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Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References

Many plant-associated microbes use secreted autoinducer molecules, including N-acylhomoserine lactones (AHLs), to regulate diverse behaviours in association with their population density (quorum sensing). Often, these responses are affected by environmental conditions, including the presence of other AHL-producing bacterial species. In addition, plant-derived metabolites, including products that arise as a direct result of the bacterial infection, may profoundly influence AHL-regulated behaviours. These plant products can interact directly and indirectly with the quorum-sensing network and can profoundly affect the quorum-sensing behaviour. Local conditions on a microscopic scale may affect signal molecule longevity, stability and accumulation, and this could be used to give information in addition to cell density. Furthermore, in many Gram-negative bacteria, AHL signalling is subservient to an additional two-component signalling system dependent upon homologues of GacS and GacA. The signal(s) to which GacS responds are not known, but recent research suggests that a self-produced ligand may be being detected. This review will focus on two well-studied examples of AHL-regulated plant-associated behaviour, Erwinia carotovora and Agrobacterium tumefaciens, to illustrate the complexity of such signalling networks.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References

Many bacteria produce small signal molecules that are secreted into the local environment and trigger specific behavioural responses in the producing population when they exceed a critical concentration threshold. Under laboratory conditions, this threshold is normally reached when the bacterial culture producing the autoinducer (AI) signal molecule reaches a critical density. At this point, the population is said to be ‘quorate’, and a synchronized change in behaviour can be triggered. For this reason, the production of and response to such autoinducers is frequently termed quorum sensing. Among Gram-negative bacteria, the best studied and possibly most common group of autoinducer signals are N-acylhomoserine lactones (AHLs). In plant-associated bacteria, AHLs are found in pathogenic, symbiotic and biological control strains, and they regulate a diverse range of phenotypes including diverse pathogenicity determinants, conjugation, rhizosphere competence and the production of antifungal metabolites (Table 1). A higher proportion of AHL-producing bacteria is found in the immediate vicinity of plant roots (the rhizosphere) than in bulk soil, suggesting a general role in rhizosphere colonization (Elasri et al., 2001) and competence, possibly through regulating mechanisms such as exopolysaccharide production, attachment and biofilm formation (von Bodman et al., 1998; Denny, 1999; De Kievit et al., 2001; Marketon et al., 2003). Culture-based experiments show that AHL sensing is generally integrated into much larger control networks with many positive and negative inputs. In addition, it is becoming apparent that some plants may be able to interfere with, and possibly respond to, the bacterial AHL signalling system (Teplitski et al., 2000; Bauer and Robinson, 2002; Mathesius et al., 2003). The challenge now is to test the current models in naturally occurring plant–bacterial interactions to determine the key regulators in the AHL signalling network during the plant–microbe interaction. This is important because, under such conditions, it is possible that AI-based signalling systems might be conveying alternative or additional information to population size (Redfield, 2002) and, although such signalling mechanisms allow the co-ordination of multicellular behaviour, this may not always be the primary function.

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Figure 1. Some of the AHLs found in plant-associated bacteria and the known phenotypes that are regulated. a.Eberl et al. (1996). b.Chin-A-Woeng et al. (2001). c.McClean et al. (1997). d.Chancey et al. (1999). e.Bainton et al. (1992). f.Lithgow et al. (2000). g.Aguilar et al. (2003). h.Zhang et al. (1993). i.Pearson et al. (1994). j.Marketon et al. (2003). ?, No function yet discovered.

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This review will discuss recent research and highlight the importance of plant-derived compounds in contributing to AHL ‘quorum-sensing’ responses. As examples of this, it will focus on the role of AHL signalling in Erwinia carotovora and Agrobacterium tumefaciens, a necro-trophic and a biotrophic plant pathogen respectively.

The AHL signalling molecules and the lux paradigm

  1. Top of page
  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References

N-acylhomoserine lactones consist of a homoserine lactone moiety (derived from amino acid metabolism via S-adenosyl methionine) linked to a variable acyl side-chain [derived from fatty acid metabolism via the appropriately charged acyl–acyl carrier protein (acyl-ACP) or acyl–coenzyme A (acyl-CoA)] (Hanzelka and Greenberg, 1996; Schaefer et al., 1996). Reflecting its origin as an intermediate in fatty acid metabolism, the acyl side-chain variations generally consist of the presence or absence of an acyl chain C3 substituent (carbonyl, hydroxyl or fully reduced) and the length of the chain, typically 4–12 carbons (Hoang et al., 2002; Watson et al., 2002). In some Rhizobium species, acyl chain lengths up to 18 carbons have been reported, and unsaturated bonds may be present (Lithgow et al., 2000; Marketon et al., 2002). AHLs with an acyl chain length of six or fewer carbons are frequently referred to as short-chain AHLs, whereas those with chain lengths of 10 or more carbons are known as long-chain AHLs. Several bacterial species produce the same AHL although, in each, it may be used to regulate different biological processes (Fig. 1).

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Figure 1. Regulation of pathogenicity determinants in E. carotovora. Transcriptional repression of some exoenzymes and of rsmB is brought about by binding of KdgR (red squares) near promoter sequences. Translational repression and RNA turnover of pathogenicity-associated gene transcripts is brought about by binding of RsmA (purple ovals) to sequences within target mRNAs. AHLs activate the expression of pathogenicity determinants in a cell density-dependent manner by a mechanism that is unclear, but which may act antagonistically to RsmA. Secreted plant cell wall-degrading enzymes (PCWDEs) act on plant tissues to release metabolites that enter the bacterium and prevent KdgR repressor binding. Translational repression is removed by the action of rsmB, which may sequester RsmA. Tran-scription of rsmB is brought about by a two-component sensor kinase, GacS/GacA (also known as ExpS/ExpA) that responds to an unidentified signal.

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AHLs were first identified in the early 1980s as autoinducers of bioluminescence in the marine bacterium Vibrio fischeri (Eberhard et al., 1981), and this system has become the archetype for AHL-mediated quorum sensing. V. fischeri can be found at either low cell densities free-living in sea water or high densities as a symbiont in specialized organs (light organ) of certain fish and squid (Ruby and Lee, 1998). Significant light production only occurs in the light organ (or in laboratory cultures) where the bacteria are present at a high density. This is because of the requirement for the build-up of the AHL signalling molecule N-(3-oxohehanoyl)-l-homoserine lactone (3-oxo-C6HSL), which is synthesized by the LuxI gene product. LuxI is initially expressed at a low level, and the small amounts of 3-oxo-C6HSL AI produced diffuse out of the cell and into the surrounding environment. In the case of the free-living bacterium, most of the autoinducer will never be encountered again. However, in the confines of the host's light organ, where there are many bacteria producing the signal molecule and where diffusion is restricted, 3-oxo-C6HSL levels rise (Boettcher and Ruby, 1995). Above a threshold level, 3-oxo-C6HSL binds to a transcriptional regulator (LuxR), which dimerizes and binds to a 20 bp palindromic sequence (the lux box) located upstream of the genes required for light production (Egland and Greenberg, 1999). Bound LuxR stimulates transcription of the lux operon and bioluminescence results. In the case of V. fischeri, transcription of luxI is also stimulated, so the levels of signal molecule rise still higher as a result of positive feedback (Shadel and Baldwin, 1991). It is generally argued that linking light production to cell density has evolved because it prevents the energetically expensive process of bioluminescence from operating under conditions in which it is of no benefit.

The finding in the early 1990s that similar autoinducer molecules regulated the production of antibiotic and pathogenicity determinants by the terrestrial plant pathogen E. carotovora was the first indication of how widespread this form of signalling would prove to be (Bainton et al., 1992). However, in E. carotovora, the mechanism by which the AHL responses are regulated deviates from the lux model.

Erwinia carotovora

  1. Top of page
  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References

Erwinia carotovora is a Gram-negative bacterial phytopathogen that is the causative agent of plant soft rots and the potato disease blackleg (stem rot). Two subspecies are recognized, Erwinia carotovora spp. carotovora (Ecc) and Erwinia carotovora spp. atroseptica (Eca); Eca has a narrower host range, being primarily a pathogen of potato (reviewed by Perombelon, 2002; Toth et al., 2003). Different laboratories have favoured various isolates of Ecc or Eca but, despite subtle differences, a unified gene regulatory network is emerging.

In Ecc strain ATCC39048, the AHL signal molecule(s), which usually consist(s) of 3-oxo-C6-HSL and/or C6-HSL, is generated by the action of the LuxI homologue, CarI (also known as ExpI). The LuxR homologue, CarR (McGowan et al., 1995), forms dimers and binds upstream of the operon encoding the enzymes for carbapenem synthesis. CarR dimers can bind to DNA in the absence of signal molecule, but binding of the autoinducer to CarR is required for transcriptional activation of these genes – possibly by inducing the formation of CarR–AHL multimers (Welch et al., 2000). This system results in carbapenem production only at high cell densities, when the concentration of 3-oxo-C6HSL reaches 0.1 µg ml−1. It has been proposed that the production of antibiotics at high cell densities has evolved as a mechanism for niche protection, excluding opportunistic bacteria from taking advantage of the nutrient-rich environment produced during an Erwinia infection (Axelrood et al., 1988). The regulation of antibiotic production by AHLs thus resembles the classic lux model. However, AHLs are also required to induce the production of secreted plant cell wall-degrading exoenzymes (PCWDEs) as well as for the expression of harpin genes and components of a type III secretion system. The type III system contributes to the pathogenicity of E. carotovora, particularly in the initial stages of infection (Rantakari et al., 2001). By analogy with non-macerating plant pathogenic pseudomonads (Deslandes et al., 2003; Jin et al., 2003), this system may secrete virulence determinant(s) directly into the plant cell. However, what the secreted substrates are and how they contribute to virulence is currently not known.

The PCWDEs and harpins are the major virulence determinants and, as a consequence, carI mutants are essentially non-pathogenic. However, although there is a dependence upon the AHL signal, this is only one of several plant and environmentally derived signals that must be integrated to regulate pathogenicity.

Exoenzyme production is unaltered in Ecc harbouring a disrupted carR gene, indicating that this is not the AHL receptor activator driving PCWDE production. A second AHL receptor-like protein is encoded by expR and is transcribed convergently with carI. However, expR mutants have normal (cited by Whitehead et al., 2002) or slightly increased (Andersson et al., 2000) PCWDEs levels, depending on the Ecc strain studied. Conversely, overexpression of ExpR results in reduced PCWDE levels and a concomitant increase in the levels of a small RNA-binding protein, RsmA. Thus, there is either an as yet unidentified AHL receptor activator, or AHLs regulate exoenzyme production via an alternative, indirect mechanism. This latter possibility has been raised by Chatterjee et al. (2002), who suggested that AHLs might act to remove a post-transcriptional block on exoenzyme gene expression. The model suggests that RsmA (regulator of secondary metabolism) binds to exoenzyme (and possibly carI) mRNAs and promotes their degradation, and that AHL signalling acts to remove this inhibitory activity. This model is supported by the fact that carI mutants (which do not produce AHLs and PCWDEs) can be restored to PCWDE production by a second mutation that inactivates rsmA (Chatterjee et al., 1995) (Fig. 1). In addition, rsmA expression in E. carotovora is stimulated by 3-oxo-C6-HSL deficiency (Chatterjee et al., 2002). RsmA has homologues in many Gram-negative bacteria (Heeb et al., 2002) and is closely related to CsrA (carbon storage regulator) of Escherichia coli, which has been shown to bind to specific mRNAs and to promote their degradation (Liu and Romeo, 1997). Like the E. coli system, RsmA activity can be blocked by the expression of a non-coding RNA transcript from rsmB (Liu et al., 1998). This transcript is initially 479 nucleotides in length and contains nine putative RsmA-binding motifs (Liu et al., 1998). These multiple binding sites may titrate out RsmA, allowing unbound mRNAs encoding PCWDEs to be translated. Interaction with RsmA appears to induce cleavage and release of a short 259 nucleotide RNA (rsmB′) that may also suppress RsmA expression by an unknown mechanism. Transcription of rsmB is regulated via a two-component sensor kinase/response regulator system consisting of ExpS and ExpA respectively (Eriksson et al., 1998; Cui et al., 2000). ExpS and ExpA appear to be equivalent to GacS and GacA (also known as LemA and GacA) found in a number of Gram-negative bacteria (Heeb and Haas, 2001). In addition to E. carotovora, homologues of this sensor kinase two-component system regulate pathogenicity in a number of pseudomonad plant pathogens including Pseudomonas marginalis CY091, P. syringae, P. viridiflava and P. aeruginosa (reviewed by Heeb and Haas, 2001). Production of secondary metabolites with antifungal activity by the rhizosphere-colonizing biological control strains P. aureofaciens 30-84 and P. chlororaphis PCL1391 is also dependent upon a LemA/GacA-regulated mechanism. In this case, GacA/GacS controls an AHL signalling system (Chancey et al., 1999; Chin-A-Woeng et al., 2001; G. V. Bloemberg, personal communication). However, GacA/GacS also regulates biological control activity in P. fluorescens strain CHA0, which does not appear to use AHL signalling (Bull et al., 2001). The ligand or stimulus to which GacS responds is unknown, but a dichloromethane-extractable low-molecular-weight compound(s) that activates GacS/GacA-dependent ex-pression of a reporter construct has been isolated from late log phase cultures of P. fluorescens CHA0 (Zuber et al., 2003). This activator was not an AHL; thus, it appears that a second autoinducer (possibly quorum sensing) system is active in many Gram-negative bacteria. It is not yet known whether all bacteria that use GacS/GacA respond to the same signal molecule or if, like AHL signalling, different bacteria produce different but related molecules. It will be interesting to determine whether E. carotovora cultures produce similar compounds capable of cross-activating the P. fluorescens CHA0 reporter. Identifying the ligand to which this sensor responds remains one of the most exciting challenges in this area.

In E. carotovora, a further level of complexity results from the fact that the expression of many of the plant cell wall-degrading enzymes is positively regulated by plant cell wall breakdown products that are generated by the action of the bacterial pectinases on the plant tissues. These products include 5-keto-4-deoxouronate, 2,5-diketo-3-deoxygluconate and 2-keto-3-deoxygluconate (KDG) (Chatterjee et al., 1985; Nasser et al., 1994). Regulation arises via dissociation of the transcriptional repressor KdgR in the presence of these metabolites (Liu et al., 1999). Binding sites for the KdgR repressor exist not only in the operators of many of the PCWDEs but also in rsmB and, thus, the initial production of pectinases leads to a further induction of virulence genes at both transcriptional and post-transcriptional levels (Hyytiainen et al., 2001) (Fig. 1).

Breakdown products released by the action of the bacterial PCWDEs also act as signalling molecules for the plant itself, indicating the presence of a pathogenic attack and triggering the hypersensitive disease response.

The quorum-sensing model proposes that placing pathogenicity-associated genes under density-dependent control provides a mechanism for avoiding the host plant's defence systems (reviewed by Whitehead et al., 2002). Treating plants with purified Ecc pectic enzymes leads to the induction of plant defence responses as a result of the release of cell wall-derived oligogalacturonide elicitors (Palva et al., 1993). Furthermore, this induction of plant defences by exoenzyme treatment confers increased resistance against subsequent Ecc infections. A similar systemic resistance to Ecc infection is also seen when the plant defence response is artificially induced by the application of salicylic acid (Palva et al., 1994), one of the plant-produced compounds involved in signalling a pathogen attack. A successful E. carotovora infection requires a relatively high inoculum (107 cells g−1 diseased tissue; Perombelon, 2002), and the progression of the disease is then a competition between bacterial multiplication and the development of plant resistance (Perombelon and Kelman, 1980). Thus, the premature production of plant cell wall-degrading enzymes at low cell densities would not give rise to a successful infection, but would result in the induction of the local and systemic plant defence response, which in turn would hamper subsequent infections. According to this model, Erwinia uses AHLs to monitor its cell density and only initiates a pathogenic attack when its population density is above a critical level, which ensures a high probability of overcoming host resistance.

In the case of potato, Erwinia can be present at relatively high levels as a latent infection, although it is not known whether the bacteria is truly dormant or if there is a slow turnover during this period. Breaking of latency is often associated with the presence of free water and anaerobic conditions. This may assist infection in two ways: there is increased leakage of cell contents, which will increase available nutrients and promote initial bacterial growth, and there is suppression of oxygen-dependent host resistance systems (Perombelon, 2002). These include free radical, phenolic and phytoalexin production as well as lignification and suberization of the cell wall. The cell wall modifications give a surprising level of protection: injection of tubers with cell-free Erwinia culture supernatants containing large quantities of PCWDEs causes tissue maceration under anaerobic but not aerobic conditions (Maher and Kelman, 1983). If AHL regulation of exoenzyme production exists as a mechanism for preventing premature infection by a small number of cells from triggering host resistance, then it would be anticipated that exposing such a subquorate group of bacteria to a level of AHLs that gave a false indication of the local population size would result in host resistance (at least in an aerobic infection). To test this, transgenic plants have been made that express bacterial AHL synthases (Fray et al., 1999; Mae et al., 2001). Mae et al. (2001) found that tobacco plants producing 3-oxo-C6HSL were less likely to be infected by an inoculum of Erwinia carotovora, although the disease progression was unaffected, and the resistance could be overcome by increasing the inoculum fourfold. However, as tobacco is not a normal host for E. carotovora, relatively high concentrations of bacteria were used for infection assays, typically between 2 × 105 and 2 × 106 colony-forming units (cfu) in a volume of 5 µl (a level that might have been predicted to exceed the quorum-sensing threshold). We have carried out similar infection studies in transgenic potato plants modified to produce 3-oxo-C6-HSL and C6-HSL (Toth et al., unpubl., cited in Fray 2002). In this system, we find that a successful stem infection can be initiated with as few as 102 cfu, a level two orders of magnitude lower than would normally be required. The fact that an infection is possible with such a low titre in the case of potato, yet is blocked by AHLs in the case of tobacco, raises questions regarding the role of quorum sensing during naturally occurring Erwinia infections. It is possible that, although the quorum-sensing threshold is apparently set above the level required for a successful infection on potato, on other, more resistant species, a larger initiating inoculum is required. In this case, setting a higher threshold for the expression of pathogenicity-associated genes may represent the best strategy for maximizing potential host range. Alternatively, the preliminary infection may not be dependent upon AHL-induced PCWDEs, but favourable conditions (a suppressed disease response and available nutrients) might allow initial bacterial proliferation. Only as nutrient availability becomes limiting in late log phase would there be a requirement for PCWDEs to release further nutrients allowing disease progression and causing the appearance of disease symptoms. If this were the case, then AHL signalling would not be a mechanism for avoiding plant defences, but simply a means of increasing nutrient supply and ensuring disease continuation. In an alterative model, Redfield (2002) has argued that most bacterial autoinducer-sensing systems have not evolved for intercellular communication and group co-ordination at all; rather, their primary function is as a means of detecting the extent of diffusion and mixing in the cell's microenvironment. In this scenario, autoinducer levels will rise above threshold levels not only when the producing population is large but also when rates of diffusion away from an individual producing cell are low, for example as a result of physical barriers encountered by a bacterium or microcolony in plant intercellular spaces.

Regardless of what information is communicated, AHL-mediated signalling is clearly important during the infection process. This makes it an attractive target for strategies that aim to limit disease severity. To date, the most effective of these has been the use of a bacterial AHL lactonase enzyme, AiiA, which inactivates AHLs by opening the lactone ring (Dong et al., 2000). Transgenic potato plants expressing AiiA showed increased resistance to Erwinia infections (Dong et al., 2001), and a measure of biological control activity was provided in co-infection assays using bacterial strains capable of degrading the AHL signal (Molina et al., 2003; Uroz et al., 2003). Given the importance of AHL signalling during pathogenesis, it might not be surprising if plants had evolved strategies to interfere with this form of bacterial signalling. One of the first responses of the plant after exposure to E. carotovora or to pectic oligomers is a rapid influx of protons into the cells around the wound site. This has the effect of raising the pH of the apoplastic fluid between cells in the immediate vicinity from < 6.4 to > 8.2 (Baker et al., 1990). AHLs are rapidly degraded after alkalization of bacterial growth medium above pH 8.0, so a similar alkalization around a wound site would be anticipated to promote AHL turnover and may thus slow or halt the production of virulence factors. Other mechanisms by which plants might interfere with AHL-directed PCWDE secretion could include the production of signal mimics, signal blockers or signal-degrading enzymes or the production of compounds that block the activity of the AHL-producing enzymes. The marine algae Delisea pulchra produces halogenated furanones, which have some structural similarity to AHLs and appear to act as inhibitors of AHL perception in vivo– preventing bacterial cell swarming and attachment responses that lead to the build-up of bacterial biofilms on the algal surfaces (Givskov et al., 1996; Gram et al., 1996). Teplitski et al. (2000) reported AHL inhibitory activities in exudates from pea seedlings. The compounds responsible have not been identified, but they preferentially partition into polar solvents (unlike the AHL molecules themselves). We have also found compounds in a number of plant extracts that have similar partitioning characteristics in aqueous solvents: inhibitory activities were particularly pronounced in fruit (grape and strawberry) extracts, but were not detected in common host plants for Ecc such as potato (Fray, 2002). Bacterial phenotypes controlled through quorum sensing are frequently regulated by additional environmental cues. In some cases, AHL responses can be modulated or over-ridden by factors such as oxygen tension, nutrient starvation, iron limitation or catabolite repression (our unpublished results). It is possible that the plant-produced compounds are indirectly altering the bacterial AHL response rather than targeting it directly but, even if this were the case, such compounds could prove to be important in determining the outcome of interactions between higher plants and a diversity of pathogenic and symbiotic bacteria. Some authors have reported evidence of plant-derived AHL mimics capable of weakly activating quorum-sensing responses (Teplitski et al., 2000). It is important that a number of different biosensors are used in such experiments as the E. coli luxABCDE sensors (Winson et al., 1998) that are most commonly used show a considerable level of background light production in late log and early stationary phase and, although less than true AHL-induced activation, this background light production can be strongly affected by subtle variations in media composition or growing conditions.

It is now apparent that AHLs can elicit a range of specific responses in a number of eukaryotes including acting as chemoattractants for the zoospore stage of the green alga Enteromorpha (which preferentially settles on AHL-producing bacterial biofilms) (Joint et al., 2002). Oxo-C12HSL has been found to have immune modulatory effects in mammals, causing murine immune systems to become less effective against bacteria (Telford et al., 1998). Mathesius et al. (2003) found extensive alterations in the proteome profiles of Medicago truncatula roots exposed to physiologically relevant levels of long-chain AHLs (generally regarded as those AHLs with a carbon chain length of 10 or more), raising the possibility that some plants may be able to respond directly to the signal molecules produced by pathogenic or beneficial plant microbes.

Agrobacterium tumefaciens

  1. Top of page
  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References

Agrobacterium tumefaciens is a Gram-negative bacteria that causes crown gall tumours in plants by transferring a fragment of DNA into the nuclear genome of the host plant. Transformed plant tissues proliferate and produce opines, which serve as carbon and nitrogen sources for the Agrobacterium. There is little evidence that AHL-mediated quorum sensing is used directly to regulate the expression of pathogenicity determinants in the case of A. tumefaciens. Here, the main role of AHL signalling found to date is in regulating the initiation of conjugation and the transfer of the tumour-inducing (Ti) virulence plasmid to a Ti-plasmidless saprophytic A. tumefaciens recipient in the tumoursphere. In E. carotovora, plant-derived cell wall breakdown products that arise as a result of bacterial infection feed back (via KdgR) to upregulate the AHL response. A. tumefaciens takes this one step further and can only respond to the AHL signal if an appropriate opine (a plant-derived infection product) is also present (Fuqua and Winans, 1996).

A successful infection by A. tumefaciens results in a section of the Ti plasmid (the T-DNA) being transferred and integrated into the host plant's nuclear genome. The T-DNA encodes genes that, when expressed by the host plant, manipulate plant hormone levels and phytohormone sensitivity, resulting in the proliferation of transformed cells that results in the characteristic crown gall tumour. In addition, the T-DNA contains genes that direct the synthesis of opines – unique condensation products of an amino acid with a ketoacid or sugar. Genes for opine catabolism are encoded on the Ti plasmid but outside the transfer DNA (T-DNA).The ability to catabolize opines is rare among other soil bacteria; thus, the induced plant tumour becomes a cell factory supplying the infecting bacteria with large quantities of a unique carbon and nitrogen source (Guyon et al., 1993).

Agrobacterial strains and the Ti plasmids that they contain are categorized according to the class of opines whose synthesis they encode. For both octopine and nopaline types, conjugation is induced when TraR (a homologue of LuxR and CarR) binds its cognate AHL (3-oxo-C8-HSL), synthesized by the traI gene product (a homologue of LuxI and CarI) (Fig. 2). In the absence of AHLs, TraR is present as insoluble monomers that are rapidly degraded by proteases. In the presence of AHLs, however, the tertiary structure of the protein is altered, and soluble TraR dimers can form (Zhu and Winans, 2001; Vannini et al., 2002). TraR dimers are resistant to proteases and are capable of transcription activation after binding to a conserved 18 bp target sequence (the tra box) in the promoters of tra and trb operons (Zhu and Winans, 1999), the product of which are required for conjugation. It has been suggested that, before AHL binding, the fat-soluble TraR monomers are predominantly found in the A. tumefaciens cytoplasmic membrane, whereas after exposure to 3-oxo-C8-HSL, AHL-containing TraR dimers are found in the cytoplasm (Qin et al., 2000). It was proposed that a membrane location for the TraR monomer may favour interaction with externally sourced AHLs. (Qin et al., 2000). TraR is not, however, constitutively expressed; in both octopine and nopaline strains, traR is placed in the same transcriptional units as genes encoding the enzymes required for catabolism of the appropriate opines. Transcription of these opine catabolism genes is induced by octopine or agrocinopines, produced by the transformed tumour cells. In the nopaline-type Ti plasmid pTiC58, traR is part of the arc operon, a group of five genes required for nopaline catabolism. Expression of the arc operon is repressed by AccR, which binds specifically to a sequence in the arc promoter in the absence of agrocinopines (an opine produced by the nopaline-type strains). This DNA-binding activity is inhibited by agrocinopines A and B, which thus serve to derepress the arc operon, inducing both opine catabolism and TraR expression (Piper et al., 1999) (Fig. 2). A similar situation exists for octopine-type Ti plasmids: here, TraR is one of 14 genes in the occ operon (the other 13 encode products for opine transport or catabolism). The occ operon is positively regulated by OccR, which binds as a tetramer to the occ promoter in the presence and absence of octopine. Octopine causes prebound OccR to undergo a conformational change that partially relaxes a high-angle DNA bend and activates transcription (Akakura and Winans, 2002a,b). The occ and arc operons appear to be unrelated; thus, similar mechanisms for placing traR under opine induction have evolved independently on two occasions. This suggests a strong evolutionary pressure to restrict bacterial conjugation to the environment of the transformed tumour cells.

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Figure 2. Regulation of conjugation in nopaline-type A. tumefaciens C58. Bacterial cells produce 3-oxo-C8-HSL but can only respond to this signal molecule if the AHL receptor, TraR, is expressed. The traR gene is located within the arc operon, the other genes of which encode enzymes for agrocinopine catabolism. The transcriptional repressor AccR (red squares) binds the arc promoter but is released in the presence of agrocinopines derived from the transformed tumour cells. TraR binds AHL and forms a dimmer complex capable of activating transcription of the tra and trb operons. The system is attenuated by the antiactivator, TraM, and by AHL-lactonase(s). In octopine-type Agrobacterial strains, transcription of traR is activated by octopine interacting with OccR, a transcriptional activator.

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Even after induction by opines and AHL-mediated dimerization, much of the newly synthesized TraR is not free to bind DNA and activate the transcription of tra and trb. This is because an 11.2 kDa antiactivator protein, TraM, associates with dimerized TraR and prevents DNA binding. Binding of TraM to TraR is independent of AHLs and occurs regardless of whether TraR is free or already bound to DNA (Luo et al., 2000). Null mutations in traM result in premature expression of tra and trb and constitutive conjugation even at low population density (Piper and Farrand, 2000). TraM has been proposed to fulfil two roles: to prevent small amounts of TraR from inducing conjugation in the absence of opines or of sufficient AHLs and, secondly, to downregulate tra and trb transcription, preventing TraR from inducing excessive levels of conjugation. This latter role is consistent with the finding that TraM can displace bound TraR, and that TraR (in the presence of AHL) activates TraM expression (Luo et al., 2000). A further method for attenuating AHL responses, and possibly of actually exiting from quorum sensing-dependent conjugation, is conferred by an AHL-lactonase, AttM (homologous to AiiA), that is expressed in stationary phase (Zhang et al., 2002) and may serve to remove the AHL signal from the local environment. A. tumefaciens C58 actually contains three genes homologous to AiiA, although only two of these have been shown to possess AHL-lactonase activity (Carlier et al., 2003), and only the expression of attM has been studied in detail. The expression of attM is suppressed by AttJ, which specifically binds to the attM promoter, until A. tumefaciens cells reach stationary phase. The trigger for derepression of attM is not clear, although it does not appear to be triggered by 3-oxo-C8-HSL (Zhang et al., 2002).

These layers of regulation suggest that AHL-mediated quorum sensing in A. tumefaciens has evolved to give a brief burst of conjugation activity when the bacterial population is at a sufficiently high density, but only when it is in the presence of bacterially transformed plant tissues that produce the cognate opine. The evolutionary advantage of this system remains unclear.

It has been proposed recently that, for many bacterial species, the primary role of secreted autoinducers is not as a means of measuring population size, but rather is used to sense local diffusion rates in the microenvironment surrounding the producing cell (Redfield, 2002). It is certainly true that the number of cells required to produce the threshold level of AHL will be dependent upon local diffusion rates, which could vary dramatically on a microscopic scale. In addition, the diffusion rate may also be affected by the length of the acyl side-chain. AHLs with chain lengths of six carbons or less (short-chain signals) appear to traverse bacterial membranes freely by passive diffusion, whereas more hydrophobic molecules, such as 3-oxo-C12HSL, with acyl chains of 12 carbons require a dedicated export system (Pearson et al., 1999). We have made transgenic plants containing plastid-targeted bacterial LuxI homologues that result in AHL production in the chloroplast compartment of the cell. When C6-HSL is made, it is readily detected at the leaf surface (presumably as a result of efficient diffusion across the chloroplast and plant cell plasma membranes) (Fray et al., 1999). However, when 3-oxo-C12-HSL is made, almost all the signal molecule is retained within the chloroplast (our unpublished results). If it is the case that short- but not long-chain AHLs can diffuse across plant membranes, then in the confines of plant tissues, the ‘quorum-sensing threshold’ will be dependent upon the number of AI-producing bacteria as well as the physical properties of the AHL molecule. Many plant-associated bacteria produce both long- and short-chain AHL molecules (Table 1). When confined by plant tissues, the relative levels of the more hydrophobic long-chain AHLs may rise because of the presence of a greater diffusion barrier, or they may decrease if their hydrophobic nature results in preferential sequestration into the host membranes. In either event, the ratio of long- to short-chain AHLs is likely to be altered, and bacteria could use this information to regulate behaviour appropriately. It is curious that the most hydrophobic AHL molecules are produced by species of Rhizobium which, during their plant-associated form, are found in multiple small nitrogen-fixing symbiosomes completely surrounded by plant-derived membranes within a host cell. However, it is currently not known whether the differentiated bacteroids within these symbiosomes are producing an AHL signal or whether such communication is reserved for the free-living form.

AHL-mediated signalling is clearly important in many plant–microbe associations. The information communicated by such autoinducers has been questioned but, in most cases, it is likely to include some aspect of population density. However, AHL levels alone are often insufficient to trigger the full range of behavioural changes, and such quorum-sensing systems form part of integrated signalling networks. Inputs from additional autoinducers, the host plant and environmental sources come together to induce the ‘quorum-sensing’ behaviour.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References

This work was supported by a Biotechnology and Biological Sciences Research Council Sir David Phillips Fellowship and European Framework Five grant QLK3-CT-2000-31759 awarded to R.G.F., both of which are gratefully acknowledged.

References

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  2. Summary
  3. Introduction
  4. The AHL signalling molecules and the lux paradigm
  5. Erwinia carotovora
  6. Agrobacterium tumefaciens
  7. Acknowledgements
  8. References
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