Contribution of folate biosynthesis to Ralstonia solanacearum proliferation in intercellular spaces

Nicotiana benthamiana, a Surrogate Host to Study Novel Virulence Mechanisms of Gram-Positive Bacteria, Clavibacter michiganensis, and C. capsici in Plants

Clavibacter michiganensis is a Gram-positive bacterium that causes bacterial canker and wilting in host plants like tomato. Two major virulence genes encoding a cellulase (celA) and a putative serine protease (pat-1) have been reported. Here we show that Nicotiana benthamiana, a commonly used model plant for studying molecular plant-pathogen interactions, is a surrogate host of C. michiganensis and C. capsici. When a low concentration of two Clavibacter species, C. michiganensis and C. capsici, were infiltrated into N. benthamiana leaves, they caused blister-like lesions closely associated with cell death and the generation of reactive oxygen species and proliferated significantly like a pathogenic bacterium. By contrast, they did not cause any disease symptoms in N. tabacum leaves. The celA and pat-1 mutants of C. michiganensis still caused blister-like lesions and cankers like the wild-type strain. When a high concentration of two Clavibacter species and two mutant strains were infiltrated into N. benthamiana leaves, all of them caused strong and rapid necrosis. However, only C. michiganensis strains, including the celA and pat-1 mutants, caused wilting symptoms when it was injected into stems. When two Clavibacter species and two mutants were infiltrated into N. tabacum leaves at the high concentration, they (except for the pat-1 mutant) caused a strong hypersensitive response. These results indicate that C. michiganensis causes blister-like lesions, canker, and wilting in N. benthamiana, and celA and pat-1 genes are not necessary for the development of these symptoms. Overall, N. benthamiana is a surrogate host of Clavibacter species, and their novel virulence factors are responsible for disease development in this plant.

'Concord' grapevine nutritional status and chlorosis rank associated with fungal and bacterial root zone microbiomes

Leaf chlorosis in vineyards is associated with reduced crop yields and quality. While iron (Fe) is understood to play a crucial role in chlorosis, total plant and soil Fe are not always indicative of chlorosis in grapevines. Physiology of chlorosis in vineyards has been well-studied, but the soil microbial consequences of and contributions to chlorosis have received little attention. We used next-generation sequencing (NGS) to examine the bacterial and fungal communities associated with grapevines demonstrating varying degrees of visual chlorosis symptoms. Additionally, chemical analyses of soils and grape leaves were used to explore the influence of plant nutritional status and soil chemistry on microbial community composition. Finally, factors influencing bacterial community composition were correlated with predicted bacterial community function. Leaf tissue magnesium (leaf Mg) concentrations and chlorosis rank were correlated with bacterial community composition as determined via dbRDA (distance-based Redundancy Analysis) using Bray-Curtis dissimilarities. Non-metric multidimensional scaling (NMDS) revealed a significant correlation between fungal community composition and soil Fe and pH, along with leaf N, Mg, and Ca (mg.kg^-1). Chlorosis rank was moderately correlated with KEGG Orthology (KO) terms associated with nitrogen (N) and carbon (C) metabolism in soils, while leaf Mg was associated with a spectrum of KO terms including glycosphingolipid biosynthesis, glycan degradation, transporters, and porphyrin and chlorophyll metabolism. Additionally, abundance of many bacterial operational taxonomic units was significantly correlated with leaf Mg, including those from the following orders: Rhodobacterales, Acidobacteriales, Opitutales, Sphingomonadales, Burkholderiales, Saprospirales, and Flavobacteriales. Our findings suggest grapevine chlorosis is interrelated with soil microbial community structure and function, plant nutrition, and soil chemistry.

Ralfuranones contribute to mushroom-type biofilm formation by Ralstonia solanacearum strain OE1-1

After invasion into intercellular spaces of tomato plants, the soil-borne, plant-pathogenic Ralstonia solanacearum strain OE1-1 forms mushroom-shaped biofilms (mushroom-type biofilms, mBFs) on tomato cells, leading to its virulence. The strain OE1-1 produces aryl-furanone secondary metabolites, ralfuranones (A, B, J, K and L), dependent on the quorum sensing (QS) system, with methyl 3-hydroxymyristate (3-OH MAME) synthesized by PhcB as a QS signal. Ralfuranones are associated with the feedback loop of the QS system. A ralfuranone productivity-deficient mutant (ΔralA) exhibited significantly reduced growth in intercellular spaces compared with strain OE1-1, losing its virulence. To analyse the function of ralfuranones in mBF formation by OE1-1 cells, we observed cell aggregates of R. solanacearum strains statically incubated in tomato apoplast fluids on filters under a scanning electron microscope. The ΔralA strain formed significantly fewer microcolonies and mBFs than strain OE1-1. Supplementation of ralfuranones A, B, J and K, but not L, significantly enhanced the development of mBF formation by ΔralA. Furthermore, a phcB- and ralA-deleted mutant (ΔphcB/ralA) exhibited less formation of mBFs than OE1-1, although a QS-deficient, phcB-deleted mutant formed mBFs similar to OE1-1. Supplementation with 3-OH MAME significantly reduced the formation of mBFs by ΔphcB/ralA. The application of each ralfuranone significantly increased the formation of mBFs by ΔphcB/ralA supplied with 3-OH MAME. Together, our findings indicate that ralfuranones are implicated not only in the development of mBFs by strain OE1-1, but also in the suppression of QS-mediated negative regulation of mBF formation.

Transcriptomes of Ralstonia solanacearum during Root Colonization of Solanum commersonii

Bacterial wilt of potatoes-also called brown rot-is a devastating disease caused by the vascular pathogen Ralstonia solanacearum that leads to significant yield loss. As in other plant-pathogen interactions, the first contacts established between the bacterium and the plant largely condition the disease outcome. Here, we studied the transcriptome of R. solanacearum UY031 early after infection in two accessions of the wild potato Solanum commersonii showing contrasting resistance to bacterial wilt. Total RNAs obtained from asymptomatic infected roots were deep sequenced and for 4,609 out of the 4,778 annotated genes in strain UY031 were recovered. Only 2 genes were differentially-expressed between the resistant and the susceptible plant accessions, suggesting that the bacterial component plays a minor role in the establishment of disease. On the contrary, 422 genes were differentially expressed (DE) in planta compared to growth on a synthetic rich medium. Only 73 of these genes had been previously identified as DE in a transcriptome of R. solanacearum extracted from infected tomato xylem vessels. Virulence determinants such as the Type Three Secretion System (T3SS) and its effector proteins, motility structures, and reactive oxygen species (ROS) detoxifying enzymes were induced during infection of S. commersonii. On the contrary, metabolic activities were mostly repressed during early root colonization, with the notable exception of nitrogen metabolism, sulfate reduction and phosphate uptake. Several of the R. solanacearum genes identified as significantly up-regulated during infection had not been previously described as virulence factors. This is the first report describing the R. solanacearum transcriptome directly obtained from infected tissue and also the first to analyze bacterial gene expression in the roots, where plant infection takes place. We also demonstrate that the bacterial transcriptome in planta can be studied when pathogen numbers are low by sequencing transcripts from infected tissue avoiding prokaryotic RNA enrichment.

The vascular plant-pathogenic bacterium Ralstonia solanacearum produces biofilms required for its virulence on the surfaces of tomato cells adjacent to intercellular spaces

The mechanism of colonization of intercellular spaces by the soil-borne and vascular plant-pathogenic bacterium Ralstonia solanacearum strain OE1-1 after invasion into host plants remains unclear. To analyse the behaviour of OE1-1 cells in intercellular spaces, tomato leaves with the lower epidermis layers excised after infiltration with OE1-1 were observed under a scanning electron microscope. OE1-1 cells formed microcolonies on the surfaces of tomato cells adjacent to intercellular spaces, and then aggregated surrounded by an extracellular matrix, forming mature biofilm structures. Furthermore, OE1-1 cells produced mushroom-type biofilms when incubated in fluids of apoplasts including intercellular spaces, but not xylem fluids from tomato plants. This is the first report of biofilm formation by R. solanacearum on host plant cells after invasion into intercellular spaces and mushroom-type biofilms produced by R. solanacearum in vitro. Sugar application led to enhanced biofilm formation by OE1-1. Mutation of lecM encoding a lectin, RS-IIL, which reportedly exhibits affinity for these sugars, led to a significant decrease in biofilm formation. Colonization in intercellular spaces was significantly decreased in the lecM mutant, leading to a loss of virulence on tomato plants. Complementation of the lecM mutant with native lecM resulted in the recovery of mushroom-type biofilms and virulence on tomato plants. Together, our findings indicate that OE1-1 produces mature biofilms on the surfaces of tomato cells after invasion into intercellular spaces. RS-IIL may contribute to biofilm formation by OE1-1, which is required for OE1-1 virulence.

Understanding the Impact of Drought on Foliar and Xylem Invading Bacterial Pathogen Stress in Chickpea

In field conditions, plants are concurrently exposed to multiple stresses, where one stressor impacts the plant's response to another stressor, and the resultant net effect of these stresses differs from individual stress response. The present study investigated the effect of drought stress on interaction of chickpea with Pseudomonas syringae pv. phaseolicola (Psp; foliar pathogen) and Ralstonia solanacearum (Rs; xylem inhabiting wilt causing pathogen), respectively, and the net-effect of combined stress on chlorophyll content and cell death. Two type of stress treatments were used to study the influence of each stress factor during combined stress, viz., imposition of drought stress followed by pathogen challenge (DP), and pathogen inoculated plants imposed with drought in course of pathogen infection (PD). Drought stress was imposed at different levels with pathogen inoculum to understand the influence of different stress intensities on stress interaction and their net impact. Drought stressed chickpea plants challenged with Psp infection (DPsp) showed reduced in planta bacterial number compared to Psp infection alone. Similarly, Rs infection of chickpea plants showed reduced in planta bacterial number under severe drought stress. Combined drought and Psp (DPsp) infected plants showed decreased cell death compared to plants infected only with Psp but the extent of cell death was similar to drought stressed plants. Similarly, chlorophyll content in plants under combined stress was similar to the individual drought stressed plants; however, the chlorophyll content was more compared to pathogen only infected plants. Under combined drought and Rs infection (DRs), cell death was similar to individual drought stress but significantly less compared to only Rs infected plants. Altogether, the study proposes that both stress interaction and net effect of combined stress could be majorly influenced by first occurring stress, for example, drought stress in DP treatment. In addition, our results indicate that the outcome of the two stress interaction in plant depends on timing of stress occurrence and nature of infecting pathogen.

A comparative analysis of methylome profiles of Campylobacter jejuni sheep abortion isolate and gastroenteric strains using PacBio data

Campylobacter jejuni is a leading cause of human gastrointestinal disease and small ruminant abortions in the United States. The recent emergence of a highly virulent, tetracycline-resistant C. jejuni subsp. jejuni sheep abortion clone (clone SA) in the United States, and that strain's association with human disease, has resulted in a heightened awareness of the zoonotic potential of this organism. Pacific Biosciences' Single Molecule, Real-Time sequencing technology was used to explore the variation in the genome-wide methylation patterns of the abortifacient clone SA (IA3902) and phenotypically distinct gastrointestinal-specific C. jejuni strains (NCTC 11168 and 81-176). Several notable differences were discovered that distinguished the methylome of IA3902 from that of 11168 and 81-176: identification of motifs novel to IA3902, genome-specific hypo- and hypermethylated regions, strain level variability in genes methylated, and differences in the types of methylation motifs present in each strain. These observations suggest a possible role of methylation in the contrasting disease presentations of these three C. jejuni strains. In addition, the methylation profiles between IA3902 and a luxS mutant were explored to determine if variations in methylation patterns could be identified that might explain the role of LuxS-dependent methyl recycling in IA3902 abortifacient potential.

Suppression of DS1 phosphatidic acid phosphatase confirms resistance to Ralstonia solanacearum in Nicotiana benthamiana

Nicotianabenthamiana is susceptible to Ralstonia solanacearum. To analyze molecular mechanisms for disease susceptibility, we screened a gene-silenced plant showing resistance to R. solanacearum, designated as DS1 (Disease suppression 1). The deduced amino acid sequence of DS1 cDNA encoded a phosphatidic acid phosphatase (PAP) 2. DS1 expression was induced by infection with a virulent strain of R. solanacearum in an hrp-gene-dependent manner. DS1 rescued growth defects of the temperature-sensitive ∆lpp1∆dpp1∆pah1 mutant yeast. Recombinant DS1 protein showed Mg(2+)-independent PAP activity. DS1 plants showed reduced PAP activity and increased phosphatidic acid (PA) content. After inoculation with R. solanacearum, DS1 plants showed accelerated cell death, over-accumulation of reactive oxygen species (ROS), and hyper-induction of PR-4 expression. In contrast, DS1-overexpressing tobacco plants showed reduced PA content, greater susceptibility to R. solanacearum, and reduced ROS production and PR-4 expression. The DS1 phenotype was partially compromised in the plants in which both DS1 and NbCoi1 or DS1 and NbrbohB were silenced. These results show that DS1 PAP may affect plant immune responses related to ROS and JA cascades via regulation of PA levels. Suppression of DS1 function or DS1 expression could rapidly activate plant defenses to achieve effective resistance against Ralstonia solanacearum.

Metabolic adaptation of Ralstonia solanacearum during plant infection: a methionine biosynthesis case study

MetE and MetH are two distinct enzymes that catalyze a similar biochemical reaction during the last step of methionine biosynthesis, MetH being a cobalamin-dependent enzyme whereas MetE activity is cobalamin-independent. In this work, we show that the last step of methionine synthesis in the plant pathogen Ralstonia solanacearum is under the transcriptional control of the master pathogenicity regulator HrpG. This control is exerted essentially on metE expression through the intermediate regulator MetR. Expression of metE is strongly and specifically induced in the presence of plant cells in a hrpG- and metR-dependent manner. metE and metR mutants are not auxotrophic for methionine and not affected for growth inside the plant but produce significantly reduced disease symptoms on tomato whereas disruption of metH has no impact on pathogenicity. The finding that the pathogen preferentially induces metE expression rather than metH in the presence of plant cells is indicative of a probable metabolic adaptation to physiological host conditions since this induction of metE occurs in an environment in which cobalamin, the required co-factor for MetH, is absent. It also shows that MetE and MetH are not functionally redundant and are deployed during specific stages of the bacteria lifecycle, the expression of metE and metH being controlled by multiple and distinct signals.

S-glycoprotein-like protein regulates defense responses in Nicotiana plants against Ralstonia solanacearum

RsRGA4 (for Ralstonia solanacearum-responsive gene A4) encodes a polypeptide similar to S-locus glycoprotein (SGP) from Brassica rapa and SGP-like proteins from Ipomoea trifida and Medicago truncatula. Therefore, we designated RsRGA4 as NtSGLP (for Nicotiana tabacum SGP-like protein) and NbSGLP (its Nicotiana benthamiana ortholog). NbSGLP is expressed in root, leaf, petal, gynoecium, and stamen. Expression of NbSGLP was strongly induced by inoculation with an avirulent strain of R. solanacearum (Rs8107) and slightly enhanced by inoculation with virulent R. solanacearum (RsOE1-1). Expression of NbSGLP was induced by inoculation with an hrpY-deficient mutant of RsOE1-1 and Rs8107. Expression was also induced by aminocyclopropane carboxylic acid and salicylic acid. Virus-induced gene silencing of NbSGLP enhanced the growth of Rs8107. Growth of RsOE1-1 and appearance of wilt symptoms were also accelerated in silenced plants. Expression of PR-1a and EREBP was reduced, and markers for basal defense, such as callose deposition and reduced vascular flow, were compromised in NbSGLP-silenced plants. Moreover, growth of Pseudomonas cichorii, Pseudomonas syringae pv tabaci, and P. syringae pv mellea was also enhanced in the silenced plants. On the other hand, silencing of NbSGLP did not interfere with the appearance of the hypersensitive response. NbSGLP was secreted in a signal peptide-dependent manner. Agrobacterium tumefaciens-mediated expression of NbSGLP induced PR-1a and EREBP expression, callose deposition, and reduced vascular flow. NbSGLP-induced callose deposition and reduced vascular flow were not observed in salicylic acid-deficient N. benthamiana NahG plants. Taken together, SGLP might have a role in the induction of basal defense in Nicotiana plants.

Arabidopsis GH3.12 (PBS3) conjugates amino acids to 4-substituted benzoates and is inhibited by salicylate

Salicylate (SA, 2-hydroxybenzoate) is a phytohormone best known for its role as a critical mediator of local and systemic plant defense responses. In response to pathogens such as Pseudomonas syringae, SA is synthesized and activates widespread gene expression. In gh3.12/pbs3 mutants of Arabidopsis thaliana, induced total SA accumulation is significantly compromised as is SA-dependent gene expression and plant defense. AtGH3 subfamily I and II members have been shown to conjugate phytohormone acyl substrates to amino acids in vitro, with this role supported by in planta analyses. Here we sought to determine the in vitro biochemical activity and kinetic properties of GH3.12/avrPphB susceptible 3 (PBS3), a member of the uncharacterized AtGH3 subfamily III. Using a novel high throughput adenylation assay, we characterized the acyl substrate preference of PBS3. We found PBS3 favors 4-substituted benzoates such as 4-aminobenzoate and 4-hydroxybenzoate, with moderate activity on benzoate and no observed activity with 2-substituted benzoates. Similar to known GH3 enzymes, PBS3 catalyzes the conjugation of specific amino acids (e.g. Glu) to its preferred acyl substrates. Kinetic analyses indicate 4-aminobenzoate and 4-hydroxybenzoate are preferred acyl substrates as PBS3 exhibits both higher affinities (apparent K_m = 153 and 459 μm, respectively) and higher catalytic efficiencies (k_cat/K_m = 0.0179 and 0.0444 μm^–1 min^–1, respectively) with these acyl substrates compared with benzoate (apparent K_m = 867 μm, k_cat/K_m = 0.0046 μm^–1 min^–1). Notably, SA specifically and reversibly inhibits PBS3 activity with an IC₅₀ of 15 μm. This suggests a general mechanism for the rapid, reversible regulation of GH3 activity and small molecule cross-talk. For PBS3, this may allow for coordination of flux through diverse chorismate-derived pathways.

An amino acid substitution at position 740 in sigma70 of Ralstonia solanacearum strain OE1-1 affects its in planta growth

Growth of Ralstonia solanacearum strain OE1-1 in roots after invasion is required for virulence. An Arg740Cys substitution in sigma(70) of OE1-1 resulted in loss of in planta growth and virulence. The negative dominance of mutant sigma(70) over the wild-type protein suggested that the amino acid substitution may affect the in planta growth of OE1-1, leading to a lack of virulence.

Virulence of plant pathogenic bacteria attenuated by degradation of fatty acid cell-to-cell signaling factors

Diffusible signal factor (DSF) is a fatty acid signal molecule involved in regulation of virulence in several Xanthomonas species as well as Xylella fastidiosa. In this study, we identified a variety of bacteria that could disrupt DSF-mediated induction of virulence factors in Xanthomonas campestris pv. campestris. While many bacteria had the ability to degrade DSF, several bacterial strains belonging to genera Bacillus, Paenibacillus, Microbacterium, Staphylococcus, and Pseudomonas were identified that were capable of particularly rapid degradation of DSF. The molecular determinants for rapid degradation of DSF in Pseudomonas spp. strain G were elucidated. Random transposon mutants of strain G lacking the ability to degrade DSF were isolated. Cloning and characterization of disrupted genes in these strains revealed that carAB, required for the synthesis of carbamoylphosphate, a precursor for pyrimidine and arginine biosynthesis is required for rapid degradation of DSF in strain G. Complementation of carAB mutants restored both pyrimidine prototrophy and DSF degradation ability of the strain G mutant. An Escherichia coli strain harboring carAB of Pseudomonas spp. strain G degrades DSF more rapidly than the parental strain, and overexpression of carAB in trans increased the ability of Pseudomonas spp. strain G to degrade as compared with the parental strain. Coinoculation of X. campestris pv. campestris with DSF-degrading bacteria into mustard and cabbage leaves reduced disease severity up to twofold compared with plants inoculated only with the pathogen. Likewise, disease incidence and severity in grape stems coinoculated with Xylella fastidiosa and DSF-degrading strains were significantly reduced compared with plants inoculated with the pathogen alone. Coinoculation of grape plants with a carAB mutant of Pseudomonas spp. strain G complemented with carAB in trans reduced disease severity as well or better than the parental strain. These results indicate that overexpression of carAB in other endophytes could be a useful strategy of biocontrol for the control of diseases caused by plant pathogens that produce DSF.

Induction of a small heat shock protein and its functional roles in Nicotiana plants in the defense response against Ralstonia solanacearum

In tobacco (Nicotiana tabacum), Ralstonia solanacearum OE1-1 (RsOE1-1) is pathogenic, whereas R. solanacearum 8107 (Rs8107) is nonpathogenic and induces the hypersensitive response (HR). To elucidate the molecular mechanisms of plant-R. solanacearum interactions, we used differential display to isolate a cDNA fragment, A6, regulated in tobacco by inoculation with RsOE1-1. The deduced amino acid sequence predicted from full-length A6-cDNA showed similarity to small heat shock proteins from Arabidopsis (Arabidopsis thaliana; hypothetical protein), Medicago truncatula, and Cucumis melo; we therefore designated A6 to correspond to Ntshsp17 (for tobacco small heat shock protein 17). Recombinant Ntshsp17 overproduced in Escherichia coli exhibited molecular chaperone function. Expression of Ntshsp17 was increased in tobacco leaves inoculated with both RsOE1-1 and Rs8107. Expression was induced by heat treatment and by treatment with aminocyclopropane carboxylic acid, hydrogen peroxide, methyl jasmonate, and salicylic acid. Ntshsp17 expression was induced by inoculation with a HR and pathogenicity gene mutant of Rs8107 that does not induce the HR, but not by Agrobacterium-mediated transient expression of INF1, an HR elicitor. In Nbshsp17-silenced plants (an Ntshsp17 ortholog in Nicotiana benthamiana), expression of ETHYLENE-RESPONSE ELEMENT-BINDING PROTEIN, PATHOGENESIS-RELATED1a (PR1a), and PR4 genes was compromised, but expression of ELONGATION FACTOR1alpha was scarcely affected. Appearance of the HR was not affected in the silenced plants. In the silenced plants, growth of Rs8107 was accelerated. Bacterial growth and wilt symptoms elicited by RsOE1-1 were also accelerated in the silenced plants. These results indicate that this small heat shock protein might have a role in HR-independent defenses in Nicotiana plants.