Genome-wide identification of fitness determinants in the Xanthomonas campestris bacterial pathogen during early stages of plant infection.

Plant diseases are an important threat to food production. While major pathogenicity determinants required for disease have been extensively studied, less is known on how pathogens thrive during host colonization, especially at early infection stages. Here, we used randomly barcoded-transposon insertion site sequencing (RB-TnSeq) to perform a genome-wide screen and identify key bacterial fitness determinants of the vascular pathogen Xanthomonas campestris pv campestris (Xcc) during infection of the cauliflower host plant (Brassica oleracea). This high-throughput analysis was conducted in hydathodes, the natural entry site of Xcc, in xylem sap and in synthetic media. Xcc did not face a strong bottleneck during hydathode infection. In total, 181 genes important for fitness were identified in plant-associated environments with functional enrichment in genes involved in metabolism but only few genes previously known to be involved in virulence. The biological relevance of 12 genes was independently confirmed by phenotyping single mutants. Notably, we show that XC_3388, a protein with no known function (DUF1631), plays a key role in the adaptation and virulence of Xcc possibly through c-di-GMP-mediated regulation. This study revealed yet unsuspected social behaviors adopted by Xcc individuals when confined inside hydathodes at early infection stages.

Two ancestral genes shaped the Xanthomonas campestris TAL effector gene repertoire.

Xanthomonas transcription activator-like effectors (TALEs) are injected inside plant cells to promote host susceptibility by enhancing transcription of host susceptibility genes. TALE-encoding (tal) genes were thought to be absent from Brassicaceae-infecting Xanthomonas campestris (Xc) genomes based on four reference genomic sequences. We discovered tal genes in 26 of 49 Xc strains isolated worldwide and used a combination of single molecule real time (SMRT) and tal amplicon sequencing to yield a near-complete description of the TALEs found in Xc (Xc TALome). The 53 sequenced tal genes encode 21 distinct DNA binding domains that sort into seven major DNA binding specificities. In silico analysis of the Brassica rapa promoterome identified a repertoire of predicted TALE targets, five of which were experimentally validated using quantitative reverse transcription polymerase chain reaction. The Xc TALome shows multiple signs of DNA rearrangements that probably drove its evolution from two ancestral tal genes. We discovered that Tal12a and Tal15a of Xcc strain Xca5 contribute together in the development of disease symptoms on susceptible B. oleracea var. botrytis cv Clovis. This large and polymorphic repertoire of TALEs opens novel perspectives for elucidating TALE-mediated susceptibility of Brassicaceae to black rot disease and for understanding the molecular processes underlying TALE evolution.

The N-Glycan cluster from Xanthomonas campestris pv. campestris: a toolbox for sequential plant N-glycan processing.

N-Glycans are widely distributed in living organisms but represent only a small fraction of the carbohydrates found in plants. This probably explains why they have not previously been considered as substrates exploited by phytopathogenic bacteria during plant infection. Xanthomonas campestris pv. campestris, the causal agent of black rot disease of Brassica plants, possesses a specific system for GlcNAc utilization expressed during host plant infection. This system encompasses a cluster of eight genes (nixE to nixL) encoding glycoside hydrolases (GHs). In this paper, we have characterized the enzymatic activities of these GHs and demonstrated their involvement in sequential degradation of a plant N-glycan using a N-glycopeptide containing two GlcNAcs, three mannoses, one fucose, and one xylose (N2M3FX) as a substrate. The removal of the α-1,3-mannose by the α-mannosidase NixK (GH92) is a prerequisite for the subsequent action of the ß-xylosidase NixI (GH3), which is involved in the cleavage of the ß-1,2-xylose, followed by the α-mannosidase NixJ (GH125), which removes the α-1,6-mannose. These data, combined to the subcellular localization of the enzymes, allowed us to propose a model of N-glycopeptide processing by X. campestris pv. campestris. This study constitutes the first evidence suggesting N-glycan degradation by a plant pathogen, a feature shared with human pathogenic bacteria. Plant N-glycans should therefore be included in the repertoire of molecules putatively metabolized by phytopathogenic bacteria during their life cycle.

Xanthomonas campestris pv. vesicatoria Secretes Proteases and Xylanases via the Xps Type II Secretion System and Outer Membrane Vesicles.

UNLABELLED: Many plant-pathogenic bacteria utilize type II secretion (T2S) systems to secrete degradative enzymes into the extracellular milieu. T2S substrates presumably mediate the degradation of plant cell wall components during the host-pathogen interaction and thus promote bacterial virulence. Previously, the Xps-T2S system from Xanthomonas campestris pv. vesicatoria was shown to contribute to extracellular protease activity and the secretion of a virulence-associated xylanase. The identities and functions of additional T2S substrates from X. campestris pv. vesicatoria, however, are still unknown. In the present study, the analysis of 25 candidate proteins from X. campestris pv. vesicatoria led to the identification of two type II secreted predicted xylanases, a putative protease and a lipase which was previously identified as a virulence factor of X. campestris pv. vesicatoria. Studies with mutant strains revealed that the identified xylanases and the protease contribute to virulence and in planta growth of X. campestris pv. vesicatoria. When analyzed in the related pathogen X. campestris pv. campestris, several T2S substrates from X. campestris pv. vesicatoria were secreted independently of the T2S systems, presumably because of differences in the T2S substrate specificities of the two pathogens. Furthermore, in X. campestris pv. vesicatoria T2S mutants, secretion of T2S substrates was not completely absent, suggesting the contribution of additional transport systems to protein secretion. In line with this hypothesis, T2S substrates were detected in outer membrane vesicles, which were frequently observed for X. campestris pv. vesicatoria. We, therefore, propose that extracellular virulence-associated enzymes from X. campestris pv. vesicatoria are targeted to the Xps-T2S system and to outer membrane vesicles. IMPORTANCE: The virulence of plant-pathogenic bacteria often depends on TS2 systems, which secrete degradative enzymes into the extracellular milieu. T2S substrates are being studied in several plant-pathogenic bacteria, including Xanthomonas campestris pv. vesicatoria, which causes bacterial spot disease in tomato and pepper. Here, we show that the T2S system from X. campestris pv. vesicatoria secretes virulence-associated xylanases, a predicted protease, and a lipase. Secretion assays with the related pathogen X. campestris pv. campestris revealed important differences in the T2S substrate specificities of the two pathogens. Furthermore, electron microscopy showed that T2S substrates from X. campestris pv. vesicatoria are targeted to outer membrane vesicles (OMVs). Our results, therefore, suggest that OMVs provide an alternative transport route for type II secreted extracellular enzymes.

Genomics and transcriptomics of Xanthomonas campestris species challenge the concept of core type III effectome.

BACKGROUND: The bacterial species Xanthomonas campestris infects a wide range of Brassicaceae. Specific pathovars of this species cause black rot (pv. campestris), bacterial blight of stock (pv. incanae) or bacterial leaf spot (pv. raphani). RESULTS: In this study, we extended the genomic coverage of the species by sequencing and annotating the genomes of strains from pathovar incanae (CFBP 1606R and CFBP 2527R), pathovar raphani (CFBP 5828R) and a pathovar formerly named barbareae (CFBP 5825R). While comparative analyses identified a large core ORFeome at the species level, the core type III effectome was limited to only three putative type III effectors (XopP, XopF1 and XopAL1). In Xanthomonas, these effector proteins are injected inside the plant cells by the type III secretion system and contribute collectively to virulence. A deep and strand-specific RNA sequencing strategy was adopted in order to experimentally refine genome annotation for strain CFBP 5828R. This approach also allowed the experimental definition of novel ORFs and non-coding RNA transcripts. Using a constitutively active allele of hrpG, a master regulator of the type III secretion system, a HrpG-dependent regulon of 141 genes co-regulated with the type III secretion system was identified. Importantly, all these genes but seven are positively regulated by HrpG and 56 of those encode components of the Hrp type III secretion system and putative effector proteins. CONCLUSIONS: This dataset is an important resource to mine for novel type III effector proteins as well as for bacterial genes which could contribute to pathogenicity of X. campestris.

Genome sequence of Xanthomonas fuscans subsp. fuscans strain 4834-R reveals that flagellar motility is not a general feature of xanthomonads.

BACKGROUND: Xanthomonads are plant-associated bacteria responsible for diseases on economically important crops. Xanthomonas fuscans subsp. fuscans (Xff) is one of the causal agents of common bacterial blight of bean. In this study, the complete genome sequence of strain Xff 4834-R was determined and compared to other Xanthomonas genome sequences. RESULTS: Comparative genomics analyses revealed core characteristics shared between Xff 4834-R and other xanthomonads including chemotaxis elements, two-component systems, TonB-dependent transporters, secretion systems (from T1SS to T6SS) and multiple effectors. For instance a repertoire of 29 Type 3 Effectors (T3Es) with two Transcription Activator-Like Effectors was predicted. Mobile elements were associated with major modifications in the genome structure and gene content in comparison to other Xanthomonas genomes. Notably, a deletion of 33 kbp affects flagellum biosynthesis in Xff 4834-R. The presence of a complete flagellar cluster was assessed in a collection of more than 300 strains representing different species and pathovars of Xanthomonas. Five percent of the tested strains presented a deletion in the flagellar cluster and were non-motile. Moreover, half of the Xff strains isolated from the same epidemic than 4834-R was non-motile and this ratio was conserved in the strains colonizing the next bean seed generations. CONCLUSIONS: This work describes the first genome of a Xanthomonas strain pathogenic on bean and reports the existence of non-motile xanthomonads belonging to different species and pathovars. Isolation of such Xff variants from a natural epidemic may suggest that flagellar motility is not a key function for in planta fitness.

The xylan utilization system of the plant pathogen Xanthomonas campestris pv campestris controls epiphytic life and reveals common features with oligotrophic bacteria and animal gut symbionts.

Xylan is a major structural component of plant cell wall and the second most abundant plant polysaccharide in nature. Here, by combining genomic and functional analyses, we provide a comprehensive picture of xylan utilization by Xanthomonas campestris pv campestris (Xcc) and highlight its role in the adaptation of this epiphytic phytopathogen to the phyllosphere. The xylanolytic activity of Xcc depends on xylan-deconstruction enzymes but also on transporters, including two TonB-dependent outer membrane transporters (TBDTs) which belong to operons necessary for efficient growth in the presence of xylo-oligosaccharides and for optimal survival on plant leaves. Genes of this xylan utilization system are specifically induced by xylo-oligosaccharides and repressed by a LacI-family regulator named XylR. Part of the xylanolytic machinery of Xcc, including TBDT genes, displays a high degree of conservation with the xylose-regulon of the oligotrophic aquatic bacterium Caulobacter crescentus. Moreover, it shares common features, including the presence of TBDTs, with the xylan utilization systems of Bacteroides ovatus and Prevotella bryantii, two gut symbionts. These similarities and our results support an important role for TBDTs and xylan utilization systems for bacterial adaptation in the phyllosphere, oligotrophic environments and animal guts.

Genomic insights into strategies used by Xanthomonas albilineans with its reduced artillery to spread within sugarcane xylem vessels.

BACKGROUND: Xanthomonas albilineans causes leaf scald, a lethal disease of sugarcane. X. albilineans exhibits distinctive pathogenic mechanisms, ecology and taxonomy compared to other species of Xanthomonas. For example, this species produces a potent DNA gyrase inhibitor called albicidin that is largely responsible for inducing disease symptoms; its habitat is limited to xylem; and the species exhibits large variability. A first manuscript on the complete genome sequence of the highly pathogenic X. albilineans strain GPE PC73 focused exclusively on distinctive genomic features shared with Xylella fastidiosa-another xylem-limited Xanthomonadaceae. The present manuscript on the same genome sequence aims to describe all other pathogenicity-related genomic features of X. albilineans, and to compare, using suppression subtractive hybridization (SSH), genomic features of two strains differing in pathogenicity. RESULTS: Comparative genomic analyses showed that most of the known pathogenicity factors from other Xanthomonas species are conserved in X. albilineans, with the notable absence of two major determinants of the "artillery" of other plant pathogenic species of Xanthomonas: the xanthan gum biosynthesis gene cluster, and the type III secretion system Hrp (hypersensitive response and pathogenicity). Genomic features specific to X. albilineans that may contribute to specific adaptation of this pathogen to sugarcane xylem vessels were also revealed. SSH experiments led to the identification of 20 genes common to three highly pathogenic strains but missing in a less pathogenic strain. These 20 genes, which include four ABC transporter genes, a methyl-accepting chemotaxis protein gene and an oxidoreductase gene, could play a key role in pathogenicity. With the exception of hypothetical proteins revealed by our comparative genomic analyses and SSH experiments, no genes potentially involved in any offensive or counter-defensive mechanism specific to X. albilineans were identified, supposing that X. albilineans has a reduced artillery compared to other pathogenic Xanthomonas species. Particular attention has therefore been given to genomic features specific to X. albilineans making it more capable of evading sugarcane surveillance systems or resisting sugarcane defense systems. CONCLUSIONS: This study confirms that X. albilineans is a highly distinctive species within the genus Xanthomonas, and opens new perpectives towards a greater understanding of the pathogenicity of this destructive sugarcane pathogen.

Genome Sequences of 17 Strains from Eight Races of Xanthomonas campestris pv. campestris.

Xanthomonas campestris pv. campestris is a group of phytopathogenic bacteria causing black rot disease on Brassicaceae crops. Here, we report on draft genome sequences of 17 strains representing eight of nine known races of this pathogen, including the pathotype strain CFBP 6865.

Xanthomonas transcriptome inside cauliflower hydathodes reveals bacterial virulence strategies and physiological adaptations at early infection stages.

Xanthomonas campestris pv. campestris (Xcc) is a seed-transmitted vascular pathogen causing black rot disease on cultivated and wild Brassicaceae. Xcc enters the plant tissues preferentially via hydathodes, which are organs localized at leaf margins. To decipher both physiological and virulence strategies deployed by Xcc during early stages of infection, the transcriptomic profile of Xcc was analysed 3 days after entry into cauliflower hydathodes. Despite the absence of visible plant tissue alterations and despite a biotrophic lifestyle, 18% of Xcc genes were differentially expressed, including a striking repression of chemotaxis and motility functions. The Xcc full repertoire of virulence factors had not yet been activated but the expression of the HrpG regulon composed of 95 genes, including genes coding for the type III secretion machinery important for suppression of plant immunity, was induced. The expression of genes involved in metabolic adaptations such as catabolism of plant compounds, transport functions, sulphur and phosphate metabolism was upregulated while limited stress responses were observed 3 days postinfection. We confirmed experimentally that high-affinity phosphate transport is needed for bacterial fitness inside hydathodes. This analysis provides information about the nutritional and stress status of bacteria during the early biotrophic infection stages and helps to decipher the adaptive strategy of Xcc to the hydathode environment.

Insights into the extracytoplasmic stress response of Xanthomonas campestris pv. campestris: role and regulation of {sigma}E-dependent activity.

Xanthomonas campestris pv. campestris is an epiphytic bacterium that can become a vascular pathogen responsible for black rot disease of crucifers. To adapt gene expression in response to ever-changing habitats, phytopathogenic bacteria have evolved signal transduction regulatory pathways, such as extracytoplasmic function (ECF) σ factors. The alternative sigma factor σ(E), encoded by rpoE, is crucial for envelope stress response and plays a role in the pathogenicity of many bacterial species. Here, we combine different approaches to investigate the role and mechanism of σ(E)-dependent activation in X. campestris pv. campestris. We show that the rpoE gene is organized as a single transcription unit with the anti-σ gene rseA and the protease gene mucD and that rpoE transcription is autoregulated. rseA and mucD transcription is also controlled by a highly conserved σ(E)-dependent promoter within the σ(E) gene sequence. The σ(E)-mediated stress response is required for stationary-phase survival, resistance to cadmium, and adaptation to membrane-perturbing stresses (elevated temperature and ethanol). Using microarray technology, we started to define the σ(E) regulon of X. campestris pv. campestris. These genes encode proteins belonging to different classes, including periplasmic or membrane proteins, biosynthetic enzymes, classical heat shock proteins, and the heat stress σ factor σ(H). The consensus sequence for the predicted σ(E)-regulated promoter elements is GGAACTN(15-17)GTCNNA. Determination of the rpoH transcription start site revealed that rpoH was directly regulated by σ(E) under both normal and heat stress conditions. Finally, σ(E) activity is regulated by the putative regulated intramembrane proteolysis (RIP) proteases RseP and DegS, as previously described in many other bacteria. However, our data suggest that RseP and DegS are not only dedicated to RseA cleavage and that the proteolytic cascade of RseA could involve other proteases.

Analysis of the xylem sap proteome of Brassica oleracea reveals a high content in secreted proteins.

Xylem plays a major role in plant development and is considered part of the apoplast. Here, we studied the proteome of Brassica oleracea cv Bartolo and compared it to the plant cell wall proteome of another Brassicaceae, the model plant Arabidopsis thaliana. B. oleracea was chosen because it is technically difficult to harvest enough A. thaliana xylem sap for proteomic analysis. We studied the whole proteome and an N-glycoproteome obtained after Concanavalin A affinity chromatography. Altogether, 189 proteins were identified by LC-MS/MS using Brassica EST and cDNA sequences. A predicted signal peptide was found in 164 proteins suggesting that most proteins of the xylem sap are secreted. Eighty-one proteins were identified in the N-glycoproteome, with 25 of them specific of this fraction, suggesting that they were concentrated during the chromatography step. All the protein families identified in this study were found in the cell wall proteomes. However, proteases and oxido-reductases were more numerous in the xylem sap proteome, whereas enzyme inhibitors were rare. The origin of xylem sap proteins is discussed. All the experimental data including the MS/MS data were made available in the WallProtDB cell wall proteomic database.

Identification and regulation of the N-acetylglucosamine utilization pathway of the plant pathogenic bacterium Xanthomonas campestris pv. campestris.

Xanthomonas campestris pv. campestris, the causal agent of black rot disease of brassicas, is known for its ability to catabolize a wide range of plant compounds. This ability is correlated with the presence of specific carbohydrate utilization loci containing TonB-dependent transporters (CUT loci) devoted to scavenging specific carbohydrates. In this study, we demonstrate that there is an X. campestris pv. campestris CUT system involved in the import and catabolism of N-acetylglucosamine (GlcNAc). Expression of genes belonging to this GlcNAc CUT system is under the control of GlcNAc via the LacI family NagR and GntR family NagQ regulators. Analysis of the NagR and NagQ regulons confirmed that GlcNAc utilization involves NagA and NagB-II enzymes responsible for the conversion of GlcNAc-6-phosphate to fructose-6-phosphate. Mutants with mutations in the corresponding genes are sensitive to GlcNAc, as previously reported for Escherichia coli. This GlcNAc sensitivity and analysis of the NagQ and NagR regulons were used to dissect the X. campestris pv. campestris GlcNAc utilization pathway. This analysis revealed specific features, including the fact that uptake of GlcNAc through the inner membrane occurs via a major facilitator superfamily transporter and the fact that this amino sugar is phosphorylated by two proteins belonging to the glucokinase family, NagK-IIA and NagK-IIB. However, NagK-IIA seems to play a more important role in GlcNAc utilization than NagK-IIB under our experimental conditions. The X. campestris pv. campestris GlcNAc NagR regulon includes four genes encoding TonB-dependent active transporters (TBDTs). However, the results of transport experiments suggest that GlcNAc passively diffuses through the bacterial envelope, an observation that calls into question whether GlcNAc is a natural substrate for these TBDTs and consequently is the source of GlcNAc for this nonchitinolytic plant-associated bacterium.

The complete genome sequence of Xanthomonas albilineans provides new insights into the reductive genome evolution of the xylem-limited Xanthomonadaceae.

BACKGROUND: The Xanthomonadaceae family contains two xylem-limited plant pathogenic bacterial species, Xanthomonas albilineans and Xylella fastidiosa. X. fastidiosa was the first completely sequenced plant pathogen. It is insect-vectored, has a reduced genome and does not possess hrp genes which encode a Type III secretion system found in most plant pathogenic bacteria. X. fastidiosa was excluded from the Xanthomonas group based on phylogenetic analyses with rRNA sequences. RESULTS: The complete genome of X. albilineans was sequenced and annotated. X. albilineans, which is not known to be insect-vectored, also has a reduced genome and does not possess hrp genes. Phylogenetic analysis using X. albilineans genomic sequences showed that X. fastidiosa belongs to the Xanthomonas group. Order of divergence of the Xanthomonadaceae revealed that X. albilineans and X. fastidiosa experienced a convergent reductive genome evolution during their descent from the progenitor of the Xanthomonas genus. Reductive genome evolutions of the two xylem-limited Xanthomonadaceae were compared in light of their genome characteristics and those of obligate animal symbionts and pathogens. CONCLUSION: The two xylem-limited Xanthomonadaceae, during their descent from a common ancestral parent, experienced a convergent reductive genome evolution. Adaptation to the nutrient-poor xylem elements and to the cloistered environmental niche of xylem vessels probably favoured this convergent evolution. However, genome characteristics of X. albilineans differ from those of X. fastidiosa and obligate animal symbionts and pathogens, indicating that a distinctive process was responsible for the reductive genome evolution in this pathogen. The possible role in genome reduction of the unique toxin albicidin, produced by X. albilineans, is discussed.

Role of the acquisition of a type 3 secretion system in the emergence of novel pathogenic strains of Xanthomonas.

Cases of emergence of novel plant-pathogenic strains are regularly reported that reduce the yields of crops and trees. However, the molecular mechanisms underlying such emergence are still poorly understood. The acquisition by environmental non-pathogenic strains of novel virulence genes by horizontal gene transfer has been suggested as a driver for the emergence of novel pathogenic strains. In this study, we tested such an hypothesis by transferring a plasmid encoding the type 3 secretion system (T3SS) and four associated type 3 secreted proteins (T3SPs) to the non-pathogenic strains of Xanthomonas CFBP 7698 and CFBP 7700, which lack genes encoding T3SS and any previously known T3SPs. The resulting strains were phenotyped on Nicotiana benthamiana using chlorophyll fluorescence imaging and image analysis. Wild-type, non-pathogenic strains induced a hypersensitive response (HR)-like necrosis, whereas strains complemented with T3SS and T3SPs suppressed this response. Such suppression depends on a functional T3SS. Amongst the T3SPs encoded on the plasmid, Hpa2, Hpa1 and, to a lesser extent, XopF1 collectively participate in suppression. Monitoring of the population sizes in planta showed that the sole acquisition of a functional T3SS by non-pathogenic strains impairs growth inside leaf tissues. These results provide functional evidence that the acquisition via horizontal gene transfer of a T3SS and four T3SPs by environmental non-pathogenic strains is not sufficient to make strains pathogenic. In the absence of a canonical effector, the sole acquisition of a T3SS seems to be counter-selective, and further acquisition of type 3 effectors is probably needed to allow the emergence of novel pathogenic strains.

AvrAC(Xcc8004), a type III effector with a leucine-rich repeat domain from Xanthomonas campestris pathovar campestris confers avirulence in vascular tissues of Arabidopsis thaliana ecotype Col-0.

Xanthomonas campestris pathovar campestris causes black rot, a vascular disease on cruciferous plants, including Arabidopsis thaliana. The gene XC1553 from X. campestris pv. campestris strain 8004 encodes a protein containing leucine-rich repeats (LRRs) and appears to be restricted to strains of X. campestris pv. campestris. LRRs are found in a number of type III-secreted effectors in plant and animal pathogens. These prompted us to investigate the role of the XC1553 gene in the interaction between X. campestris pv. campestris and A. thaliana. Translocation assays using the hypersensitive-reaction-inducing domain of X. campestris pv. campestris AvrBs1 as a reporter revealed that XC1553 is a type III effector. Infiltration of Arabidopsis leaf mesophyll with bacterial suspensions showed no differences between the wild-type strain and an XC1553 gene mutant; both strains induced disease symptoms on Kashmir and Col-0 ecotypes. However, a clear difference was observed when bacteria were introduced into the vascular system by piercing the central vein of leaves. In this case, the wild-type strain 8004 caused disease on the Kashmir ecotype, but not on ecotype Col-0; the XC1553 gene mutant became virulent on the Col-0 ecotype and still induced disease on the Kashmir ecotype. Altogether, these data show that the XC1553 gene, which was renamed avrAC(Xcc8004), functions as an avirulence gene whose product seems to be recognized in vascular tissues.

What can bacterial genome research teach us about bacteria-plant interactions?

Biological research is changing dramatically. Genomic and post-genomic research is responsible for the accumulation of enormous datasets, which allow the formation of holistic views of the organisms under investigation. In the field of microbiology, bacteria represent ideal candidates for this new development. It is relatively easy to sequence the genomes of bacteria, to analyse their transcriptomes and to collect information at the proteomic level. Genome research on symbiotic, pathogenic and associative bacteria is providing important information on bacteria-plant interactions, especially on type-III secretion systems (TTSS) and their role in the interaction of bacteria with plants.

Using Ecology, Physiology, and Genomics to Understand Host Specificity in Xanthomonas.

How pathogens coevolve with and adapt to their hosts are critical to understanding how host jumps and/or acquisition of novel traits can lead to new disease emergences. The Xanthomonas genus includes Gram-negative plant-pathogenic bacteria that collectively infect a broad range of crops and wild plant species. However, individual Xanthomonas strains usually cause disease on only a few plant species and are highly adapted to their hosts, making them pertinent models to study host specificity. This review summarizes our current understanding of the molecular basis of host specificity in the Xanthomonas genus, with a particular focus on the ecology, physiology, and pathogenicity of the bacterium. Despite our limited understanding of the basis of host specificity, type III effectors, microbe-associated molecular patterns, lipopolysaccharides, transcriptional regulators, and chemotactic sensors emerge as key determinants for shaping host specificity.

Genome Sequences of the Race 1 and Race 4 Xanthomonas campestris pv. campestris Strains CFBP 1869 and CFBP 5817.

Xanthomonas campestris pv. campestris is the causal agent of black rot on Brassicaceae. The draft genome sequences of strains CFBP 1869 and CFBP 5817 have been determined and are the first ones corresponding to race 1 and race 4 strains, which have a predominant agronomic and economic impact on cabbage cultures worldwide.

The Decoy Substrate of a Pathogen Effector and a Pseudokinase Specify Pathogen-Induced Modified-Self Recognition and Immunity in Plants.

In plants, host response to pathogenic microbes is driven both by microbial perception and detection of modified-self. The Xanthomonas campestris effector protein AvrAC/XopAC uridylylates the Arabidopsis BIK1 kinase to dampen basal resistance and thereby promotes bacterial virulence. Here we show that PBL2, a paralog of BIK1, is similarly uridylylated by AvrAC. However, in contrast to BIK1, PBL2 uridylylation is specifically required for host recognition of AvrAC to trigger immunity, but not AvrAC virulence. PBL2 thus acts as a decoy and enables AvrAC detection. AvrAC recognition also requires the RKS1 pseudokinase of the ZRK family and the NOD-like receptor ZAR1, which is known to recognize the Pseudomonas syringae effector HopZ1a. ZAR1 forms a stable complex with RKS1, which specifically recruits PBL2 when the latter is uridylylated by AvrAC, triggering ZAR1-mediated immunity. The results illustrate how decoy substrates and pseudokinases can specify and expand the capacity of the plant immune system.