Xanthomonas campestris sensor kinase HpaS co-opts the orphan response regulator VemR to form a branched two-component system that regulates motility

Design and Synthesis of Mandelic Acid Derivatives for Suppression of Virulence via T3SS against Citrus Canker

Citrus canker, a highly contagious bacterial disease caused by Xanthomonas citri subsp. citri (Xcc), poses a substantial threat to citrus crops, leading to serious reductions in fruit yield and economic losses. Most commonly used bactericides against Xcc lead to the rapid development of resistant subpopulations. Therefore, it is imperative to create novel drugs, such as type III secretion system (T3SS) inhibitors, that specifically target bacterial virulence factors rather than bacterial viability. In our study, we designed and synthesized a series of mandelic acid derivatives including 2-mercapto-1,3,4-thiazole. Seven substances were found to reduce the level of transcription of hpa1 without affecting bacterial viability. In vivo bioassays indicated that compound F9 significantly inhibited hypersensitive response and pathogenicity. RT-qPCR assays showed that compound F9 visibly suppressed the expression of Xcc T3SS-related genes as well as citrus canker susceptibility gene CsLOB1. Furthermore, the combination with compound F9 and quorum-quenching bacteria HN-8 can also obviously alleviate canker symptoms.

Innovative Strategy for the Control of Citrus Canker: Inhibitors Targeting the Type III Secretion System of Xanthomonas citri Subsp. citri

To find potential type III secretion system (T3SS) inhibitors against citrus canker caused by Xanthomonas citri subsp. citri (Xcc), a new series of 5-phenyl-2-furan carboxylic acid derivatives stitched with 2-mercapto-1,3,4-thiadiazole were designed and synthesized. Among the 30 compounds synthesized, 14 compounds significantly inhibited the promoter activity of a harpin gene hpa1. Eight of the 14 compounds did not affect the growth of Xcc, but significantly reduced the hypersensitive response (HR) of tobacco and decreased the pathogenicity of Xcc on citrus plants. Subsequent studies have demonstrated that these inhibitory molecules effectively suppress the T3SS of Xcc and significantly impair the pathogen's ability to subvert citrus immunity, resulting in a reduction in the level of disease progression. As a result, our work has identified a series of potentially attractive agents for the control of citrus canker.

Xanthomonas campestris VemR enhances the transcription of the T3SS key regulator HrpX via physical interaction with HrpG

VemR is a response regulator of the two-component signalling systems (TCSs). It consists solely of a receiver domain. Previous studies have shown that VemR plays an important role in influencing the production of exopolysaccharides and exoenzymes, cell motility, and virulence of Xanthomonas campestris pv. campestris (Xcc). However, whether VemR is involved in the essential pathogenicity determinant type III secretion system (T3SS) is unclear. In this work, we found by transcriptome analysis that VemR modulates about 10% of Xcc genes, which are involved in various cellular processes including the T3SS. Further experiments revealed that VemR physically interacts with numerous proteins, including the TCS sensor kinases HpaS and RavA, and the TCS response regulator HrpG, which directly activates the transcription of HrpX, a key regulator controlling T3SS expression. It has been demonstrated previously that HpaS composes a TCS with HrpG or VemR to control the expression of T3SS or swimming motility, while RavA and VemR form a TCS to control the expression of flagellar genes. Mutation analysis and in vitro transcription assay revealed that phosphorylation might be essential for the function of VemR and phosphorylated VemR could significantly enhance the activation of hrpX transcription by HrpG. We infer that the binding of VemR to HrpG can modulate the activity of HrpG to the hrpX promoter, thereby enhancing hrpX transcription. Although further studies are required to validate this inference and explore the detailed functional mechanism of VemR, our findings provide some insights into the complex regulatory cascade of the HpaS/RavA-VemR/HrpG-HrpX signal transduction system in the control of T3SS.

Anti-Virulence Strategy of Novel Dehydroabietic Acid Derivatives: Design, Synthesis, and Antibacterial Evaluation

Anti-virulence strategies are attractive and interesting strategies for controlling bacterial diseases because virulence factors are fundamental to the infection process of numerous serious phytopathogenics. To extend the novel anti-virulence agents, a series of dehydroabietic acid (DAA) derivatives decorated with amino alcohol unit were semi-synthesized based on structural modification of the renewable natural DAA and evaluated for their antibacterial activity against Xanthomonas oryzae pv. oryzae (Xoo), Xanthomonas axonopodis pv. citri (Xac), and Pseudomonas syringae pv. actinidiae (Psa). Compound 2b showed the most promising antibacterial activity against Xoo with an EC50 of 2.7 μg mL-1. Furthermore, compound 2b demonstrated remarkable control effectiveness against bacterial leaf blight (BLB) in rice, with values of 48.6% and 61.4% for curative and protective activities. In addition, antibacterial behavior suggested that compound 2b could suppress various virulence factors, including EPS, biofilm, swimming motility, and flagella. Therefore, the current study provided promising lead compounds for novel bactericides discovery by inhibiting bacterial virulence factors.

HpaP divergently regulates the expression of hrp genes in Xanthomonas oryzae pathovars oryzae and oryzicola

The bacterial pathogens Xanthomonas oryzae pathovars oryzae (Xoo) and oryzicola (Xoc) cause leaf blight and leaf streak diseases on rice, respectively. Pathogenesis is largely defined by the virulence genes harboured in the pathogen genome. Recently, we demonstrated that the protein HpaP of the crucifer pathogen Xanthomonas campestris pv. campestris is an enzyme with both ATPase and phosphatase activities, and is involved in regulating the synthesis of virulence factors and the induction of the hypersensitive response (HR). In this study, we investigated the role of HpaP homologues in Xoo and Xoc. We showed that HpaP is required for full virulence of Xoo and Xoc. Deletion of hpaP in Xoo and Xoc led to a reduction in virulence and alteration in the production of virulence factors, including extracellular polysaccharide and cell motility. Comparative transcriptomics and reverse transcription-quantitative PCR assays revealed that in XVM2 medium, a mimic medium of the plant environment, the expression levels of hrp genes (for HR and pathogenicity) were enhanced in the Xoo hpaP deletion mutant compared to the wild type. By contrast, in the same growth conditions, hrp gene expression was decreased in the Xoc hpaP deletion mutant compared to the wild type. However, an opposite expression pattern was observed when the pathogens grew in planta, where the expression of hrp genes was reduced in the Xoo hpaP mutant but increased in the Xoc hpaP mutant. These findings indicate that HpaP plays a divergent role in Xoo and Xoc, which may lead to the different infection strategies employed by these two pathogens.

Transcriptional regulator Sar regulates the multiple secretion systems in Xanthomonas oryzae

Xanthomonas oryzae pv. oryzae (Xoo) is a notorious plant pathogen that causes leaf blight of rice cultivars. The pathogenic bacteria possess numerous transcriptional regulators to regulate various biological processes, such as pathogenicity in the host plant. Our previous study identified a new master regulator PXO_RS20790 that is involved in pathogenicity for Xoo against the host rice. However, the molecular functions of PXO_RS20790 are still unclear. Here, we demonstrate that transcriptional regulator Sar (PXO_RS20790) regulates multiple secretion systems. The RNA-sequencing analysis, bacterial one-hybrid assay, and electrophoretic mobility shift assay revealed that Sar enables binding of the promoters of the T1SS-related genes, the avirulence gene, raxX, and positively regulates these genes' expression. Meanwhile, we found that Sar positively regulated the T6SS-1 clusters but did not regulate the T6SS-2 clusters. Furthermore, we revealed that only T6SS-2 is involved in interbacterial competition. We also indicated that Sar could bind the promoters of the T3SS regulators, hrpG and hrpX, to activate these two genes' transcription. Our findings revealed that Sar is a crucial regulator of multiple secretion systems and virulence.

The MinCDE Cell Division System Participates in the Regulation of Type III Secretion System (T3SS) Genes, Bacterial Virulence, and Motility in Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial leaf blight (BLB) in rice, which is one of the most severe bacterial diseases in rice in some Asian countries. The type III secretion system (T3SS) of Xoo encoded by the hypersensitive response and pathogenicity (hrp) genes is essential for its pathogenicity in host rice. Here, we identified the Min system (MinC, MinD, and MinE), a negative regulatory system for bacterial cell division encoded by minC, minD, and minE genes, which is involved in negative regulation of hrp genes (hrpB1 and hrpF) in Xoo. We found that the deletion of minC, minD, and minCDE resulted in enhanced hrpB1 and hrpF expression, which is dependent on two key hrp regulators HrpG and HrpX. The minC, minD, and minCDE mutants exhibited elongated cell lengths, and the classic Min system-defective cell morphology including minicells and short filamentations. Mutation of minC in Xoo resulted in significantly impaired virulence in host rice, swimming motility, and enhanced biofilm formation. Our transcriptome profiling also indicated some virulence genes were differentially expressed in the minC mutants. To our knowledge, this is the first report about the Min system participating in the regulation of T3SS expression. It sheds light on the understanding of Xoo virulence mechanisms.

Xanthomonas oryzae Pv. oryzicola Response Regulator VemR Is Co-opted by the Sensor Kinase CheA for Phosphorylation of Multiple Pathogenicity-Related Targets

Two-component systems (TCSs) (cognate sensor histidine kinase/response regulator pair, HK/RR) play a crucial role in bacterial adaptation, survival, and productive colonization. An atypical orphan single-domain RR VemR was characterized by the non-vascular pathogen Xanthomonas oryzae pv. oryzicola (Xoc) is known to cause bacterial leaf streak (BLS) disease in rice. Xoc growth and pathogenicity in rice, motility, biosynthesis of extracellular polysaccharide (EPS), and the ability to trigger HR in non-host tobacco were severely compromised in the deletion mutant strain RΔvemR as compared to the wild-type strain RS105. Site-directed mutagenesis and phosphotransfer experiments revealed that the conserved aspartate (D⁵⁶) residue within the stand-alone phosphoacceptor receiver (REC) domain is essential for phosphorelay and the regulatory activity of Xoc VemR. Yeast two-hybrid (Y2H) and co-immunoprecipitation (co-IP) data identified CheA as the HK co-opting the RR VemR for phosphorylation. Affinity proteomics identified several downstream VemR-interacting proteins, such as 2-oxoglutarate dehydrogenase (OGDH), DNA-binding RR SirA, flagellar basal body P-ring formation protein FlgA, Type 4a pilus retraction ATPase PilT, stress-inducible sensor HK BaeS, septum site-determining protein MinD, cytoskeletal protein CcmA, and Type III and VI secretion system proteins HrpG and Hcp, respectively. Y2H and deletion mutant analyses corroborated that VemR interacted with OGDH, SirA, FlgA, and HrpG; thus, implicating multi-layered control of diverse cellular processes including carbon metabolism, motility, and pathogenicity in the rice. Physical interaction between VemR and HrpG suggested cross-talk interaction between CheA/VemR- and HpaS/HrpG-mediated signal transduction events orchestrating the hrp gene expression.

McvR, a single domain response regulator regulates motility and virulence in the plant pathogen Xanthomonas campestris

Signal transduction pathways mediated by sensor histidine kinases and cognate response regulators control a variety of physiological processes in response to environmental conditions in most bacteria. Comparatively little is known about the mechanism(s) by which single-domain response regulators (SD-RRs), which lack a dedicated output domain but harbour a phosphoryl receiver domain, exert their various regulatory effects in bacteria. Here we have examined the role of the SD-RR proteins encoded by the phytopathogen Xanthomonas campestris pv. campestris (Xcc). We describe the identification and characterization of a SD-RR protein named McvR (motility, chemotaxis, and virulence-related response regulator) that is required for virulence and motility regulation in Xcc. Deletion of the mcvR open reading frame caused reduced motility, chemotactic movement, and virulence in Xcc. Global transcriptome analyses revealed the McvR had a broad regulatory role and that most motility and pathogenicity genes were down-regulated in the mcvR mutant. Bacterial two-hybrid and protein pull-down assays revealed that McvR did not physically interact with components of the bacterial flagellum but interacts with other SD-RR proteins (like CheY) and the subset of DNA-binding proteins involved in gene regulation. Site-directed mutagenesis and phosphor-transfer experiments revealed that the aspartyl residue at position 55 of the receiver domain is important for phosphorylation and the regulatory activity of McvR protein. Taken together, the findings describe a previously unrecognized class of SD-RR protein that contributes to the regulation of motility and virulence in Xcc.

The Methyltransferase HemK Regulates the Virulence and Nutrient Utilization of the Phytopathogenic Bacterium Xanthomonas citri Subsp. citri

Citrus canker, caused by the bacterium Xanthomonas citri subsp. citri (Xcc), seriously affects fruit quality and yield, leading to significant economic losses around the world. Understanding the mechanism of Xcc virulence is important for the effective control of Xcc infection. In this report, we investigate the role of a protein named HemK in the regulation of the virulence traits of Xcc. The hemK gene was deleted in the Xcc jx-6 background, and the ΔhemK mutant phenotypically displayed significantly decreased motility, biofilm formation, extracellular enzymes, and polysaccharides production, as well as increased sensitivity to oxidative stress and high temperatures. In accordance with the role of HemK in the regulation of a variety of virulence-associated phenotypes, the deletion of hemK resulted in reduced virulence on citrus plants as well as a compromised hypersensitive response on a non-host plant, Nicotiana benthamiana. These results indicated that HemK is required for the virulence of Xcc. To characterize the regulatory effect of hemK deletion on gene expression, RNA sequencing analysis was conducted using the wild-type Xcc jx-6 strain and its isogenic ΔhemK mutant strain, grown in XVM2 medium. Comparative transcriptome analysis of these two strains revealed that hemK deletion specifically changed the expression of several virulence-related genes associated with the bacterial secretion system, chemotaxis, and quorum sensing, and the expression of various genes related to nutrient utilization including amino acid metabolism, carbohydrate metabolism, and energy metabolism. In conclusion, our results indicate that HemK plays an essential role in virulence, the regulation of virulence factor synthesis, and the nutrient utilization of Xcc.

The RavA/VemR two-component system plays vital regulatory roles in the motility and virulence of Xanthomonas campestris

Xanthomonas campestris pv. campestris (Xcc) can cause black rot in cruciferous plants worldwide. Two-component systems (TCSs) are key for bacterial adaptation to various environments, including hosts. VemR is a TCS response regulator and crucial for Xcc motility and virulence. Here, we report that RavA is the cognate histidine kinase (HK) of VemR and elucidate the signalling pathway by which VemR regulates Xcc motility and virulence. Genetic analysis showed that VemR is epistatic to RavA. Using bacterial two-hybrid experiments and pull-down and phosphorylation assays, we found that RavA can interact with and phosphorylate VemR, suggesting that RavA is the cognate HK of VemR. In addition, we found that RpoN2 and FleQ are epistatic to VemR in regulating bacterial motility and virulence. In vivo and in vitro experiments demonstrated that VemR interacts with FleQ but not with RpoN2. RavA/VemR regulates the expression of the flagellin-encoding gene fliC by activating the transcription of the rpoN2-vemR-fleQ and flhF-fleN-fliA operons. In summary, our data show that the RavA/VemR TCS regulates FleQ activity and thus influences the expression of motility-related genes, thereby affecting Xcc motility and virulence. The identification of this novel signalling pathway will deepen our understanding of Xcc-plant interactions.

A HU-like protein is required for full virulence in Xanthomonas campestris pv. campestris

Bacteria harbour several abundant small DNA-binding proteins known as nucleoid-associated proteins (NAPs) that contribute to the structure of the bacterial nucleoid as well as to gene regulation. Although the function of NAPs as global transcriptional regulators has been comprehensively studied in the model organism Escherichia coli, their regulatory functions in other bacteria remain relatively poorly understood. Xanthomonas campestris pv. campestris (Xcc) is a gram-negative bacterium that causes black rot disease in almost all members of the crucifer family. In previous work, we demonstrated that a Fis homologue protein, which we named Fis-like protein (Flp), contributes to the regulation of virulence, type III secretion, and a series of other phenotypes in Xcc. Here we have examined the role of XC_1355, which is predicted to encode a DNA-binding protein belonging to the HU family herein named HU-like protein (Hlp). We show that mutation of XC_1355 in Xcc reduces the virulence, extracellular polysaccharide production, and cell motility, but has no effect on the production of extracellular enzymes and induction of the hypersensitive response. These data together with transcriptome analysis indicate that hlp is a previously uncharacterized gene involved in virulence that has partially overlapping and complementary functions with flp in Xcc, although the two regulators have opposite effects on the expression of genes involved in type III secretion. The findings add to our understanding of the complex regulatory pathways that act to regulate virulence in Xcc.