The Micacocidin Production-Related RSc1806 Deletion Alters the Quorum Sensing-Dependent Gene Regulation of Ralstonia pseudosolanacearum Strain OE1-1.

The soil-borne phytopathogenic gram-negative bacterium Ralstonia solanacearum species complex (RSSC) produces staphyloferrin B and micacocidin as siderophores that scavenge for trivalent iron (Fe3+) in the environment, depending on the intracellular divalent iron (Fe2+) concentration. The staphyloferrin B-deficient mutant reportedly retains its virulence, but the relationship between micacocidin and virulence remains unconfirmed. To elucidate the effect of micacocidin on RSSC virulence, we generated the micacocidin productivity-deficient mutant (ΔRSc1806) that lacks RSc1806, which encodes a putative polyketide synthase/non-ribosomal peptide synthetase, using the RSSC phylotype I Ralstonia pseudosolanacearum strain OE1-1. When incubated in the condition without Fe2+, ΔRSc1806 showed significantly lower Fe3+-scavenging activity, compared with OE1-1. Until 8 days after inoculation on tomato plants, ΔRSc1806 was not virulent, similar to the mutant (ΔphcA) missing phcA, which encodes the LysR-type transcriptional regulator PhcA that regulates the expression of the genes responsible for quorum sensing (QS)-dependent phenotypes including virulence. The transcriptome analysis revealed that RSc1806 deletion significantly altered the expression of more than 80% of the PhcA-regulated genes in the mutant grown in medium with or without Fe2+. Among the PhcA-regulated genes, the transcript levels of the genes whose expression was affected by the deletion of RSc1806 were strongly and positively correlated between the ΔRSc1806 and the phcA-deletion mutant. Furthermore, the deletion of RSc1806 significantly modified QS-dependent phenotypes, similar to the effects of the deletion of phcA. Collectively, our findings suggest that the deletion of micacocidin production-related RSc1806 alters the regulation of PhcA-regulated genes responsible for QS-dependent phenotypes including virulence as well as Fe3+-scavenging activity. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Phosphatidylinositol-phospholipase C3 negatively regulates the hypersensitive response via complex signaling with MAP kinase, phytohormones, and reactive oxygen species in Nicotiana benthamiana.

Phospholipid signaling plays important roles in plant immune responses. Here, we focused on two phospholipase C3 (PLC3) orthologs in the Nicotiana benthamiana genome, NbPLC3-1 and NbPLC3-2. We generated NbPLC3-1 and NbPLC3-2-double-silenced plants (NbPLC3s-silenced plants). In NbPLC3s-silenced plants challenged with Ralstonia solanacearum 8107, induction of hypersensitive response (HR)-related cell death and bacterial population reduction was accelerated, and the expression level of Nbhin1, a HR marker gene, was enhanced. Furthermore, the expression levels of genes involved in salicylic acid and jasmonic acid signaling drastically increased, reactive oxygen species production was accelerated, and NbMEK2-induced HR-related cell death was also enhanced. Accelerated HR-related cell death was also observed by bacterial pathogens Pseudomonas cichorii, P. syringae, bacterial AvrA, oomycete INF1, and TMGMV-CP with L1 in NbPLC3s-silenced plants. Although HR-related cell death was accelerated, the bacterial population was not reduced in double NbPLC3s and NbCoi1-suppressed plants nor in NbPLC3s-silenced NahG plants. HR-related cell death acceleration and bacterial population reduction resulting from NbPLC3s-silencing were compromised by the concomitant suppression of either NbPLC3s and NbrbohB (respiratory oxidase homolog B) or NbPLC3s and NbMEK2 (mitogen activated protein kinase kinase 2). Thus, NbPLC3s may negatively regulate both HR-related cell death and disease resistance through MAP kinase- and reactive oxygen species-dependent signaling. Disease resistance was also regulated by NbPLC3s through jasmonic acid- and salicylic acid-dependent pathways.

The behavior of Ralstonia pseudosolanacearum strain OE1-1 and morphological changes of cells in tomato roots.

The soil-borne Gram-negative ß-proteobacterium Ralstonia solanacearum species complex (RSSC) infects tomato roots through the wounds where secondary roots emerge, infecting xylem vessels. Because it is difficult to observe the behavior of RSSC by a fluorescence-based microscopic approach at high magnification, we have little information on its behavior at the root apexes in tomato roots. To analyze the infection route of a strain of phylotype I of RSSC, R. pseudosolanacearum strain OE1-1, which invades tomato roots through the root apexes, we first developed an in vitro pathosystem using 4 day-old-tomato seedlings without secondary roots co-incubated with the strain OE1-1. The microscopic observation of toluidine blue-stained longitudinal semi-thin resin sections of tomato roots allowed to detect attachment of the strain OE1-1 to surfaces of the meristematic and elongation zones in tomato roots. We then observed colonization of OE1-1 in intercellular spaces between epidermis and cortex in the elongation zone, and a detached epidermis in the elongation zone. Furthermore, we observed cortical and endodermal cells without a nucleus and with the cell membrane pulling away from the cell wall. The strain OE1-1 next invaded cell wall-degenerated cortical cells and formed mushroom-shaped biofilms to progress through intercellular spaces of the cortex and endodermis, infecting pericycle cells and xylem vessels. The deletion of egl encoding ß-1,4-endoglucanase, which is one of quorum sensing (QS)-inducible plant cell wall-degrading enzymes (PCDWEs) secreted via the type II secretion system (T2SS) led to a reduced infectivity in cortical cells. Furthermore, the QS-deficient and T2SS-deficient mutants lost their infectivity in cortical cells and the following infection in xylem vessels. Taking together, infection of OE1-1, which attaches to surfaces of the meristematic and elongation zones, in cortical cells of the elongation zone in tomato roots, dependently on QS-inducible PCDWEs secreted via the T2SS, leads to its subsequent infection in xylem vessels.

Phosphatidylinositol-phospholipase C2 regulates pattern-triggered immunity in Nicotiana benthamiana.

Phospholipid signaling plays an important role in plant immune responses against phytopathogenic bacteria in Nicotiana benthamiana. Here, we isolated two phospholipase C2 (PLC2) orthologs in the N. benthamiana genome, designated as PLC2-1 and 2-2. Both NbPLC2-1 and NbPLC2-2 were expressed in most tissues and were induced by infiltration with bacteria and flg22. NbPLC2-1 and NbPLC2-2 (NbPLC2s) double-silenced plants showed a moderately reduced growth phenotype. The induction of the hypersensitive response was not affected, but bacterial growth and the appearance of bacterial wilt were accelerated in NbPLC2s-silenced plants when they were challenged with a virulent strain of Ralstonia solanacearum that was compatible with N. benthamiana. NbPLC2s-silenced plants showed reduced expression levels of NbPR-4, a marker gene for jasmonic acid signaling, and decreased jasmonic acid and jasmonoyl-L-isoleucine contents after inoculation with R. solanacearum. The induction of pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) marker genes was reduced in NbPLC2s-silenced plants after infiltration with R. solanacearum or Pseudomonas fluorescens. Accordingly, the resistance induced by flg22 was compromised in NbPLC2s-silenced plants. In addition, the expression of flg22-induced PTI marker genes, the oxidative burst, stomatal closure, and callose deposition were all reduced in the silenced plants. Thus, NbPLC2s might have important roles in pre- and post-invasive defenses, namely in the induction of PTI.

Studies on the biosynthesis of ralfuranones in Ralstonia solanacearum.

Ralfuranones, aryl-furanone secondary metabolites, are involved in the virulence of Ralstonia solanacearum in solanaceous plants. Ralfuranone I (6) has been suggested as a biosynthetic precursor for other ralfuranones; however, this conversion has not been confirmed. We herein investigate the biosynthesis of ralfuranones using feeding experiments with ralfuranone I (6) and its putative metabolite, ralfuranone B (2). The results obtained demonstrated that the biosynthesis of ralfuranones proceeded in enzymatic and non-enzymatic manners.

Methyl 3-Hydroxymyristate, a Diffusible Signal Mediating phc Quorum Sensing in Ralstonia solanacearum.

Ralstonia solanacearum, a plant pathogenic bacterium causing "bacterial wilt" on crops, uses a quorum sensing (QS) system consisting of phc regulatory elements to control its virulence. Methyl 3-hydroxypalmitate (3-OH PAME) was previously identified as the QS signal in strain AW1. However, 3-OH PAME has not been reportedly detected from any other strains, and this suggests that they produce another unknown QS signal. Here we identify (R)-methyl 3-hydroxymyristate [(R)-3-OH MAME] as a new QS signal that regulates the production of virulence factors and secondary metabolites. (R)-3-OH MAME was synthesized by the methyltransferase PhcB and sensed by the histidine kinase PhcS. The phylogenetic trees of these proteins from R. solanacearum strains were divided into two groups, according to their QS signal types--(R)-3-OH MAME or (R)-3-OH PAME. These results demonstrate that (R)-3-OH MAME is another crucial QS signal and highlight the unique evolution of QS systems in R. solanacearum.

Involvement of ralfuranone production in the virulence of Ralstonia solanacearum OE1-1.

Ralstonia solanacearum causes a destructive disease called "bacterial wilt" in numerous plant species. Its virulence is controlled by the transcriptional regulator PhcA, the activity of which is, in turn, regulated in a cell-density dependent manner, termed quorum sensing. We herein described the identification and characterization of ralfuranones J-L, new PhcA-regulated secondary metabolites, and the known derivatives, ralfuranones A and B, from R. solanacearum strain OE1-1. Their structures were determined by spectroscopic and chemical methods. These ralfuranones were also detected in vascular exudates from host plants infected with OE1-1. Deletion of ralA, which encodes an enzyme for ralfuranone biosynthesis, reduced the virulence of OE1-1 in tomato plants. Virulence was restored by complementation of the ralA gene. The results suggest that ralfuranones play important roles in the virulence of OE1-1.

Complete genome sequence of Japanese RSSC phylotype-I strains infecting different host plants.

Ralstonia solanacearum species complex (RSSC) shows a broad host range and is classified into four phylotypes. To compare type III effectors, we have determined the complete genome sequences of several RSSC strains, especially phylotype-I strains isolated in Japan, with different host specificity.

A translationally controlled tumor protein negatively regulates the hypersensitive response in Nicotiana benthamiana.

We have been isolating and characterizing Ralstonia solanacearum-responsive genes (RsRGs) in Nicotiana plants. In this study we focused on RsRG308, which we renamed NbTCTP (N. benthamiana translationally controlled tumor protein) because it encodes a polypeptide showing similarity to translationally controlled tumor proteins. Induction of the hypersensitive response (HR) was accelerated in NbTCTP-silenced N. benthamiana plants challenged with R. solanacearum 8107 (Rs8107). The Rs8107 population decreased significantly, whereas hin1 gene expression was enhanced in the silenced plant. Accelerated induction of HR was observed in NbTCTP-silenced plants inoculated with Pseudomonas cichorii and P. syringae pv. syringae. Silencing of NbTCTP also accelerated the induction of HR cell death by Agrobacterium-mediated transient expression of HR inducers, such as AvrA, BAX, INF1 and NbMEK2(DD). NbTCTP silencing enhanced NbrbohB- and NbMEK2-mediated reactive oxygen species production, leading to HR. Transient expression of both the full-length sequence and the Bcl-xL domain of NbTCTP decreased HR cell death induced by Agrobacterium-mediated transient expression of HR inducers. NbTCTP-silenced plants also showed slightly dwarf phenotypes. Therefore, NbTCTP might have a role in cell death regulation during HR to fine-tune programmed cell death-associated plant defense responses.

Functional analysis of Ralstonia solanacearum PrhG regulating the hrp regulon in host plants.

Genes in the hrp regulon encode component proteins of the type III secretion system and are essential for the pathogenicity of Ralstonia solanacearum. The hrp regulon is controlled by HrpB. We isolated several genes regulating hrpB expression from the Japanese strain OE1-1 using minitransposon mutagenesis. Among them, we mainly focused on two genes, hrpG and prhG, which are the positive regulators of hrpB. Although the global virulence regulator PhcA negatively regulated hrpG expression via prhIR, it positively regulated prhG expression. We further investigated the contrasting regulation of hrpG and prhG by PhcA and speculated that R. solanacearum may switch from HrpG to PrhG for hrpB activation in a cell density-dependent manner. Although the prhG mutant proliferated similarly to the wild-type in leaf intercellular spaces and in xylem vessels of the host plants, it was less virulent than the wild-type. The expression of the popA operon, which belongs to the hrp regulon, was significantly reduced in the prhG mutant by more than half in the leaf intercellular spaces and more than two-thirds in the xylem vessels when compared with the wild-type.

Phosphatidylinositol-phospholipase C4 suppresses the hypersensitive response of Nicotiana benthamiana.

Phospholipid signaling plays an important role in plant immune responses. Here, we isolated two phospholipase C4 (PLC4) orthologs in the Nicotiana benthamiana genome, designated as N. benthamiana PLC4-1 and PLC4-2 (NbPLC4-1 and NbPLC4-2). We created NbPLC4-1- and NbPLC4-2- silenced plants. Induction of the hypersensitive response (HR), including HR cell death and bacterial population reduction, was accelerated in both NbPLC4-1- and NbPLC4-2-silenced plants challenged with N. benthamiana-incompatible Ralstonia solanacearum 8107. The NbPLC4-1- and NbPLC4-2-silenced plants also showed enhanced expression of Nbhin1, a HR marker gene. Expressions of genes for salicylic acid (SA) and jasmonic acid (JA) signaling were drastically increased in NbPLC4-1- and NbPLC4-2-silenced plants by R. solanacearum inoculation. In addition, NbPLC4-1 and NbPLC4-2 silencing triggered reactive oxygen species (ROS) hyper-production. These results suggest that NbPLC4s are closely associated with JA, SA, and ROS signaling and act as negative regulators of the HR in N. benthamiana.

Suppressed expression of ErbB3-binding protein 1 (EBP1) genes compromised the hypersensitive response cell death in Nicotiana benthamiana.

Target of rapamycin (TOR) regulates essential processes associated with plant growth, development, and cell death by modulating metabolic activities and translation in response to environmental signals. The ATP-competitive TOR inhibitor AZD8055 suppressed the hypersensitive response (HR) cell death in Nicotiana benthamiana infected with the incompatible Ralstonia solanacearum. The induced expression of the HR marker gene hin1 was also inhibited by the AZD8055 treatment. To further clarify the mechanisms underlying TOR-regulated HR cell death, we focused on TOR-related ErbB3-binding protein 1 (EBP1) in N. benthamiana (NbEBP1). We found four EBP1 orthologs in the N. benthamiana genome. The expression levels of all four EBP1 orthologs in N. benthamiana were up-regulated by the R. solanacearum infection. The silencing of the four NbEBP1 orthologs suppressed the induction of HR cell death, hin1 expression, and the production of reactive oxygen species. These results suggest that the TOR signaling pathway helps regulate HR cell death along with reactive oxygen species-related signaling in N. benthamiana.

Complete Genome Sequences of Ralstonia solanacearum Strains Isolated from Infected Ginger Plants.

We present the complete genome sequences of Ralstonia solanacearum strains isolated from ginger plants. Strains MAFF 211471, MAFF 211479, MAFF 211491, MAFF 301560, MAFF 241647, and MAFF 241648 contain 69, 64, 65, 69, 72, and 64 type III effector genes, respectively.

The transcription regulator ChpA affects the global transcriptome including quorum sensing-dependent genes in Ralstonia pseudosolanacearum strain OE1-1.

The gram-negative plant-pathogenic ß-proteobacterium Ralstonia pseudosolanacearum strain OE1-1 produces methyl 3-hydroxymyristate as a quorum sensing (QS) signal through methyltransferase PhcB and senses the chemical via the sensor histidine kinase PhcS. This leads to activation of the LysR family transcription regulator PhcA, which regulates the genes (QS-dependent genes) responsible for QS-dependent phenotypes, including virulence. The transcription regulator ChpA, which possesses a response regulator receiver domain and also a hybrid sensor histidine kinase/response regulator phosphore-acceptor domain but lacks a DNA-binding domain, is reportedly involved in QS-dependent biofilm formation and virulence of R. pseudosolanacearum strain GMI1000. To explore the function of ChpA in QS of OE1-1, we generated a chpA-deletion mutant (ΔchpA) and revealed that the chpA deletion leads to significantly altered QS-dependent phenotypes. Furthermore, ΔchpA exhibited a loss in its infectivity in xylem vessels of tomato plant roots, losing virulence on tomato plants, similar to the phcA-deletion mutant (ΔphcA). Transcriptome analysis showed that the transcript levels of phcB, phcQ, phcR, and phcA in ΔchpA were comparable to those in OE1-1. However, the transcript levels of 89.9% and 88.9% of positively and negatively QS-dependent genes, respectively, were significantly altered in ΔchpA compared with OE1-1. Furthermore, the transcript levels of these genes in ΔchpA were positively correlated with those in ΔphcA. Together, our results suggest that ChpA is involved in the regulation of these QS-dependent genes, thereby contributing to the behaviour in host plant roots and virulence of OE1-1.

ß-1,4-Cellobiohydrolase is involved in full expression of phcA, contributing to the feedback loop in quorum sensing of Ralstonia pseudosolanacearum strain OE1-1.

After infecting roots of tomato plants, the gram-negative bacterium Ralstonia pseudosolanacearum strain OE1-1 activates quorum sensing (QS) to induce production of plant cell wall-degrading enzymes, such as ß-1,4-endoglucanase (Egl) and ß-1,4-cellobiohydrolase (CbhA), via the LysR family transcriptional regulator PhcA and then invades xylem vessels to exhibit virulence. The phcA-deletion mutant (ΔphcA) exhibits neither the ability to infect xylem vessels nor virulence. Compared with strain OE1-1, the egl-deletion mutant (Δegl) exhibits lower cellulose degradation activity, lower infectivity in xylem vessels, and reduced virulence. In this study, we analysed functions of CbhA other than cell wall degradation activity that are involved in the virulence of strain OE1-1. The cbhA-deletion mutant (ΔcbhA) lacked the ability to infect xylem vessels and displayed loss of virulence, similar to ΔphcA, but exhibited less reduced cellulose degradation activity compared with Δegl. Transcriptome analysis revealed that the phcA expression levels in ΔcbhA were significantly lower than in OE1-1, with significantly altered expression of more than 50% of PhcA-regulated genes. Deletion of cbhA led to a significant change in QS-dependent phenotypes, similar to the effects of phcA deletion. Complementation of ΔcbhA with native cbhA or transformation of this mutant with phcA controlled by a constitutive promoter recovered its QS-dependent phenotypes. The expression level of phcA in ΔcbhA-inoculated tomato plants was significantly lower than in strain OE1-1-inoculated plants. Our results collectively suggest that CbhA is involved in the full expression of phcA, thereby contributing to the QS feedback loop and virulence of strain OE1-1.

Functional differentiation in the leucine-rich repeat domains of closely related plant virus-resistance proteins that recognize common avr proteins.

The N' gene of Nicotiana sylvestris and L genes of Capsicum plants confer the resistance response accompanying the hypersensitive response (HR) elicited by tobamovirus coat proteins (CP) but with different viral specificities. Here, we report the identification of the N' gene. We amplified and cloned an N' candidate using polymerase chain reaction primers designed from L gene sequences. The N' candidate gene was a single 4143 base pairs fragment encoding a coiled-coil nucleotide-binding leucine-rich repeat (LRR)-type resistance protein of 1,380 amino acids. The candidate gene induced the HR in response to the coexpression of tobamovirus CP with the identical specificity as reported for N'. Analysis of N'-containing and tobamovirus-susceptible N. tabacum accessions supported the hypothesis that the candidate is the N' gene itself. Chimera analysis between N' and L(3) revealed that their LRR domains determine the spectrum of their tobamovirus CP recognition. Deletion and mutation analyses of N' and L(3) revealed that the conserved sequences in their C-terminal regions have important roles but contribute differentially to the recognition of common avirulence proteins. The results collectively suggest that Nicotiana N' and Capsicum L genes, which most likely evolved from a common ancestor, differentiated in their recognition specificity through changes in the structural requirements for LRR function.

Genetic basis for the hierarchical interaction between Tobamovirus spp. and L resistance gene alleles from different pepper species.

The pepper L gene conditions the plant's resistance to Tobamovirus spp. Alleles L(1), L(2), L(3), and L(4) confer a broadening spectra of resistance to different virus pathotypes. In this study, we report the genetic basis for the hierarchical interaction between L genes and Tobamovirus pathotypes. We cloned L(3) using map-based methods, and L(1), L(1a), L(1c), L(2), L(2b), and L(4) using a homology-based method. L gene alleles encode coiled-coil, nucleotide-binding, leucine-rich repeat (LRR)-type resistance proteins with the ability to induce resistance response to the viral coat protein (CP) avirulence effectors by themselves. Their different recognition spectra in original pepper species were reproduced in an Agrobacterium tumefaciens-mediated transient expression system in Nicotiana benthamiana. Chimera analysis with L(1), which showed the narrowest recognition spectrum, indicates that the broader recognition spectra conferred by L(2), L(2b), L(3), and L(4) require different subregions of the LRR domain. We identified a critical amino acid residue for the determination of recognition spectra but other regions also influenced the L genes' resistance spectra. The results suggest that the hierarchical interactions between L genes and Tobamovirus spp. are determined by the interaction of multiple subregions of the LRR domain of L proteins with different viral CP themselves or some protein complexes including them.

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.

Trigalactosyldiacylglycerol 3 protein orthologs are required for basal disease resistance in Nicotiana benthamiana.

Phosphatidic acid plays an important role in Nicotiana benthamiana immune responses against phytopathogenic bacteria. We analyzed the contributions of endoplasmic reticulum-derived chloroplast phospholipids, including phosphatidic acid, to the resistance of N. benthamiana against Ralstonia solanacearum. Here, we focused on trigalactosyldiacylglycerol 3 (TGD3) protein as a candidate required for phosphatidic acid signaling. On the basis of Arabidopsis thaliana TGD3 sequences, we identified two putative TGD3 orthologs in the N. benthamiana genome, NbTGD3-1 and NbTGD3-2. To address the role of TGD3s in plant defense responses, we created double NbTGD3-silenced plants using virus-induced gene silencing. The NbTGD3-silenced plants showed a moderately reduced growth phenotype. Bacterial growth and the appearance of bacterial wilt disease were accelerated in NbTGD3-silenced plants, compared with control plants, challenged with R. solanacearum. The NbTGD3-silenced plants showed reduced both expression of allene oxide synthase that encoded jasmonic acid biosynthetic enzyme and NbPR-4, a marker gene for jasmonic acid signaling, after inoculation with R. solanacearum. Thus, NbTGD3-mediated endoplasmic reticulum-chloroplast lipid transport might be required for jasmonic acid signaling-mediated basal disease resistance in N. benthamiana.

PhcQ mainly contributes to the regulation of quorum sensing-dependent genes, in which PhcR is partially involved, in Ralstonia pseudosolanacearum strain OE1-1.

The gram-negative plant-pathogenic ß-proteobacterium Ralstonia pseudosolanacearum strain OE1-1 produces methyl 3-hydroxymyristate as a quorum sensing (QS) signal via the methyltransferase PhcB and senses the chemical through the sensor histidine kinase PhcS. This leads to functionalization of the LysR family transcriptional regulator PhcA, regulating QS-dependent genes responsible for the QS-dependent phenotypes including virulence. The phc operon consists of phcB, phcS, phcR, and phcQ, with the latter two encoding regulator proteins with a receiver domain and a histidine kinase domain and with a receiver domain, respectively. To elucidate the function of PhcR and PhcQ in the regulation of QS-dependent genes, we generated phcR-deletion and phcQ-deletion mutants. Though the QS-dependent phenotypes of the phcR-deletion mutant were largely unchanged, deletion of phcQ led to a significant change in the QS-dependent phenotypes. Transcriptome analysis coupled with quantitative reverse transcription-PCR and RNA-sequencing revealed that phcB, phcK, and phcA in the phcR-deletion and phcQ-deletion mutants were expressed at similar levels as in strain OE1-1. Compared with strain OE1-1, expression of 22.9% and 26.4% of positively and negatively QS-dependent genes, respectively, was significantly altered in the phcR-deletion mutant. However, expression of 96.8% and 66.9% of positively and negatively QS-dependent genes, respectively, was significantly altered in the phcQ-deletion mutant. Furthermore, a strong positive correlation of expression of these QS-dependent genes was observed between the phcQ-deletion and phcA-deletion mutants. Our results indicate that PhcQ mainly contributes to the regulation of QS-dependent genes, in which PhcR is partially involved.