Genetic evidence for direct sensing of phenolic compounds by the VirA protein of Agrobacterium tumefaciens

Virulence and Ecology of Agrobacteria in the Context of Evolutionary Genomics

Among plant-associated bacteria, agrobacteria occupy a special place. These bacteria are feared in the field as agricultural pathogens. They cause abnormal growth deformations and significant economic damage to a broad range of plant species. However, these bacteria are revered in the laboratory as models and tools. They are studied to discover and understand basic biological phenomena and used in fundamental plant research and biotechnology. Agrobacterial pathogenicity and capability for transformation are one and the same and rely on functions encoded largely on their oncogenic plasmids. Here, we synthesize a substantial body of elegant work that elucidated agrobacterial virulence mechanisms and described their ecology. We review findings in the context of the natural diversity that has been recently unveiled for agrobacteria and emphasize their genomics and plasmids. We also identify areas of research that can capitalize on recent findings to further transform our understanding of agrobacterial virulence and ecology.

Agrobacterium fabrum gene atu1420 regulates the pathogenicity by affecting the degradation of growth- and virulence-associated phenols

Agrobacterium fabrum is a phytopathogen that causes the crown gall disease. Some plant-derived molecules, e.g. phenols, directly affect A. fabrum-plant interactions. Here, we characterize a phenolic catabolism-related gene, atu1420, that affects the pathogenicity of A. fabrum. Atu1420 is predicted to be an O-demethylase with high structural homology to Sphingomonas paucimobilis LigM. The HPLC-UV analysis showed that atu1420 affected the degradation of acetosyringone (AS). The deletion of atu1420 gene significantly enhanced the AS-induced virulence (vir) gene expression. atu1420 was shown to relieve the inhibitory effect of vanillic acid on the AS-induced vir gene expression and the growth of A. fabrum. The expression of atu1420 and the degradation of AS in A. fabrum C58 was up-regulated by the addition of indole acetic acid (IAA). The inhibitory effect of IAA on the AS-induced vir gene expression was partially relieved by the deletion of atu1420 gene, indicating that accelerating the degradation of AS is one of the ways that IAA inhibits vir genes induction. Furthermore, atu1420 mutant produced more pronounced tumors on kalanchoe leaves than the wild-type strain. These findings reveal the role of atu1420 in A. fabrum-host interactions and will broaden our understanding of the regulatory network of the interactions.

Phenolic signals for prehaustorium formation in Striga hermonthica

Striga hermonthica is a root parasitic plant that causes considerable crop yield losses. To parasitize host plants, parasitic plants develop a specialized organ called the haustorium that functions in host invasion and nutrient absorption. The initiation of a prehaustorium, the primitive haustorium structure before host invasion, requires the perception of host-derived compounds, collectively called haustorium-inducing factors (HIFs). HIFs comprise quinones, phenolics, flavonoids and cytokinins for S. hermonthica; however, the signaling pathways from various HIFs leading to prehaustorium formation remain largely uncharacterized. It has been proposed that quinones serve as direct signaling molecules for prehaustorium induction and phenolic compounds originating from the host cell wall are the oxidative precursors, but the overlap and distinction of their downstream signaling remain unknown. Here we show that quinone and phenolic-triggered prehaustorium induction in S. hermonthica occurs through partially divergent signaling pathways. We found that ASBr, an inhibitor of acetosyringone in virulence gene induction in the soil bacterium Agrobacterium, compromised prehaustorium formation in S. hermonthica. In addition, LGR-991, a competitive inhibitor of cytokinin receptors, inhibited phenolic-triggered but not quinone-triggered prehaustorium formation, demonstrating divergent signaling pathways of phenolics and quinones for prehaustorium formation. Comparisons of genome-wide transcriptional activation in response to either phenolic or quinone-type HIFs revealed markedly distinct gene expression patterns specifically at the early initiation stage. While quinone DMBQ triggered rapid and massive transcriptional changes in genes at early stages, only limited numbers of genes were induced by phenolic syringic acid. The number of genes that are commonly upregulated by DMBQ and syringic acid is gradually increased, and many genes involved in oxidoreduction and cell wall modification are upregulated at the later stages by both HIFs. Our results show kinetic and signaling differences in quinone and phenolic HIFs, providing useful insights for understanding how parasitic plants interpret different host signals for successful parasitism.

Genetic factors governing bacterial virulence and host plant susceptibility during Agrobacterium infection

Several species of the Agrobacterium genus represent unique bacterial pathogens able to genetically transform plants, by transferring and integrating a segment of their own DNA (T-DNA, transferred DNA) in their host genome. Whereas in nature this process results in uncontrolled growth of the infected plant cells (tumors), this capability of Agrobacterium has been widely used as a crucial tool to generate transgenic plants, for research and biotechnology. The virulence of Agrobacterium relies on a series of virulence genes, mostly encoded on a large plasmid (Ti-plasmid, tumor inducing plasmid), involved in the different steps of the DNA transfer to the host cell genome: activation of bacterial virulence, synthesis and export of the T-DNA and its associated proteins, intracellular trafficking of the T-DNA and effector proteins in the host cell, and integration of the T-DNA in the host genomic DNA. Multiple interactions between these bacterial encoded proteins and host factors occur during the infection process, which determine the outcome of the infection. Here, we review our current knowledge of the mechanisms by which bacterial and plant factors control Agrobacterium virulence and host plant susceptibility.

Regulatory Networks of the T4SS Control: From Host Cell Sensing to the Biogenesis and the Activity during the Infection

Delivery of effectors, DNA or proteins, that hijack host cell processes to the benefit of bacteria is a mechanism widely used by bacterial pathogens. It is achieved by complex effector injection devices, the secretion systems, among which Type 4 Secretion Systems (T4SSs) play a key role in bacterial virulence of numerous animal and plant pathogens. Considerable progress has recently been made in the structure-function analyses of T4SSs. Nevertheless, the signals and processes that trigger machine assembly and activity during infection, as well as those involved in substrate recognition and transfer, are complex and still poorly understood. In this review, we aim at summarizing the last updates of the knowledge on signaling pathways that regulate the biogenesis and the activity of T4SSs in important bacterial pathogens.

The MexE/MexF/AmeC Efflux Pump of Agrobacterium tumefaciens and Its Role in Ti Plasmid Virulence Gene Expression

Expression of the tumor-inducing (Ti) plasmid virulence genes of Agrobacterium tumefaciens is required for the transfer of DNA from the bacterium into plant cells, ultimately resulting in the initiation of plant tumors. The vir genes are induced as a result of exposure to certain phenol derivatives, monosaccharides, and low pH in the extracellular milieu. The soil, as well as wound sites on a plant-the usual site of the virulence activity of this bacterium-can contain these signals, but vir gene expression in the soil would be a wasteful utilization of energy. This suggests that mechanisms may exist to ensure that vir gene expression occurs only at the higher concentrations of inducers typically found at a plant wound site. In a search for transposon-mediated mutations that affect sensitivity for the virulence gene-inducing activity of the phenol, 3,5-dimethoxy-4-hydroxyacetophenone (acetosyringone [AS]), an RND-type efflux pump homologous to the MexE/MexF/OprN pump of Pseudomonas aeruginosa was identified. Phenotypes of mutants carrying an insertion or deletion of pump components included hypersensitivity to the vir-inducing effects of AS, hypervirulence in the tobacco leaf explant virulence assay, and hypersensitivity to the toxic effects of chloramphenicol. Furthermore, the methoxy substituents on the phenol ring of AS appear to be critical for recognition as a pump substrate. These results support the hypothesis that the regulation of virulence gene expression is integrated with cellular activities that elevate the level of plant-derived inducers required for induction so that this occurs preferentially, if not exclusively, in a plant environment.IMPORTANCE Expression of genes controlling the virulence activities of a bacterial pathogen is expected to occur preferentially at host sites vulnerable to that pathogen. Host-derived molecules that induce such activities in the plant pathogen Agrobacterium tumefaciens are found in the soil, as well as in the plant. Here, we tested the hypothesis that mechanisms exist to suppress the sensitivity of Agrobacterium species to a virulence gene-inducing molecule by selecting for mutant bacteria that are hypersensitive to its inducing activity. The mutant genes identified encode an efflux pump whose proposed activity increases the concentration of the inducer necessary for vir gene expression; this pump is also involved in antibiotic resistance, demonstrating a relationship between cellular defense activities and the control of virulence in Agrobacterium.

Pathways of DNA Transfer to Plants from Agrobacterium tumefaciens and Related Bacterial Species

Genetic transformation of host plants by Agrobacterium tumefaciens and related species represents a unique model for natural horizontal gene transfer. Almost five decades of studying the molecular interactions between Agrobacterium and its host cells have yielded countless fundamental insights into bacterial and plant biology, even though several steps of the DNA transfer process remain poorly understood. Agrobacterium spp. may utilize different pathways for transferring DNA, which likely reflects the very wide host range of Agrobacterium. Furthermore, closely related bacterial species, such as rhizobia, are able to transfer DNA to host plant cells when they are provided with Agrobacterium DNA transfer machinery and T-DNA. Homologs of Agrobacterium virulence genes are found in many bacterial genomes, but only one non-Agrobacterium bacterial strain, Rhizobium etli CFN42, harbors a complete set of virulence genes and can mediate plant genetic transformation when carrying a T-DNA-containing plasmid.

A critical review on use of Agrobacterium rhizogenes and their associated binary vectors for plant transformation

Agrobacterium rhizogenes, along with A. tumefaciens, has been used to affect genetic transformation in plants for many years. Detailed studies conducted in the past have uncovered the basic mechanism of foreign gene transfer and the implication of Ri/Ti plasmids in this process. A number of reviews exist describing the usage of binary vectors with A. tumefaciens, but no comprehensive account of the numerous binary vectors employed with A. rhizogenes and their successful applications has been published till date. In this review, we recollect a brief history of development of Ri-plasmid/Ri-T-DNA based binary vectors systems and their successful implementation with A. rhizogenes for different applications. The modification of native Ri plasmid to introduce foreign genes followed by development of binary vector using Ri plasmid and how it facilitated rapid and feasible genetic manipulation, earlier impossible with native Ri plasmid, have been discussed. An important milestone was the development of inducible plant expressing promoter systems which made expression of toxic genes in plant systems possible. The successful application of binary vectors in conjunction with A. rhizogenes in gene silencing and genome editing studies which are relatively newer developments, demonstrating the amenability and adaptability of hairy roots systems to make possible studying previously intractable research areas have been summarized in the present review.

Host lignin composition affects haustorium induction in the parasitic plants Phtheirospermum japonicum and Striga hermonthica

Parasitic plants in the family Orobanchaceae are destructive weeds of agriculture worldwide. The haustorium, an essential parasitic organ used by these plants to penetrate host tissues, is induced by host-derived phenolic compounds called haustorium-inducing factors (HIFs). The origin of HIFs remains unknown, although the structures of lignin monomers resemble that of HIFs. Lignin is a natural phenylpropanoid polymer, commonly found in secondary cell walls of vascular plants. We therefore investigated the possibility that HIFs are derived from host lignin. Various lignin-related phenolics, quinones and lignin polymers, together with nonhost and host plants that have different lignin compositions, were tested for their haustorium-inducing activity in two Orobanchaceae species, a facultative parasite, Phtheirospermum japonicum, and an obligate parasite, Striga hermonthica. Lignin-related compounds induced haustoria in P. japonicum and S. hermonthica with different specificities. High concentrations of lignin polymers induced haustorium formation. Treatment with laccase, a lignin degradation enzyme, promoted haustorium formation at low concentrations. The distinct lignin compositions of the host and nonhost plants affected haustorium induction, correlating with the response of the different parasitic plants to specific types of lignin-related compounds. Our study provides valuable insights into the important roles of lignin biosynthesis and degradation in the production of HIFs.

Agrobacterium-mediated plant transformation: biology and applications

Plant genetic transformation heavily relies on the bacterial pathogen Agrobacterium tumefaciens as a powerful tool to deliver genes of interest into a host plant. Inside the plant nucleus, the transferred DNA is capable of integrating into the plant genome for inheritance to the next generation (i.e. stable transformation). Alternatively, the foreign DNA can transiently remain in the nucleus without integrating into the genome but still be transcribed to produce desirable gene products (i.e. transient transformation). From the discovery of A. tumefaciens to its wide application in plant biotechnology, numerous aspects of the interaction between A. tumefaciens and plants have been elucidated. This article aims to provide a comprehensive review of the biology and the applications of Agrobacterium-mediated plant transformation, which may be useful for both microbiologists and plant biologists who desire a better understanding of plant transformation, protein expression in plants, and plant-microbe interaction.

Bacterial pathogenesis of plants: future challenges from a microbial perspective: Challenges in Bacterial Molecular Plant Pathology

Plant infection is a complicated process. On encountering a plant, pathogenic microorganisms must first adapt to life on the epiphytic surface, and survive long enough to initiate an infection. Responsiveness to the environment is critical throughout infection, with intracellular and community-level signal transduction pathways integrating environmental signals and triggering appropriate responses in the bacterial population. Ultimately, phytopathogens must migrate from the epiphytic surface into the plant tissue using motility and chemotaxis pathways. This migration is coupled with overcoming the physical and chemical barriers to entry into the plant apoplast. Once inside the plant, bacteria use an array of secretion systems to release phytotoxins and protein effectors that fulfil diverse pathogenic functions (Fig. ) (Melotto and Kunkel, ; Phan Tran et al., ). As our understanding of the pathways and mechanisms underpinning plant pathogenicity increases, a number of central research challenges are emerging that will profoundly shape the direction of research in the future. We need to understand the bacterial phenotypes that promote epiphytic survival and surface adaptation in pathogenic bacteria. How do these pathways function in the context of the plant-associated microbiome, and what impact does this complex microbial community have on the onset and severity of plant infections? The huge importance of bacterial signal transduction to every stage of plant infection is becoming increasingly clear. However, there is a great deal to learn about how these signalling pathways function in phytopathogenic bacteria, and the contribution they make to various aspects of plant pathogenicity. We are increasingly able to explore the structural and functional diversity of small-molecule natural products from plant pathogens. We need to acquire a much better understanding of the production, deployment, functional redundancy and physiological roles of these molecules. Type III secretion systems (T3SSs) are important and well-studied contributors to bacterial disease. Several key unanswered questions will shape future investigations of these systems. We need to define the mechanism of hierarchical and temporal control of effector secretion. For successful infection, effectors need to interact with host components to exert their function. Advanced biochemical, proteomic and cell biological techniques will enable us to study the function of effectors inside the host cell in more detail and on a broader scale. Population genomics analyses provide insight into evolutionary adaptation processes of phytopathogens. The determination of the diversity and distribution of type III effectors (T3Es) and other virulence genes within and across pathogenic species, pathovars and strains will allow us to understand how pathogens adapt to specific hosts, the evolutionary pathways available to them, and the possible future directions of the evolutionary arms race between effectors and molecular plant targets. Although pathogenic bacteria employ a host of different virulence and proliferation strategies, as a result of the space constraints, this review focuses mainly on the hemibiotrophic pathogens. We discuss the process of plant infection from the perspective of these important phytopathogens, and highlight new approaches to address the outstanding challenges in this important and fast-moving field.

Early signaling events induced by the peptide elicitor PIP-1 necessary for acetosyringone accumulation in tobacco cells

A peptide elicitor PIP-1 induces defense-related secondary metabolites such as phytoalexin capsidiol in tobacco cells. In this study, we identified one of other metabolites induced by PIP-1 as acetosyringone. Unlike capsidiol accumulation that requires long-term stimulation with PIP-1, acetosyringone was induced by short-term stimulation with PIP-1. The importance of NADPH oxidase in the acetosyringone induction was also demonstrated.

Adaptation of the Agrobacterium tumefaciens VirG response regulator to activate transcription in plants

The Agrobacterium tumefaciens VirG response regulator of the VirA/VirG two-component system was adapted to function in tobacco protoplasts. The subcellular localization of VirG and VirA proteins transiently expressed in onion cells was determined using GFP fusions. Preliminary studies using Gal4DBD-VP16 fusions with VirG and Escherichia coli UhpA, and NarL response regulators indicated compatibility of these bacterial proteins with the eukaryotic transcriptional apparatus. A strong transcriptional activator based on tandem activation domains from the Drosophila fushi tarazu and Herpes simplex VP16 was created. Selected configurations of the two-site Gal4-vir box GUS reporters were activated by chimeric effectors dependent on either the yeast Gal4 DNA-binding domain or that of VirG. Transcriptional induction of the GUS reporter was highest for the VirE19-element promoter with both constitutive and wild-type VirG-tandem activation domain effectors. Multiple VirE19 elements increased the reporter activity proportionately, indicating that the VirG DNA binding domain was functional in plants. The VirG constitutive-Q-VP16 effector was more active than the VirG wild-type. In both the constitutive and wild-type forms of VirG, Q-VP16 activated transcription of the GUS reporter best when located at the C-terminus, i.e. juxtaposed to the VirG DNA binding domain. These results demonstrate the possibility of using DNA binding domains from bacterial response regulators and their cognate binding elements in the engineering of plant gene expression.

Collection of Apoplastic Fluids from Arabidopsis thaliana Leaves

The leaf apoplast comprises the extracellular continuum outside cell membranes. A broad range of processes take place in the apoplast, including intercellular signaling, metabolite transport, and plant-microbe interactions. To study these processes, it is essential to analyze the metabolite content in apoplastic fluids. Due to the fragile nature of leaf tissues, it is a challenge to obtain apoplastic fluids from leaves. Here, methods to collect apoplastic washing fluid and guttation fluid from Arabidopsis thaliana leaves are described.

A Rhizobium radiobacter Histidine Kinase Can Employ Both Boolean AND and OR Logic Gates to Initiate Pathogenesis

The molecular logic gates that regulate gene circuits are necessarily intricate and highly regulated, particularly in the critical commitments necessary for pathogenesis. We now report simple AND and OR logic gates to be accessible within a single protein receptor. Pathogenesis by the bacterium Rhizobium radiobacter is mediated by a single histidine kinase, VirA, which processes multiple small molecule host signals (phenol and sugar). Mutagenesis analyses converged on a single signal integration node, and finer functional analyses revealed that a single residue could switch VirA from a functional AND logic gate to an OR gate where each of two signals activate independently. Host range preferences among natural strains of R. radiobacter correlate with these gate logic strategies. Although the precise mechanism for the signal integration node requires further analyses, long-range signal transmission through this histidine kinase can now be exploited for synthetic signaling circuits.

Enhancing T-DNA Transfer Efficiency in Barley (Hordeum vulgare L.) Cells Using Extracellular Cellulose and Lectin

A major limitation of transforming barley tissues by Agrobacterium tumefaciens is the low frequency of T-DNA transfer due to recalcitrance of barley as a host. The effect of extracellular cellulose and lectin on Agrobacterium transformation efficiency was investigated in this study. Barley callus cultures were transformed with the AGL1 strain containing the vector pBI121 in the presence of 10 mg mL(-1) cellulose or 0.001, 0.05 and 0.1 mg mL(-1) lectin. Addition of cellulose significantly (P ≤ 0.05) increased the number of GUS spots by 50 % compared to standard conditions in the presence of only 200 μM acetosyringone (AS). Frequency of G418-resistant aggregates on the surfaces of callus cultures was 29 and 71.5 %, following AS and AS + cellulose treatments, respectively, after 4 weeks of selection. Presence of 0.05 or 0.1 mg mL(-1) lectin also increased the number of GUS spots and frequency of G418-resistant cells in the selection period, but the increase in blue spots was not significant. We examined the effect of lectin and cellulose on bacterial attachment to callus tissues. Both cellulose and lectin were found to have a significant positive effect on the numbers of bacteria attached to barley callus. Epifluorescence microscopy revealed that Agrobacterium cells had accumulated in the scaffolds of irregular fibrous cellulose with a mean particle size of 200 μm. Expression of nptII in transformed callus lines confirmed the stable transformation of the gene. Our study showed for the first time the binding of Agrobacterium cells to fibrous cellulose and also demonstrated how polysaccharides and glycoproteins can be used to improve T-DNA transfer in monocotyledon transformation procedures.

Adrenergic antagonists restrict replication of Legionella

Legionella pneumophila is a facultative intracellular bacterium, which upon inhalation can cause a potentially fatal pneumonia termed Legionnaires' disease. The opportunistic pathogen grows in environmental amoebae and mammalian macrophages within a unique membrane-bound compartment, the 'Legionella-containing vacuole'. Bacteria are exposed to many environmental cues including small signalling molecules from eukaryotic cells. A number of pathogenic bacteria sense and respond to catecholamine hormones, such as adrenalin and noradrenalin, a process mediated via the QseBC two-component system in some bacteria. In this study, we examined the effect of adrenergic compounds on L. pneumophila, and discovered that the adrenergic receptor antagonists benoxathian, naftopidil, propranolol and labetalol, as well as the QseC sensor kinase inhibitor LED209, reduced the growth of L. pneumophila in broth or amoebae, while replication in macrophages was enhanced. Growth restriction was common to members of the genus Legionella and Mycobacterium, and was observed for L. pneumophila in the replicative but not stationary phase of the biphasic life cycle. Deletion of the L. pneumophila qseBC genes indicated that growth inhibition by adrenergics or LED209 is mediated only to a minor extent by this two-component system, implying the presence of other adrenergic sensing systems. This study identifies adrenergic molecules as novel inhibitors of extra- and intracellular growth of Legionella and reveals LED209 as a potential lead compound to combat infections with Legionella or Mycobacterium spp.

Plant phenolic compounds and oxidative stress: integrated signals in fungal-plant interactions

Upon invasion of a host, fungal pathogens are exposed to a variety of stresses. Plants release reactive oxygen species, and mount a variety of preformed and induced chemical defenses. Phenolic compounds are one example: they are ubiquitous in plants, and an invading pathogen encounters them already at the leaf surface, or for soil-borne pathogens, in the rhizosphere. Phenolic and related aromatic compounds show varying degrees of toxicity to cells. Some compounds are quite readily metabolized, and others less so. It was known already from classical studies that phenolic substrates induce the expression of the enzymes for their degradation. Recently, the ability to degrade phenolics was shown to be a virulence factor. Conversely, phenolic compounds can increase the effectiveness of antifungals. Phenolics are known antioxidants, yet they have been shown to elicit cellular responses that would usually be triggered to counter oxidant stress. Here, we review the evidence for a connection between the fungal response to phenolics as small-molecule signals, and the response to oxidants. The connections proposed here should enable genetic screens to identify specific fungal receptors for plant phenolics. Furthermore, understanding how the pathogen detects plant phenolic compounds as a stress signal may facilitate new antifungal strategies.

Agrobacterium tumefaciens responses to plant-derived signaling molecules

As a special phytopathogen, Agrobacterium tumefaciens infects a wide range of plant hosts and causes plant tumors also known as crown galls. The complexity of Agrobacterium-plant interaction has been studied for several decades. Agrobacterium pathogenicity is largely attributed to its evolved capabilities of precise recognition and response to plant-derived chemical signals. Agrobacterium perceives plant-derived signals to activate its virulence genes, which are responsible for transferring and integrating its Transferred DNA (T-DNA) from its Tumor-inducing (Ti) plasmid into the plant nucleus. The expression of T-DNA in plant hosts leads to the production of a large amount of indole-3-acetic acid (IAA), cytokinin (CK), and opines. IAA and CK stimulate plant growth, resulting in tumor formation. Agrobacterium utilizes opines as nutrient sources as well as signals in order to activate its quorum sensing (QS) to further promote virulence and opine metabolism. Intriguingly, Agrobacterium also recognizes plant-derived signals including γ-amino butyric acid and salicylic acid (SA) to activate quorum quenching that reduces the level of QS signals, thereby avoiding the elicitation of plant defense and preserving energy. In addition, Agrobacterium hijacks plant-derived signals including SA, IAA, and ethylene to down-regulate its virulence genes located on the Ti plasmid. Moreover, certain metabolites from corn (Zea mays) also inhibit the expression of Agrobacterium virulence genes. Here we outline the responses of Agrobacterium to major plant-derived signals that impact Agrobacterium-plant interactions.

Optimization and utilization of Agrobacterium-mediated transient protein production in Nicotiana

Agrobacterium-mediated transient protein production in plants is a promising approach to produce vaccine antigens and therapeutic proteins within a short period of time. However, this technology is only just beginning to be applied to large-scale production as many technological obstacles to scale up are now being overcome. Here, we demonstrate a simple and reproducible method for industrial-scale transient protein production based on vacuum infiltration of Nicotiana plants with Agrobacteria carrying launch vectors. Optimization of Agrobacterium cultivation in AB medium allows direct dilution of the bacterial culture in Milli-Q water, simplifying the infiltration process. Among three tested species of Nicotiana, N. excelsiana (N. benthamiana × N. excelsior) was selected as the most promising host due to the ease of infiltration, high level of reporter protein production, and about two-fold higher biomass production under controlled environmental conditions. Induction of Agrobacterium harboring pBID4-GFP (Tobacco mosaic virus-based) using chemicals such as acetosyringone and monosaccharide had no effect on the protein production level. Infiltrating plant under 50 to 100 mbar for 30 or 60 sec resulted in about 95% infiltration of plant leaf tissues. Infiltration with Agrobacterium laboratory strain GV3101 showed the highest protein production compared to Agrobacteria laboratory strains LBA4404 and C58C1 and wild-type Agrobacteria strains at6, at10, at77 and A4. Co-expression of a viral RNA silencing suppressor, p23 or p19, in N. benthamiana resulted in earlier accumulation and increased production (15-25%) of target protein (influenza virus hemagglutinin).

The role of the secondary cell wall in plant resistance to pathogens

Plant resistance to pathogens relies on a complex network of constitutive and inducible defensive barriers. The plant cell wall is one of the barriers that pathogens need to overcome to successfully colonize plant tissues. The traditional view of the plant cell wall as a passive barrier has evolved to a concept that considers the wall as a dynamic structure that regulates both constitutive and inducible defense mechanisms, and as a source of signaling molecules that trigger immune responses. The secondary cell walls of plants also represent a carbon-neutral feedstock (lignocellulosic biomass) for the production of biofuels and biomaterials. Therefore, engineering plants with improved secondary cell wall characteristics is an interesting strategy to ease the processing of lignocellulosic biomass in the biorefinery. However, modification of the integrity of the cell wall by impairment of proteins required for its biosynthesis or remodeling may impact the plants resistance to pathogens. This review summarizes our understanding of the role of the plant cell wall in pathogen resistance with a focus on the contribution of lignin to this biological process.

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.

Effect of plant phenolic compounds on biofilm formation by Pseudomonas aeruginosa

In the natural environment, bacteria predominantly exist in matrix-enclosed multicellular communities associated with various surfaces, referred to as biofilms. Bacteria in biofilms are extremely resistant to antibacterial agents thus causing serious problems for antimicrobial therapy. In this study, we showed that different plant phenolic compounds, at concentrations that did not or weakly suppressed bacterial growth, increased the capacity of Pseudomonas aeruginosa PAO1 to form biofilms. Biofilm formation of P. aeruginosa PAO1 was enhanced 3- to 7-fold under the action of vanillin and epicatechin, and 2- to 2.5-fold in the presence of 4-hydroxybenzoic, gallic, cinnamic, sinapic, ferulic, and chlorogenic acids. At higher concentrations, these compounds displayed an inhibiting effect. Similar experiments carried out for comparison with Agrobacterium tumefaciens C58 showed the same pattern. Vanillin, 4-hydroxybenzoic, and gallic acids at concentrations within the range of 40 to 400 μg/mL increased the production of N-3-oxo-dodecanoyl-homoserine lactone in P. aeruginosa PAO1 which suggests a possible relationship between stimulation of biofilm formation and Las Quorum Sensing system of this bacterium. Using biosensors to detect N-acyl-homoserine lactones (AHL), we demonstrated that the plant phenolics studied did not mimic AHLs.

Bile salt-induced intermolecular disulfide bond formation activates Vibrio cholerae virulence

To be successful pathogens, bacteria must often restrict the expression of virulence genes to host environments. This requires a physical or chemical marker of the host environment as well as a cognate bacterial system for sensing the presence of a host to appropriately time the activation of virulence. However, there have been remarkably few such signal-sensor pairs identified, and the molecular mechanisms for host-sensing are virtually unknown. By directly applying a reporter strain of Vibrio cholerae, the causative agent of cholera, to a thin layer chromatography (TLC) plate containing mouse intestinal extracts, we found two host signals that activate virulence gene transcription. One of these was revealed to be the bile salt taurocholate. We then show that a set of bile salts cause dimerization of the transmembrane transcription factor TcpP by inducing intermolecular disulfide bonds between cysteine (C)-207 residues in its periplasmic domain. Various genetic and biochemical analyses led us to propose a model in which the other cysteine in the periplasmic domain, C218, forms an inhibitory intramolecular disulfide bond with C207 that must be isomerized to form the active C207-C207 intermolecular bond. We then found bile salt-dependent effects of these cysteine mutations on survival in vivo, correlating to our in vitro model. Our results are a demonstration of a mechanism for direct activation of the V. cholerae virulence cascade by a host signal molecule. They further provide a paradigm for recognition of the host environment in pathogenic bacteria through periplasmic cysteine oxidation.

The roles of bacterial and host plant factors in Agrobacterium-mediated genetic transformation

The genetic transformation of plants mediated by Agrobacterium tumefaciens represents an essential tool for both fundamental and applied research in plant biology. For a successful infection, culminating in the integration of its transferred DNA (T-DNA) into the host genome, Agrobacterium relies on multiple interactions with host-plant factors. Extensive studies have unraveled many of such interactions at all major steps of the infection process: activation of the bacterial virulence genes, cell-cell contact and macromolecular translocation from Agrobacterium to host cell cytoplasm, intracellular transit of T-DNA and associated proteins (T-complex) to the host cell nucleus, disassembly of the T-complex, T-DNA integration, and expression of the transferred genes. During all these processes, Agrobacterium has evolved to control and even utilize several pathways of host-plant defense response. Studies of these Agrobacterium-host interactions substantially enhance our understanding of many fundamental cellular biological processes and allow improvements in the use of Agrobacterium as a gene transfer tool for biotechnology.

Chemical signaling between plants and plant-pathogenic bacteria

Studies of chemical signaling between plants and bacteria in the past have been largely confined to two models: the rhizobial-legume symbiotic association and pathogenesis between agrobacteria and their host plants. Recent studies are beginning to provide evidence that many plant-associated bacteria undergo chemical signaling with the plant host via low-molecular-weight compounds. Plant-produced compounds interact with bacterial regulatory proteins that then affect gene expression. Similarly, bacterial quorum-sensing signals result in a range of functional responses in plants. This review attempts to highlight current knowledge in chemical signaling that takes place between pathogenic bacteria and plants. This chemical communication between plant and bacteria, also referred to as interkingdom signaling, will likely become a major research field in the future, as it allows the design of specific strategies to create plants that are resistant to plant pathogens.

Identification of two feruloyl esterases in Dickeya dadantii 3937 and induction of the major feruloyl esterase and of pectate lyases by ferulic acid

The plant-pathogenic bacterium Dickeya dadantii (formerly Erwinia chrysanthemi) produces a large array of plant cell wall-degrading enzymes. Using an in situ detection test, we showed that it produces two feruloyl esterases, FaeD and FaeT. These enzymes cleave the ester link between ferulate and the pectic or xylan chains. FaeD and FaeT belong to the carbohydrate esterase family CE10, and they are the first two feruloyl esterases to be identified in this family. Cleavage of synthetic substrates revealed strong activation of FaeD and FaeT by ferulic acid. The gene faeT appeared to be weakly expressed, and its product, FaeT, is a cytoplasmic protein. In contrast, the gene faeD is strongly induced in the presence of ferulic acid, and FaeD is an extracellular protein secreted by the Out system, responsible for pectinase secretion. The product of the adjacent gene faeR is involved in the positive control of faeD in response to ferulic acid. Moreover, ferulic acid acts in synergy with polygalacturonate to induce pectate lyases, the main virulence determinant of soft rot disease. Feruloyl esterases dissociate internal cross-links in the polysaccharide network of the plant cell wall, suppress the polysaccharide esterifications, and liberate ferulic acid, which contributes to the induction of pectate lyases. Together, these effects of feruloyl esterases could facilitate soft rot disease caused by pectinolytic bacteria.

The fungal pathogen Cochliobolus heterostrophus responds to maize phenolics: novel small molecule signals in a plant-fungal interaction

The transcription factor ChAP1 of the fungal pathogen of maize, Cochliobolus heterostrophus, responds to oxidative stress by migration to the nucleus and activation of antioxidant genes. Phenolic and related compounds found naturally in the host also trigger nuclear localization of ChAP1, but only slight upregulation of some antioxidant genes. ChAP1 thus senses phenolic compounds without triggering a strong antioxidant response. We therefore searched for genes whose expression is regulated by phenolic compounds and/or ChAP1. The C. heterostrophus genome contains a cluster of genes for metabolism of phenolics. One such gene, catechol dioxygenase CCHD1, was induced at least 10-fold by caffeic and coumaric acids. At high phenolic concentrations (≥ 1.6 mM), ChAP1 is needed for maximum CCHD1 expression. At micromolar levels of phenolics CCHD1 is as strongly induced in chap1 mutants as in the wild type. The pathogen thus detects phenolics by at least two signalling pathways: one causing nuclear retention of ChAP1, and another triggering induction of CCHD1 expression. The low concentrations required for induction of CCHD1 indicate fungal receptors for plant phenolics. Symbiotic and pathogenic bacteria are known to detect phenolics, and our findings generalize this to a eukaryotic pathogen. Phenolics and related compounds thus provide a ubiquitous plant-derived signal.

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

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

p-Coumaric acid decarboxylase from Lactobacillus plantarum: structural insights into the active site and decarboxylation catalytic mechanism

p-Coumaric acid decarboxylases (PDCs) catalyze the nonoxidative decarboxylation of hydroxycinnamic acids to generate the corresponding vinyl derivatives. Despite the biotechnological relevance of PDCs in food industry, their catalytic mechanism remains largely unknown. Here, we report insights into the structural basis of catalysis for the homodimeric PDC from Lactobacillus plantarum (LpPDC). The global fold of LpPDC is based on a flattened beta-barrel surrounding an internal cavity. Crystallographic and functional analyses of single-point mutants of residues located within this cavity have permitted identifying a potential substrate-binding pocket and also to provide structural evidences for rearrangements of surface loops so that they can modulate the accessibility to the active site. Finally, combination of the structural and functional data with in silico results enables us to propose a two-step catalytic mechanism for decarboxylation of p-coumaric acid by PDCs where Glu71 is involved in proton transfer, and Tyr18 and Tyr20 are involved in the proper substrate orientation and in the release of the CO(2) product.

Phenolic acids act as signaling molecules in plant-microbe symbioses

Phenolic acids are the main polyphenols made by plants. These compounds have diverse functions and are immensely important in plant-microbe interactions/symbiosis. Phenolic compounds act as signaling molecules in the initiation of legumerhizobia symbioses, establishment of arbuscular mycorrhizal symbioses and can act as agents in plant defense. Flavonoids are a diverse class of polyphenolic compounds that have received considerable attention as signaling molecules involved in plant-microbe interactions compared to the more widely distributed, simple phenolic acids; hydroxybenzoic and hydroxycinnamic acids, which are both derived from the general phenylpropanoid pathway. This review describes the well-known roles attributed to phenolic compounds as nod gene inducers of legume-rhizobia symbioses, their roles in induction of the GmGin1 gene in fungus for establishment of arbuscular mycorrhizal symbiosis, their roles in inducing vir gene expression in Agrobacterium, and their roles as defense molecules operating against soil borne pathogens that could have great implications for rhizospheric microbial ecology. Amongst plant phenolics we have a lack of knowledge concerning the roles of phenolic acids as signaling molecules beyond the relatively well-defined roles of flavonoids. This may be addressed through the use of plant mutants defective in phenolic acids biosynthesis or knock down target genes in future investigations.

Role of CheY1 and CheY2 in the chemotaxis of A. tumefaciens toward acetosyringone

Agrobacterium tumefaciens has a chemtaxis operon, which includes orf1, orf2, cheY1, cheA, cheR, cheB, cheY2, orf9, and orf10. In-frame deletions of cheY1 and cheY2 were constructed and the chemosensory behavior of the mutants was examined on swarm plates and in a chemotaxis assay toward acetosyringone. The cheY2 mutant (C1/delY2) showed impaired chemotactic capabilities in both swarming and chemotaxis assays. The effect of lacking CheY1 on chemotaxis is less severe than that of CheY2, under the conditions studied.

Ethylene production in plants during transformation suppresses vir gene expression in Agrobacterium tumefaciens

Ethylene evolution from plants inhibits Agrobacterium-mediated genetic transformation, but the mechanism is little understood. In this study, the possible role of ethylene in Agrobacterium-mediated genetic transformation was clarified. It was tested whether or not plant ethylene sensitivity affected genetic transformation; the sensitivity might regulate bacterial growth during co-cultivation and vir gene expression in Agrobacterium tumefaciens. For these experiments, melon (Cucumis melo) was used, in which ethylene sensitivity was controlled by chemicals, and Arabidopsis ethylene-insensitive mutants. Agrobacterium-mediated genetic transformation was inhibited in ethylene-sensing melon, whereas, in Arabidopsis ethylene-insensitive mutant, it was enhanced. However, the ethylene sensitivity did not affect bacterial growth. vir gene expression was inhibited by application of plant exudate from ethylene-sensitive plants. The inhibitory effect of the ethylene sensitivity on genetic transformation relieved the activation of vir gene expression in A. tumefaciens with vir gene inducer molecule (acetosyringone, AS) or A. tumefaciens mutant strain which has constitutive vir gene expression. These results indicate that ethylene evolution from a plant inoculated with A. tumefaciens inhibited vir gene expression in A. tumefaciens through the ethylene signal transduction in the plant, and, as a result, Agrobacterium-mediated genetic transformation was inhibited.

Agrobacterium tumefaciens and plant cell interactions and activities required for interkingdom macromolecular transfer

Host recognition and macromolecular transfer of virulence-mediating effectors represent critical steps in the successful transformation of plant cells by Agrobacterium tumefaciens. This review focuses on bacterial and plant-encoded components that interact to mediate these two processes. First, we examine the means by which Agrobacterium recognizes the host, via both diffusible plant-derived chemicals and cell-cell contact, with emphasis on the mechanisms by which multiple host signals are recognized and activate the virulence process. Second, we characterize the recognition and transfer of protein and protein-DNA complexes through the bacterial and plant cell membrane and wall barriers, emphasizing the central role of a type IV secretion system-the VirB complex-in this process.

Sensing by bacterial regulatory systems in host and non-host environments

Free-living organisms have the ability to gauge their surroundings and modify their gene expression patterns in ways that help them cope with new environments. Here we discuss the physiological significance of recent reports describing the ability of the Salmonella typhimurium PhoP/PhoQ two-component system to recognize and respond to host-derived antimicrobial peptides.

Indoleacetic acid, a product of transferred DNA, inhibits vir gene expression and growth of Agrobacterium tumefaciens C58

Agrobacterium tumefaciens induces crown gall tumors by transferring a piece of its tumor-inducing plasmid into plant cells. This transferred DNA encodes the synthesis of indole acetic acid (IAA) and cytokinin, and their overproduction results in tumor formation. The transfer is initiated by a two-component regulatory system, VirA/G recognizing plant signal molecules in the plant rhizosphere and activating a regulon on the tumor-inducing plasmid, which is required for the processing and transfer of DNA and protein. Although a great deal is known about vir gene activation, nothing is known about whether or how the vir gene regulon is inactivated after plant cell transformation. Presumably, just as a mechanism exists for activating the vir gene regulon only when a plant is in the immediate environment, a mechanism should exist for inactivating the same regulon once it has fulfilled its mission to transferred DNA into plant cells. We now show that IAA inactivates vir gene expression by competing with the inducing phenolic compound acetosyringone for interaction with VirA. IAA does not inhibit the vir genes in cells containing a constitutive sensor virA locus, which does not require any signal molecules to become phosphorylated. At higher concentrations, IAA inhibits the growth of Agrobacterium and many other plant-associated bacteria but not the growth of bacteria that occupy other ecological niches. These observations provide the missing link in the cycle of vir gene activation and inactivation.

Agrobacterium T-DNA-mediated integration and gene replacement in the brown rot pathogen Monilinia fructicola

A transformation system utilizing Agrobacterium tumefaciens was developed for targeted gene disruption in Monilinia fructicola, a fungal pathogen that causes brown rot disease of stone fruits. Transformation with a vector containing the neomycin phosphotransferase II (nptII) cassette flanked with 4 kb cutinase gene (Mfcut1) flanking sequences resulted in an average of 13 transformants per 10(5) spores. When assayed by PCR and DNA blot analyses, more than 50% of the transformants recovered had integrated in the targeted Mfcut1 locus. Both target-gene-specific and non-specific integrations carried direct (head-to-tail) repeat T-DNA integrations. Sequence analysis of these T-DNA integrations revealed that 26 bp of the T-DNA right border were missing at the junctions between direct repeats in all cases. The recombination event during non-specific T-DNA integration in this fungus was unlike that reported in Agrobacterium-mediated transformation in plants.

The coexistence of symbiosis and pathogenicity-determining genes in Rhizobium rhizogenes strains enables them to induce nodules and tumors or hairy roots in plants

Bacteria belonging to the family Rhizobiaceae may establish beneficial or harmful relationships with plants. The legume endosymbionts contain nod and nif genes responsible for nodule formation and nitrogen fixation, respectively, whereas the pathogenic strains carry vir genes responsible for the formation of tumors or hairy roots. The symbiotic and pathogenic strains currently belong to different species of the genus Rhizobium and, until now, no strains able to establish symbiosis with legumes and also to induce tumors or hairy roots in plants have been reported. Here, we report for the first time the occurrence of two rhizobial strains (163C and ATCC11325T) belonging to Rhizobium rhizogenes able to induce hairy roots or tumors in plants and also to nodulate Phaseolus vulgaris under natural environmental conditions. Symbiotic plasmids (pSym) containing nod and nif genes and pTi- or pRi-type plasmids containing vir genes were found in these strains. The nodD and nifH genes of the strains from this study are phylogenetically related to those of Sinorhizobium strains nodulating P. vulgaris. The virA and virB4 genes from strain 163C are phylogenetically related to those of R. tumefaciens C58, whereas the same genes from strain ATCC 11325T are related to those of hairy root-inducing strains. These findings may be of high relevance for the better understanding of plant-microbe interactions and knowledge of rhizobial phylogenetic history.

Recognition of antimicrobial peptides by a bacterial sensor kinase

PhoQ is a membrane bound sensor kinase important for the pathogenesis of a number of Gram-negative bacterial species. PhoQ and its cognate response regulator PhoP constitute a signal-transduction cascade that controls inducible resistance to host antimicrobial peptides. We show that enzymatic activity of Salmonella typhimurium PhoQ is directly activated by antimicrobial peptides. A highly acidic surface of the PhoQ sensor domain participates in both divalent-cation and antimicrobial-peptide binding as a first step in signal transduction across the bacterial membrane. Identification of PhoQ signaling mutants, binding studies with the PhoQ sensor domain, and structural analysis of this domain can be incorporated into a model in which antimicrobial peptides displace divalent cations from PhoQ metal binding sites to initiate signal transduction. Our findings reveal a molecular mechanism by which bacteria sense small innate immune molecules to initiate a transcriptional program that promotes bacterial virulence.

Detection of and response to signals involved in host-microbe interactions by plant-associated bacteria

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.

Involvement of acetosyringone in plant-pathogen recognition

In this study, acetosyringone was identified as one of the major extracellular phenolics in tobacco suspension cells and was shown to have bioactive properties that influence early events in plant-bacterial pathogenesis. In our model system, tobacco cell suspensions treated with bacterial isolate Pseudomonas syringae WT (HR+) undergo a resistant interaction characterized by a burst in oxygen uptake several hours after inoculation. When the extracellular concentration of acetosyringone in tobacco cell suspensions was supplemented with exogenous acetosyringone, the burst in oxygen uptake occurred as much as 1.5h earlier. The exogenous acetosyringone had no effect on tobacco suspensions undergoing susceptible interactions with Pseudomonas tabaci or a non-resistant interaction with a near-isogenic mutant derivative of isolate P. syringae WT (HR+). Resistant interactions with isolate P. syringae WT (HR+) also produce an oxidative burst which oxidizes the extracellular acetosyringone. This study demonstrates that acetosyringone, and likely other extracellular phenolics, may have bioactive characteristics that can influence plant-bacterial pathogenesis.

Cloning, deletion, and characterization of PadR, the transcriptional repressor of the phenolic acid decarboxylase-encoding padA gene of Lactobacillus plantarum

Lactobacillus plantarum displays a substrate-inducible padA gene encoding a phenolic acid decarboxylase enzyme (PadA) that is considered a specific chemical stress response to the inducing substrate. The putative regulator of padA was located in the padA locus based on its 52% identity with PadR, the padA gene transcriptional regulator of Pediococcus pentosaceus (L. Barthelmebs, B. Lecomte, C. Diviès, and J.-F. Cavin, J. Bacteriol. 182:6724-6731, 2000). Deletion of the L. plantarum padR gene clearly demonstrates that the protein it encodes is the transcriptional repressor of divergently oriented padA. The padR gene is cotranscribed with a downstream open reading frame (ORF1), the product of which may belong to a group of universal stress proteins (Usp). The padR deletion mutant overexpressed padA constitutively, and the padA promoter appears to be tightly regulated in this bacterium. Gel mobility shift assays using the padA gene promoter region and purified PadR expressed in Escherichia coli indicated that operator DNA binding by PadR was not eliminated by addition of p-coumarate. Gel mobility shift assays using partially purified extracts of native PadR protein from both phenolic acid-induced and noninduced L. plantarum cells demonstrate that inactivation of PadR by phenolic acids requires the integrity of L. plantarum and mediation by a specific protein absent in E. coli.

Reexamining the role of the accessory plasmid pAtC58 in the virulence of Agrobacterium tumefaciens strain C58

Isogenic strains of Agrobacterium tumefaciens carrying pTiC58, pAtC58, or both were constructed and assayed semiquantitatively and quantitatively for virulence and vir gene expression to study the effect of the large 542-kb accessory plasmid, pAtC58, on virulence. Earlier studies indicate that the att (attachment) genes of A. tumefaciens are crucial in the ability of this soil phytopathogen to infect susceptible host plants. Mutations in many att genes, notably attR and attD, rendered the strain avirulent. These genes are located on pAtC58. Previous work also has shown that derivatives of the wild-type strain C58 cured of pAtC58 are virulent as determined by qualitative virulence assays and, hence, pAtC58 was described as nonessential for virulence. We show here that the absence of pAtC58 in pTiC58-containing strains results in reduced virulence but that disruption of the attR gene does not result in avirulence or a reduction in virulence. Our studies indicate that pAtC58 has a positive effect on vir gene induction as revealed by immunoblot analysis of Vir proteins and expression of a PvirB::lacZ fusion.

Agrobacterium-mediated plant transformation: the biology behind the "gene-jockeying" tool

Agrobacterium tumefaciens and related Agrobacterium species have been known as plant pathogens since the beginning of the 20th century. However, only in the past two decades has the ability of Agrobacterium to transfer DNA to plant cells been harnessed for the purposes of plant genetic engineering. Since the initial reports in the early 1980s using Agrobacterium to generate transgenic plants, scientists have attempted to improve this "natural genetic engineer" for biotechnology purposes. Some of these modifications have resulted in extending the host range of the bacterium to economically important crop species. However, in most instances, major improvements involved alterations in plant tissue culture transformation and regeneration conditions rather than manipulation of bacterial or host genes. Agrobacterium-mediated plant transformation is a highly complex and evolved process involving genetic determinants of both the bacterium and the host plant cell. In this article, I review some of the basic biology concerned with Agrobacterium-mediated genetic transformation. Knowledge of fundamental biological principles embracing both the host and the pathogen have been and will continue to be key to extending the utility of Agrobacterium for genetic engineering purposes.

Comparative genomic analysis of plant-associated bacteria

This review deals with a comparative analysis of seven genome sequences from plant-associated bacteria. These are the genomes of Agrobacterium tumefaciens, Mesorhizobium loti, Sinorhizobium meliloti, Xanthomonas campestris pv campestris, Xanthomonas axonopodis pv citri, Xylella fastidiosa, and Ralstonia solanacearum. Genome structure and the metabolism pathways available highlight the compromise between the genome size and lifestyle. Despite the recognized importance of the type III secretion system in controlling host compatibility, its presence is not universal in all necrogenic pathogens. Hemolysins, hemagglutinins, and some adhesins, previously reported only for mammalian pathogens, are present in most organisms discussed. Different numbers and combinations of cell wall degrading enzymes and genes to overcome the oxidative burst generally induced by the plant host are characterized in these genomes. A total of 19 genes not involved in housekeeping functions were found common to all these bacteria.

Vibrio cholerae-induced cellular responses of polarized T84 intestinal epithelial cells are dependent on production of cholera toxin and the RTX toxin

To study the utility of in vitro-polarized intestinal cell monolayers for modeling Vibrio cholerae-host cell interactions, we added live V. cholerae bacteria to the apical surfaces of polarized T84 cell monolayers and monitored changes in electrical properties. We found that both classical and El Tor strains produce cholera toxin after addition to the monolayer, but induction is most likely due to medium components rather than bacterium-cell interactions. We also found that the RTX toxin is produced by El Tor strains. This toxin caused a loss of the barrier function of the paracellular tight junction that was measured as a decrease in transepithelial resistance. This decrease occurred when bacteria were added to either the apical or basolateral surfaces, indicating that the RTX toxin receptor is expressed on both surfaces. These results are discussed with regard to the applicability of the polarized T84 cell monolayers as an in vitro model of host-pathogen interactions.