Relationship between Symptom Development and Actual Sites of Infection in Leaves of Anthurium Inoculated with a Bioluminescent Strain of Xanthomonas campestris pv. dieffenbachiae

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.

From macro to micro: a combined bioluminescence-fluorescence approach to monitor bacterial localization

Bacterial bioluminescence is widely used to study the spatiotemporal dynamics of bacterial populations and gene expression in vivo at a population level but cannot easily be used to study bacterial activity at the level of individual cells. In this study, we describe the development of a new library of mini‐Tn7‐lux and lux::eyfp reporter constructs that provide a wide range of lux expression levels, and which combine the advantages of both bacterial bioluminescence and fluorescent proteins to bridge the gap between macro‐ and micro‐scale imaging techniques. We demonstrate that a dual bioluminescence‐fluorescence approach using the lux operon and eYFP can be used to monitor bacterial movement in plants both macro‐ and microscopically and demonstrate that Pseudomonas syringae pv phaseolicola can colonize the leaf vascular system and systemically infect leaves of common bean (Phaseolus vulgaris). We also show that bacterial bioluminescence can be used to study the impact of plant immune responses on bacterial multiplication, viability and spread within plant tissues. The constructs and approach described in this study can be used to study the spatiotemporal dynamics of bacterial colonization and to link population dynamics and cellular interactions in a wide range of biological contexts.

Mangroves in the Leaves: Anatomy, Physiology, and Immunity of Epithemal Hydathodes

Hydathodes are organs found on aerial parts of a wide range of plant species that provide almost direct access for several pathogenic microbes to the plant vascular system. Hydathodes are better known as the site of guttation, which is the release of droplets of plant apoplastic fluid to the outer leaf surface. Because these organs are only described through sporadic allusions in the literature, this review aims to provide a comprehensive view of hydathode development, physiology, and immunity by compiling a historic and contemporary bibliography. In particular, we refine the definition of hydathodes.We illustrate their important roles in the maintenance of plant osmotic balance, nutrient retrieval, and exclusion of deleterious chemicals from the xylem sap. Finally, we present our current understanding of the infection of hydathodes by adapted vascular pathogens and the associated plant immune responses.

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.

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.

Genetic basis of resistance to systemic infection by Xanthomonas axonopodis pv. dieffenbachiae in Anthurium

The genetic basis of systemic resistance to bacterial blight disease (blight) of anthurium (Anthurium andraeanum) caused by Xanthomonas axonopodis pv. dieffenbachiae was investigated in progenies of 53 crosses involving 31 parent cultivars using segregation analysis. Inoculation of parents and progenies was achieved by injecting the petiole base of the most recent fully expanded leaf with 100 microl of 10(9) colony forming units per ml of the blight pathogen (strain X4gfp) transformed with the green fluorescent protein (GFP) gene. The time to death and the presence or absence of GFP fluorescence on newly emerging leaves was monitored over a period of 30 weeks after inoculation (WAI), on an individual plant basis. The expected resistance to susceptible ratios based on a digenic model involving two dominant genes, designated A and B, interacting according to a duplicate recessive epistasis model fitted the observed segregation ratios in the crosses. Based on the segregation ratios obtained, the parental cultivars were assigned plausible genotypes. There were significant differences (P < 0.001) in time to death following inoculation between the various genotypic designations. Cultivars with genotypes AABB, AABb, AaBB, and AaBb died within 10 WAI and designated as susceptible; AAbb and aaBB died from 18.8 to 25.6 WAI and were designated as moderately resistant; and Aabb, aaBb, and aabb produced resistant phenotypes. There was also some evidence for dosage effect especially in the highly resistant category. Hence, (AABb = AaBB = AaBb) < (aaBB = AAbb) < Aabb = aaBb = aabb). An approach to fixing resistance to blight in anthurium is discussed.

Bleeding sap and old wood are the two main sources of contamination of merging organs of vine plants by Xylophilus ampelinus, the causal agent of bacterial necrosis

The spatial distribution of vine plants contaminated by Xylophilus ampelinus, the agent responsible for bacterial necrosis, was studied over a 5-year period within two vineyards in the Cognac area. Both vineyards were planted with Vitis vinifera cv. Ugni blanc but were different in age and agronomic location. The emission of X. ampelinus in contaminated bleeding sap was observed during vine sprouting. Contaminated bleeding sap is an important source of inoculum for external contamination due to the high susceptibility of young merging shoots to the pathogen. X. ampelinus emission by bleeding sap was not affected by the age of the plants or the location of the vineyards. However, its emission was irregular with time, and it varied between two fruit canes from individual plants and between plants as well as between years. Moreover, the two vineyards appeared to be entirely contaminated. Consequently, the behavior of the pathogen is not predictable. The distribution of the pathogen inside vine plant organs was analyzed through the four growing seasons. The old wood was contaminated throughout the year and constituted a stock inoculum for endophytic contamination of crude sap during the winter and the spring. Despite the fact that most of the young green shoots were contaminated in May, X.ampelinus was not found in green shoots in June and September, refuting the hypothesis of an epiphytic life of the pathogen under natural conditions. Although all plants were entirely contaminated in both vineyards, symptoms were rare and were observed on different plants each year.

Specific detection of Xanthomonas axonopodis pv. dieffenbachiae in anthurium (Anthurium andreanum) tissues by nested PCR

Efficient control of Xanthomonas axonopodis pv. dieffenbachiae, the causal agent of anthurium bacterial blight, requires a sensitive and reliable diagnostic tool. A nested PCR test was developed from a sequence-characterized amplified region marker identified by randomly amplified polymorphic DNA PCR for the detection of X. axonopodis pv. dieffenbachiae. Serological and pathogenicity tests were performed concurrently with the nested PCR test with a large collection of X. axonopodis pv. dieffenbachiae strains that were isolated worldwide and are pathogenic to anthurium and/or other aroids. The internal primer pair directed amplification of the expected product (785 bp) for all 70 X. axonopodis pv. dieffenbachiae strains pathogenic to anthurium tested and for isolates originating from syngonium and not pathogenic to anthurium. This finding is consistent with previous studies which indicated that there is a high level of relatedness between strains from anthurium and strains from syngonium. Strains originating from the two host genera can be distinguished by restriction analysis of the amplification product. No amplification product was obtained with 98 strains of unrelated phytopathogenic bacteria or saprophytic bacteria from the anthurium phyllosphere, except for a weak signal obtained for one X. axonopodis pv. allii strain. Nevertheless, restriction enzyme analysis permitted the two pathovars to be distinguished. The detection threshold obtained with pure cultures or plant extracts (10(3) CFU ml(-1)) allowed detection of the pathogen from symptomless contaminated plants. This test could be a useful diagnostic tool for screening propagation stock plant material and for monitoring international movement of X. axonopodis pv. dieffenbachiae.

Effect of temperature on the incubation period and leaf colonization in bacterial blight of anthurium

ABSTRACT Effect of temperature on leaf colonization in anthurium blight was studied using a bioluminescent strain of Xanthomonas campestris pv. dieffenbachiae. In a susceptible cultivar, colonization of leaf tissues (monitored by detection of bioluminescence) and symptom development (assessed visually) advanced rapidly at higher temperatures. For a susceptible cultivar, there was a linear relationship between degree-days and percent leaf area colonized by the pathogen, indicating that leaf colonization in a susceptible cultivar was a direct function of the cumulative effect of temperature. The degree-day intercept of the regression line represented the time from inoculation to detection of bioluminescence, and the slope indicated the increase of leaf colonization per degree-day. There also was a linear relationship between the logarithm of degree-days and the logarithm of percent leaf area showing visible symptoms in a susceptible cultivar. The degree-day intercept of this relationship represented the incubation period (about 500 degree-days). The degree-days required to detect bioluminescence was not considerably different between susceptible and resistant cultivars. However, the subsequent rates of leaf colonization were significantly lower for a resistant cultivar than for a susceptible cultivar in all temperature regimes. The results suggest that multiplication of the pathogen in the leaf tissues is optimized in the susceptible cultivar. In contrast, in the resistant cultivar, the defense mechanisms overshadow the temperature effect. The differential response to temperatures may be an additional indicator of cultivar susceptibility.

Comparisons of single versus multiple bacterial species on biological control of anthurium blight

ABSTRACT Effects of single versus multiple biological control agents (BCAs) on suppression of bacterial blight of anthurium were studied using a bioluminescent strain (V108LRUH1) of Xanthomonas campestris pv. dieffenbachiae. When five BCAs (GUT3, GUT4, GUT5, GUT6, and GUT9) were coinoculated in various combinations with V108LRUH1 into filter-sterilized guttation fluids of anthurium plants, a mixture of all five strains or four strains without GUT9 was most inhibitory to V108LRUH1. None of the individual BCAs inhibited V108LRUH1 in the guttation fluid. When BCAs were sprayed at congruent with10(8) CFU/ml on the foliage of a susceptible cultivar 1 day prior to inoculation with V108LRUH1, GUT6 alone and any mixtures containing GUT6 were highly effective in suppressing wound invasion and subsequent leaf infection by V108LRUH1. When tested on several cultivars that differed in susceptibility to the disease, the mixture of five strains or four strains without GUT9 consistently suppressed leaf infection regardless of the cultivars. In some cultivars, BCAs completely suppressed both wound and hydathode invasion by V108LRUH1, resulting in no infection in many leaves. These results indicate that application of bacterial mixtures provides anthurium cultivars with bacterial communities suppressive to X. campestris pv. dieffenbachiae. The results also suggest that selecting an effective mixture of BCAs first and then removing ineffective strains may be a better general approach to finding the most effective BCAs than finding individual strains and combining them.

Suppression of bacterial blight by a bacterial community isolated from the guttation fluids of anthuriums

Growth and survival of Xanthomonas campestris pv. dieffenbachiae in guttation fluids (xylem sap exuded from leaf margins) of anthuriums were suppressed by several bacterial strains indigenous to leaves of various anthurium cultivars. Inhibition of growth was not observed in filter-sterilized guttation fluids and was restored to original levels only by reintroducing specific mixtures of bacteria into filter-sterilized guttation fluids. The inhibitory effect was related to the species in the bacterial community rather than to the total numbers of bacteria in the guttation fluids. One very effective bacterial community consisted of five species isolated from inhibitory guttation fluids of two susceptible anthurium cultivars. The individual strains in this community had no effect on the pathogen, but the mixture was inhibitory to X. campestris pv. dieffenbachiae in guttation fluids. The populations of the individual strains remained near the initial inoculum levels for at least 14 days. The effect of the five inhibitory strains on reducing disease in susceptible anthurium plants was tested by using a bioluminescent strain of X. campestris pv. dieffenbachiae to monitor the progression of disease in leaves nondestructively. Invasion of the pathogen through hydathodes at leaf margins was reduced by applying the strain mixture to the leaves. When the strain mixture was applied directly to wounds created on the leaf margins, the pathogen failed to invade through the wounds. This bacterial community has potential for biological control of anthurium blight.

Differential Susceptibility of Anthurium Cultivars to Bacterial Blight in Foliar and Systemic Infection Phases

Susceptibility of anthurium cultivars to systemic infection by the bacterial blight pathogen, Xanthomonas campestris pv. dieffenbachiae, was examined using a bioengineered bioluminescent strain (V108LRUH1) and compared with susceptibility to foliar infection. Eight cultivars with different levels of susceptibility to foliar infection were evaluated for their susceptibility to systemic infection. Petioles of second youngest leaves cut near the main stem were inoculated with strain V108LRUH1, and subsequent movement of this bacterium into other petioles was monitored by observing bioluminescence from the plants. The actual extent of systemic movement was determined by reisolating V108LRUH1 from dissected segments of the remaining petioles. In susceptible cultivars, the pathogen advanced very rapidly and nearly reached the distal end of petioles. In resistant cultivars, the pathogen was detected in none (or very few) of the petiole segments. However, the susceptibility ranking among the tested cultivars for systemic infection did not always correspond to the ranking determined for foliar infection: i.e., one cultivar that was susceptible to foliar infection was highly resistant to systemic infection, and vice versa. This suggests that cultivar susceptibility of anthuriums to bacterial blight may differ depending on the phase of disease progression, and thus evaluation for both disease phases is essential for complete understanding of cultivar susceptibility.