PrhG, a transcriptional regulator responding to growth conditions, is involved in the control of the type III secretion system regulon in Ralstonia solanacearum

Reshaping the Primary Cell Wall: Dual Effects on Plant Resistance to Ralstonia solanacearum and Heat Stress Response

Temperature elevation drastically affects plant defense responses to Ralstonia solanacearum and inhibits the major source of resistance in Arabidopsis thaliana, which is mediated by the receptor pair RRS1-R/RPS4. In this study, we refined a previous genome-wide association (GWA) mapping analysis by using a local score approach and detected the primary cell wall CESA3 gene as a major gene involved in plant response to R. solanacearum at both 27°C and an elevated temperature, 30°C. We functionally validated CESA3 as a susceptibility gene involved in resistance to R. solanacearum at both 27 and 30°C through a reverse genetic approach. We provide evidence that the cesa3mre1 mutant enhances resistance to bacterial disease and that resistance is associated with an alteration of root cell morphology conserved at elevated temperatures. However, even by forcing the entry of the bacterium to bypass the primary cell wall barrier, the cesa3mre1 mutant still showed enhanced resistance to R. solanacearum with delayed onset of bacterial wilt symptoms. We demonstrated that the cesa3mre1 mutant had constitutive expression of the defense-related gene VSP1, which is upregulated at elevated temperatures, and that during infection, its expression level is maintained higher than in the wild-type Col-0. In conclusion, this study reveals that alteration of the primary cell wall by mutating the cellulose synthase subunit CESA3 contributes to enhanced resistance to R. solanacearum, remaining effective under heat stress. We expect that these results will help to identify robust genetic sources of resistance to R. solanacearum in the context of global warming. [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.

Cell wall-mediated root development is targeted by a soil-borne bacterial pathogen to promote infection

Plant pathogens manipulate host development, facilitating colonization and proliferation. Ralstonia solanacearum is a soil-borne bacterial pathogen that penetrates roots and colonizes plants through the vascular system, causing wilting and death. Here, we find that RipAC, an effector protein from R. solanacearum, alters root development in Arabidopsis, promoting the formation of lateral roots and root hairs. RipAC interacts with CELLULOSE SYNTHASE (CESA)-INTERACTIVE PROTEIN 1 (CSI1), which regulates the activity of CESA complexes at the plasma membrane. RipAC disrupts CESA-CSI1 interaction, leading to a reduction in cellulose content, root developmental alterations, and a promotion of bacterial pathogenicity. We find that CSI1 also associates with the receptor kinase FERONIA, forming a complex that negatively regulates immunity in roots; this interaction, however, is not affected by RipAC. Our work reveals a bacterial virulence strategy that selectively affects the activities of a host target, promoting anatomical alterations that facilitate infection without causing activation of immunity.

Positive regulation of the PhcB neighbouring regulator PrhX on expression of the type III secretion system and pathogenesis in Ralstonia solanacearum

Ralstonia solanacearum PhcB and PhcA control a quorum-sensing (QS) system that globally regulates expression of about one third of all genes, including pathogenesis genes. The PhcB-PhcA QS system positively regulates the production of exopolysaccharide (EPS) and negatively regulates hrp gene expression, which is crucial for the type III secretion system (T3SS). Both EPS and the T3SS are essential for pathogenicity. The gene rsc2734 is located upstream of a phcBSR operon and annotated as a response regulator of a two-component system. Here, we demonstrated that RSc2734, hereafter named PrhX, positively regulated hrp gene expression via a PrhA-PrhIR-PrhJ-HrpG signalling cascade. Moreover, PrhX was crucial for R. solanacearum to invade host roots and grow in planta naturally. prhX expression was independent of the PhcB-PhcA QS system. PrhX did not affect the expression of phcB and phcA and the QS-dependent phenotypes, such as EPS production and biofilm formation. Our results provide novel insights into the complex regulatory network of the T3SS and pathogenesis in R. solanacearum.

Transcriptomic profiling reveals host-specific evolutionary pathways promoting enhanced fitness in the plant pathogen Ralstonia pseudosolanacearum

The impact of host diversity on the genotypic and phenotypic evolution of broad-spectrum pathogens is an open issue. Here, we used populations of the plant pathogen Ralstonia pseudosolanacearum that were experimentally evolved on five types of host plants, either belonging to different botanical families or differing in their susceptibility or resistance to the pathogen. We investigated whether changes in transcriptomic profiles, associated with or independent of genetic changes, could occur during the process of host adaptation, and whether transcriptomic reprogramming was dependent on host type. Genomic and transcriptomic variations were established for 31 evolved clones that showed better fitness in their experimental host than the ancestral clone. Few genomic polymorphisms were detected in these clones, but significant transcriptomic variations were observed, with a large number of differentially expressed genes (DEGs). In a very clear way, a group of genes belonging to the network of regulation of the bacterial virulence such as efpR, efpH or hrpB, among others, were deregulated in several independent evolutionary lineages and appeared to play a key role in the transcriptomic rewiring observed in evolved clones. A double hierarchical clustering based on the 400 top DEGs for each clone revealed 2 major patterns of gene deregulation that depend on host genotype, but not on host susceptibility or resistance to the pathogen. This work therefore highlights the existence of two major evolutionary paths that result in a significant reorganization of gene expression during adaptive evolution and underscore clusters of co-regulated genes associated with bacterial adaptation on different host lines.

PhcX Is a LqsR-family response regulator that contributes to Ralstonia solanacearum virulence and regulates multiple virulence factors

IMPORTANCE

The bacterial wilt caused by the soil-borne phytopathogen Ralstonia solanacearum is one of the most destructive crop diseases. To achieve a successful infection, R. solanacearum has evolved an intricate regulatory network to orchestrate the expression of an arsenal of virulence factors and fine-tune the allocation of energy. However, despite the wealth of knowledge gained in the past decades, many players and connections are still missing from the network. The importance of our study lies in the identification of PhcX, a novel conserved global regulator with critical roles in modulating the virulence and metabolism of R. solanacearum. PhcX affects many well-characterized regulators and exhibits contrasting modes of regulation from the central regulator PhcA on a variety of virulence-associated traits and genes. Our findings add a valuable piece to the puzzle of how the pathogen regulates its proliferation and infection, which is critical for understanding its pathogenesis and developing disease control strategies.

An atypical NLR gene confers bacterial wilt susceptibility in Arabidopsis

Quantitative disease resistance (QDR) remains the most prevalent form of plant resistance in crop fields and wild habitats. Genome-wide association studies (GWAS) have proved to be successful in deciphering the quantitative genetic basis of complex traits such as QDR. To unravel the genetics of QDR to the devastating worldwide bacterial pathogen Ralstonia solanacearum, we performed a GWAS by challenging a highly polymorphic local mapping population of Arabidopsis thaliana with four R. solanacearum type III effector (T3E) mutants, identified as key pathogenicity determinants after a first screen on an A. thaliana core collection of 25 accessions. Although most quantitative trait loci (QTLs) were highly specific to the identity of the T3E mutant (ripAC, ripAG, ripAQ, and ripU), we finely mapped a common QTL located on a cluster of nucleotide-binding domain and leucine-rich repeat (NLR) genes that exhibited structural variation. We functionally validated one of these NLRs as a susceptibility factor in response to R. solanacearum, named it Bacterial Wilt Susceptibility 1 (BWS1), and cloned two alleles that conferred contrasting levels of QDR. Further characterization indicated that expression of BWS1 leads to suppression of immunity triggered by different R. solanacearum effectors. In addition, we showed a direct interaction between BWS1 and RipAC T3E, and BWS1 and SUPPRESSOR OF G2 ALLELE OF skp1 (SGT1b), the latter interaction being suppressed by RipAC. Together, our results highlight a putative role for BWS1 as a quantitative susceptibility factor directly targeted by the T3E RipAC, mediating negative regulation of the SGT1-dependent immune response.

Ill Communication: Host Metabolites as Virulence-Regulating Signals for Plant-Pathogenic Bacteria

Plant bacterial pathogens rely on host-derived signals to coordinate the deployment of virulence factors required for infection. In this review, I describe how diverse plant-pathogenic bacteria detect and respond to plant-derived metabolic signals for the purpose of virulence gene regulation. I highlight examples of how pathogens perceive host metabolites through membrane-localized receptors as well as intracellular response mechanisms. Furthermore, I describe how individual strains may coordinate their virulence using multiple distinct host metabolic signals, and how plant signals may positively or negatively regulate virulence responses. I also describe how plant defenses may interfere with the perception of host metabolites as a means to dampen pathogen virulence. The emerging picture is that recognition of host metabolic signals for the purpose of virulence gene regulation represents an important primary layer of interaction between pathogenic bacteria and host plants that shapes infection outcomes.

A bacterial type III effector targets plant vesicle-associated membrane proteins

The soilborne bacterial pathogen Ralstonia solanacearum is one of the most destructive plant pathogens worldwide, and its infection process involves the manipulation of numerous plant cellular functions. In this work, we found that the R. solanacearum effector protein RipD partially suppressed different levels of plant immunity triggered by R. solanacearum elicitors, including specific responses triggered by pathogen-associated molecular patterns and secreted effectors. RipD localized in different subcellular compartments in plant cells, including vesicles, and its vesicular localization was enriched in cells undergoing R. solanacearum infection, suggesting that this specific localization may be particularly relevant during infection. Among RipD-interacting proteins, we identified plant vesicle-associated membrane proteins (VAMPs). We also found that overexpression of Arabidopsis thaliana VAMP721 and VAMP722 in Nicotiana benthamiana leaves promoted resistance to R. solanacearum, and this was abolished by the simultaneous expression of RipD, suggesting that RipD targets VAMPs to contribute to R. solanacearum virulence. Among proteins secreted in VAMP721/722-containing vesicles, CCOAOMT1 is an enzyme required for lignin biosynthesis, and mutation of CCOAOMT1 enhanced plant susceptibility to R. solanacearum. Altogether our results reveal the contribution of VAMPs to plant resistance against R. solanacearum and their targeting by a bacterial effector as a pathogen virulence strategy.

Widespread Bradyrhizobium distribution of diverse Type III effectors that trigger legume nodulation in the absence of Nod factor

The establishment of the rhizobium-legume symbiosis is generally based on plant perception of Nod factors (NFs) synthesized by the bacteria. However, some Bradyrhizobium strains can nodulate certain legume species, such as Aeschynomene spp. or Glycine max, independently of NFs, and via two different processes that are distinguished by the necessity or not of a type III secretion system (T3SS). ErnA is the first known type III effector (T3E) triggering nodulation in Aeschynomene indica. In this study, a collection of 196 sequenced Bradyrhizobium strains was tested on A. indica. Only strains belonging to the photosynthetic supergroup can develop a NF-T3SS-independent symbiosis, while the ability to use a T3SS-dependent process is found in multiple supergroups. Of these, 14 strains lacking ernA were tested by mutagenesis to identify new T3Es triggering nodulation. We discovered a novel T3E, Sup3, a putative SUMO-protease without similarity to ErnA. Its mutation in Bradyrhizobium strains NAS96.2 and WSM1744 abolishes nodulation and its introduction in an ernA mutant of strain ORS3257 restores nodulation. Moreover, ectopic expression of sup3 in A. indica roots led to the formation of spontaneous nodules. We also report three other new T3Es, Ubi1, Ubi2 and Ubi3, which each contribute to the nodulation capacity of strain LMTR13. These T3Es have no homology to known proteins but share with ErnA three motifs necessary for ErnA activity. Together, our results highlight an unsuspected distribution and diversity of T3Es within the Bradyrhizobium genus that may contribute to their symbiotic efficiency by participating in triggering legume nodulation.

Insights into the metabolic specificities of pathogenic strains from the Ralstonia solanacearum species complex

All the strains grouped under the species Ralstonia solanacearum represent a species complex responsible for many diseases on agricultural crops throughout the world. The strains have different lifestyles and host range. Here, we investigated whether specific metabolic pathways contribute to strain diversification. To this end, we carried out systematic comparisons on 11 strains representing the diversity of the species complex. We reconstructed the metabolic network of each strain from its genome sequence and looked for the metabolic pathways differentiating the different reconstructed networks and, by extension, the different strains. Finally, we conducted an experimental validation by determining the metabolic profile of each strain with the Biolog technology. Results revealed that the metabolism is conserved between strains, with a core metabolism composed of 82% of the pan-reactome. The three species composing the species complex could be distinguished according to the presence/absence of some metabolic pathways, in particular, one involving salicylic acid degradation. Phenotypic assays revealed that the trophic preferences on organic acids and several amino acids such as glutamine, glutamate, aspartate, and asparagine are conserved between strains. Finally, we generated mutants lacking the quorum-sensing-dependent regulator PhcA in four diverse strains, and we showed that the phcA-dependent trade-off between growth and production of virulence factors is conserved across the R. solanacearum species complex. IMPORTANCE Ralstonia solanacearum is one of the most important threats to plant health worldwide, causing disease on a very large range of agricultural crops such as tomato or potato. Behind the R. solanacearum name are hundreds of strains with different host range and lifestyle, classified into three species. Studying the differences between strains allows to better apprehend the biology of the pathogens and the specificity of some strains. None of the published genomic comparative studies have focused on the metabolism of the strains so far. We developed a new bioinformatic pipeline to build high-quality metabolic networks and used a combination of metabolic modeling and high-throughput phenotypic Biolog microplates to look for the metabolic differences between 11 strains across the three species. Our study revealed that genes encoding enzymes are overall conserved, with few variations between strains. However, more variations were observed when considering substrate usage. These variations probably result from regulation rather than the presence or absence of enzymes in the genome.

LysM receptor-like kinases involved in immunity perceive lipo-chitooligosaccharides in mycotrophic plants

Symbiotic microorganisms such as arbuscular mycorrhizal fungi (AMF) produce both conserved microbial molecules that activate plant defense and lipo-chitooligosaccharides (LCOs) that modulate plant defense. Beside a well-established role of LCOs in the activation of a signaling pathway required for AMF penetration in roots, LCO perception and defense modulation during arbuscular mycorrhiza is not well understood. Here we show that members of the LYRIIIA phylogenetic group from the multigenic Lysin Motif Receptor-Like Kinase family have a conserved role in dicotyledons as modulators of plant defense and regulate AMF colonization in the Solanaceae species Nicotiana benthamiana. Interestingly, these proteins have a high-affinity for LCOs in plant species able to form a symbiosis with AMF but have lost this property in species that have lost this ability. Our data support the hypothesis that LYRIIIA proteins modulate plant defense upon LCO perception to facilitate AMF colonization in mycotrophic plant species and that only their role in plant defense, but not their ability to be regulated by LCOs, has been conserved in non-mycotrophic plants.

NpPP2-B10, an F-Box-Nictaba Gene, Promotes Plant Growth and Resistance to Black Shank Disease Incited by Phytophthora nicotianae in Nicotiana tabacum

Black shank, a devastating disease affecting tobacco production worldwide, is caused by Phytophthora nicotianae. However, few genes related to Phytophthora resistance have been reported in tobacco. Here, we identified NpPP2-B10, a gene strongly induced by P. nicotianae race 0, with a conserved F-box motif and Nictaba (tobacco lectin) domain, in the highly resistant tobacco species Nicotiana plumbaginifolia. NpPP2-B10 is a typical F-box-Nictaba gene. When it was transferred into the black shank-susceptible tobacco cultivar 'Honghua Dajinyuan', it was found to promote resistance to black shank disease. NpPP2-B10 was induced by salicylic acid, and some resistance-related genes (NtPR1, NtPR2, NtCHN50, and NtPAL) and resistance-related enzymes (catalase and peroxidase) were significantly upregulated in the overexpression lines after infection with P. nicotianae. Furthermore, we showed that NpPP2-B10 actively regulated the tobacco seed germination rate, growth rate, and plant height. The erythrocyte coagulation test of purified NpPP2-B10 protein showed that NpPP2-B10 had plant lectin activity, and the lectin content in the overexpression lines was significantly higher than that in the WT, which could lead to accelerated growth and improved resistance of tobacco. SKP1 is an adaptor protein of the E3 ubiquitin ligase SKP1, Cullin, F-box (SCF) complex. We demonstrated that NpPP2-B10 could interact with the NpSKP1-1A gene in vivo and in vitro through yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC), indicating that NpPP2-B10 likely participates in the plant immune response by mediating the ubiquitin protease pathway. In conclusion, our study provides some important insights concerning NpPP2-B10-mediated regulation of tobacco growth and resistance.

Functional characterization of two 3-dehydroquinases of AroQ1 and AroQ2 in the shikimate pathway and expression of genes for the type III secretion system in Ralstonia solanacearum

The shikimate pathway is a general route for the biosynthesis of aromatic amino acids (AAAs) in many microorganisms. A 3-dehydroquinase, AroQ, controls the third step of the shikimate pathway that catalyzes the formation of 3-dehydroquinate from 3-dehydroshikimate via a trans-dehydration reaction. Ralstonia solanacearum harbors two 3-dehydroquinases, AroQ1 and AroQ2, sharing 52% similarity in amino acids. Here, we demonstrated that two 3-dehydroquinases, AroQ1 and AroQ2, are essential for the shikimate pathway in R. solanacearum. The growth of R. solanacearum was completely diminished in a nutriment-limited medium with the deletion of both aroQ1 and aroQ2, while substantially impaired in planta. The aroQ1/2 double mutant was able to replicate in planta but grew slowly, which was ~4 orders of magnitude less than the parent strain to proliferate to the maximum cell densities in tomato xylem vessels. Moreover, the aroQ1/2 double mutant failed to cause disease in tomato and tobacco plants, whereas the deletion of either aroQ1 or aroQ2 did not alter the growth of R. solanacearum or pathogenicity on host plants. Supplementary shikimic acid (SA), an important intermediate of the shikimate pathway, substantially restored the diminished or impaired growth of aroQ1/2 double mutant in a limited medium or inside host plants. The necessity of AroQ1 and AroQ2 on the pathogenicity of solanacearum toward host plants was partially due to insufficient SA inside host plants. Moreover, the deletion of both aroQ1 and aroQ2 significantly impaired the expression of genes for the type III secretion system (T3SS) both in vitro and in planta. Its involvement in the T3SS was mediated through the well-characterized PrhA signaling cascade and was independent of growth deficiency under nutrient-limited conditions. Taken together, R. solanacearum 3-dehydroquinases play important roles in bacterial growth, the expression of the T3SS, and pathogenicity in host plants. These results could extend our insights into the understanding of the biological function of AroQ and the sophisticated regulation of the T3SS in R. solanacearum.

Functional characterization of a gamma-glutamyl phosphate reductase ProA in proline biosynthesis and promoting expression of type three secretion system in Ralstonia solanacearum

Ralstonia solanacearum RSc2741 has been predicted as a gamma-glutamyl phosphate reductase ProA catalyzing the second reaction of proline formation from glutamate. Here, we experimentally demonstrated that proA mutants were proline auxotrophs that failed to grow in a minimal medium, and supplementary proline, but not glutamate, fully restored the diminished growth, confirming that ProA is responsible for the biosynthesis of proline from glutamate in R. solanacearum. ProA was previously identified as one of the candidates regulating the expression of genes for type three secretion system (T3SS), one of the essential pathogenicity determinants of R. solanacearum. Supplementary proline significantly enhanced the T3SS expression both in vitro and in planta, indicating that proline is a novel inducer of the T3SS expression. Deletion of proA substantially impaired the T3SS expression both in vitro and in planta even under proline-supplemented conditions, indicating that ProA plays additional roles apart from proline biosynthesis in promoting the expression of the T3SS genes. It was further revealed that the involvement of ProA in the T3SS expression was mediated through the pathway of PrhG-HrpB. Both the proA mutants and the wild-type strain grew in the intercellular spaces of tobacco leaves, while their ability to invade and colonize tobacco xylem vessels was substantially impaired, which was about a 1-day delay for proA mutants to successfully invade xylem vessels and was about one order of magnitude less than the wild-type strain to proliferate to the maximum densities in xylem vessels. It thus resulted in substantially impaired virulence of proA mutants toward host tobacco plants. The impaired abilities of proA mutants to invade and colonize xylem vessels were not due to possible proline insufficiency in the rhizosphere soil or inside the plants. All taken together, these results extend novel insights into the understanding of the biological function of ProA and sophisticated regulation of the T3SS and pathogenicity in R. solanacearum.

The NEL Family of Bacterial E3 Ubiquitin Ligases

Some pathogenic or symbiotic Gram-negative bacteria can manipulate the ubiquitination system of the eukaryotic host cell using a variety of strategies. Members of the genera Salmonella, Shigella, Sinorhizobium, and Ralstonia, among others, express E3 ubiquitin ligases that belong to the NEL family. These bacteria use type III secretion systems to translocate these proteins into host cells, where they will find their targets. In this review, we first introduce type III secretion systems and the ubiquitination process and consider the various ways bacteria use to alter the ubiquitin ligation machinery. We then focus on the members of the NEL family, their expression, translocation, and subcellular localization in the host cell, and we review what is known about the structure of these proteins, their function in virulence or symbiosis, and their specific targets.

An anthranilic acid-responsive transcriptional regulator controls the physiology and pathogenicity of Ralstonia solanacearum

Quorum sensing (QS) is widely employed by bacterial cells to control gene expression in a cell density-dependent manner. A previous study revealed that anthranilic acid from Ralstonia solanacearum plays a vital role in regulating the physiology and pathogenicity of R. solanacearum. We reported here that anthranilic acid controls the important biological functions and virulence of R. solanacearum through the receptor protein RaaR, which contains helix-turn-helix (HTH) and LysR substrate binding (LysR_substrate) domains. RaaR regulates the same processes as anthranilic acid, and both are present in various bacterial species. In addition, anthranilic acid-deficient mutant phenotypes were rescued by in trans expression of RaaR. Intriguingly, we found that anthranilic acid binds to the LysR_substrate domain of RaaR with high affinity, induces allosteric conformational changes, and then enhances the binding of RaaR to the promoter DNA regions of target genes. These findings indicate that the components of the anthranilic acid signaling system are distinguished from those of the typical QS systems. Together, our work presents a unique and widely conserved signaling system that might be an important new type of cell-to-cell communication system in bacteria.

Bacterial wilt caused by Ralstonia solanacearum is one of the most widespread, harmful and destructive plant diseases in the world. Our previous study showed that the pathogenic bacterium R. solanacearum uses anthranilic acid to regulate the important biological functions, virulence and the production of quorum sensing signals. Here, we show that RaaR, a transcriptional regulator from R. solanacearum, was first identified to regulate the same phenotypes as anthranilic acid. Anthranilic acid binds to the LysR_substrate domain of RaaR and enhances the regulatory activity of RaaR to control the target gene expression, including the QS signal synthase encoding genes, phcB and solI. Both the anthranilic acid synthase TrpEG and the response regulator RaaR are present in diverse bacteria, suggesting that the anthranilic acid-type signaling system is widespread. Together, our work describes a system where a pathogen uses a single protein to control the bacterial physiology and pathogenesis by responding to anthranilic acid.

A CysB regulator positively regulates cysteine synthesis, expression of type III secretion system genes, and pathogenicity in Ralstonia solanacearum

A syringe-like type III secretion system (T3SS) plays essential roles in the pathogenicity of Ralstonia solanacearum, which is a causal agent of bacterial wilt disease on many plant species worldwide. Here, we characterized functional roles of a CysB regulator (RSc2427) in R. solanacearum OE1-1 that was demonstrated to be responsible for cysteine synthesis, expression of the T3SS genes, and pathogenicity of R. solanacearum. The cysB mutants were cysteine auxotrophs that failed to grow in minimal medium but grew slightly in host plants. Supplementary cysteine substantially restored the impaired growth of cysB mutants both in minimal medium and inside host plants. Genes of cysU and cysI regulons have been annotated to function for R. solanacearum cysteine synthesis; CysB positively regulated expression of these genes. Moreover, CysB positively regulated expression of the T3SS genes both in vitro and in planta through the PrhG to HrpB pathway, whilst impaired expression of the T3SS genes in cysB mutants was independent of growth deficiency under nutrient-limited conditions. CysB was also demonstrated to be required for exopolysaccharide production and swimming motility, which contribute jointly to the host colonization and infection process of R. solanacearum. Thus, CysB was identified here as a novel regulator on the T3SS expression in R. solanacearum. These results provide novel insights into understanding of various biological functions of CysB regulators and complex regulatory networks on the T3SS in R. solanacearum.

Involvement of a FAD-Linked Oxidase RSc0454 for Expression of the Type III Secretion System and Pathogenicity in Ralstonia solanacearum

Ralstonia solanacearum RSc0454 is predicted as a FAD-linked oxidase based on protein homologies, while it contains distinct domains of lactate dehydrogenase and succinate dehydrogenase. A previous study demonstrated that RSc0454 exhibits lactate dehydrogenase activity using pyruvate and NADH as substrates, and is essential for pathogenicity of R. solanacearum. Here, we genetically characterized involvement of RSc0454 on bacterial growth and expression of genes for the type III secretion system (T3SS, a pathogenicity determinant) in R. solanacearum. The RSc0454 mutant grew normally in rich medium but grew faintly in host plants, and failed to grow in minimal medium. Supplementary succinate but not lactate could substantially restore some phenotypes of RSc0454 mutants, including faint growth in host plants, diminished growth in the minimal medium, and lost pathogenicity toward host plants. Expression of T3SS genes is directly controlled by a master regulator, HrpB, and hrpB expression is positively regulated by HrpG and PrhG in parallel ways. Deletion of RSc0454 substantially reduced expression levels of hrpB and T3SS both in vitro and in planta. Moreover, RSc0454 is revealed to be required for the T3SS expression via HrpG and PrhG, although through some novel pathway, and impaired expression of these genes was not due to growth deficiency of RSc0454 mutants. RSc0454 is suggested to be important for redox balance inside cells, and supplementary NADH partially restored diminished growth of the RSc0454 mutant in the minimal medium only in the presence of succinate at some moderate concentrations, indicating that the unbalanced redox in the RSc0454 mutant might be responsible for its diminished growth in the minimal medium. Taken together, these results provide novel insights into the understanding of various biological functions of this FAD-linked oxidase RSc0454 and involvement of the redox balance on expression of the T3SS in R. solanacearum.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

The GABA transaminase GabT is required for full virulence of Ralstonia solanacearum in tomato

The utilization of plant gamma-aminobutyric acid (GABA) is essential for the pathogenicity of the bacterial plant pathogen Ralstonia solanacearum. A knockout mutant in the GABA transaminase-encoding gene gabT is unable to utilize GABA as a nutrient and its ability to cause disease in plants is strongly compromised. However, the proximity of the gabD gene (encoding a succinate semialdehyde dehydrogenase) in the same operon raises the question of whether an impact on the gabD gene underlies or contributes to the virulence attenuation of the ΔgabT mutant. In this work, we use genetic complementation to show that the expression of the gabT gene is able to rescue the impaired virulence of the ΔgabT knockout mutant in tomato plants, confirming that the gabT-encoded GABA transaminase is indeed required for full virulence of R. solanacearum in a natural host plant.

The Same against Many: AtCML8, a Ca2+ Sensor Acting as a Positive Regulator of Defense Responses against Several Plant Pathogens

Calcium signals are crucial for the activation and coordination of signaling cascades leading to the establishment of plant defense mechanisms. Here, we studied the contribution of CML8, an Arabidopsis calmodulin-like protein in response to Ralstonia solanacearum and to pathogens with different lifestyles, such as Xanthomonas campestris pv. campestris and Phytophtora capsici. We used pathogenic infection assays, gene expression, RNA-seq approaches, and comparative analysis of public data on CML8 knockdown and overexpressing Arabidopsis lines to demonstrate that CML8 contributes to defense mechanisms against pathogenic bacteria and oomycetes. CML8 gene expression is finely regulated at the root level and manipulated during infection with Ralstonia, and CML8 overexpression confers better plant tolerance. To understand the processes controlled by CML8, genes differentially expressed at the root level in the first hours of infection have been identified. Overexpression of CML8 also confers better tolerance against Xanthomonas and Phytophtora, and most of the genes differentially expressed in response to Ralstonia are differentially expressed in these different pathosystems. Collectively, CML8 acts as a positive regulator against Ralstonia solanaceraum and against other vascular or root pathogens, suggesting that CML8 is a multifunctional protein that regulates common downstream processes involved in the defense response of plants to several pathogens.

A bacterial effector protein uncovers a plant metabolic pathway involved in tolerance to bacterial wilt disease

Bacterial wilt caused by the soil-borne plant pathogen Ralstonia solanacearum is a devastating disease worldwide. Upon plant colonization, R. solanacearum replicates massively, causing plant wilting and death; collapsed infected tissues then serve as a source of inoculum. In this work, we show that the plant metabolic pathway mediated by pyruvate decarboxylases (PDCs) contributes to plant tolerance to bacterial wilt disease. Arabidopsis and tomato plants respond to R. solanacearum infection by increasing PDC activity, and plants with deficient PDC activity are more susceptible to bacterial wilt. Treatment with either pyruvic acid or acetic acid (substrate and product of the PDC pathway, respectively) enhances plant tolerance to bacterial wilt disease. An effector protein secreted by R. solanacearum, RipAK, interacts with PDCs and inhibits their oligomerization and enzymatic activity. Collectively, our work reveals a metabolic pathway involved in plant resistance to biotic and abiotic stresses, and a bacterial virulence strategy to promote disease and the completion of the pathogenic life cycle.

Convergent Rewiring of the Virulence Regulatory Network Promotes Adaptation of Ralstonia solanacearum on Resistant Tomato

The evolutionary and adaptive potential of a pathogen is a key determinant for successful host colonization and proliferation but remains poorly known for most of the pathogens. Here, we used experimental evolution combined with phenotyping, genomics, and transcriptomics to estimate the adaptive potential of the bacterial plant pathogen Ralstonia solanacearum to overcome the quantitative resistance of the tomato cultivar Hawaii 7996. After serial passaging over 300 generations, we observed pathogen adaptation to within-plant environment of the resistant cultivar but no plant resistance breakdown. Genomic sequence analysis of the adapted clones revealed few genetic alterations, but we provide evidence that all but one were gain of function mutations. Transcriptomic analyses revealed that even if different adaptive events occurred in independently evolved clones, there is convergence toward a global rewiring of the virulence regulatory network as evidenced by largely overlapping gene expression profiles. A subset of four transcription regulators, including HrpB, the activator of the type 3 secretion system regulon and EfpR, a global regulator of virulence and metabolic functions, emerged as key nodes of this regulatory network that are frequently targeted to redirect the pathogen’s physiology and improve its fitness in adverse conditions. Significant transcriptomic variations were also detected in evolved clones showing no genomic polymorphism, suggesting that epigenetic modifications regulate expression of some of the virulence network components and play a major role in adaptation as well.

Insights into the Polyhydroxybutyrate Biosynthesis in Ralstonia solanacearum Using Parallel 13C Tracers and Comparative Genome Analysis

Bacterial accumulation of poly(3-hydroxybutyrate) [P(3HB)] is a metabolic strategy often adopted to cope with challenging surroundings. Ralstonia solanacearum, a phytopathogen, seems to be an ideal candidate with inherent ability to accumulate this biodegradable polymer of high industrial relevance. This study is focused on investigating the metabolic networks that channel glucose into P(3HB) using comparative genome analysis, ¹³C tracers, microscopy, gas chromatography-mass spectrometry (GC-MS), and proton nuclear magnetic resonance (¹H NMR). Comparative genome annotation of 87 R. solanacearum strains confirmed the presence of a conserved P(3HB) biosynthetic pathway genes in the chromosome. Parallel ¹³C glucose feeding ([1-¹³C], [1,2-¹³C]) analysis mapped the glucose oxidation to 3-hydroxybutyrate (3HB), the metabolic precursor of P(3HB) via the Entner-Doudoroff pathway (ED pathway), potentially to meet the NADPH demands. Fluorescence microscopy, GC-MS, and ¹H NMR analysis further confirmed the ability of R. solanacearum to accumulate P(3HB) granules. In addition, it is demonstrated that the carbon/nitrogen (C/N) ratio influences the P(3HB) yields, thereby highlighting the need to further optimize the bioprocessing parameters. This study provided key insights into the biosynthetic abilities of R. solanacearum as a promising P(3HB) producer.

The Tn7-Based Genomic Integration Is Dependent on an attTn7 Box in the glms Gene and Is Site-Specific With Monocopy in Ralstonia solanacearum Species Complex

The Tn7-based genomic integration system enables direct insertion of foreign gene elements into the chromosome downstream of glms in many bacteria species. The glms gene is greatly conserved in Ralstonia solanacearum species complex (RSSC), while its downstream regions are mostly different in the RSSC. Here, we provided genetic evidence to validate that this Tn7 integration is dependent on a conserved 30-bp motif in the glms, called an attTn7 box, and artificial attTn7 boxes elsewhere are competent for the Tn7 integration, which is further confirmed to be simultaneous downstream of both original and artificial attTn7 boxes, using PCR. With the whole-genome resequencing on 500 Tn7-colonies, the Tn7 integration was confirmed to be site- specific at 25 bp downstream of glms with monocopy as a chromosome of the RSSC. Characteristic of a monocopy in a chromosome enables the Tn7-based complementation to fully restore phenotypes of mutants to those of parent strains that are advantageous rather than those based on plasmids with low-copy numbers. The Tn7-based genomic integration system provides a generally applicable and versatile genetic tool for studies of complementation, pathogenesis, overexpression, and in-vivo promoter activity assays with monocopy in the RSSC.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

StMBF1c positively regulates disease resistance to Ralstonia solanacearum via it's primary and secondary upregulation combining expression of StTPS5 and resistance marker genes in potato

Multiprotein bridging factor 1 (MBF1) is a transcription coactivator that has a general defense response to pathogens. However, the regulatory mechanisms of MBF1 resistance bacterial wilt remain largely unknown. Here, the role of StMBF1c in potato resistance to Ralstonia solanacearum infection was characterized. qRT-PCR assays indicated that StMBF1c could was elicited by SA, MJ and ABA and the time-course expression pattern of the StMBF1c gene induced by R. solanacearum was found to be twice significant upregulated expression during the early and middle stages of bacterial wilt. Combined with the co-expression analysis of disease-resistant marker genes, gain-of-function and loss-of-function assays demonstrated that StMBF1c was associated with defence priming. Overexpression or silencing the MBF1c could enhance plants resistance or sensitivity to R. solanacearum through inducing or reducing NPR and PR genes related to SA signal pathway. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) experiment results confirmed the interaction of StMBF1c with StTPS5 which played a key role in ABA signal pathway in potato. It is speculated that by combining StTPS5 and resistance marker genes, StMBF1c is activated twice to participate in potato bacterial wilt resistance, in which EPI, PTI involved.

Unravelling physiological signatures of tomato bacterial wilt and xylem metabolites exploited by Ralstonia solanacearum

The plant pathogen Ralstonia solanacearum uses plant resources to intensely proliferate in xylem vessels and provoke plant wilting. We combined automatic phenotyping and tissue/xylem quantitative metabolomics of infected tomato plants to decipher the dynamics of bacterial wilt. Daily acquisition of physiological parameters such as transpiration and growth were performed. Measurements allowed us to identify a tipping point in bacterial wilt dynamics. At this tipping point, the reached bacterial density brutally disrupts plant physiology and rapidly induces its death. We compared the metabolic and physiological signatures of the infection with drought stress, and found that similar changes occur. Quantitative dynamics of xylem content enabled us to identify glutamine (and asparagine) as primary resources R. solanacearum consumed during its colonization phase. An abundant production of putrescine was also observed during the infection process and was strongly correlated with in planta bacterial growth. Dynamic profiling of xylem metabolites confirmed that glutamine is the favoured substrate of R. solanacearum. On the other hand, a triple mutant strain unable to metabolize glucose, sucrose and fructose appears to be only weakly reduced for in planta growth and pathogenicity.

Arabidopsis cell wall composition determines disease resistance specificity and fitness

Plant cell walls are complex structures subject to dynamic remodeling in response to developmental and environmental cues and play essential functions in disease resistance responses. We tested the specific contribution of plant cell walls to immunity by determining the susceptibility of a set of Arabidopsis cell wall mutants (cwm) to pathogens with different parasitic styles: a vascular bacterium, a necrotrophic fungus, and a biotrophic oomycete. Remarkably, most cwm mutants tested (29/34; 85.3%) showed alterations in their resistance responses to at least one of these pathogens in comparison to wild-type plants, illustrating the relevance of wall composition in determining disease-resistance phenotypes. We found that the enhanced resistance of cwm plants to the necrotrophic and vascular pathogens negatively impacted cwm fitness traits, such as biomass and seed yield. Enhanced resistance of cwm plants is not only mediated by canonical immune pathways, like those modulated by phytohormones or microbe-associated molecular patterns, which are not deregulated in the cwm tested. Pectin-enriched wall fractions isolated from cwm plants triggered immune responses in wild-type plants, suggesting that wall-mediated defensive pathways might contribute to cwm resistance. Cell walls of cwm plants show a high diversity of composition alterations as revealed by glycome profiling that detect specific wall carbohydrate moieties. Mathematical analysis of glycome profiling data identified correlations between the amounts of specific wall carbohydrate moieties and disease resistance phenotypes of cwm plants. These data support the relevant and specific function of plant wall composition in plant immune response modulation and in balancing disease resistance/development trade-offs.

A complex network of additive and epistatic quantitative trait loci underlies natural variation of Arabidopsis thaliana quantitative disease resistance to Ralstonia solanacearum under heat stress

Plant immunity is often negatively impacted by heat stress. However, the underlying molecular mechanisms remain poorly characterized. Based on a genome-wide association mapping approach, this study aims to identify in Arabidopsis thaliana the genetic bases of robust resistance mechanisms to the devastating pathogen Ralstonia solanacearum under heat stress. A local mapping population was phenotyped against the R. solanacearum GMI1000 strain at 27 and 30 °C. To obtain a precise description of the genetic architecture underlying natural variation of quantitative disease resistance (QDR), we applied a genome-wide local score analysis. Alongside an extensive genetic variation found in this local population at both temperatures, we observed a playful dynamics of quantitative trait loci along the infection stages. In addition, a complex genetic network of interacting loci could be detected at 30 °C. As a first step to investigate the underlying molecular mechanisms, the atypical meiotic cyclin SOLO DANCERS gene was validated by a reverse genetic approach as involved in QDR to R. solanacearum at 30 °C. In the context of climate change, the complex genetic architecture underlying QDR under heat stress in a local mapping population revealed candidate genes with diverse molecular functions.

A Bacterial Effector Protein Hijacks Plant Metabolism to Support Pathogen Nutrition

Many bacterial plant pathogens employ a type III secretion system to inject effector proteins within plant cells to suppress plant immunity. Whether and how effector proteins also co-opt plant metabolism to support extensive bacterial replication remains an open question. Here, we show that Ralstonia solanacearum, the causal agent of bacterial wilt disease, secretes the effector protein RipI, which interacts with plant glutamate decarboxylases (GADs) to alter plant metabolism and support bacterial growth. GADs are activated by calmodulin and catalyze the biosynthesis of gamma-aminobutyric acid (GABA), an important signaling molecule in plants and animals. RipI promotes the interaction of GADs with calmodulin, enhancing the production of GABA. R. solanacearum is able to replicate efficiently using GABA as a nutrient, and both RipI and plant GABA contribute to a successful infection. This work reveals a pathogenic strategy to hijack plant metabolism for the biosynthesis of nutrients that support microbial growth during plant colonization.

KatE From the Bacterial Plant Pathogen Ralstonia solanacearum Is a Monofunctional Catalase Controlled by HrpG That Plays a Major Role in Bacterial Survival to Hydrogen Peroxide

Ralstonia solanacearum is the causative agent of bacterial wilt disease on a wide range of plant species. Besides the numerous bacterial activities required for host invasion, those involved in the adaptation to the plant environment are key for the success of infection. R. solanacearum ability to cope with the oxidative burst produced by the plant is likely one of the activities required to grow parasitically. Among the multiple reactive oxygen species (ROS)-scavenging enzymes predicted in the R. solanacearum GMI1000 genome, a single monofunctional catalase (KatE) and two KatG bifunctional catalases were identified. In this work, we show that these catalase activities are active in bacterial protein extracts and demonstrate by gene disruption and mutant complementation that the monofunctional catalase activity is encoded by katE. Different strategies were used to evaluate the role of KatE in bacterial physiology and during the infection process that causes bacterial wilt. We show that the activity of the enzyme is maximal during exponential growth in vitro and this growth-phase regulation occurs at the transcriptional level. Our studies also demonstrate that katE expression is transcriptionally activated by HrpG, a central regulator of R. solanacearum induced upon contact with the plant cells. In addition, we reveal that even though both KatE and KatG catalase activities are induced upon hydrogen peroxide treatment, KatE has a major effect on bacterial survival under oxidative stress conditions and especially in the adaptive response of R. solanacearum to this oxidant. The katE mutant strain also exhibited differences in the structural characteristics of the biofilms developed on an abiotic surface in comparison to wild-type cells, but not in the overall amount of biofilm production. The role of catalase KatE during the interaction with its host plant tomato is also studied, revealing that disruption of this gene has no effect on R. solanacearum virulence or bacterial growth in leave tissues, which suggests a minor role for this catalase in bacterial fitness in planta. Our work provides the first characterization of the R. solanacearum catalases and identifies KatE as a bona fide monofunctional catalase with an important role in bacterial protection against oxidative stress.

An immune receptor complex evolved in soybean to perceive a polymorphic bacterial flagellin

In both animals and plants, the perception of bacterial flagella by immune receptors elicits the activation of defence responses. Most plants are able to perceive the highly conserved epitope flg22 from flagellin, the main flagellar protein, from most bacterial species. However, flagellin from Ralstonia solanacearum, the causal agent of the bacterial wilt disease, presents a polymorphic flg22 sequence (flg22Rso) that avoids perception by all plants studied to date. In this work, we show that soybean has developed polymorphic versions of the flg22 receptors that are able to perceive flg22Rso. Furthermore, we identify key residues responsible for both the evasion of perception by flg22Rso in Arabidopsis and the gain of perception by the soybean receptors. Heterologous expression of the soybean flg22 receptors in susceptible plant species, such as tomato, enhances resistance to bacterial wilt disease, demonstrating the potential of these receptors to enhance disease resistance in crop plants.

Functional Characterization of RsRsgA for Ribosome Biosynthesis and Expression of the Type III Secretion System in Ralstonia solanacearum

RsgA plays an important role in maturation of 30S subunit in many bacteria that assists in the release of RbfA from the 30S subunit during a late stage of ribosome biosynthesis. Here, we genetically characterized functional roles of RsgA in Ralstonia solanacearum, hereafter designated RsRsgA. Deletion of R. solanacearum rsgA or rbfA resulted in distinct deficiency of 16S ribosomal RNA, significantly slowed growth in broth medium, and diminished growth in nutrient-limited medium, which are similar as phenotypes of rsgA mutants and rbfA mutants of Escherichia coli and other bacteria. Our gene-expression studies revealed that RsRsgA is important for expression of genes encoding the type III secretion system (T3SS) (a pathogenicity determinant of R. solanacearum) both in vitro and in planta. Compared with the wild-type R. solanacearum strain, proliferation of the rsgA and rbfA mutants in tobacco leaves was significantly impaired, while they failed to migrate into tobacco xylem vessels from infiltrated leaves, and hence, these two mutants failed to cause any bacterial wilt disease in tobacco plants. It was further revealed that rsgA expression was highly enhanced under nutrient-limited conditions compared with that in broth medium and RsRsgA affects T3SS expression through the PrhN-PrhG-HrpB pathway. Moreover, expression of a subset of type III effectors was substantially impaired in the rsgA mutant, some of which are responsible for R. solanacearum GMI1000 elicitation of a hypersensitive response (HR) in tobacco leaves, while RsRsgA is not required for HR elicitation of GMI1000 in tobacco leaves. All these results provide novel insights into understanding various biological functions of RsgA proteins and complex regulation on the T3SS in R. solanacearum.

Expression of Ralstonia solanacearum type III secretion system is dependent on a novel type 4 pili (T4P) assembly protein (TapV) but is T4P independent

Type IV pili (T4P) are virulence factors in various pathogenic bacteria of animals and plants that play important roles in twitching motility, swimming motility, biofilm formation, and adhesion to host cells. Here, we genetically characterized functional roles of a putative T4P assembly protein TapV (Rsc1986 in reference strain GMI1000) and its homologue Rsp0189, which shares 58% amino acid identity with TapV, in Ralstonia solanacearum. Deletion of tapV, but not rsp0189, resulted in significantly impaired twitching motility, swimming motility, and adhesion to tomato roots, which are consistent as phenotypes of the pilA mutant (a known R. solanacearum T4P-deficient mutant). However, unlike the pilA mutant, the tapV mutant produced more biofilm than the wild-type strain. Our gene expression studies revealed that TapV, but not Rsp0189, is important for expression of a type III secretion system (T3SS, a pathogenicity determinant of R. solanacearum) both in vitro and in planta, but it is T4P independent. We further revealed that TapV affected the T3SS expression via the PhcA-TapV-PrhG-HrpB pathway, consistent with previous reports that PhcA positively regulates expression of pilA and prhG. Moreover, deletion of tapV, but not rsp0189, significantly impaired the ability to migrate into and colonize xylem vessels of host plants, but there was no alteration in intercellular proliferation of R. solanacearum in tobacco leaves, which is similar to the pilA mutant. The tapV mutant showed significantly impaired virulence in host plants. This is the first report on the impact of T4P components on the T3SS, providing novel insights into our understanding of various biological functions of T4P and the complex regulatory pathway of T3SS in R. solanacearum.

HpaP Sequesters HrpJ, an Essential Component of Ralstonia solanacearum Virulence That Triggers Necrosis in Arabidopsis

The Gram-negative bacterium Ralstonia solanacearum, the causal agent of bacterial wilt, is a worldwide major crop pathogen whose virulence strongly relies on a type III secretion system (T3SS). This extracellular apparatus allows the translocation of proteins, called type III effectors (T3Es), directly into the host cells. To date, very few data are available in plant-pathogenic bacteria concerning the role played by type III secretion (T3S) regulators at the posttranslational level. We have demonstrated that HpaP, a putative T3S substrate specificity switch protein of R. solanacearum, controls T3E secretion. To better understand the role of HpaP on T3S control, we analyzed the secretomes of the GMI1000 wild-type strain as well as the hpaP mutant using a mass spectrometry experiment (liquid chromatography tandem mass spectrometry). The secretomes of both strains appeared to be very similar and highlighted the modulation of the secretion of few type III substrates. Interestingly, only one type III-associated protein, HrpJ, was identified as specifically secreted by the hpaP mutant. HrpJ appeared to be an essential component of the T3SS, essential for T3S and pathogenicity. We further showed that HrpJ is specifically translocated in planta by the hpaP mutant and that HrpJ can physically interact with HpaP. Moreover, confocal microscopy experiments demonstrated a cytoplasmic localization for HrpJ once in planta. When injected into Arabidopsis thaliana leaves, HrpJ is able to trigger a necrosis on 16 natural accessions. A genome-wide association mapping revealed a major association peak with 12 highly significant single-nucleotide polymorphisms located on a plant acyl-transferase.

Modulation of Quorum Sensing as an Adaptation to Nodule Cell Infection during Experimental Evolution of Legume Symbionts

Over millions of years, changes have occurred in regulatory circuitries in response to genome reorganization and/or persistent changes in environmental conditions. How bacteria optimize regulatory circuitries is crucial to understand bacterial adaptation. Here, we analyzed the experimental evolution of the plant pathogen Ralstonia solanacearum into legume symbionts after the transfer of a natural plasmid encoding the essential mutualistic genes. We showed that the Phc quorum sensing system required for the virulence of the ancestral bacterium was reconfigured to improve intracellular infection of root nodules induced by evolved Ralstonia A single mutation in either the PhcB autoinducer synthase or the PhcQ regulator of the sensory cascade tuned the kinetics of activation of the central regulator PhcA in response to cell density so that the minimal stimulatory concentration of autoinducers needed for a given response was increased. Yet, a change in the expression of a PhcA target gene was observed in infection threads progressing in root hairs, suggesting early programming for the late accommodation of bacteria in nodule cells. Moreover, this delayed switch to the quorum sensing mode decreased the pathogenicity of the ancestral strain, illustrating the functional plasticity of regulatory systems and showing how a small modulation in signal response can produce drastic changes in bacterial lifestyle.IMPORTANCE Rhizobia are soil bacteria from unrelated genera able to form a mutualistic relationship with legumes. Bacteria induce the formation of root nodules, invade nodule cells, and fix nitrogen to the benefit of the plant. Rhizobial lineages emerged from the horizontal transfer of essential symbiotic genes followed by genome remodeling to activate and/or optimize the acquired symbiotic potential. This evolutionary scenario was replayed in a laboratory evolution experiment in which the plant pathogen Ralstonia solanacearum successively evolved the capacities to nodulate Mimosa pudica and poorly invade, then massively invade, nodule cells. In some lines, the improvement of intracellular infection was achieved by mutations modulating a quorum sensing regulatory system of the ancestral strain. This modulation that affects the activity of a central regulator during the earliest stages of symbiosis has a huge impact on late stages of symbiosis. This work showed that regulatory rewiring is the main driver of this pathogeny-symbiosis transition.

Involvement of a PadR regulator PrhP on virulence of Ralstonia solanacearum by controlling detoxification of phenolic acids and type III secretion system

Ralstonia solanacearum can metabolize ferulic acid (FA) and salicylic acid (SA), two representative phenolic acids, to protect it from toxicity of phenolic acids. Here, we genetically demonstrated a novel phenolic acid decarboxylase regulator (PadR)-like regulator PrhP as a positive regulator on detoxification of SA and FA in R. solanacearum. Although the ability to degrade SA and FA enhances the infection process of R. solanacearum toward host plants, PrhP greatly contributes to the infection process besides degradation of SA and FA. Our results from the growth assay, promoter activity assay, RNA-seq and qRT-PCR revealed that PrhP plays multiple roles in the virulence of R. solanacearum: (1) positively regulates expression of genes for degradation of SA and FA; (2) positively regulates expression of genes encoding type III secretion system (T3SS) and type III effectors both in vitro and in planta; (3) positively regulates expression of many virulence-related genes, such as the flagella, type IV pili and cell wall degradation enzymes; and (4) is important for the extensive proliferation in planta. The T3SS is one of the essential pathogenicity determinants in many pathogenic bacteria, and PrhP positively regulates its expression mediated with the key regulator HrpB but through some novel pathway to HrpB in R. solanacearum. This is the first report on PadR regulators to regulate the T3SS and it could improve our understanding of the various biological functions of PadR regulators and the complex regulatory pathway on T3SS in R. solanacearum.

Fabrication of pH-Sensitive Tetramycin Releasing Gel and Its Antibacterial Bioactivity against Ralstonia solanacearum

Ralstonia solanacearum (R. solanacearum)-induced bacterial wilt of the nightshade family causes a great loss in agricultural production annually. Although there has been some efficient pesticides against R. solanacearum, inaccurate pesticide releasing according to the onset time of bacterial wilt during the use of pesticides still hinders the disease management efficiency. Herein, on the basis of the soil pH change during R. solanacearum growth, and pH sensitivity of the Schiff base structure, a pH-sensitive oxidized alginate-based double-crosslinked gel was fabricated as a pesticide carrier. The gel was prepared by crosslinking oxidized sodium alginate (OSA) via adipic dihydrazide (ADH) and Ca2+. After loading tetramycin into the gel, it showed a pH-dependent pesticide releasing behavior and anti-bacterial activity against R. solanacearum. Further study also showed that the inhibition rate of the tetramycin-loaded gel was higher than that of industrial pesticide difenoconazole. This work aimed to reduce the difficulty of pesticide administration in the high incidence period of bacterial wilt and we believe it has a great application potential in nightshade production.

Spontaneous mutations in a regulatory gene induce phenotypic heterogeneity and adaptation of Ralstonia solanacearum to changing environments

An evolution experiment with the bacterial plant pathogen Ralstonia solanacearum revealed that several adaptive mutations conferring enhanced fitness in plants arose in the efpR gene encoding a regulator of virulence and metabolic functions. In this study, we found that an efpR mutant systematically displays colonies with two morphotypes: the type S ('smooth', similar to the wild type) and the type EV ('efpR variant'). We demonstrated that the efpH gene, a homologue of efpR, plays a key role in the control of phenotypic heterogeneity, the ΔefpR-ΔefpH double mutant being stably locked into the EV type. Using mixed infection assays, we demonstrated that the type EV is metabolically more proficient than the type S and displays fitness gain in specific environments, whereas the type S has a better fitness into the plant environment. We provide evidence that this efpR-dependent phenotypic heterogeneity is a general feature of strains of the R. solanacearum species complex and could occur in natural conditions. This study highlights the potential role of phenotypic heterogeneity in this plant pathogen as an adaptive trait to changing environments.

Functional Characterization of Two Putative DAHP Synthases of AroG1 and AroG2 and Their Links With Type III Secretion System in Ralstonia solanacearum

Type three secretion system (T3SS) is essential for Ralstonia solanacearum to cause disease in host plants and we previously screened AroG1 as a candidate with impact on the T3SS expression. Here, we focused on two putative DAHP synthases of AroG1 and AroG2, which control the first step of the shikimate pathway, a common route for biosynthesis of aromatic amino acids (AAA), to characterize their functional roles and possible links with virulence in R. solanacearum. Deletion of aroG1/2 or aroG1, but not aroG2, significantly impaired the T3SS expression both in vitro and in planta, and the impact of AroG1 on T3SS was mediated with a well-characterized PrhA signaling cascade. Virulence of the aroG1/2 or aroG1 mutants was completely diminished or significantly impaired in tomato and tobacco plants, but not the aroG2 mutants. The aroG1/2 mutants failed to grow in limited medium, but grew slowly in planta. This significantly impaired growth was also observed in the aroG1 mutants both in planta and limited medium, but not in aroG2 mutants. Complementary aroG1 significantly restored the impaired or diminished bacterial growth, T3SS expression and virulence. Supplementary AAA or shikimic acid, an important intermediate of the shikimate pathway, significantly restored diminished growth in limited medium. The promoter activity assay showed that expression of aroG1 and aroG2 was greatly increased to 10-20-folder higher levels with deletion of the other. All these results demonstrated that both AroG1 and AroG2 are involved in the shikimate pathway and cooperatively essential for AAA biosynthesis in R. solanacearum. The AroG1 plays a major role on bacterial growth, T3SS expression and pathogenicity, while the AroG2 is capable to partially carry out the function of AroG1 in the absence of AroG1.

NtPR1a regulates resistance to Ralstonia solanacearum in Nicotiana tabacum via activating the defense-related genes

Pathogenesis-related proteins (PRs) are associated with the development of systemic acquired resistance (SAR) against further infection enforced by fungi, bacteria and viruses. PR1a is the first PR-1 member that could be purified and characterized. Previous studies have reported its role in plants' resistance system against oomycete pathogens. However, the role of PR1a in Solanaceae plants against the bacterial wilt pathogen Ralstonia solanacearum remains unclear. To assess roles of NtPR1a in tobacco responding to R. solanacearum, we performed overexpression experiments in Yunyan 87 plants (a susceptible tobacco cultivar). The results illuminated that overexpression of NtPR1a contributed to improving resistance to R. solanacearum in tobacco Yunyan 87. Specifically speaking, NtPR1a gene could be induced by exogenous hormones like salicylic acid (SA) and pathogenic bacteria R. Solanacearum. Moreover, NtPR1a-overexpressing tobacco significantly reduced multiple of R. solanacearum and inhibited the development of disease symptoms compared with wild-type plants. Importantly, overexpression of NtPR1a activated a series of defense-related genes expression, including the hypersensitive response (HR)-associated genes NtHSR201 and NtHIN1, SA-, JA- and ET-associated genes NtPR2, NtCHN50, NtPR1b, NtEFE26, and Ntacc oxidase, and detoxification-associated gene NtGST1. In summary, our results suggested that NtPR1a-enhanced tobacco resistance to R. solanacearum may be mainly dependent on activation of the defense-related genes.

In Vitro and In Vivo Secretion/Translocation Assays to Identify Novel Ralstonia solanacearum Type 3 Effectors

Phytopathogenic bacteria have evolved multiple strategies to infect plants. Like many gram-negative bacteria, Ralstonia solanacearum, the causal agent of bacterial wilt, possesses a specialized protein secretion machinery to deliver effector proteins directly into the host cells. This type 3 secretion system (T3SS) and the bacterial proteins translocated, called type 3 effectors (T3Es), constitute the main pathogenicity determinants of the R. solanacearum species complex (RSSC). Up to 113 orthologous groups defining T3E genes have been identified among the RSSC strains sequenced to date. The increasing number of R. solanacearum genomic sequences available still expands the number of T3E candidates which require experimental validation. Here, we describe in vitro (type 3 secretion) and in vivo (type 3 translocation based on CyaA' reporter gene) methods to identify and validate type 3-dependent delivery of proteins of interest highlighted as candidate T3Es. We also present protocols to generate dedicated vectors and R. solanacearum transformation to perform these experiments.

Control of primary metabolism by a virulence regulatory network promotes robustness in a plant pathogen

Robustness is a key system-level property of living organisms to maintain their functions while tolerating perturbations. We investigate here how a regulatory network controlling multiple virulence factors impacts phenotypic robustness of a bacterial plant pathogen. We reconstruct a cell-scale model of Ralstonia solanacearum connecting a genome-scale metabolic network, a virulence macromolecule network, and a virulence regulatory network, which includes 63 regulatory components. We develop in silico methods to quantify phenotypic robustness under a broad set of conditions in high-throughput simulation analyses. This approach reveals that the virulence regulatory network exerts a control of the primary metabolism to promote robustness upon infection. The virulence regulatory network plugs into the primary metabolism mainly through the control of genes likely acquired via horizontal gene transfer, which results in a functional overlay with ancestral genes. These results support the view that robustness may be a selected trait that promotes pathogenic fitness upon infection.

A putative LysR-type transcriptional regulator PrhO positively regulates the type III secretion system and contributes to the virulence of Ralstonia solanacearum

LysR-type transcriptional regulators (LTTRs) are ubiquitous and abundant amongst bacteria and control a variety of cellular processes. Here, we investigated the effect of Rsc1880 (a putative LTTR, hereafter designated as PrhO) on the pathogenicity of Ralstonia solanacearum. Deletion of prhO substantially reduced the expression of the type III secretion system (T3SS) both in vitro and in planta, and resulted in significantly impaired virulence in tomato and tobacco plants. Complementary prhO completely restored the reduced virulence and T3SS expression to that of the wild-type. Moreover, PrhO-dependent T3SS and virulence were conserved amongst R. solanacearum species. However, deletion of prhO did not alter biofilm formation, swimming mobility and in planta growth. The expression of some type III effectors was significantly reduced in prhO mutants, but the hypersensitive response was not affected in tobacco leaves. Consistent with the key regulatory role of HrpB on T3SS, PrhO positively regulated the T3SS through HrpB. Furthermore, PrhO regulated hrpB expression via two close paralogues, HrpG and PrhG, which are two-component response regulators and positively regulate hrpB expression in a parallel manner. However, deletion of prhO did not alter the expression of phcA, prhJ and prhN, which are also involved in hrpB regulation. In addition, PrhO was expressed in a cell density-dependent manner, but negatively repressed by itself. No regulation was observed for HrpB, PhcA and PrhN on prhO expression. Taken together, we genetically demonstrated that PrhO is a novel virulence regulator of R. solanacearum, which positively regulates T3SS expression through HrpG, PrhG and HrpB and contributes to virulence.

The Ralstonia solanacearum type III effector RipAY targets plant redox regulators to suppress immune responses

The subversion of plant cellular functions is essential for bacterial pathogens to proliferate in host plants and cause disease. Most bacterial plant pathogens employ a type III secretion system to inject type III effector (T3E) proteins inside plant cells, where they contribute to the pathogen-induced alteration of plant physiology. In this work, we found that the Ralstonia solanacearum T3E RipAY suppresses plant immune responses triggered by bacterial elicitors and by the phytohormone salicylic acid. Further biochemical analysis indicated that RipAY associates in planta with thioredoxins from Nicotiana benthamiana and Arabidopsis. Interestingly, RipAY displays γ-glutamyl cyclotransferase (GGCT) activity to degrade glutathione in plant cells, which is required for the reported suppression of immune responses. Given the importance of thioredoxins and glutathione as major redox regulators in eukaryotic cells, RipAY activity may constitute a novel and powerful virulence strategy employed by R. solanacearum to suppress immune responses and potentially alter general redox signalling in host cells.

Recruitment of a Lineage-Specific Virulence Regulatory Pathway Promotes Intracellular Infection by a Plant Pathogen Experimentally Evolved into a Legume Symbiont

Ecological transitions between different lifestyles, such as pathogenicity, mutualism and saprophytism, have been very frequent in the course of microbial evolution, and often driven by horizontal gene transfer. Yet, how genomes achieve the ecological transition initiated by the transfer of complex biological traits remains poorly known. Here, we used experimental evolution, genomics, transcriptomics and high-resolution phenotyping to analyze the evolution of the plant pathogen Ralstonia solanacearum into legume symbionts, following the transfer of a natural plasmid encoding the essential mutualistic genes. We show that a regulatory pathway of the recipient R. solanacearum genome involved in extracellular infection of natural hosts was reused to improve intracellular symbiosis with the Mimosa pudica legume. Optimization of intracellular infection capacity was gained through mutations affecting two components of a new regulatory pathway, the transcriptional regulator efpR and a region upstream from the RSc0965-0967 genes of unknown functions. Adaptive mutations caused the downregulation of efpR and the over-expression of a downstream regulatory module, the three unknown genes RSc3146-3148, two of which encoding proteins likely associated to the membrane. This over-expression led to important metabolic and transcriptomic changes and a drastic qualitative and quantitative improvement of nodule intracellular infection. In addition, these adaptive mutations decreased the virulence of the original pathogen. The complete efpR/RSc3146-3148 pathway could only be identified in the genomes of the pathogenic R. solanacearum species complex. Our findings illustrate how the rewiring of a genetic network regulating virulence allows a radically different type of symbiotic interaction and contributes to ecological transitions and trade-offs.

Involvement of NpdA, a Putative 2-Nitropropane Dioxygenase, in the T3SS Expression and Full Virulence in Ralstonia solanacearum OE1-1

Previously, we isolated several genes that potentially affected the expression of type III secretion system (T3SS) in Ralstonia solanacearum OE1-1. Here, we focused on the rsp0316, which encodes a putative 2-nitropropane dioxygenase (hereafter designated NpdA). The deletion of npdA substantially reduced the T3SS expression and virulence in OE1-1, and the complementation with functional NpdA could completely restore its reduced T3SS expression and virulence to that of wild type. The NpdA was highly conserved among diverse R. solanacearum species and the NpdA-dependent expression of T3SS was not specific to OE1-1 strain, but not the virulence. The NpdA was important for the T3SS expression in planta, while it was not required for the bacterial growth in planta. Moreover, the NpdA was not required for the elicitation of hypersensitive response (HR) of R. solanacearum strains in tobacco leaves. The T3SS in R. solanacearum is directly controlled by the AraC-type transcriptional regulator HrpB and regulated by a complex regulation network. The NpdA affected the T3SS expression mediated with HrpB but through some novel pathway. All these results from genetic studies demonstrate that NpdA is a novel factor for the T3SS expression in diverse R. solanacearum species in medium, but specifically for the T3SS expression in strain OE1-1 in planta. And the NpdA-dependent expression of T3SS in planta plays an important role in pathogenicity of R. solanacearum OE1-1 in host plants.

A prophage tail-like protein is deployed by Burkholderia bacteria to feed on fungi

Some bacteria can feed on fungi, a phenomenon known as mycophagy. Here we show that a prophage tail-like protein (Bg_9562) is essential for mycophagy in Burkholderia gladioli strain NGJ1. The purified protein causes hyphal disintegration and inhibits growth of several fungal species. Disruption of the Bg_9562 gene abolishes mycophagy. Bg_9562 is a potential effector secreted by a type III secretion system (T3SS) and is translocated into fungal mycelia during confrontation. Heterologous expression of Bg_9562 in another bacterial species, Ralstonia solanacearum, confers mycophagous ability in a T3SS-dependent manner. We propose that the ability to feed on fungi conferred by Bg_9562 may help the bacteria to survive in certain ecological niches. Furthermore, considering its broad-spectrum antifungal activity, the protein may be potentially useful in biotechnological applications to control fungal diseases.Some bacteria can feed on live fungi through unclear mechanisms. Here, the authors show that a T3SS-secreted protein, which is homologous to phage tail proteins, allows a Burkholderia gladioli strain to kill and feed on various fungal species.

Quantitative Disease Resistance under Elevated Temperature: Genetic Basis of New Resistance Mechanisms to Ralstonia solanacearum

In the context of climate warming, plants will be facing an increased risk of epidemics as well as the emergence of new highly aggressive pathogen species. Although a permanent increase of temperature strongly affects plant immunity, the underlying molecular mechanisms involved are still poorly characterized. In this study, we aimed to uncover the genetic bases of resistance mechanisms that are efficient at elevated temperature to the Ralstonia solanacearum species complex (RSSC), one of the most harmful phytobacteria causing bacterial wilt. To start the identification of quantitative trait loci (QTLs) associated with natural variation of response to R. solanacearum, we adopted a genome wide association (GWA) mapping approach using 176 worldwide natural accessions of Arabidopsis thaliana inoculated with the R. solanacearum GMI1000 strain. Following two different procedures of root-inoculation (root apparatus cut vs. uncut), plants were grown either at 27 or 30°C, with the latter temperature mimicking a permanent increase in temperature. At 27°C, the RPS4/RRS1-R locus was the main QTL of resistance detected regardless of the method of inoculation used. This highlights the power of GWA mapping to identify functionally important loci for resistance to the GMI1000 strain. At 30°C, although most of the accessions developed wilting symptoms, we identified several QTLs that were specific to the inoculation method used. We focused on a QTL region associated with response to the GMI1000 strain in the early stages of infection and, by adopting a reverse genetic approach, we functionally validated the involvement of a strictosidine synthase-like 4 (SSL4) protein that shares structural similarities with animal proteins known to play a role in animal immunity.

HpaB-Dependent Secretion of Type III Effectors in the Plant Pathogens Ralstonia solanacearum and Xanthomonas campestris pv. vesicatoria

Plant pathogenic bacteria exerts their pathogenicity through the injection of large repertoires of type III effectors (T3Es) into plant cells, a mechanism controlled in part by type III chaperones (T3Cs). In Ralstonia solanacearum, the causal agent of bacterial wilt, little is known about the control of type III secretion at the post-translational level. Here, we provide evidence that the HpaB and HpaD proteins do act as bona fide R. solanacearum class IB chaperones that associate with several T3Es. Both proteins can dimerize but do not interact with each other. After screening 38 T3Es for direct interactions, we highlighted specific and common interacting partners, thus revealing the first picture of the R. solanacearum T3C-T3E network. We demonstrated that the function of HpaB is conserved in two phytopathogenic bacteria, R. solanacearum and Xanthomonas campestris pv. vesicatoria (Xcv). HpaB from Xcv is able to functionally complement a R. solanacearum hpaB mutant for hypersensitive response elicitation on tobacco plants. Likewise, Xcv is able to translocate a heterologous T3E from R. solanacearum in an HpaB-dependent manner. This study underlines the central role of the HpaB class IB chaperone family and its potential contribution to the bacterial plasticity to acquire and deliver new virulence factors.