncogenes in a 11.4 kb Xba I fragment which shows a general organization similar to its pTiC58 counterpart. Sequence analysis of the 4.7 kb right part of this fragment allowed us to identify the pTi82.139 ipt, 6b and nos coding sequences. pTi82.139 lacks the 6a gene, which lies between the ipt and 6b genes in pTiC58. The intervening region between the 6b and the nos genes contains an additional ORF with homology to ORF 21 (transcript 3') from the TR-DNA of octopine strain pTi15955.

Organization and functional analysis of three T-DNAs from the vitopine Ti plasmid pTiS4

The vitopine Ti plasmid pTiS4 of Agrobacterium vitis has an unusual T-DNA organization. The pTiS4 oncogenes, localized by screening selected pTiS4 clones for growth-inducing activity, are localized on three T-DNAs, whereas in all other characterized Ti plasmids one or two T-DNAs are found. The nucleotide sequences and predicted amino acid sequences of the pTiS4 oncogenes set them apart from the corresponding genes from other Ti or Ri plasmids. The oncogenes induce the same type of reaction on various test plants as the well-known pTiAch5 oncogenes but the pTiS4 ipt gene induces considerably more shoots than its Ach5 homologue. We have also identified the gene coding for vitopine synthase as well as a vitopine synthase pseudogene. Both sequences show homology to the octopine synthase gene. In terms of both nucleotide sequence and overall organization, the pTiS4 T-DNAs appear to be only distantly related to previously characterized T-DNAs.

Expression of a chimaeric heat-shock-inducible Agrobacterium 6b oncogene in Nicotiana rustica

The T-6b gene of Agrobacterium tumefaciens strain Tm4 induces tumours on Nicotiana rustica by an as yet unknown mechanism. These tumours cannot be regenerated into normal plants. To study the effect of the T-6b gene product on normal plant cells, the T-6b gene was placed under control of the Drosophila melanogaster hsp70 heat-shock promoter and introduced into N. rustica. Progeny of an hsp70-T-6b transformant developed into normal plants. The inducibility of the hsp70-T-6b construct was shown by northern analysis and by heat-shock-dependent growth alterations on the level of whole seedlings. Upon wounding at normal temperature conditions hsp70-T-6b plants formed small tumours on leaves and stems. Grafts between transformed plants and normal plants led to a wound callus which remained limited to transformed tissues, indicating that the T-6b gene product does not diffuse. Protoplasts of hsp70-T-6b plants divided in the same way as control protoplasts under standard culture conditions. However, when protoplast cultures were started in the absence of hormones, normal cells rapidly lost their sensitivity towards hormones, whereas hsp70-T-6b cells remained sensitive for a significantly longer period. Thus, the T-6b gene product alters hormone sensitivity during the initial phases of protoplast culture.

35S-beta-glucuronidase gene blocks biological effects of cotransferred iaa genes

The iaaM and iaaH genes of Agrobacterium tumefaciens and Agrobacterium rhizogenes play an important role in crown gall and hairy root disease. The iaaM gene codes for tryptophan monooxygenase which converts tryptophan into indole-3-acetamide (IAM). IAM is converted into the auxin indole-3-acetic acid (IAA) by indoleacetamide hydrolase, encoded by the iaaH gene. In functional studies on the activity of the iaa genes of the TB region of the A. tumefaciens biotype III strain Tm4, the frequently used 35S-beta-glucuronidase (35S-UidA or GUS) marker gene was found to inhibit IAA synthesis and root induction encoded by the TB iaa genes. To exert this inhibition, the 35S-UidA gene must be cotransferred with the iaaH gene. The 35S promoter alone is sufficient to cause the inhibitory effect.

Organization of the agropine synthesis region of the T-DNA of the Ri plasmid from Agrobacterium rhizogenes

The agropine/mannopine synthesis region of the TR region of the Ri plasmid of Agrobacterium rhizogenes strain A4 was localized on the basis of sequence similarity with probes from Ti plasmids of Agrobacterium tumefaciens and analysis of transposon insertions. The nucleotide sequence of the right part of the TR-DNA of pRiA4, encompassing the three genes involved in mannityl-opine synthesis, was determined and compared to the sequence of the corresponding region of the octopine-type Ti plasmid pTi15955. The organization of this region is strongly conserved between Ri and Ti plasmids, but the similarity is restricted to the coding sequences: no homology was detected in the 5' and 3' flanking sequences. The mas1' and ags proteins are the most conserved, showing more than 68% amino acid conservation, whereas the mas2' proteins are only 59% identical. Significant G/C content and codon usage differences are observed between pTi15955 and pRiA4. An open reading frame strongly similar to that of bacterial repressors is situated immediately to the right of the TR region.

Agrobacterium tumefaciens T-DNA gene 6b stimulates rol-induced root formation, permits growth at high auxin concentrations and increases root size

All Agrobacterium tumefaciens strains studied up to now transfer an active 6b gene to plant cells. However, the role of this gene in natural tumour induction is unknown. Various effects of 6b on plant cell growth have been described, but the precise mechanism by which 6b causes these effects has not been elucidated. Earlier experiments indicated that the 6b gene might increase auxin sensitivity as do the A. rhizogenes rol genes. The 6b gene from Tm4 (T-6b) was therefore compared with the rolB and rolABC genes. Although T-6b was unable to induce root formation, it strongly interfered with root induction and root elongation. In rolABC/T-6b coinfection experiments on carrots, T-6b-transformed cells stimulated root outgrowth of rolABC-transformed cells, indicating that the biologically active T-6b product is diffusible. Carrot rolABC roots containing the T-6b gene rapidly developed into unorganized calli. Nicotiana rustica roots with rolABC and T-6b continued their development, but became very large. Fragments of such roots formed callus at alpha-naphthaleneacetic acid concentrations which inhibited growth of rolABC and normal root fragments, suggesting that the role of 6b genes in natural tumour induction may be to reduce the inhibitory effects of high auxin levels and to keep cells in an undifferentiated state.