Extracellular virulence factors of Pseudomonas solanacearum: role in disease and regulation of expression

Investigation of antioxidant and anticancer activities of unsaturated oligo-galacturonic acids produced by pectinase of Streptomyces hydrogenans YAM1

Pectin, a diverse carbohydrate polymer in plants consists of a core of α-1,4-linked D-galacturonic acid units, includes a vast portion of fruit and agricultural wastes. Using the wastes to produce beneficial compounds is a new approach to control the negative environmental impacts of the accumulated wastes. In the present study, we report a pectinase producing bacterium Streptomyces hydrogenans YAM1 and evaluate antioxidative and anticancer effects of the oligosaccharides obtained from pectin degradation. The production of oligosaccharides due to pectinase activity was detected by thin layer chromatography (TLC) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Our results revealed that S. hydrogenans YAM1 can degrade pectin to unsaturated pectic oligo-galacturonic acids (POS) with approximately 93% radical scavenging activity in 20 mg/mL which it is more than 50% of the same concentration of pectin. Flow cytometric analysis revealed that MCF-7 cells viability decreased more than 32 and 92% following treatment with 6 and 20 mg/mL POS after 24 h, respectively. It is suggested that pectin degradation by S. hydrogenans YAM1 is not only a new approach to produce highly active compounds from fruit wastes, but also is an effective method to remove fibrous pollutants from different environments.

Phylotype and sequevar of Ralstonia solanacearum which causes bacterial wilt in Curcuma alismatifolia Gagnep

Bacterial wilt of Curcuma alismatifolia (Patumma) caused by Ralstonia solanacearum is a major disease affecting the quality of rhizome exports. Traditionally, R. solanacearum is classified into five races based on differences in host range and six biovars based on biochemical properties. Recently a classification scheme based on phylotypes and sequevars was presented by the scientific community as a tool for determining phylogenetic relationships within R. solanacearum. This study used traditional and molecular methods to identify R. solanacearum strains from Patumma. All the strains were identified as biovar 4. A phylotype-specific multiplex PCR-based phylotyping of all the isolates detected the phylotype I-specific amplicon of 144 bp and the R. solanacearum-specific 281 bp amplicon. Phylogenetic analyses of endoglucanase (egl) sequences clustered all three strains of Patumma into phylotype I, sequevar 48 with reference strains M2 and M6. The study determined that the R. solanacearum strains from Patumma belong to biovar 4, phylotype I that originated from Asia, and sequevar 48.

SIGNIFICANCE AND IMPACT OF THE STUDY

Phylotype and sequevar of Ralstonia solanacearum were associated with geographic region and geographic distribution. This is the first study to identify phylotype and sequevar of R. solanacearum from Patumma in Chiang Mai, Thailand. This will be useful for study of disease epidemiology and could help management for control of bacterial wilt diseases in this host.

Plant genome complexity may be a factor limiting in situ the transfer of transgenic plant genes to the phytopathogen Ralstonia solanacearum

The development of natural competence by bacteria in situ is considered one of the main factors limiting transformation-mediated gene exchanges in the environment. Ralstonia solanacearum is a plant pathogen that is also a naturally transformable bacterium that can develop the competence state during infection of its host. We have attempted to determine whether this bacterium could become the recipient of plant genes. We initially demonstrated that plant DNA was released close to the infecting bacteria. We constructed and tested various combinations of transgenic plants and recipient bacteria to show that the effectiveness of such transfers was directly related to the ratio of the complexity of the plant genome to the number of copies of the transgene.

Control of Virulence and Pathogenicity Genes of Ralstonia Solanacearum by an Elaborate Sensory Network

Ralstonia solanacearum causes a lethal bacterial wilt disease of diverse plants. It invades the xylem vessels of roots and disseminates into the stem where it multiplies and wilts by excessive exopolysaccharide production. Many of its key extracytoplasmic virulence and pathogenicity factors are transcriptionally controlled by an extensive network of distinct, interacting signal transduction pathways. The core of this sensory network is the five-gene Phc system that regulates exopolysaccharide, cell-wall-degrading exoenzymes, and other factors in response to a self-produced signal molecule that monitors the pathogen's growth status and environment. Four additional environmentally responsive two-component systems work independently and with the Phc system to fine-tune virulence gene expression. Another critical system is Prh which transduces plant cell-derived signals through a six-gene cascade to activate deployment of the Type III secretion pathway encoded by the hrp pathogenicity genes. Here I summarize knowledge about the regulated targets, signal transduction mechanisms, and crosstalk between Phc, Prh, and other systems. I also provide insight into why R. solanacearum has evolved such a sophisticated sensory apparatus, and how it functions in disease.

Genetic characterization of RRS1, a recessive locus in Arabidopsis thaliana that confers resistance to the bacterial soilborne pathogen Ralstonia solanacearum

The soilborne, vascular pathogen Ralstonia solanacearum, the causative agent of bacterial wilt, was shown to infect a range of Arabidopsis thaliana accessions. The pathogen was capable of infecting the Col-5 accession in an hrp-dependent manner, following root inoculation. Elevated bacterial population levels were found in leaves of Col-5, 4 to 5 days after root inoculation by the GMI1000 strain. Bacteria were found predominantly in the xylem vessels and spread systematically throughout the plant. The Nd-1 accession of A. thaliana was resistant to the GMI1000 strain of R. solanacearum. Bacterial concentrations detected in leaves of Nd-1, inoculated with an hrp+ strain of R. solanacearum, were only slightly higher than those detected in the susceptible accession, Col-5, following inoculation with a strain whose hrp gene cluster was deleted. Leaf inoculation of the GMI1000 strain on the resistant accession Nd-1 induced the formation of lesions in the older leaves of the rosette whereas the same strain of R. solanacearum provoked complete wilting of Col-5. Resistance to strain GMI1000 of R. solanacearum segregated as a simply inherited recessive trait in a genetic cross between Col-5 and Nd-1. F9 recombinant inbred lines generated between these two accessions were used to map a locus, RRS1, that was the major determinant of resistance between restriction fragment length polymorphism markers mi83 and mi61 on chromosome V. This region of the A. thaliana genome is known to contain many other pathogen recognition capabilities.

Joint transcriptional control of xpsR, the unusual signal integrator of the Ralstonia solanacearum virulence gene regulatory network, by a response regulator and a LysR-type transcriptional activator

Ralstonia (Pseudomonas) solanacearum is a soil-borne phytopathogen that causes a wilting disease of many important crops. It makes large amounts of the exopolysaccharide EPS I, which it requires for efficient colonization, wilting, and killing of plants. Transcription of the eps operon, encoding biosynthetic enzymes for EPS I, is controlled by a unique and complex sensory network that responds to multiple environmental signals. This network is comprised of the novel transcriptional activator XpsR, three distinct two-component regulatory systems (VsrAD, VsrBC, and PhcSR), and the LysR-type regulator PhcA, which is under the control of PhcSR. Here we show that the xpsR promoter (PxpsR) is simultaneously controlled by PhcA and VsrD, permitting XpsR to act like a signal integrator, simultaneously coordinating signal input into the eps promoter from both VsrAD and PhcSR. Additionally, we used in vivo expression analysis and in vitro DNA binding assays with substitution and deletion mutants of PxpsR to show the following. (i) PhcA primarily interacts with a typical 14-bp LysR-type consensus sequence around position -77, causing a sixfold activation of PxpsR; a weaker, less-defined binding site between -183 and -239 likely enhances PhcA binding and activation via the -77 site another twofold. (ii) Full 70-fold activation of PxpsR requires the additional interaction of the VsrD response regulator (or its surrogate) with a 14-bp dyadic sequence centered around -315 where it enhances activation (and possibly binding) by PhcA; however, VsrD alone cannot activate PxpsR. (iii) Increasing the distance between the putative VsrD binding site from that of PhcA by up to 232 bp did not dramatically affect PxpsR activation or regulation.

An exo-poly-alpha-D-galacturonosidase, PehB, is required for wild-type virulence of Ralstonia solanacearum

Ralstonia solanacearum, which causes bacterial wilt disease of many plant species, produces several extracellular plant cell wall-degrading enzymes that are suspected virulence factors. These include a previously described endopolygalacturonase (PG), PehA, and two exo-PGs. A gene encoding one of the exo-PGs, pehB, was cloned from R. solanacearum K60. The DNA fragment specifying PehB contained a 2,103-bp open reading frame that encodes a protein of 74.2 kDa with a typical N-terminal signal sequence. The cloned pehB gene product cleaves polygalacturonic acid into digalacturonic acid units. The amino acid sequence of pehB resembles that of pehX, an exo-PG gene from Erwinia chrysanthemi, with 47.2% identity at the amino acid level. PehB also has limited similarity to plant exo-PGs from Zea mays and Arabidopsis thaliana. The chromosomal pehB genes in R. solanacearum wild-type strain K60 and in an endo-PG PehA- strain were replaced with an insertionally inactivated copy of pehB. The resulting mutants were deficient in the production of PehB and of both PehA and PehB, respectively. The pehB mutant was significantly less virulent than the wild-type strain in eggplant virulence assays using a soil inoculation method. However, the pehA mutant was even less virulent, and the pehA pehB double mutant was the least virulent of all. These results suggest that PehB is required for a wild-type level of virulence in R. solanacearum although its individual role in wilt disease development may be minor. Together with endo-PG PehA, however, PehB contributes substantially to the virulence of R. solanacearum.

A two-component system in Ralstonia (Pseudomonas) solanacearum modulates production of PhcA-regulated virulence factors in response to 3-hydroxypalmitic acid methyl ester

Expression of virulence factors in Ralstonia solanacearum is controlled by a complex regulatory network, at the center of which is PhcA, a LysR family transcriptional regulator. We report here that expression of phcA and production of PhcA-regulated virulence factors are affected by products of the putative operon phcBSR(Q). phcB is required for production of an extracellular factor (EF), tentatively identified as the fatty acid derivative 3-hydroxypalmitic acid methyl ester (3-OH PAME), but a biochemical function for PhcB could not be deduced from DNA sequence analysis. The other genes in the putative operon are predicted to encode proteins homologous to members of two-component signal transduction systems: PhcS has amino acid similarity to histidine kinase sensors, whereas PhcR and OrfQ are similar to response regulators. PhcR is quite unusual because its putative output domain strongly resembles the histidine kinase domain of a sensor protein. Production of the PhcA-regulated factors exopolysaccharide I, endoglucanase, and pectin methyl esterase was reduced 10- to 100-fold only in mutants with a nonpolar insertion in phcB [which express phcSR(Q) in the absence of the EF]; simultaneously, expression of phcA was reduced fivefold. Both a wild-type phenotype and phcA expression were restored by addition of 3-OH PAME to growing cultures. Mutants with polar insertions in phcB or lacking the entire phcBSR(Q) region produced wild-type levels of PhcA-regulated virulence factors. The genetic data suggest that PhcS and PhcR function together to regulate expression of phcA, but the biochemical mechanism for this is unclear. At low levels of the EF, it is likely that PhcS phosphorylates PhcR, and then PhcR interacts either with PhcA (which is required for full expression of phcA) or an unknown component of the signal cascade to inhibit expression of phcA. When the EF reaches a threshold concentration, we suggest that it reduces the ability of PhcS to phosphorylate PhcR, resulting in increased expression of phcA and production of PhcA-regulated factors.

A complex network regulates expression of eps and other virulence genes of Pseudomonas solanacearum

We have discovered an unusual and complex regulatory network used by the phytopathogen Pseudomonas solanacearum to control transcription of eps, which encodes for production of its primary virulence factor, the exopolysaccharide EPS I. The major modules of this network were shown to be three separate signal transduction systems: PhcA, a LysR-type transcriptional regulator, an dual two-component regulatory systems, VsrA/VsrD and VsrB/VsrC. Using lacZ fusions and RNA analysis, we found that both PhcA and VsrA/VsrD control transcription of another network component, xpsR, which in turn acts in conjunction with vsrB/vsrC to increase transcription of the eps promoter by > 25-fold. Moreover, gel shift DNA binding assays showed that PhcA specifically binds to the xpsR promoter region. Thus, the unique XpsR protein interconnects the three signal transduction systems, forming a network for convergent control of EPS I in simultaneous response to multiple environmental inputs. In addition, we demonstrate that each individual signaling system of the network also acts independently to divergently regulate other unique sets of virulence factors. The purpose of this complex network may be to allow this phytopathogen to both coordinately or independently regulate diverse virulence factors in order to cope with the dynamic situations and conditions encountered during interactions with plants.

Phenotype conversion in Pseudomonas solanacearum due to spontaneous inactivation of PhcA, a putative LysR transcriptional regulator

Phenotype conversion (PC) in Pseudomonas solanacearum is the coordinated change in production of extracellular polysaccharide and a variety of extracellular proteins, some of which contribute to virulence. Although PC is normally spontaneous, it is mimicked by transposon inactivation of the phcA locus (S. M. Brumbley and T. P. Denny, J. Bacteriol. 172:5677-5685, 1990). The DNA sequence of a 1.8-kb region from strain AW1 that contains phcA revealed one open reading frame that should encode a polypeptide of 38.6 kDa. The PhcA protein produced in Escherichia coli by using a T7 RNA polymerase expression system was of the predicted size. The deduced amino acid sequence of PhcA is similar to that of some members of the LysR transcriptional activator gene family, especially in the amino terminus, where a putative helix-turn-helix DNA-binding motif was identified. An analogous allele (phcA1) was cloned from the spontaneous PC mutant strain AW1-PC and found to be nonfunctional in complementation studies. When phcA1 was expressed in E. coli, the PhcA1 protein was 35.5 kDa, 3 kDa smaller than PhcA. Sequence analysis of phcA1 and chimeric constructs of phcA and phcA1 confirmed that PhcA1 is truncated by a 2-bp insertion 147 nucleotides upstream of the carboxyl terminus of PhcA. Southern blot analysis of 10 additional independently isolated PC mutants of strain AW1 revealed that two strains have larger insertions (0.2 and 1.0 kb) within phcA. These results suggest that phcA encodes a DNA-binding protein that regulates the transcription of one or more of the genes involved in P. solanacearum virulence and that spontaneous PC can be attributed to one of several different insertions within this locus.