A plant-inducible gene of Xanthomonas campestris pv. campestris encodes an exocellular component required for growth in the host and hypersensitivity on nonhosts

Gene expression changes throughout the life cycle allow a bacterial plant pathogen to persist in diverse environmental habitats

Bacterial pathogens exhibit a remarkable ability to persist and thrive in diverse ecological niches. Understanding the mechanisms enabling their transition between habitats is crucial to control dissemination and potential disease outbreaks. Here, we use Ralstonia solanacearum, the causing agent of the bacterial wilt disease, as a model to investigate pathogen adaptation to water and soil, two environments that act as bacterial reservoirs, and compare this information with gene expression in planta. Gene expression in water resembled that observed during late xylem colonization, with an intriguing induction of the type 3 secretion system (T3SS). Alkaline pH and nutrient scarcity-conditions also encountered during late infection stages-were identified as the triggers for this T3SS induction. In the soil environment, R. solanacearum upregulated stress-responses and genes for the use of alternate carbon sources, such as phenylacetate catabolism and the glyoxylate cycle, and downregulated virulence-associated genes. We proved through gain- and loss-of-function experiments that genes associated with the oxidative stress response, such as the regulator OxyR and the catalase KatG, are key for bacterial survival in soil, as their deletion cause a decrease in culturability associated with a premature induction of the viable but non culturable state (VBNC). This work identifies essential factors necessary for R. solanacearum to complete its life cycle and is the first comprehensive gene expression analysis in all environments occupied by a bacterial plant pathogen, providing valuable insights into its biology and adaptation to unexplored habitats.

Programmed cell death in Xanthomonas axonopodis pv. glycines is associated with modulation of gene expression resulting in altered states of motility, biofilm and virulence

One of the foremost report of apoptosis-like programmed cell death (PCD) came from Xanthomonas axonopodis pv. glycines (Xag), which displayed rapid post-exponential cell death in PCD inducing media (PIM) but not in a non-inducing media (PNIM). The current study aims to decipher for the first time, the advantages of the existence of PCD in this phytopathogenic microorganism. Analysis of RNA-seq under inducing and non-inducing conditions, revealed differential expression of a number of genes related to key physiology of Xag, such as, motility, xanthan biosynthesis and export as well as virulence. A PCD negative mutant Xag M42 displayed diminished virulence and a contrasting transcriptome pattern. In vitro experiments revealed that under PCD inducing condition, Xag produced negligible xanthan gum as well as extracellular amylase, displayed enhanced swarming motility, released copious e-DNA and formed scanty biofilm. Lack of 'diffusible signalling factor' production was eliminated as possible reason for PCD-induction. Altogether, it appears that, in planta existence of the pathogen metabolically resembles PNIM, and on being transferred to PIM, the cells experience oxidative stress and circumvents it by adopting PCD as an altruistic response. Survival of the remaining population is encouraged by upregulating motility, detachment from the fragile biofilm to achieve dispersal.

Bacterial quorum sensing facilitates Xanthomonas campesteris pv. campestris invasion of host tissue to maximize disease symptoms

Quorum sensing (QS) helps the Xanthomonas group of phytopathogens to infect several crop plants. The vascular phytopathogen Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot disease on Brassicaceae leaves, where a typical v-shaped lesion spans both vascular and mesophyll regions with progressive leaf chlorosis. Recently, the role of QS has been elucidated during Xcc early infection stages. However, a detailed insight into the possible role of QS-regulated bacterial invasion in host chlorophagy during late infection stages remains elusive. In this study, using QS-responsive whole-cell bioreporters of Xcc, we present a detailed chronology of QS-facilitated Xcc colonization in the mesophyll region of cabbage (Brassica oleracea) leaves. We report that QS-enabled localization of Xcc to parenchymal chloroplasts triggers leaf chlorosis and promotion of systemic infection. Our results indicate that the QS response in the Xanthomonas group of vascular phytopathogens maximizes their population fitness across host tissues to trigger stage-specific host chlorophagy and establish a systemic infection.

The HrpG/HrpX Regulon of Xanthomonads-An Insight to the Complexity of Regulation of Virulence Traits in Phytopathogenic Bacteria

Bacteria of the genus Xanthomonas cause a wide variety of economically important diseases in most crops. The virulence of the majority of Xanthomonas spp. is dependent on secretion and translocation of effectors by the type 3 secretion system (T3SS) that is controlled by two master transcriptional regulators HrpG and HrpX. Since their discovery in the 1990s, the two regulators were the focal point of many studies aiming to decipher the regulatory network that controls pathogenicity in Xanthomonas bacteria. HrpG controls the expression of HrpX, which subsequently controls the expression of T3SS apparatus genes and effectors. The HrpG/HrpX regulon is activated in planta and subjected to tight metabolic and genetic regulation. In this review, we cover the advances made in understanding the regulatory networks that control and are controlled by the HrpG/HrpX regulon and their conservation between different Xanthomonas spp.

Blue-light perception by epiphytic Pseudomonas syringae drives chemoreceptor expression, enabling efficient plant infection

Adaptation and efficient colonization of the phyllosphere are essential processes for the switch to an epiphytic stage in foliar bacterial pathogens. Here, we explore the interplay among light perception and global transcriptomic alterations in epiphytic populations of the hemibiotrophic pathogen Pseudomonas syringae pv. tomato DC3000 (PsPto) following contact with tomato leaves. We found that blue-light perception by PsPto on leaf surfaces is required for optimal colonization. Blue light triggers the activation of metabolic activity and increases the transcript levels of five chemoreceptors through the function of light oxygen voltage and BphP1 photoreceptors. The inactivation of PSPTO_1008 and PSPTO_2526 chemoreceptors causes a reduction in virulence. Our results indicate that during PsPto interaction with tomato plants, light perception, chemotaxis, and virulence are highly interwoven processes.

Allelochemical catechol comprehensively impedes bacterial blight of rice caused by Xanthomonas oryzae pv. oryzae

Xanthomonas oryzae pv. oryzae (Xoo) induces bacterial leaf blight (BLB), is known to be the most devastating disease of rice. The present investigation for the first time explains the antibacterial, anti-biofilm, and antivirulence potential of the simplest allelochemical catechol. Bacterial viability and growth are significantly reducing in catechol treatment. Further study also reveals that catechol also inhibits primary attachment and preformed biofilm of Xoo even at half MIC concentration. The half MIC concentration of catechol also induce a significant decrease in virulence factors like swimming, swarming, exopolysaccharide, and xanthomonadin production. Next, we investigate the possible antibacterial mode of action of catechol against Xoo. Results show that, the catechol caused oxidative stress and targets cell membrane for its antibacterial activity. Whereas, in silico study reveals that, catechol binds with the catalytic domain of XanA protein and this may be consider as a reason for antibiofilm activity of catechol. Moreover, in virulence assay on rice plants, we observe significant decrement in lesion length in catechol and Xoo co-treated rice leaves as compared with only Xoo treated leaves. All the results clearly show, allelochemical catechol to be a potential compound for the antibacterial, anti-biofilm, and antivirulence agent against Xoo and consequently mitigating the BLB disease advancement in rice.

Impact of fluazinam on morphological and physiological characteristics of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a necrotrophic and filamentous fungus with a broad host range. Fluazinam is a pyridinamine fungicide with a broad spectrum of antifungal activity and had a strong inhibition effect on mycelial growth of S. sclerotiorum populations. But the impact of fluazinam on morphological and physiological characteristics of S. sclerotiorum is little known. In this study, the EC₅₀ values of fluazinam to three strains of S. sclerotiorum (CZ17S, YZ55S and SA42S) were 0.0084, 0.007, 0.0065 μg/ml respectively. After fluazinam treatment, hyphae of S. sclerotiorum became thinner, hyphal offshoot of top increased, the distance between one septum and another became shorter, cell membrane permeability increased markedly, exopolysaccharide (EPS) content and oxalic acid content decreased significantly, peroxidase (POD) activity increased significantly and mycelial respiration was inhibited. While the number and dry weight of sclerotia, glycerol content in the mycelia did not significantly change. In protective activity assay on detached rapeseed leaves, application of fluazinam at 40 μg/ml and 80 μg/ml, the control efficacy reached to 41.4% and 100%, respectively. In curative activity assay, application of fluazinam at 100 μg/ml, the control efficacy reached to 61.09%. In the same concentration, protective activity of fluazinam against S. sclerotiorum was higher than curative activity. These results will contribute to us on evaluating the potential of the fungicide fluazinam for management of Sclerotinia stem rot and understanding the mode of action of fluazinam against S. sclerotiorum.

Pharmacological characteristics of the novel fungicide pyrisoxazole against Sclerotinia sclerotiorum

Pyrisoxazole is a pyridine compound of demethylation inhibitor fungicides. In this study, baseline sensitivity of Sclerotinia sclerotiorum to pyrisoxazole was determined using 166 strains from the oilseed rape fields in 2014, 2015 and 2016. The EC₅₀ values for mycelial growth inhibition ranged from 0.0214 to 0.5443 μg mL^-1, with a mean EC₅₀ value of 0.2329 ± 0.1048 μg mL^-1 and were normally distributed. The EC₅₀ values had no significant difference among three populations from 2014, 2015, and 2016. There was no correlation with sensitivity between pyrisoxazole and carbendazim or iprodione. After treated with pyrisoxazole, we observed increased cell membrane permeability, and decreased exopolysaccharide and oxalic acid production, which can contribute to reduced virulence of S. sclerotiorum and lead to failure of disease infection. Protective and curative activity tests showed that pyrisoxazole exhibited excellent protective and curative activity against S. sclerotiorum in oilseed rape, and protective activity was better than curative activity. Compared with the currently used fungicides, pyrisoxazole not only exhibited excellent control efficacy on Sclerotinia stem rot, but also dramatically reduced the doses of fungicides in the field trials. Overall, these data provide more references for revealing pharmacological effect of pyrisoxazole against S. sclerotiorum and managing Sclerotinia stem rot on oilseed rape caused by benzimidazole- and dicarboximide-resistant populations.

Mining of potential drug targets through the identification of essential and analogous enzymes in the genomes of pathogens of Glycine max, Zea mays and Solanum lycopersicum

Pesticides are one of the most widely used pest and disease control measures in plant crops and their indiscriminate use poses a direct risk to the health of populations and environment around the world. As a result, there is a great need for the development of new, less toxic molecules to be employed against plant pathogens. In this work, we employed an in silico approach to study the genes coding for enzymes of the genomes of three commercially important plants, soybean (Glycine max), tomato (Solanum lycopersicum) and corn (Zea mays), as well as 15 plant pathogens (4 bacteria and 11 fungi), focusing on revealing a set of essential and non-homologous isofunctional enzymes (NISEs) that could be prioritized as drug targets. By combining sequence and structural data, we obtained an initial set of 568 cases of analogy, of which 97 were validated and further refined, revealing a subset of 29 essential enzymatic activities with a total of 119 different structural forms, most belonging to central metabolic routes, including the carbohydrate metabolism, the metabolism of amino acids, among others. Further, another subset of 26 enzymatic activities possess a tertiary structure specific for the pathogen, not present in plants, men and Apis mellifera, which may be of importance for the development of specific enzymatic inhibitors against plant diseases that are less harmful to humans and the environment.

Anti-biofilm and anti-virulence potential of 3,7-dimethyloct-6-enal derived from Citrus hystrix against bacterial blight of rice caused by Xanthomonas oryzae pv. oryzae

The present investigation for the first time explains the anti biofilm and anti virulence potential of Kaffir lime oil (KLO) and its major constituent, Citronellal (3,7-dimethyloct-6-enal) against Xanthomonas oryzae pv. oryzae, causal organism of bacterial blight disease of rice. KLO at 500 ppm showed potential activity against X. oryzae pv. oryzae. Among the major components identified, citronellal (CIT) at 75 μM concentration was found to significantly inhibit biofilm along with the swimming and swarming potential of X. oryzae pv. oryzae. In contrary, CIT did not affect the metabolic status and growth kinetics of the bacterial cells. Gene expression analysis showed down regulation in motA, cheD, cheY, flgF, gumC, xylanase, endogluconase, cellulose, cellobiosidase, virulence and rpfF transcript levels by citronellal treatment. However, an insignificant effect of 75 μM CIT treatment was observed on motB, flgE, pilA, estY, pglA, protease and lytic genes expression. Finally, the observations recorded were in confirmity with the virulence leaf clip test as lesion length was significantly decreased (39%) in CIT treatment as compared to the control leaves inoculated with only X. oryzae pv. oryzae. Overall, the findings obtained advocate the use of CIT for promising anti biofilm and anti virulence activity which in turn can be used for managing the blight disease in rice.

Frontiers for research on the ecology of plant‐pathogenic bacteria: fundamentals for sustainability

Methods to ensure the health of crops owe their efficacy to the extent to which we understand the ecology and biology of environmental microorganisms and the conditions under which their interactions with plants lead to losses in crop quality or yield. However, in the pursuit of this knowledge, notions of the ecology of plant‐pathogenic microorganisms have been reduced to a plant‐centric and agro‐centric focus. With increasing global change, i.e. changes that encompass not only climate, but also biodiversity, the geographical distribution of biomes, human demographic and socio‐economic adaptations and land use, new plant health problems will emerge via a range of processes influenced by these changes. Hence, knowledge of the ecology of plant pathogens will play an increasingly important role in the anticipation and response to disease emergence. Here, we present our opinion on the major challenges facing the study of the ecology of plant‐pathogenic bacteria. We argue that the discovery of markedly novel insights into the ecology of plant‐pathogenic bacteria is most likely to happen within a framework of more extensive scales of space, time and biotic interactions than those that currently guide much of the research on these bacteria. This will set a context that is more propitious for the discovery of unsuspected drivers of the survival and diversification of plant‐pathogenic bacteria and of the factors most critical for disease emergence, and will set the foundation for new approaches to the sustainable management of plant health. We describe the contextual background of, justification for and specific research questions with regard to the following challenges:

Development of terminology to describe plant–bacterial relationships in terms of bacterial fitness.

Definition of the full scope of the environments in which plant‐pathogenic bacteria reside or survive.

Delineation of pertinent phylogenetic contours of plant‐pathogenic bacteria and naming of strains independent of their presumed life style.

Assessment of how traits of plant‐pathogenic bacteria evolve within the overall framework of their life history.

Exploration of possible beneficial ecosystem services contributed to by plant‐pathogenic bacteria.

A set of PCRs for rapid identification and characterization of Pseudomonas syringae phylogroups

AIMS

The aim of this study was to develop a rapid PCR-based method for the specific detection of individual phylogroups of the Pseudomonas syringae complex.

METHODS AND RESULTS

Seven primer pairs were developed by analysing whole genomes of 54 Ps. syringae strains. The specificity and sensitivity of these primer pairs were assessed on 236 strains from a large and comprehensive Ps. syringae collection. The method was also validated by characterizing the phylogenetic diversity of 174 putative Ps. syringae isolates from kiwifruit and apricot orchards of southeastern France.

CONCLUSION

Our PCR-based method allows for the detection and characterization of nine of the 13 Ps. syringae phylogroups (phylogroups 1, 2, 3, 4, 7, 8, 9, 10 and 13).

SIGNIFICANCE AND IMPACT OF THE STUDY

To date, phylogenetic affiliation within the Ps. syringae complex was only possible by sequencing housekeeping genes. Here, we propose a rapid PCR-based method for the detection of specific phylogroups of the Ps. syringae complex. Furthermore, for the first time we reveal the presence of Ps. syringae strains belonging to phylogroups 10 and 13 as epiphytes on plants, whereas they had previously only been observed in aquatic habitats.

Real time live imaging of phytopathogenic bacteria Xanthomonas campestris pv. campestris MAFF106712 in 'plant sweet home'

Xanthomonas is one of the most widespread phytobacteria, causing diseases on a variety of agricultural plants. To develop novel control techniques, knowledge of bacterial behavior inside plant cells is essential. Xanthomonas campestris pv. campestris, a vascular pathogen, is the causal agent of black rot on leaves of Brassicaceae, including Arabidopsis thaliana. Among the X. campestris pv. campestris stocks in the MAFF collection, we selected XccMAFF106712 as a model compatible pathogen for the A. thaliana reference ecotype Columbia (Col-0). Using modified green fluorescent protein (AcGFP) as a reporter, we observed real time XccMAFF106712 colonization in planta with confocal microscopy. AcGFP-expressing bacteria colonized the inside of epidermal cells and the apoplast, as well as the xylem vessels of the vasculature. In the case of the type III mutant, bacteria colonization was never detected in the xylem vessel or apoplast, though they freely enter the xylem vessel through the wound. After 9 days post inoculation with XccMAFF106712, the xylem vessel became filled with bacterial aggregates. This suggests that Xcc colonization can be divided into main four steps, (1) movement in the xylem vessel, (2) movement to the next cell, (3) adhesion to the host plant cells, and (4) formation of bacterial aggregates. The type III mutant abolished at least steps (1) and (2). Better understanding of Xcc colonization is essential for development of novel control techniques for black rot.

Xanthomonas campestris pv. campestris (cause of black rot of crucifers) in the genomic era is still a worldwide threat to brassica crops

BACKGROUND

Xanthomonas campestris pv. campestris (Xcc) (Pammel) Dowson is a Gram-negative bacterium that causes black rot, the most important disease of vegetable brassica crops worldwide. Intensive molecular investigation of Xcc is gaining momentum and several whole genome sequences are available.

TAXONOMY

Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadacea; Genus Xanthomonas; Species X. campestris.

HOST RANGE AND SYMPTOMS

Xcc can cause disease in a large number of species of Brassicaceae (ex-Cruciferae), including economically important vegetable Brassica crops and a number of other cruciferous crops, ornamentals and weeds, including the model plant Arabidopsis thaliana. Black rot is a systemic vascular disease. Typical disease symptoms include V-shaped yellow lesions starting from the leaf margins and blackening of the veins.

RACE STRUCTURE, PATHOGENESIS AND EPIDEMIOLOGY

Collections of Xcc isolates have been differentiated into physiological races based on the response of several brassica species lines. Black rot is a seed-borne disease. The disease is favoured by warm, humid conditions and can spread rapidly from rain dispersal and irrigation water.

DISEASE CONTROL

The control of black rot is difficult and relies on the use of pathogen-free planting material and the elimination of other potential inoculum sources (infected crop debris and cruciferous weeds). Major gene resistance is very rare in B. oleracea (brassica C genome). Resistance is more readily available in other species, including potentially useful sources of broad-spectrum resistance in B. rapa and B. carinata (A and BC genomes, respectively) and in the wild relative A. thaliana.

GENOME

The reference genomes of three isolates have been released. The genome consists of a single chromosome of approximately 5 100 000 bp, with a GC content of approximately 65% and an average predicted number of coding DNA sequences (CDS) of 4308.

IMPORTANT GENES IDENTIFIED

Three different secretion systems have been identified and studied in Xcc. The gene clusters xps and xcs encode a type II secretion system and xps genes have been linked to pathogenicity. The role of the type IV secretion system in pathogenicity is still uncertain. The hrp gene cluster encodes a type III secretion system that is associated with pathogenicity. An inventory of candidate effector genes has been assembled based on homology with known effectors. A range of other genes have been associated with virulence and pathogenicity, including the rpf, gum and wxc genes involved in the regulation of the synthesis of extracellular degrading enzymes, xanthan gum and lipopolysaccharides.

USEFUL WEBSITE

http://www.xanthomonas.org/

A luminescent reporter evidences active expression of Ralstonia solanacearum type III secretion system genes throughout plant infection

Although much is known about the signals that trigger transcription of virulence genes in plant pathogens, their prevalence and timing during infection are still unknown. In this work, we address these questions by analysing expression of the main pathogenicity determinants in the bacterial pathogen Ralstonia solanacearum. We set up a quantitative, non-invasive luminescent reporter to monitor in planta transcription from single promoters in the bacterial chromosome. We show that the new reporter provides a real-time measure of promoter output in vivo - either after re-isolation of pathogens from infected plants or directly in situ - and confirm that the promoter controlling exopolysaccharide (EPS) synthesis is active in bacteria growing in the xylem. We also provide evidence that hrpB, the master regulator of type III secretion system (T3SS) genes, is transcribed in symptomatic plants. Quantitative RT-PCR assays demonstrate that hrpB and type III effector transcripts are abundant at late stages of plant infection, suggesting that their function is required throughout disease. Our results challenge the widespread view in R. solanacearum pathogenicity that the T3SS, and thus injection of effector proteins, is only active to manipulate plant defences at the first stages of infection, and that its expression is turned down when bacteria reach high cell densities and EPS synthesis starts.

Pantoea stewartii subsp. stewartii produces an endoglucanase that is required for full virulence in sweet corn

Pantoea stewartii subsp. stewartii, a xylem-dwelling bacterium, is the causal agent of Stewart's wilt and blight of sweet corn. The goal of this study was to characterize the only gene in the P. stewartii subsp. stewartii genome predicted to encode an endoglucanase (EGase); this gene was designated engY. Culture supernatants from P. stewartii subsp. stewartii and Escherichia coli expressing recombinant EngY protein possessed both EGase and xylanase activities. Deletion of engY abolished EGase and xylanase activity, demonstrating that EngY appears to be the major EGase or xylanase produced by P. stewartii subsp. stewartii. Most importantly, our results show that EngY contributes to movement in the xylem and disease severity during the wilting phase of Stewart's wilt but is not required for water-soaked lesion formation.

Colonization of tomato seedlings by bioluminescent Clavibacter michiganensis subsp. michiganensis under different humidity regimes

Tomato bacterial canker, caused by Clavibacter michiganensis subsp. michiganensis, is transmitted by infected or infested seed and mechanically from plant to plant. Wounds occurring during seedling production and crop maintenance facilitate the dissemination of the pathogen. However, the effects of environmental factors on C. michiganensis subsp. michiganensis translocation and growth as an endophyte have not been fully elucidated. A virulent, stable, constitutively bioluminescent C. michiganensis subsp. michiganensis strain BL-Cmm 17 coupled with an in vivo imaging system allowed visualization of the C. michiganensis subsp. michiganensis colonization process in tomato seedlings in real time. The dynamics of bacterial infection in seedlings through wounds were compared under low (45%) and high (83%) relative humidity. Bacteria multiplied rapidly in cotyledon petioles remaining after clip inoculation and moved in the stem toward both root and shoot. Luminescent signals were also observed in tomato seedling roots over time, and root development was reduced in inoculated plants maintained under both humidity regimes. Wilting was more severe in seedlings under high-humidity regimes. A strong positive correlation between light intensity and bacterial population in planta suggests that bioluminescent C. michiganensis subsp. michiganensis strains will be useful in evaluating the efficacy of bactericides and host resistance.

Type III secretion-dependent host defence elicitation and type III secretion-independent growth within leaves by Xanthomonas campestris pv. campestris

In many plant-bacterial interactions, loss of the type III secretion system (T3SS) severely reduces bacterial growth, symptom causation and suppression of defences in host plants. In the present study of Xanthomonas campestris pv. campestris (Xcc), Xcc strain B305 grew better than strain B186 in Arabidopsis thaliana after hydathode inoculation, and B305 strains mutated to the loss of T3SS (ΔhrcC and/or ΔhrpE; also ΔhrcCΔflgBC) grew similarly to wild-type B305 in Arabidopsis leaves. Unlike Xcc strain B186, wild-type B305 was relatively inefficient in secreting the exogenous T3S effector AvrBsT, but ΔhrcC and/or ΔhrpE attenuated the disease symptoms caused by Xcc B305, showing that the partially compromised T3SS of this strain still promotes necrotic leaf symptoms. In contrast with the T3SS-dependent defence suppression that has been observed for some other plant pathogenic bacteria, the Xcc B186 and B305 wild-type strains (which are virulent on Arabidopsis) caused greater elicitation of host PR-1 and PR-5 expression and callose deposition in comparison with their respective T3SS mutants. A defence-suppressing/virulence-enhancing activity of the Xcc T3SS effector suite was detectable when co-inoculation with wild-type Xcc B186 increased the growth of ΔhrcC Xcc, but this activity did not prevent the above defence elicitation. Experiments using T3SS mutants and Arabidopsis fls2 mutants suggested that FLS2 does not play a prominent role in restriction of the examined Xcc strains. However, ectopic overexpression of the Pseudomonas syringae effector AvrPto promoted in planta growth of wild-type and ΔhrcC Xcc. In summary, the T3SS components or effector suite from virulent Xcc strains elicit some host defence responses, but suppress other defences and stimulate more severe disease symptoms, AvrPto-disruptable elements other than FLS2 apparently contribute to the host restriction of Xcc, and in some virulent Xcc strains the T3SS is not absolutely required for wild-type levels of bacterial growth within the plant.

Comparison of the Behavior of Epiphytic Fitness Mutants of Pseudomonas syringae under Controlled and Field Conditions

The epiphytic fitness of four Tn5 mutants of Pseudomonas syringae that exhibited reduced epiphytic fitness in the laboratory was evaluated under field conditions. The mutants differed more from the parental strain under field conditions than under laboratory conditions in their survival immediately following inoculation onto bean leaves and in the size of the epiphytic populations that they established, demonstrating that their fitness was reduced more under field conditions than in the laboratory. Under both conditions, the four mutants exhibited distinctive behaviors. One mutant exhibited particularly large population decreases and short half-lives following inoculation but grew epiphytically at near-wild-type rates, while the others exhibited reduced survival only in the warmest, driest conditions tested and grew epiphytically at reduced rates or, in the case of one mutant, not at all. The presence of the parental strain, B728a, did not influence the survival or growth of three of the mutants under field conditions; however, one mutant, an auxotroph, established larger populations in the presence of B728a than in its absence, possibly because of cross-feeding by B728a in planta. Experiments with B728a demonstrated that established epiphytic populations survived exposure of leaves to dry conditions better than newly inoculated cells did and that epiphytic survival was not dependent on the cell density in the inoculum. Three of the mutants behaved similarly to two nonpathogenic strains of P. syringae, suggesting that the mutants may be altered in traits that are missing or poorly expressed in naturally occurring nonpathogenic epiphytes.

Survival, Growth, and Localization of Epiphytic Fitness Mutants of Pseudomonas syringae on Leaves

Among 82 epiphytic fitness mutants of a Pseudomonas syringae pv. syringae strain that were characterized in a previous study, 4 mutants were particularly intolerant of the stresses associated with dry leaf surfaces. These four mutants each exhibited distinctive behaviors when inoculated onto and into plant leaves. For example, while none showed measurable growth on dry potato leaf surfaces, they grew to different population sizes in the intercellular spaces of bean leaves and on dry bean leaf surfaces, and one mutant appeared incapable of growth in both environments although it grew well on moist bean leaves. The presence of the parental strain did not influence the survival of the mutants immediately following exposure of leaves to dry, high-light incubation conditions, suggesting that the reduced survival of the mutants did not result from an inability to produce extracellular factors in planta. On moist bean leaves that were colonized by either a mutant or the wild type, the proportion of the total epiphytic population that was located in sites protected from a surface sterilant was smaller for the mutants than for the wild type, indicating that the mutants were reduced in their ability to locate, multiply in, and/or survive in such protected sites. This reduced ability was only one of possibly several factors contributing to the reduced epiphytic fitness of each mutant. Their reduced fitness was not specific to the host plant bean, since they also exhibited reduced fitness on the nonhost plant potato; the functions altered in these strains are thus of interest for their contribution to the general fitness of bacterial epiphytes.

Phenotypic Switching Affecting Chemotaxis, Xanthan Production, and Virulence in Xanthomonas campestris

The chemotaxis towards sucrose and yeast extract of nine strains of Xanthomonas campestris representing pathovars campestris, armoraciae, translucens, vesicatoria, and pelargonii was analyzed by using swarm plates. Unexpectedly, each of these strains formed small or reduced swarms typical of nonmotile or nonchemotactic bacteria. With time, however, chemotactic cells appeared on the swarm plates as blebs of bacteria. These cells were strongly chemotactic and were concomitantly deficient in exopolysaccharide production. The switch from the wild type (exopolysaccharide producing and nonchemotactic) to the swarmer type (exopolysaccharide deficient and chemotactic) appeared irreversible ex planta in bacteriological medium. However, in radish leaves swarmer-type strains of X. campestris pv. campestris were able to revert to the wild type. Swarmer-type derivatives of two X. campestris pv. campestris wild-type isolates showed reduced virulence and growth in the host plants cauliflower and radish. However, exocellular complementation of X. campestris pv. campestris Hrp (nonpathogenic) mutant was achieved by coinoculation with a swarmer-type strain.

Bioluminescence imaging of Clavibacter michiganensis subsp. michiganensis infection of tomato seeds and plants

Clavibacter michiganensis subsp. michiganensis is a Gram-positive bacterium that causes wilting and cankers, leading to severe economic losses in commercial tomato production worldwide. The disease is transmitted from infected seeds to seedlings and mechanically from plant to plant during seedling production, grafting, pruning, and harvesting. Because of the lack of tools for genetic manipulation, very little is known regarding the mechanisms of seed and seedling infection and movement of C. michiganensis subsp. michiganensis in grafted plants, two focal points for application of bacterial canker control measures in tomato. To facilitate studies on the C. michiganensis subsp. michiganensis movement in tomato seed and grafted plants, we isolated a bioluminescent C. michiganensis subsp. michiganensis strain using the modified Tn1409 containing a promoterless lux reporter. A total of 19 bioluminescent C. michiganensis subsp. michiganensis mutants were obtained. All mutants tested induced a hypersensitive response in Mirabilis jalapa and caused wilting of tomato plants. Real-time colonization studies of germinating seeds using a virulent, stable, constitutively bioluminescent strain, BL-Cmm17, showed that C. michiganensis subsp. michiganensis aggregated on hypocotyls and cotyledons at an early stage of germination. In grafted seedlings in which either the rootstock or scion was exposed to BL-Cmm17 via a contaminated grafting knife, bacteria were translocated in both directions from the graft union at higher inoculum doses. These results emphasize the use of bioluminescent C. michiganensis subsp. michiganensis to help better elucidate the C. michiganensis subsp. michiganensis-tomato plant interactions. Further, we demonstrated the broader applicability of this tool by successful transformation of C. michiganensis subsp. nebraskensis with Tn1409::lux. Thus, our approach would be highly useful to understand the pathogenesis of diseases caused by other subspecies of the agriculturally important C. michiganensis.

Two Xanthomonas extracellular polygalacturonases, PghAxc and PghBxc, are regulated by type III secretion regulators HrpX and HrpG and are required for virulence

Xanthomonas campestris pv. campestris, the causal agent of black rot disease, produces a suite of extracellular cell-wall degrading enzymes (CWDE) that are involved in bacterial virulence. Polygalacturonase (PG) is an important CWDE and functions to degrade the pectic layers of plant cell walls. Although previous studies have documented the virulence functions of PG in Erwinia and Ralstonia species, the regulation of PG genes still needs to be elucidated. In this study, we identified two novel PG genes (pghAxc and pghBxc) encoding functional PG from X. campestris pv. campestris 8004. The expressions of these two PG genes are regulated by the type III secretion regulators HrpX and HrpG and the global regulator Clp. These PG genes could be efficiently induced in planta and were required for the full virulence of X. campestris pv. campestris to Arabidopsis. In addition, these PG were confirmed to be secreted via the type II secretion system in an Xps-dependent manner.

The Xanthomonas oryzae pv. oryzae eglXoB endoglucanase gene is required for virulence to rice

Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of bacterial leaf blight, a serious disease of rice worldwide. A Tn5-based transposon randomly insertional mutant library was previously constructed. By screening mutants against susceptible rice cultivar IR24, four mutants were identified with reduced virulence on rice plants and were found to have Tn5 transposon inserted at an endo-1,4-beta-D glucanase (E.C. 3.2.1.4) gene eglXoB. In planta growth analysis indicated that multiplication of the mutants in rice leaves was greatly reduced comparing to the Xoo wild-type strain. Reverse transcriptase polymerase chain reaction (RT-PCR) showed that the expression of eglXoB was induced in planta. Genetic complementation of these mutants with a functional eglXoB gene restored both virulence and in planta growth, suggesting that the eglXoB gene was required for virulence. Ectopic expression of eglXoB in Escherichia coli demonstrated its endoglucanase activity. Otherwise, the growth of the mutants in synthetic medium containing cellulose as the sole sugar source was not affected. Data of this study suggested eglXoB gene is required for pathogenesis of rice bacterial blight disease.

Mutagenesis of all eight avr genes in Xanthomonas campestris pv. campestris had no detected effect on pathogenicity, but one avr gene affected race specificity

Suppression subtractive hybridization (SSH) was used to identify genes present in the systemic crucifer black rot pathogen Xanthomonas campestris pv. campestris 528T but missing from the nonsystemic crucifer leaf spot pathogen, X. campestris pv. armoraciae 417. Among the DNA fragments unique to 528T was Xcc2109, one of eight putative avr genes identified in the published 528T genome (NC_003902). Individual and sequential deletion, insertion mutations, or both of all eight 528T avr gene loci were made, but no change in pathogenicity was observed with any combination of avr mutations, including a strain with all eight avr genes deleted. However, insertion or deletion mutants affecting the Xcc2109 locus lost avirulence (i.e., became virulent) on Florida Mustard, an X. campestris pv. campestris race-determining, differential host. The Xcc2109 open reading frame as annotated was cloned and found to be nonfunctional. A longer gene, encompassing Xcc2109 and here designated avrXccFM, was cloned and found to complement the Xcc2109 mutants and to confer avirulence to two additional wild-type X. campestris pv. campestris strains, thereby changing their races. Resistance in Florida Mustard to 528T strains carrying avrXccFM occurred without a typical hypersensitive response (HR) on leaves, although a vascular HR was observed in seedlings.

The genome sequence of Xanthomonas oryzae pathovar oryzae KACC10331, the bacterial blight pathogen of rice

The nucleotide sequence was determined for the genome of Xanthomonas oryzae pathovar oryzae (Xoo) KACC10331, a bacterium that causes bacterial blight in rice (Oryza sativa L.). The genome is comprised of a single, 4 941 439 bp, circular chromosome that is G + C rich (63.7%). The genome includes 4637 open reading frames (ORFs) of which 3340 (72.0%) could be assigned putative function. Orthologs for 80% of the predicted Xoo genes were found in the previously reported X.axonopodis pv. citri (Xac) and X.campestris pv. campestris (Xcc) genomes, but 245 genes apparently specific to Xoo were identified. Xoo genes likely to be associated with pathogenesis include eight with similarity to Xanthomonas avirulence (avr) genes, a set of hypersensitive reaction and pathogenicity (hrp) genes, genes for exopolysaccharide production, and genes encoding extracellular plant cell wall-degrading enzymes. The presence of these genes provides insights into the interactions of this pathogen with its gramineous host.

The molecular genetics of virulence of Xanthomonas campestris

Bacteria belonging to the genus Xanthomonas are important pathogens of many plants, and their virulence appears to be due primarily to secreted and surface compounds that could increase host nutrient loss, or avoid or suppress unfavorable conditions in the host. Type II and III secretory pathways are essential for virulence. Some individual extracellular enzymes (type II-secretion dependent) affect final bacterial population levels, whereas some avirulence gene products (type III-secretion dependent) affect virulence by altering host metabolism. Avr proteins, probably secreted via a pilus, can also be recognized by host resistance gene products. Virulence is also associated with bacterial surface polysaccharides, which may help to avoid host defense responses, and regulatory gene systems, which can control virulence gene expression.

Colonization of Vitis vinifera by a green fluorescence protein-labeled, gfp-marked strain of Xylophilus ampelinus, the causal agent of bacterial necrosis of grapevine

The dynamics of Xylophilus ampelinus were studied in Vitis vinifera cv. Ugni blanc using gfp-marked bacterial strains to evaluate the relative importance of epiphytic and endophytic phases of plant colonization in disease development. Currently, bacterial necrosis of grapevine is of economic importance in vineyards in three regions in France: the Cognac, Armagnac, and Die areas. This disease is responsible for progressive destruction of vine shoots, leading to their death. We constructed gfp-marked strains of the CFBP2098 strain of X. ampelinus for histological studies. We studied the colonization of young plants of V. vinifera cv. Ugni blanc by X. ampelinus after three types of artificial contamination in a growth chamber and in a greenhouse. (i) After wounding of the stem and inoculation, the bacteria progressed down to the crown through the xylem vessels, where they organized into biofilms. (ii) When the bacteria were forced into woody cuttings, they rarely colonized the emerging plantlets. Xylem vessels could play a key role in the multiplication and conservation of the bacteria, rather than being a route for plant colonization. (iii) When bacterial suspensions were sprayed onto the plants, bacteria progressed in two directions: both in emerging organs and down to the crown, thus displaying the importance of epiphytic colonization in disease development.

Identification and Origin of Xanthomonas campestris pv. campestris Races and Related Pathovars

ABSTRACT One hundred sixty-four isolates of Xanthomonas campestris pv. campestris and other X. campestris pathovars known to infect cruciferous hosts (X. campestris pvs. aberrans, raphani, armoraciae, and incanae) were inoculated onto a differential series of Brassica spp. to determine both pathogenicity to brassicas and race. Of these, 144 isolates were identified as X. campestris pv. campestris and grouped into six races, with races 1 (62%) and 4 (32%) being predominant. Other races were rare. The remaining 20 isolates from brassicas and other cruciferous hosts were either nonpathogenic or very weakly pathogenic on the differential series and could not be race-typed. Five of these isolates, from the ornamental crucifers wallflower (Cheiranthus cheiri), stock (Matthiola incana) and candytuft (Iberis sp.), showed clear evidence of pathovar-like specificity to the hosts of origin. A gene-for-gene model based on the interaction of four avirulence genes in X. campestris pv. campestris races and four matching resistance genes in the differential hosts is proposed. Knowledge of the race structure and worldwide distribution of races is fundamental to the search for sources of resistance and for the establishment of successful resistance breeding programs.

Cloning and molecular characterization of hrpX from Xanthomonas axonopodis pv. citri

The hrpX gene of plant pathogenic Xanthomonas species is essential for pathogenicity on host plants and to cause hypersensitive reaction on non-host plants. We cloned and analyzed a hrpX homologue, designated hrpXct, of X. axonopodis pv. citri, a pathogen of citrus canker. The open reading frame of hrpXct has 1431 bp in nucleotides which has a coding capacity of 476 amino acid residues with a molecular mass of 52.4 kDa. The predicted amino acid sequence of HrpXct has 90% identity to the AraC family type transcriptional activator protein HrpXc of X. campestris pv. campestris, 95% to HrpXo of X. oryzae pv. oryzae and 97% to X. vesicatoria. These findings clearly indicate and confirm that the structure of the hrpX genes in plant pathogenic Xanthomonas species is highly conserved.

Role of the Hrp type III protein secretion system in growth of Pseudomonas syringae pv. syringae B728a on host plants in the field

hrp genes are reportedly required for pathogenicity in Pseudomonas syringae pv. syringae (Pss) and other phytopathogenic bacterial species. A subset of these genes encodes a type III secretion system through which virulence factors are thought to be delivered to plant cells. In this study, we sought to better understand the role that hrp genes play in interactions of Pss with its host as they occur naturally under field conditions. Population sizes of hrp mutants with defects in genes that encode components of the Hrp secretion system (DeltahrcC::nptII and hrpJ:: OmegaSpc) and a protein secreted via the system (DeltahrpZ::nptII) were similar to B728a on germinating seeds. However, phyllosphere (i.e., leaf) population sizes of the hrcC and hrpJ secretion mutants, but not the hrpZ mutant, were significantly reduced relative to B728a. Thus, the Hrp type III secretion system, but not HrpZ, plays an important role in enabling Pss to flourish in the phyllosphere, but not the spermosphere. The hrcC and hrpJ mutants caused brown spot lesions on primary leaves at a low frequency when they were inoculated onto seeds at the time of planting. Pathogenic reactions also were found when the hrp secretion mutants were co-infiltrated into bean leaves with a non-lesion-forming gacS mutant of B728a. In both cases, the occurrence of disease was associated with elevated population sizes of the hrp secretion mutants. The role of the Hrp type III secretion system in pathogenicity appears to be largely mediated by its requirement for growth of Pss in the phyllosphere. Without growth, disease does not occur.

Bacterial colonization of leaves: a spectrum of strategies

ABSTRACT Bacteria associated with plant leaves, or phyllobacteria, probably employ a range of colonization strategies. Steps in these colonization strategies include modification of the leaf habitat, aggregation, ingression, and egression. Considerable evidence indicates that bacteria can modify their environment to enhance their colonization of plants, such as by increasing local nutrient concentrations or by producing a layer of extracellular polysaccharides. This local habitat modification may occur on the surface of leaves, as well as in the leaf interior, and may be enhanced by the formation of bacterial aggregates. The conspicuous presence of bacterial aggregates on leaves and the finding that the behavior of bacteria on plants varies in a density-dependent manner indicate the potential importance of cooperative interactions among phyllobacteria. Such cooperative interactions may occur among both homogeneous and heterogeneous populations, thus influencing the development of microbial communities. While the sites commonly colonized by most phyllobacteria have not been unambiguously identified, there is strong circumstantial evidence that a sizable proportion of cells, particularly of phytopathogenic strains, are localized within "protected sites" on plants. The likelihood that these protected sites are located in the interior of leaves indicates that phytopathogenic bacteria have access to more resources and greater protection from stresses associated with the leaf surface than bacteria that are restricted to the leaf surface. The internal and external leaf-associated populations probably form a continuum due to the processes of ingression and egression. For a specific pathogen, however, the extent of egression that occurs prior to disease induction is likely to influence the success of disease predictions based on external population size, i.e., the number of bacteria in leaf washings. In this review, we illustrate the complexity of the ecology of leaf-associated bacteria and propose a model of leaf colonization that emphasizes the common elements in bacterial colonization strategies, as well as allows for distinct behavior of different phyllobacterial species.

Unusual structure of the tonB-exb DNA region of Xanthomonas campestris pv. campestris: tonB, exbB, and exbD1 are essential for ferric iron uptake, but exbD2 is not

The nucleotide sequence of a 3.6-kb HindIII-SmaI DNA fragment of Xanthomonas campestris pv. campestris revealed four open reading frames which, based on sequence homologies, were designated tonB, exbB, exbD1, and exbD2. Analysis of translational fusions to alkaline phosphatase and beta-galactosidase confirmed that the TonB, ExbB, ExbD1, and ExbD2 proteins are anchored in the cytoplasmic membrane. The TonB protein of X. campestris pv. campestris lacks the conserved (Glu-Pro)n and (Lys-Pro)m repeats but harbors a 13-fold repeat of proline residues. By mutational analysis, the tonB, exbB, and exbD1 genes were shown to be essential for ferric iron import in X. campestris pv. campestris. In contrast, the exbD2 gene is not involved in the uptake of ferric iron.

The role of hrp genes during plant-bacterial interactions

hrp genes control the ability of phytopathogenic bacteria to cause disease and to elicit hypersensitive reactions on resistant plants. Genetic and biochemical studies have demonstrated that Hrp proteins are components of Type III secretion systems, regulatory proteins, proteinaceous elicitors of the hypersensitive reaction, and enzymes needed for synthesis of periplasmic glucans. Significantly, Type III secretion systems are involved with the secretion of pathogenicity proteins in bacterial pathogens of animals. The transcriptional activation of a number of bacterial avirulence (avr) genes is controlled by Hrp regulatory proteins, and recent experimental evidence suggests that Avr proteins may be transported by Hrp secretion systems. It has also been hypothesized that pathogenicity and/or virulence gene products exit bacterial phytopathogens via Hrp pathways. Thus, hrp genes may be one of the most important groups of genes found in phytopathogenic bacteria in relationship to pathogenicity and host range.

Heterologous growth phase- and temperature-dependent expression and H2O2 toxicity protection of a superoxide-inducible monofunctional catalase gene from Xanthomonas oryzae pv. oryzae

Catalase is an important protective enzyme against H2O2 toxicity. Here, we report the characterization of a Xanthomonas oryzae pv. oryzae catalase gene (katX). The gene was localized and its nucleotide sequence was determined. The gene codes for a 77-kDa polypeptide. The deduced katX amino acid sequence shares regions of high identity with other monofunctional catalases in a range of organisms from bacteria to eukaryotes. The transcriptional regulation of katX was atypical of bacterial monofunctional kat genes. Northern (RNA) analysis showed that katX transcription was highly induced by treatments with low concentrations of menadione, a superoxide generator, and methyl methanesulfonate, a mutagen. It was only weakly induced by H2O2. Unlike in other bacteria, a high level of catalase in Xanthomonas spp. provided protection from the growth-inhibitory and killing effects of H2O2 but not from those of organic peroxides and superoxide generators. Unexpectedly, heterologous expression of katX in Escherichia coli was both growth phase and temperature dependent. Catalase activity in E. coli kat mutants harboring katX on an expression vector was detectable only when the cells entered the stationary phase of growth and at 28 degrees C. The patterns of transcription regulation, heterologous expression, and physiological function of katX are different from previously studied bacterial kat genes.

HrpXv, an AraC-type regulator, activates expression of five of the six loci in the hrp cluster of Xanthomonas campestris pv. vesicatoria

hrp genes, basic pathogenicity genes of the pepper and tomato pathogen Xanthomonas campestris pv. vesicatoria, are regulated dependent on environmental conditions. We isolated the hrpXv gene, which was found to be outside the large hrp cluster comprising the six loci hrpA to hrpF. The predicted HrpXv protein is 476 amino acids long and has a molecular mass of 52.5 kDa. HrpX is highly conserved among xanthomonads and is a member of the AraC family of regulatory proteins. An hrpXv insertion mutant has a typical hrp phenotype and no longer allows induction of the five hrp loci hrpB to hrpF in the new hrp induction medium XVM2, indicating that HrpXv is the positive regulator of these loci. An hrpXv mutant could be partially complemented by the related hrpB gene of Burkholderia solanacearum, the protein product of which shows 40 and 58% amino acid identity and similarity, respectively, to HrpXv. The hrpXv gene itself has a low basal level of expression that is enhanced in XVM2. Expression of hrpXv as well as that of the hrpA locus is independent of the hrpXv gene. The transcription start site of hrpXv was mapped. Comparison between the hrpXv promoter and the corresponding region of the hrpXc gene from X. campestris pv. campestris revealed sequence conservation up to position -84. A putative helix-turn-helix motif in the C-terminal region of HrpXv and its possible interaction with a conserved hrp promoter element, the plant-inducible promoter box, are discussed.

Bacterial avirulence genes

Although more than 30 bacterial avirulence genes have been cloned and characterized, the function of the gene products in the elictitation of resistance is unknown in all cases but one. The product of avrD from Pseudomonas syringae pv. glycinea likely functions indirectly to elicit resistance in soybean, that is, evidence suggests the gene product is an enzyme involved in elicitor production. In most if not all cases, bacterial avirulence gene function is dependent on interactions with the hypersensitive response and pathogenicity (hrp) genes. Many hrp genes are similar to genes involved in delivery of pathogenicity factors in mammalian bacterial pathogens. Thus, analogies between mammalian and plant pathogens may provide needed clues to elucidate how virulence gene products control induction of resistance.

Conservation of the hypersensitivity-pathogenicity regulatory gene hrpX of Xanthomonas campestris and X. oryzae

The hrpX gene is essential for pathogenicity of Xanthomonas species. Loss of hrpX by mutation results in the loss of pathogenicity and a gain in the ability of Xanthomonas to cause the hypersensitive response in their respective host plants, suggesting that hrpX confers a means to evade this host defense response. The function of HrpX protein was predicted by sequencing of hrpXc and hrpXo from X. campestris pv. campestris and X. oryzae, respectively. The predicted amino acid sequences of the protein encoded by these respective genes revealed similarities (45.96%) to the HrpB protein of Burkholderia solanacearum, which has sequence identity to the transcriptional activator VirF of Yersinia enterocolitica and AraC of Escherichia coli. Thus, HrpX may regulate Xanthomonas virulence genes since a putative DNA binding domain present in the carboxyl terminal half of HrpX is highly conserved among HrpB, VirF and AraC and since over-expression of the carboxyl terminal half of HrpX in E. coli is lethal.

Specific genomic fingerprints of phytopathogenic Xanthomonas and Pseudomonas pathovars and strains generated with repetitive sequences and PCR

DNA primers corresponding to conserved motifs in bacterial repetitive (REP, ERIC, and BOX) elements and PCR were used to show that REP-, ERIC-, and BOX-like DNA sequences are widely distributed in phytopathogenic Xanthomonas and Pseudomonas strains. REP-, ERIC, and BOX-PCR (collectively known as rep-PCR) were used to generate genomic fingerprints of a variety of Xanthomonas and Pseudomonas isolates and to identify pathovars and strains that were previously not distinguishable by other classification methods. Analogous rep-PCR-derived genomic fingerprints were generated from purified genomic DNA, colonies on agar plates, liquid cultures, and directly from lesions on infected plants. REP, ERIC, and BOX-PCR-generated fingerprints of specific Xanthomonas and Pseudomonas strains were found to yield similar conclusions wtih regard to the identity of and relationship between these strains. This suggests that the distribution of REP-, ERIC, and BOX-like sequences in these strains is a reflection of their genomic structure. Thus, the rep-PCR technique appears to be a rapid, simple, and reproducible method to identify and classify Xanthomonas and Pseudomonas strains, and it may be a useful diagnostic tool for these important plant pathogens.