Genetic analysis of crown gall: fine structure map of the T-DNA by site-directed mutagenesis

Agrobacterium tumefaciens: a Transformative Agent for Fundamental Insights into Host-Microbe Interactions, Genome Biology, Chemical Signaling, and Cell Biology

Agrobacterium tumefaciens incites the formation of readily visible macroscopic structures known as crown galls on plant tissues that it infects. Records from biologists as early as the 17th century noted these unusual plant growths and began examining the basis for their formation. These studies eventually led to isolation of the infectious agent, A. tumefaciens, and decades of study revealed the remarkable mechanisms by which A. tumefaciens causes crown gall through stable horizontal genetic transfer to plants. This fundamental discovery generated a barrage of applications in the genetic manipulation of plants that is still under way. As a consequence of the intense study of A. tumefaciens and its role in plant disease, this pathogen was developed as a model for the study of critical processes that are shared by many bacteria, including host perception during pathogenesis, DNA transfer and toxin secretion, bacterial cell-cell communication, plasmid biology, and more recently, asymmetric cell biology and composite genome coordination and evolution. As such, studies of A. tumefaciens have had an outsized impact on diverse areas within microbiology and plant biology that extend far beyond its remarkable agricultural applications. In this review, we attempt to highlight the colorful history of A. tumefaciens as a study system, as well as current areas that are actively demonstrating its value and utility as a model microorganism.

The Mystery of Cancer Resistance: A Revelation Within Nature

Cancer, a disease due to uncontrolled cell proliferation is as ancient as multicellular organisms. A 255-million-years-old fossilized forerunner mammal gorgonopsian is probably the oldest evidence of cancer, to date. Cancer seems to have evolved by adapting to the microenvironment occupied by immune sentinel, modulating the cellular behavior from cytotoxic to regulatory, acquiring resistance to chemotherapy and surviving hypoxia. The interaction of genes with environmental carcinogens is central to cancer onset, seen as a spectrum of cancer susceptibility among human population. Cancer occurs in life forms other than human also, although their exposure to environmental carcinogens can be different. Role of genetic etiology in cancer in multiple species can be interesting with regard to not only cancer susceptibility, but also genetic conservation and adaptation in speciation. The widely used model organisms for cancer research are mouse and rat which are short-lived and reproduce rapidly. Research in these cancer prone animal models has been valuable as these have led to cancer therapy. However, another rewarding area of cancer research can be the cancer-resistant animal species. The Peto's paradox and G-value paradox are evident when natural cancer resistance is observed in large mammals, like elephant and whale, small rodents viz. Naked Mole Rat and Blind Mole Rat, and Bat. The cancer resistance remains to be explored in other small or large and long-living animals like giraffe, camel, rhinoceros, water buffalo, Indian bison, Shire horse, polar bear, manatee, elephant seal, walrus, hippopotamus, turtle and tortoise, sloth, and squirrel. Indeed, understanding the molecular mechanisms of avoiding neoplastic transformation across various life forms can be potentially having translational value for human cancer management. Adapted and Modified from (Hanahan and Weinberg 2011).

The Dawn of Plant Molecular Biology: How Three Key Methodologies Paved the Way

The adoption of Arabidopsis thaliana in the 1980s as a universal plant model finally enabled researchers to adopt and take full advantage of the molecular biology tools and methods developed in the bacterial and animal fields since the early 1970s. It further brought the plant sciences up to speed with other research fields, which had been employing widely accepted model organisms for decades. In parallel with this major development, the concurrent establishment of the plant transformation methodology and the description of the cauliflower mosaic virus (CaMV) 35S promoter enabled scientists to create robust transgenic plant lines for the first time, thereby providing a valuable tool for studying gene function. The ability to create transgenic plants launched the plant biotechnology sector, with Monsanto and Plant Genetic Systems developing the first herbicide- and pest-tolerant plants, initiating a revolution in the agricultural industry. Here I review the major developments over a less than 10-year span and demonstrate how they complemented each other to trigger a revolution in plant molecular biology and launch an era of unprecedented progress for the whole plant field. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

Opine biosynthesis in naturally transgenic plants: Genes and products

The plant pathogen Agrobacterium transfers DNA into plant cells by a specific transfer mechanism. Expression of this transferred DNA or T-DNA leads to crown gall tumors or abnormal, hairy roots and the synthesis of specific compounds, called opines. Opines are produced from common plant metabolites like sugars, amino acids and α-keto acids, which are combined into different low molecular weight structures by T-DNA-encoded opine synthase enzymes. Opines can be converted back by Agrobacterium into the original metabolites and used for agrobacterial growth. Recently it has been discovered that about 7% of Angiosperms carry T-DNA-like sequences. These result from ancient Agrobacterium transformation events, followed by spontaneous regeneration of transformed cells into natural genetically transformed organisms (nGMOs). Nearly all nGMOs identified up to date carry opine synthesis genes, several of these are intact and potentially encode opine synthesis. So far, only tobacco and cuscuta have been demonstrated to contain opines. Whereas opines from crown gall and hairy root tissues have been studied for over 60 years, those from the nGMOs remain to be explored.

Mapping Reaction-Diffusion Networks at the Plant Wound Site With Pathogens

The rich collection of microbes colonizing the plant root making up the rhizosphere function as a multigenomic organ for nutrient distribution. The extent to which its dynamic mutualistic cellular order depends on morphogenic signaling, while likely, remains unknown. We have shown that reaction-diffusion chemical networks constructed with model plant and bacterial metabolites can mimic processes ranging from oxidative burst kinetics to traveling waves and extracellular stationary state reaction-diffusion networks for spatiotemporal ordering of the rhizosphere. Plant parasites and pathogens can be limited by host attachment require dynamic informational networks and continue to provide insight into what controls the rhizosphere. Here we take advantage of Agrobacterium tumefaciens, a plant pathogen with a gated receptor that requires simultaneous perception of two plant metabolites. Genetic manipulations have created receptors allowing each metabolite concentration to be correlated with pathogen behavior. The development of the florescent strains used here provide initial maps of the reaction-diffusion dynamics existing in the rhizosphere, revealing significant differences in the signaling landscape of host and non-host plants before and after wounding, specifically highlighting networks that may inform rhizosphere organization.

Plant tumors: a hundred years of study

The review provides information on the mechanisms underlying the development of spontaneous and pathogen-induced tumors in higher plants. The activation of meristem-specific regulators in plant tumors of various origins suggests the meristem-like nature of abnormal plant hyperplasia. Plant tumor formation has more than a century of research history. The study of this phenomenon has led to a number of important discoveries, including the development of the Agrobacterium-mediated transformation technique and the discovery of horizontal gene transfer from bacteria to plants. There are two main groups of plant tumors: pathogen-induced tumors (e.g., tumors induced by bacteria, viruses, fungi, insects, etc.), and spontaneous ones, which are formed in the absence of any pathogen in plants with certain genotypes (e.g., interspecific hybrids, inbred lines, and mutants). The causes of the transition of plant cells to tumor growth are different from those in animals, and they include the disturbance of phytohormonal balance and the acquisition of meristematic characteristics by differentiated cells. The aim of this review is to discuss the mechanisms underlying the development of most known examples of plant tumors.

Presence of an Agrobacterium-Type Tumor-Inducing Plasmid in Neorhizobium sp. NCHU2750 and the Link to Phytopathogenicity

The genus Agrobacterium contains a group of plant-pathogenic bacteria that have been developed into an important tool for genetic transformation of eukaryotes. To further improve this biotechnology application, a better understanding of the natural genetic variation is critical. During the process of isolation and characterization of wild-type strains, we found a novel strain (i.e., NCHU2750) that resembles Agrobacterium phenotypically but exhibits high sequence divergence in several marker genes. For more comprehensive characterization of this strain, we determined its complete genome sequence for comparative analysis and performed pathogenicity assays on plants. The results demonstrated that this strain is closely related to Neorhizobium in chromosomal organization, gene content, and molecular phylogeny. However, unlike the characterized species within Neorhizobium, which all form root nodules with legume hosts and are potentially nitrogen-fixing mutualists, NCHU2750 is a gall-forming pathogen capable of infecting plant hosts across multiple families. Intriguingly, this pathogenicity phenotype could be attributed to the presence of an Agrobacterium-type tumor-inducing plasmid in the genome of NCHU2750. These findings suggest that these different lineages within the family Rhizobiaceae are capable of transitioning between ecological niches by having novel combinations of replicons. In summary, this work expanded the genomic resources available within Rhizobiaceae and provided a strong foundation for future studies of this novel lineage. With an infectivity profile that is different from several representative Agrobacterium strains, this strain may be useful for comparative analysis to better investigate the genetic determinants of host range among these bacteria.

Agrobacterial tumors interfere with nodulation and demonstrate the expression of nodulation-induced CLE genes in pea

CLAVATA (CLV) system including CLV1-like kinase and CLE-peptides is the part of the AON (autoregulation of nodulation) that controls nodule number in legume plants. Moreover, CLV system plays a key role in meristems, where it regulates the expression of WOX genes in organizing centers. Recently, we found that WOX5 homolog in pea is also expressed in nodules and in tumors induced by Agrobacterium tumefaciens. Based on this, we hypothesized that both nodules and agrobacterial tumors may be regulated by and may trigger the same components of AON, including the same WOX and CLV genes. Here, we found that pea plants with agrobacterial tumors induced prior to rhizobial inoculation had reduced the number of nodules. This effect was absent in pea sym29 mutant defective in CLV1-like kinase, the key component of AON. That suggests that agrobacterial tumors may produce a signal activating CLV1-like kinase and thereby decrease the nodule number. Since CLE peptides are known to act upstream of CLV1-like kinase, expression analysis of CLE genes has been performed both in developing nodules and tumors. Overall, 45 CLE genes were identified, and among them nine nodulation-induced CLEs were found in pea. In agrobacterial tumors, no expression of nodule-specific CLE genes the homologues of which inhibit nodulation in other legumes was observed. However, increased expression of two other nodulation-induced CLE genes was observed in agrobacterial tumors, suggesting that CLE genes are expressed in tumors that may still contribute to autoregulatory processes suppressing nodulation.

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.

Use of chimeric type IV secretion systems to define contributions of outer membrane subassemblies for contact-dependent translocation

Recent studies have shown that conjugation systems of Gram-negative bacteria are composed of distinct inner and outer membrane core complexes (IMCs and OMCCs, respectively). Here, we characterized the OMCC by focusing first on a cap domain that forms a channel across the outer membrane. Strikingly, the OMCC caps of the Escherichia coli pKM101 Tra and Agrobacterium tumefaciens VirB/VirD4 systems are completely dispensable for substrate transfer, but required for formation of conjugative pili. The pKM101 OMCC cap and extended pilus also are dispensable for activation of a Pseudomonas aeruginosa type VI secretion system (T6SS). Chimeric conjugation systems composed of the IMC_pKM101 joined to OMCCs from the A. tumefaciens VirB/VirD4, E. coli R388 Trw, and Bordetella pertussis Ptl systems support conjugative DNA transfer in E. coli and trigger P. aeruginosa T6SS killing, but not pilus production. The A. tumefaciens VirB/VirD4 OMCC, solved by transmission electron microscopy, adopts a cage structure similar to the pKM101 OMCC. The findings establish that OMCCs are highly structurally and functionally conserved - but also intrinsically conformationally flexible - scaffolds for translocation channels. Furthermore, the OMCC cap and a pilus tip protein coregulate pilus extension but are not required for channel assembly or function.

Direct fluorescence detection of VirE2 secretion by Agrobacterium tumefaciens

VirE2 is a ssDNA binding protein essential for virulence in Agrobacterium tumefaciens. A tetracysteine mutant (VirE2-TC) was prepared for in vitro and in vivo fluorescence imaging based on the ReAsH reagent. VirE2-TC was found to be biochemically active as it binds both ssDNA and the acidic secretion chaperone VirE1. It was also biologically functional in complementing virE2 null strains transforming Arabidopsis thaliana roots and Nicotiana tabacum leaves. In vitro experiments demonstrated a two-color fluorescent complex using VirE2-TC/ReAsH and Alexa Fluor 488 labeled ssDNA. In vivo, fluorescent VirE2-TC/ReAsH was detected in bacteria and in plant cells at time frames relevant to transformation.

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.

A new QRT-PCR assay designed for the differentiation between elements provided from Agrobacterium sp. in GMOs plant events and natural Agrobacterium sp. bacteria

The question asked in the present work was how to differentiate between contamination of field samples with and GM plants contained sequences provided from this bacterium in order to avoid false positives in the frame of the detection and the quantification of GMO. For this, new set of primers and corresponding TaqMan Minor Groove Binder (MGB) probes were designed to target Agrobacterium sp. using the tumor-morphology-shooty gene (TMS1). Final standard curves were calculated for each pathogen by plotting the threshold cycle value against the bacterial number (log (colony forming units) per milliliter) via linear regression. The method designed was highly specific and sensitive, with a detection limit of 10CFU/ml. No significant cross-reaction was observed. Results from this study showed that TaqMan real-time PCR, is potentially an effective method for the rapid and reliable quantification of Agrobacterium sp. in samples containing GMO or non GMO samples.

Regulation of oncogene expression in T-DNA-transformed host plant cells

Virulent Agrobacterium tumefaciens strains integrate their T-DNA into the plant genome where the encoded agrobacterial oncogenes are expressed and cause crown gall disease. Essential for crown gall development are IaaH (indole-3-acetamide hydrolase), IaaM (tryptophan monooxygenase) and Ipt (isopentenyl transferase), which encode enzymes for the biosynthesis of auxin (IaaH, IaaM) and cytokinin (Ipt). Although these oncogenes are well studied as the tumor-inducing principle, nothing is known about the regulation of oncogene expression in plant cells. Our studies show that the intergenic regions (IGRs) between the coding sequences (CDS) of the three oncogenes function as promoters in plant cells. These promoters possess a eukaryotic sequence organization and cis-regulatory elements for the binding of plant transcription factors. WRKY18, WRKY40, WRKY60 and ARF5 were identified as activators of the Ipt promoter whereas IaaH and IaaM is constitutively expressed and no transcription factor further activates their promoters. Consistent with these results, the wrky triple mutant plants in particular, develops smaller crown galls than wild-type and exhibits a reduced Ipt transcription, despite the presence of an intact ARF5 gene. WRKY40 and WRKY60 gene expression is induced by A. tumefaciens within a few hours whereas the ARF5 gene is transcribed later during crown gall development. The WRKY proteins interact with ARF5 in the plant nucleus, but only WRKY40 together with ARF5 synergistically boosts the activation of the Ipt promoter in an auxin-dependent manner. From our data, we propose that A. tumefaciens initially induces WRKY40 gene expression as a pathogen defense response of the host cell. The WRKY protein is recruited to induce Ipt expression, which initiates cytokinin-dependent host cell division. With increasing auxin levels triggered by ubiquitous expression of IaaH and IaaM, ARF5 is activated and interacts with WRKY40 to potentiate Ipt expression and balance cytokinin and auxin levels for further cell proliferation.

Crown gall development requires the expression of agrobacterial genes in the plant host. These genes are transferred by the T-DNA of the plant pathogen Agrobacterium tumefaciens and include the oncogenes IaaH, IaaM and Ipt, which, according to the tumor-inducing principle, are essential for crown gall development. The oncogenes are involved in auxin and cytokinin production. This results, when at appropriate hormone ratios, in enhanced cell proliferation. The T-DNA transformation process and the encoded oncogene enzymes have been intensively studied, but knowledge of oncogene expression in plant cells and the regulatory host factors is missing. We set out to fill this gap, providing evidence that expression of the Ipt gene is host-cell controlled, whereas the IaaH and IaaM genes are ubiquitously expressed at low levels in T-DNA transformed tissue. This is achieved by A. tumefaciens, which first hijacks transcription factors of the plant pathogen response pathway to activate Ipt oncogene expression and initiates cell proliferation. With increasing auxin levels during the infection process, a transcription factor of the auxin-signaling pathway is recruited, potentiating Ipt gene expression. Thus, for crown gall development, two host-signaling pathways are combined through the interaction of transcription factors that adjust the ratio of cytokinin to auxin.

Initiation of spontaneous tumors in radish (Raphanus sativus): Cellular, molecular and physiological events

In plant meristems, the balance of cell proliferation and differentiation is maintained by phytohormones, specifically auxin and cytokinin, as well as transcription factors. Changing of the cytokinin/auxin balance in plants may lead to developmental abnormalities, and in particular, to the formation of tumors. The examples of spontaneous tumor formation in plants include tumors formed on the roots of radish (Raphanus sativus) inbred lines. Previously, it was found that the cytokinin/auxin ratio is altered in radish tumors. In this study, a detailed histological analysis of spontaneous radish tumors was performed, revealing a possible mechanism of tumor formation, namely abnormal cambial activity. The analysis of cell proliferation patterns revealed meristematic foci in radish tumors. By using a fusion of an auxin-responsive promoter (DR5) and a reporter gene, the involvement of auxin in developmental processes in tumors was shown. In addition, the expression of the root meristem-specific WUSCHEL-related homeobox 5 (WOX5) gene was observed in cells adjacent to meristematic foci. Taken together, the results of the present study show that tumor tissues share some characteristics with root apical meristems, including the presence of auxin-response maxima in meristematic foci with adjacent cells expressing WOX5.

Ecological dynamics and complex interactions of Agrobacterium megaplasmids

As with many pathogenic bacteria, agrobacterial plant pathogens carry most of their virulence functions on a horizontally transmissible genetic element. The tumor-inducing (Ti) plasmid encodes the majority of virulence functions for the crown gall agent Agrobacterium tumefaciens. This includes the vir genes which drive genetic transformation of host cells and the catabolic genes needed to utilize the opines produced by infected plants. The Ti plasmid also encodes, an opine-dependent quorum sensing system that tightly regulates Ti plasmid copy number and its conjugal transfer to other agrobacteria. Many natural agrobacteria are avirulent, lacking the Ti plasmid. The burden of harboring the Ti plasmid depends on the environmental context. Away from diseased hosts, plasmid costs are low but the benefit of the plasmid is also absent. Consequently, plasmidless genotypes are favored. On infected plants the costs of the Ti plasmid can be very high, but balanced by the opine benefits, locally favoring plasmid bearing cells. Cheating derivatives which do not incur virulence costs but can benefit from opines are favored on infected plants and in most other environments, and these are frequently isolated from nature. Many agrobacteria also harbor an At plasmid which can stably coexist with a Ti plasmid. At plasmid genes are less well characterized but in general facilitate metabolic activities in the rhizosphere and bulk soil, such as the ability to breakdown plant exudates. Examination of A. tumefaciens C58, revealed that harboring its At plasmid is much more costly than harboring it's Ti plasmid, but conversely the At plasmid is extremely difficult to cure. The interactions between these co-resident plasmids are complex, and depend on environmental context. However, the presence of a Ti plasmid appears to mitigate At plasmid costs, consistent with the high frequency with which they are found together.

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.

Historical account on gaining insights on the mechanism of crown gall tumorigenesis induced by Agrobacterium tumefaciens

The plant tumor disease known as crown gall was not called by that name until more recent times. Galls on plants were described by Malpighi (1679) who believed that these extraordinary growth are spontaneously produced. Agrobacterium was first isolated from tumors in 1897 by Fridiano Cavara in Napoli, Italy. After this bacterium was recognized to be the cause of crown gall disease, questions were raised on the mechanism by which it caused tumors on a variety of plants. Numerous very detailed studies led to the identification of Agrobacterium tumefaciens as the causal bacterium that cleverly transferred a genetic principle to plant host cells and integrated it into their chromosomes. Such studies have led to a variety of sophisticated mechanisms used by this organism to aid in its survival against competing microorganisms. Knowledge gained from these fundamental discoveries has opened many avenues for researchers to examine their primary organisms of study for similar mechanisms of pathogenesis in both plants and animals. These discoveries also advanced the genetic engineering of domesticated plants for improved food and fiber.

Plant responses to Agrobacterium tumefaciens and crown gall development

Agrobacterium tumefaciens causes crown gall disease on various plant species by introducing its T-DNA into the genome. Therefore, Agrobacterium has been extensively studied both as a pathogen and an important biotechnological tool. The infection process involves the transfer of T-DNA and virulence proteins into the plant cell. At that time the gene expression patterns of host plants differ depending on the Agrobacterium strain, plant species and cell-type used. Later on, integration of the T-DNA into the plant host genome, expression of the encoded oncogenes, and increase in phytohormone levels induce a fundamental reprogramming of the transformed cells. This results in their proliferation and finally formation of plant tumors. The process of reprogramming is accompanied by altered gene expression, morphology and metabolism. In addition to changes in the transcriptome and metabolome, further genome-wide ("omic") approaches have recently deepened our understanding of the genetic and epigenetic basis of crown gall tumor formation. This review summarizes the current knowledge about plant responses in the course of tumor development. Special emphasis is placed on the connection between epigenetic, transcriptomic, metabolomic, and morphological changes in the developing tumor. These changes not only result in abnormally proliferating host cells with a heterotrophic and transport-dependent metabolism, but also cause differentiation and serve as mechanisms to balance pathogen defense and adapt to abiotic stress conditions, thereby allowing the coexistence of the crown gall and host plant.

Agrobacterium: nature's genetic engineer

Agrobacterium was identified as the agent causing the plant tumor, crown gall over 100 years ago. Since then, studies have resulted in many surprising observations. Armin Braun demonstrated that Agrobacterium infected cells had unusual nutritional properties, and that the bacterium was necessary to start the infection but not for continued tumor development. He developed the concept of a tumor inducing principle (TIP), the factor that actually caused the disease. Thirty years later the TIP was shown to be a piece of a tumor inducing (Ti) plasmid excised by an endonuclease. In the next 20 years, most of the key features of the disease were described. The single-strand DNA (T-DNA) with the endonuclease attached is transferred through a type IV secretion system into the host cell where it is likely coated and protected from nucleases by a bacterial secreted protein to form the T-complex. A nuclear localization signal in the endonuclease guides the transferred strand (T-strand), into the nucleus where it is integrated randomly into the host chromosome. Other secreted proteins likely aid in uncoating the T-complex. The T-DNA encodes enzymes of auxin, cytokinin, and opine synthesis, the latter a food source for Agrobacterium. The genes associated with T-strand formation and transfer (vir) map to the Ti plasmid and are only expressed when the bacteria are in close association with a plant. Plant signals are recognized by a two-component regulatory system which activates vir genes. Chromosomal genes with pleiotropic functions also play important roles in plant transformation. The data now explain Braun's old observations and also explain why Agrobacterium is nature's genetic engineer. Any DNA inserted between the border sequences which define the T-DNA will be transferred and integrated into host cells. Thus, Agrobacterium has become the major vector in plant genetic engineering.

Large deletions in the pAtC58 megaplasmid of Agrobacterium tumefaciens can confer reduced carriage cost and increased expression of virulence genes

The accessory plasmid pAtC58 of the common laboratory strain of Agrobacterium tumefaciens confers numerous catabolic functions and has been proposed to play a role in virulence. Genomic sequencing of evolved laboratory strains of A. tumefaciens revealed the presence of multiple deletion events in the At plasmid, with reductions in plasmid size ranging from 25% to 30% (115–194 kb). Flanking both ends of the sites of these deletions is a short-nucleotide repeat sequence that is in a single copy in the deleted plasmids, characteristic of a phage- or transposon-mediated deletion event. This repeat sequence is widespread throughout the C58 genome, but concentrated on the At plasmid, suggesting its frequency to be nonrandom. In this study, we assess the prevalence of the larger of these deletions in multiple C58 derivatives and characterize its functional significance. We find that in addition to elevating virulence gene expression, this deletion is associated with a significantly reduced carriage cost to the cell. These observations are a clear demonstration of the dynamic nature of the bacterial genome and suggest a mechanism for genetic plasticity of these costly but otherwise stable plasmids. Additionally, this phenomenon could be the basis for some of the dramatic recombination events so ubiquitous within and among megaplasmids.

Silencing Agrobacterium oncogenes in transgenic grapevine results in strain-specific crown gall resistance

KEY MESSAGE

Grapevine rootstock transformed with an Agrobacterium oncogene-silencing transgene was resistant to certain Agrobacterium strains but sensitive to others. Thus, genetic diversity of Agrobacterium oncogenes may limit engineering crown gall resistance.

ABSTRACT

Crown gall disease of grapevine induced by Agrobacterium vitis or Agrobacterium tumefaciens causes serious economic losses in viticulture. To establish crown gall-resistant lines, somatic proembryos of Vitis berlandieri × V. rupestris cv. 'Richter 110' rootstock were transformed with an oncogene-silencing transgene based on iaaM and ipt oncogene sequences from octopine-type, tumor-inducing (Ti) plasmid pTiA6. Twenty-one transgenic lines were selected, and their transgenic nature was confirmed by polymerase chain reaction (PCR). These lines were inoculated with two A. tumefaciens and three A. vitis strains. Eight lines showed resistance to octopine-type A. tumefaciens A348. Resistance correlated with the expression of the silencing genes. However, oncogene silencing was mostly sequence specific because these lines did not abolish tumorigenesis by A. vitis strains or nopaline-type A. tumefaciens C58.

The role of RAR1 in Agrobacterium-mediated plant transformation

RAR 1 is identified as a critical protein involved in plant innate immunity. We investigated the role of RAR 1 in Agrobacterium-mediated plant transformation based on the previous findings that accessory proteins associated with the E3 ligase complex such as SGT1, which tightly interacts with RAR 1, play a role in the transformation process. RAR1 gene silencing in Nicotiana benthamiana and Arabidopsis rar1 mutant analyses suggested that RAR1 is required for early stages of Agrobacterium-mediated plant transformation. This finding further illustrates that RAR 1, along with SGT1, that serve as a HSP90 co-chaperone is important for Agrobacterium-mediated plant transformation.

The role of RAR1 in Agrobacterium-mediated plant transformation

RAR 1 is identified as a critical protein involved in plant innate immunity. We investigated the role of RAR 1 in Agrobacterium-mediated plant transformation based on the previous findings that accessory proteins associated with the E3 ligase complex such as SGT1, which tightly interacts with RAR 1, play a role in the transformation process. RAR1 gene silencing in Nicotiana benthamiana and Arabidopsis rar1 mutant analyses suggested that RAR1 is required for early stages of Agrobacterium-mediated plant transformation. This finding further illustrates that RAR 1, along with SGT1, that serve as a HSP90 co-chaperone is important for Agrobacterium-mediated plant transformation.

Ectopic localization of auxin and cytokinin in tobacco seedlings by the plant-oncogenic AK-6b gene of Agrobacterium tumefaciens AKE10

The oncogenic 6b gene of Agrobacterium tumefaciens induces a number of morphological and metabolic alterations in plants. Although molecular functions associated with the 6b genes have been proposed, including auxin transport, sugar transport, transcriptional regulation, and miRNA metabolism, so far an unequivocal conclusion has not been obtained. We investigated the association between auxin accumulation and tumor development of the tobacco seedlings expressing the AK-6b gene under the control of the dexamethasone-inducible promoter. Indole-3-acetic acid (IAA) localization was examined by immunochemical staining with monoclonal antibody against IAA and by histochemical analysis using the IAA-specific induced construct, DR5::GUS (β-glucuronidase). Both procedures indicated that IAA preferentially accumulated in the tumorous protrusions as well as in newly developing vascular bundles in the tumors. Furthermore, true leaves also showed abaxial IAA localization, leading to altered leaves in which the adaxial and abaxial identities were no longer evident. Co-localization of cytokinin and auxin in the abaxial tumors was verified by immunochemical staining with an antibody against cytokinin. Treatment of AK-6b-seedlings with N-1-naphthylphthalamic acid, an inhibitor of polar auxin transport, promoted the morphological severity of phenotypes, whereas 1-naphthoxyacetic acid, a specific auxin influx carrier inhibitor, induced tumor regression on cotyledons and new tumorous proliferations on hypocotyls. Prominent accumulation of both auxin and cytokinin was observed in both regressed and newly developing tumors. We suggest from these results that modulation of auxin/cytokinin localization as a result of AK-6b gene expression is responsible for the tumorous proliferation.

A putative transmembrane leucine zipper of agrobacterium VirB10 is essential for t-pilus biogenesis but not type IV secretion

The Agrobacterium tumefaciens VirB/VirD4 type IV secretion system is composed of a translocation channel and an extracellular T pilus. Bitopic VirB10, the VirB7 lipoprotein, and VirB9 interact to form a cell envelope-spanning structural scaffold termed the "core complex" that is required for the assembly of both structures. The related pKM101-encoded core complex is composed of 14 copies each of these VirB homologs, and the transmembrane (TM) α helices of VirB10-like TraF form a 55-Å-diameter ring at the inner membrane. Here, we report that the VirB10 TM helix possesses two types of putative dimerization motifs, a GxxxA (GA4) motif and two leucine (Leu1, Leu2) zippers. Mutations in the Leu1 motif disrupted T-pilus biogenesis, but these or other mutations in the GA4 or Leu2 motif did not abolish substrate transfer. Replacement of the VirB10 TM domain with a nondimerizing poly-Leu/Ala TM domain sequence also blocked pilus production but not substrate transfer or formation of immunoprecipitable complexes with the core subunits VirB7 and VirB9 and the substrate receptor VirD4. The VirB10 TM helix formed weak homodimers in Escherichia coli, as determined with the TOXCAT assay, whereas replacement of the VirB10 TM helix with the strongly dimerizing TM helix from glycophorin A blocked T-pilus biogenesis in A. tumefaciens. Our findings support a model in which VirB10's TM helix contributes to the assembly or activity of the translocation channel as a weakly self-interacting membrane anchor but establishes a heteromeric TM-TM helix interaction via its Leu1 motif that is critical for T-pilus biogenesis.

Several components of SKP1/Cullin/F-box E3 ubiquitin ligase complex and associated factors play a role in Agrobacterium-mediated plant transformation

• Successful genetic transformation of plants by Agrobacterium tumefaciens requires the import of bacterial T-DNA and virulence proteins into the plant cell that eventually form a complex (T-complex). The essential components of the T-complex include the single stranded T-DNA, bacterial virulence proteins (VirD2, VirE2, VirE3 and VirF) and associated host proteins that facilitate the transfer and integration of T-DNA. The removal of the proteins from the T-complex is likely achieved by targeted proteolysis mediated by VirF and the plant ubiquitin proteasome complex. • We evaluated the involvement of the host SKP1/culin/F-box (SCF)-E3 ligase complex and its role in plant transformation. Gene silencing, mutant screening and gene expression studies suggested that the Arabidopsis homologs of yeast SKP1 (suppressor of kinetochore protein 1) protein, ASK1 and ASK2, are required for Agrobacterium-mediated plant transformation. • We identified the role for SGT1b (suppressor of the G2 allele of SKP1), an accessory protein that associates with SCF-complex, in plant transformation. We also report the differential expression of many genes that encode F-box motif containing SKP1-interacting proteins (SKIP) upon Agrobacterium infection. • We speculate that these SKIP genes could encode the plant specific F-box proteins that target the T-complex associated proteins for polyubiquitination and subsequent degradation by the 26S proteasome.

Antibegomoviral activity of the agrobacterial virulence protein VirE2

Mungbean yellow mosaic geminivirus (MYMV) causes severe yellow mosaic disease in blackgram, mungbean, Frenchbean, pigeonpea, soybean and mothbean. We attempted to induce resistance against this virus using the transcriptional activator protein gene deleted in the C-terminal activation domain (TrAP-∆AD) and Agrobacterium tumefaciens virE2. MYMV is known to replicate in agroinoculated tobacco leaf discs. Three transgenic tobacco plants which harboured a truncated MYMV transcriptional activator protein gene and two tobacco plants transformed with the octopine type A. tumefaciens virE2 gene were agroinoculated with an A. tumefaciens strain which harboured the partial dimers of both DNA A and DNA B of MYMV. The level of viral DNA accumulation in leaf discs of transgenic plants correlated inversely to the level of the MYMV TrAP-∆AD transcript. Two VirE2-transgenic plants, which complemented tumorigenesis of a virE2 mutant A. tumefaciens strain, effectively reduced MYMV DNA accumulation in the leaf disc agroinoculation assay.

An Arabidopsis thaliana ABC transporter that confers kanamycin resistance in transgenic plants does not endow resistance to Escherichia coli

Concerns have been raised about potential horizontal gene transfer (HGT) of antibiotic resistance markers (ARMs) from transgenic plants to bacteria of medical and environmental importance. All ARMs used in transgenic plants have been bacterial in origin, but it has been recently shown that an Arabidopsis thaliana ABC transporter, Atwbc19, confers kanamycin resistance when overexpressed in transgenic plants. Atwbc19 was evaluated for its ability to transfer kanamycin resistance to Escherichia coli, a kanamycin‐sensitive model bacterium, under simulated HGT, staged by subcloning Atwbc19 under the control of a bacterial promoter, genetically transforming to kanamycin‐sensitive bacteria, and assessing if resistance was conferred as compared with bacteria harbouring nptII, the standard kanamycin resistance gene used to produce transgenic plants. NptII provided much greater resistance than Atwbc19 and was significantly different from the no‐plasmid control at low concentrations. Atwbc19 was not significantly different from the no‐plasmid control at higher concentrations. Even though HGT risks are considered low with nptII, Atwbc19 should have even lower risks, as its encoded protein is possibly mistargeted in bacteria.

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.

Evidence for VirB4-mediated dislocation of membrane-integrated VirB2 pilin during biogenesis of the Agrobacterium VirB/VirD4 type IV secretion system

Agrobacterium VirB2 pilin is required for assembly of the VirB/VirD4 type IV secretion system (T4SS). The propilin is processed by signal sequence cleavage and covalent linkage of the N and C termini, and the cyclized pilin integrates into the inner membrane (IM) as a pool for assembly of the secretion channel and T pilus. Here, by use of the substituted cysteine accessibility method (SCAM), we defined the VirB2 IM topology and then identified distinct contributions of the T4SS ATPase subunits to the pilin structural organization. Labeling patterns of Cys-substituted pilins exposed to the membrane-impermeative, thiol-reactive reagent 3-(N-maleimidopropionyl)biocytin (MPB) supported a topology model in which two hydrophobic stretches comprise transmembrane domains, an intervening hydrophilic loop (residues 90 to 94) is cytoplasmic, and the hydrophilic N and C termini joined at residues 48 and 121 form a periplasmic loop. Interestingly, the VirB4 ATPase, but not a Walker A nucleoside triphosphate (NTP) binding motif mutant, induced (i) MPB labeling of Cys94, a residue that in the absence of the ATPase is located in the cytoplasmic loop, and (ii) release of pilin from the IM upon osmotic shock. These findings, coupled with evidence for VirB2-VirB4 complex formation by coimmunoprecipitation, support a model in which VirB4 functions as a dislocation motor to extract pilins from the IM during T4SS biogenesis. The VirB11 ATPase functioned together with VirB4 to induce a structural change in the pilin that was detectable by MPB labeling, suggestive of a role for VirB11 as a modulator of VirB4 dislocase activity.

Transgenic plants as tools to study the molecular organization of plant genes

Transgenic plants are generated in nature by Agrobacterium tumefaciens, a pathogen that produces disease through the transfer of some of its own DNA into susceptible plants. The genes are carried on a plasmid. Much has been learned about how the plasmid is transferred, how the plasmid-borne genes are organized, regulated, and expressed, and how the bacteria's pathogenic effects are produced. The A. tumefaciens plasmid has been manipulated for use as a general vector for the transfer of specific segments of foreign DNA of interest (from plants and other sources) into plants; the activities of various genes and their regulation by enhancer and silencer sequences have been assessed. Future uses of the vector (or others like it that have different host ranges) by the agriculture industry are expected to aid in moving into vulnerable plants specific genes that will protect them from such killers as nonselective herbicides, insects, and viruses.

Rhizobium meliloti genes required for nodule development are related to chromosomal virulence genes in Agrobacterium tumefaciens

Symbiotically essential genes have been identified in Rhizobium meliloti that are structurally and functionally related to chromosomal virulence (chv) genes of Agrobacterium tumefaciens. Homologous sequences also exist in the genomes of other fast-growing rhizobia including Rhizobium trifolii, Rhizobium leguminosarum, and Rhizobium phaseoli. In Agrobacterium, the chvA and chvB loci are known to be essential for oncogenic transformation of dicotyledonous plants and for attachment to plant cells [Douglas, C. J., Staneloni, R. J., Rubin, R. A. & Nester, E. W. (1985) J. Bacteriol. 64, 102-106], and the chvB locus has been implicated in the production of (1-->2)-beta-glucan, a unique exopolysaccharide component [Puvanesarajah, V., Schell, F. M., Stacey, G., Douglas, C. J. & Nester, E. W. (1985) J. Bacteriol. 164, 102-106]. Site-directed transposon insertion mutants in the chvA and chvB-equivalent regions of R. meliloti are symbiotically defective. Mutants in the chvB-equivalent region have been examined in detail and have been found to induce the formation of nodule-like structures on alfalfa that are devoid of bacteroids, lack infection threads, and cannot fix nitrogen. Such mutants fluoresce normally in the presence of Calcofluor, a histochemical stain for beta-linked polysaccharides, and produce normal amounts of total exopolysaccharide. The Rhizobium loci have been designated ndv because of their requirement for nodule development.

T-DNA organization in tumor cultures and transgenic plants of the monocotyledon Asparagus officinalis

Asparagus officinalis was the first monocotyledonous plant from which hormone-independent and opine-producing crown gall tissue could be isolated. We confirm by DNA hybridization that tumor lines obtained after infection of this plant by Agrobacterium strains harboring wild-type nopaline and octopine tumor-inducing (Ti) plasmids are stably transformed and contain transferred DNA (T-DNA) segments identical to the T-DNA found in dicotyledonous plants. We have also infected Asparagus with a nononcogenic T-DNA vector that carries a chimeric aminoglycoside phosphotransferase [NOS-APH(3')II] gene and selected transformed tissues on kanamycin-containing medium. The transformed status of these tissues was then confirmed by DNA hybridization. From these calli we regenerated kanamycin-resistant shoots that were subsequently rooted. Thus we report the isolation of transgenic monocotyledonous plants engineered via the Agrobacterium vector system.

Hairy roots are more sensitive to auxin than normal roots

Responses to auxin of Lotus corniculatus root tips or protoplasts transformed by Agrobacterium rhizogenes strains 15834 and 8196 were compared to those of their normal counterparts. Three different types of experiments were performed, involving long-term, medium-term, or short-term responses to a synthetic auxin, 1-naphthaleneacetic acid. Root tip elongation, proton excretion by root tips, and transmembrane electrical potential difference of root protoplasts were measured as a function of exogenous auxin concentration. The sensitivity of hairy root tips or protoplasts to exogenous auxin was found to be 100-1000 times higher than that of untransformed material.

Identification of a cloned cytokinin biosynthetic gene

A small region of the Ti plasmid (the tmr locus), thought to be involved in phytohormone metabolism in Agrobacterium tumefaciens-transformed plant tissue, was cloned and expressed in Escherichia coli. By enzyme assay, the tmr locus was shown to encode isopentenyltransferase, an enzyme that catalyzes the first step in cytokinin biosynthesis.

Cytokinin gene fused with a strong promoter enhances shoot organogenesis and zeatin levels in transformed plant cells

The isopentenyltransferase (ipt) gene associated with cytokinin biosynthesis in plants was cloned from a tumor-inducing plasmid carried by Agrobacterium tumefaciens and placed under the control of promoters of differing activities, the cauliflower mosaic virus 35S promoter and the nopaline synthase promoter. These promoter-gene constructs were introduced into wounded Nicotiana stems, leaf pieces, and cucumber seedlings by A. tumefaciens infection. Shoots were observed in the infection site on all responding genotypes of Nicotiana plants infected with the 35S promoter construct (35S-ipt), whereas only 41% responded similarly to infection with the unmodified gene. Furthermore, shoots were observed 19 days after infection with the 35S-ipt gene but not until 28 to 45 days with the unaltered ipt gene. Shoots were more numerous (>40) on galls incited by 35S-ipt and were up to 6 times taller than shoots induced by the native gene. On Cucumis (cucumber), shoots were observed only on galls incited by the 35S-ipt construct. These galls were on the average 7.5 times larger than those incited by the nopaline synthase promoter construct (NOS-ipt) or the unmodified ipt gene. Zeatin and zeatinriboside concentrations averaged 23 times greater in the 35S-ipt transformed shoots than in ones transformed with the native ipt gene. These results suggest that a more active promoter on the ipt gene can enhance or change the morphogenic potential of transformed plant cells by increasing their endogenous cytokinin levels.

Dual promoter of Agrobacterium tumefaciens mannopine synthase genes is regulated by plant growth hormones

Temporal and spacial distribution of mannopine synthase (mas) promoter activity was determined throughout the development of transgenic tobacco plants using bacterial luciferase luxA and luxB as reporter genes. Luciferase activity was determined by luminometry in vitro and visualized by computer-enhanced single-photon video imaging in vivo. The activity of the mas dual promoters increased basipetally in developing plants and was wound-inducible in leaf and stem tissue. Hormone bioassays with isolated plant tissues and tumors deficient in the transferred DNA (T-DNA)-encoded genes iaaM, iaaH, and ipt indicated that activity of the mas dual promoters is regulated by auxin and enhanced by cytokinin in both differentiated and tumorous plant cells.

Complementation of Agrobacterium tumefaciens tumor-inducing aux mutants by genes from the T(R)-region of the Ri plasmid of Agrobacterium rhizogenes

In this paper we provide information indicating that the agropine-type root-inducing (Ri) plasmid pRi1855 of Agrobacterium rhizogenes contains functional genes for auxin production (aux) in the right transferred DNA (T-DNA) region (T(R)-region). These genes were cloned and introduced into the T-region of the tumor-inducing (Ti) plasmids of mutants of Agrobacterium tumefaciens carrying an aux mutation. Depending on the Ri aux gene present, the oncogenicity of the Ti aux-1 and/or aux-2 mutations was restored, showing that the Ri aux genes are able to complement the Ti aux genes. Agrobacterium strains with an agropine-type Ri plasmid not only cause hairy root on certain plant species, but they also induce tumors on other plant species. In this paper it is shown that a mutation in either of the aux genes in the Ri plasmid leads to a total loss of tumorigenicity and a strongly diminished rhizogenicity of the host bacterium, revealing that the aux genes are important for tumor and root induction. Agrobacterium strains containing the T(R)-region but not the T(L) (left)-region of the Ri plasmid are still tumorigenic on certain plant species but are no longer capable of hairy-root induction.

Rapid assay of foreign gene expression in leaf discs transformed by Agrobacterium tumefaciens: Role of T-DNA borders in the transfer process

We have developed a sensitive leaf disc transformation procedure for studying early and/or transient T-DNA expression during Agrobacterium tumefaciens-mediated transformation of plant cells. Using this system, we have examined the function of T-DNA border sequences on the early expression of T-DNA genes and on the stable integration of those genes in infected cells. Deletion of the right border from the T-DNA appears to permit transfer of T-DNA genes from the tumor-inducing (Ti) plasmid but greatly reduces the frequency of their stable integration. A binary vector has been constructed to permit examination of T-DNA border function in trans to the Ti plasmid. In this situation, a single T-DNA border is necessary for early expression of T-DNA genes and is sufficient for stable integration in any orientation.

Cytokinin/auxin balance in crown gall tumors is regulated by specific loci in the T-DNA

Insertion of the transposon Tn5 into the T-region of the octopine Ti plasmid of Agrobacterium tumefaciens gives rise to crown gall tumors having altered morphology. Three loci within the T-DNA that control tumor morphology have been detected [Garfinkel, D. J., Simpson, R. B., Ream, L. W., White, F. F., Gordon, M. P. & Nester, E. W. (1981) Cell 27, 143-153]. They influence tumor size (tml), production of roots (tmr), or production of shoots (tms). Cytokinin and auxin levels in such mutant tumors were examined by HPLC/radioimmunoassay and HPLC/fluorescence assay, respectively. Free indoleacetic acid levels (in pmol/g) were: uninfected tobacco stem tissues, 128; wild-type A348 tumors, 295; tml mutant tumors, 307; tmr mutant tumors, 129; and tms mutant tumors, 70. Average trans-ribosylzeatin levels were correspondingly: 0.97, 48, 40, 0.54, and 1,400 pmol/g. trans-Ribosylzeatin/indoleacetic acid ratios were as high as 24 in shoot-producing tumors and as low as 0.003 in root-producing tumors. The evidence strongly suggests that tumor phytohormone levels are determined by genes in the T-DNA.

Short direct repeats flank the T-DNA on a nopaline Ti plasmid

Crown gall disease results from the insertion of a segment of the Agrobacterium Ti plasmid, called T-DNA, into host plant nuclear DNA. We have subjected to sequence analysis the border regions of pTi T37 (ends of T-DNA) and one left T-DNA/plant DNA border fragment isolated from BT37 tobacco teratoma by molecular cloning. These sequence studies, taken together with published sequence of a right T-DNA/plant DNA border fragment, allowed us to identify the positions of left and right borders at the DNA sequence level. Comparison of left and right border regions of the Ti plasmid revealed a "core" direct repeat of 13 of 14 bases (12 contiguous) precisely at the borders of T-DNA. An extended repeat of 21 of 25 bases overlaps this core repeat. T-DNA on the Ti plasmid exhibits no longer direct or inverted repeats in the border regions, based on Southern hybridization studies. The physical structure of T-DNA differs from that of known prokaryotic and eukaryotic transposable elements but bears a structural resemblance to the prophage of bacteriophage lambda.

Agroinfiltration reduces ABA levels and suppresses Pseudomonas syringae-elicited salicylic acid production in Nicotiana tabacum

BACKGROUND

Agrobacterium tumefaciens strain GV3101 (pMP90) is widely used in transient gene expression assays, including assays to study pathogen effectors and plant disease resistance mechanisms. However, inoculation of A. tumefaciens GV3101 into Nicotiana tabacum (tobacco) leaves prior to infiltration with pathogenic and non-host strains of Pseudomonas syringae results in suppression of macroscopic symptoms when compared with leaves pre-treated with a buffer control.

METHODOLOGY/FINDINGS

To gain further insight into the mechanistic basis of symptom suppression by A. tumefaciens we examined the effect of pre-treatment with A. tumefaciens on the growth of P. syringae, the production of the plant signalling molecules salicylic acid (SA) and abscisic acid (ABA), and the presence of callose deposits. Pre-treatment with A. tumefaciens reduced ABA levels, P. syringae multiplication and P. syringae-elicited SA and ABA production, but promoted increased callose deposition. However, pre-treatment with A. tumefaciens did not suppress necrosis or SA production in leaves inoculated with the elicitor HrpZ.

CONCLUSIONS/SIGNIFICANCE

Collectively, these results show that inoculation of N. tabacum leaves with A. tumefaciens alters plant hormone levels and plant defence responses to P. syringae, and demonstrate that researchers should consider the impact of A. tumefaciens on plant signal transduction when using A. tumefaciens-mediated transient expression assays to investigate ABA-regulated processes or pathogenicity and plant defence mechanisms.

Small heat-shock protein HspL is induced by VirB protein(s) and promotes VirB/D4-mediated DNA transfer in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a Gram-negative plant-pathogenic bacterium that causes crown gall disease by transferring and integrating its transferred DNA (T-DNA) into the host genome. We characterized the chromosomally encoded alpha-crystallin-type small heat-shock protein (alpha-Hsp) HspL, which was induced by the virulence (vir) gene inducer acetosyringone (AS). The transcription of hspL but not three other alpha-Hsp genes (hspC, hspAT1, hspAT2) was upregulated by AS. Further expression analysis in various vir mutants suggested that AS-induced hspL transcription is not directly activated by the VirG response regulator but rather depends on the expression of VirG-activated virB genes encoding components of the type IV secretion system (T4SS). Among the 11 virB genes encoded by the virB operon, HspL protein levels were reduced in strains with deletions of virB6, virB8 or virB11. VirB protein accumulation but not virB transcription levels were reduced in an hspL deletion mutant early after AS induction, implying that HspL may affect the stability of individual VirB proteins or of the T4S complex directly or indirectly. Tumorigenesis efficiency and the VirB/D4-mediated conjugal transfer of an IncQ plasmid RSF1010 derivative between A. tumefaciens strains were reduced in the absence of HspL. In conclusion, increased HspL abundance is triggered in response to certain VirB protein(s) and plays a role in optimal VirB protein accumulation, VirB/D4-mediated DNA transfer and tumorigenesis.

Interaction between Agrobacterium tumefaciens oncoprotein 6b and a tobacco nucleolar protein that is homologous to TNP1 encoded by a transposable element of Antirrhinum majus

When gene 6b on the T-DNA of Agrobacterium tumefaciens is transferred to plant cells, its expression causes plant hormone-independent division of cells in in vitro culture and abnormal cell growth, which induces various morphological defects in 6b-expressing transgenic Arabidopsis thaliana and Nicotiana tabacum plants. Protein 6b localizes to the nuclei, a requirement for the abnormal cell growth, and binds to a tobacco nuclear protein called NtSIP1 and histone H3. In addition, 6b has histone chaperone-like activity in vitro and affects the expression of various plant genes, including cell division-related genes and meristem-related class 1 KNOX homeobox genes, in transgenic Arabidopsis. Here, we report that 6b binds to a newly identified protein NtSIP2, whose amino acid sequence is predicted to be 30% identical and 51% similar to that of the TNP1 protein encoded by the transposon Tam1 of Antirrhinum majus. Immunolocalization analysis using anti-T7 antibodies showed nucleolar localization of most of the T7 epitope-tagged NtSIP2 proteins. A similar analysis with the T7-tagged 6b protein also showed subnucleolar as well as nuclear localization of the 6b protein. These results suggest the involvement of 6b along with NtSIP2 in certain molecular processes in the nucleolus as well as the nucleoplasm.

Opine-based Agrobacterium competitiveness: dual expression control of the agrocinopine catabolism (acc) operon by agrocinopines and phosphate levels

Agrobacterium tumefaciens strain C58 can transform plant cells to produce and secrete the sugar-phosphate conjugate opines agrocinopines A and B. The bacterium then moves in response to the opines and utilizes them as exclusive sources of carbon, energy, and phosphate via the functions encoded by the acc operon. These privileged opine-involved activities contribute to the formation of agrobacterial niches in the environment. We found that the expression of the acc operon is induced by agrocinopines and also by limitation of phosphate. The main promoter is present in front of the first gene, accR, which codes for a repressor. This operon structure enables efficient repression when opine levels are low. The promoter contains two putative operators, one overlapping the -10 sequence and the other in the further upstream from it; two partly overlapped putative pho boxes between the two operators; and two consecutive transcription start sites. DNA fragments containing either of the operators bound purified repressor AccR in the absence of agrocinopines but not in the presence of the opines, demonstrating the on-off switch of the promoter. Induction of the acc operon can occur under low-phosphate conditions in the absence of agrocinopines and further increases when the opines also are present. Such opine-phosphate dual regulatory system of the operon may ensure maximum utilization of agrocinopines when available and thereby increase the chances of agrobacterial survival in the highly competitive environment with limited general food sources.