Effect of phenotypic plasticity on epiphytic survival and colonization by Pseudomonas syringae

Dining in Blue Light Impairs the Appetite of Some Leaf Epiphytes

Background: The phyllosphere is subjected to fluctuating abiotic conditions. This study examined the phenotypic plasticity (PP) of four selected non-phototrophic phyllosphere bacteria [control strain: Pseudomonas sp. DR 5-09; Pseudomonas agarici, Bacillus thuringiensis serovar israeliensis (Bti), and Streptomyces griseoviridis (SG)] regarding their respiration patterns and surfactant activity as affected by light spectrum and nutrient supply.

Methods: The PP of the strains was examined under four light regimes [darkness (control); monochromatic light-emitting diodes (LED) at 460 nm (blue) and 660 nm (red); continuously polychromatic white LEDs], in the presence of 379 substrates and conditions.

Results: Light treatment affected the studied bacterial strains regarding substrate utilization (Pseudomonas strains > SG > Bti). Blue LEDs provoked the most pronounced impact on the phenotypic reaction norms of the Pseudomonas strains and Bti. The two Gram-positive strains Bti and SG, respectively, revealed inconsistent biosurfactant formation in all cases. Biosurfactant formation by both Pseudomonas strains was supported by most substrates incubated in darkness, and blue LED exposure altered the surface activity profoundly. Blue and white LEDs enhanced biofilm formation in PA in highly utilized C-sources. Putative blue light receptor proteins were found in both Pseudomonas strains, showing 91% similarity with the sequence from NCBI accession number WP_064119393.

Conclusion: Light quality–nutrient interactions affect biosurfactant activity and biofilm formation of some non-phototrophic phyllosphere bacteria and are, thus, crucial for dynamics of the phyllosphere microbiome.

Seed biopriming with plant growth promoting rhizobacteria: a review

Beneficial microbes are applied to the soil and plant tissues directly or through seed inoculation, whereas soil application is preferred when there is risk of inhibitors or antagonistic microbes on the plant tissues. Insufficient survival of the microorganisms, hindrance in application of fungicides to the seeds and exposure to heat and sunlight in subsequent seed storage in conventional inoculation methods force to explore appropriate and efficient bacterial application method. Seed priming, where seeds are hydrated to activate metabolism without actual germination followed by drying, increases the germination, stand establishment and stress tolerance in different crops. Seed priming with living bacterial inoculum is termed as biopriming that involves the application of plant growth promoting rhizobacteria. It increases speed and uniformity of germination; also ensures rapid, uniform and high establishment of crops; and hence improves harvest quality and yield. Seed biopriming allows the bacteria to enter/adhere the seeds and also acclimatization of bacteria in the prevalent conditions. This review focuses on methods used for biopriming, and also the role in improving crop productivity and stress tolerance along with prospects of this technology. The comparison of methods being followed is also reviewed proposing biopriming as a promising technique for application of beneficial microbes to the seeds.

Distribution of Agrobacterium vitis in Grapevines and Its Relevance to Pathogen Elimination

Agrobacterium vitis, the cause of crown gall disease on grapevine, survives internally in vines and can be spread in cuttings for propagation. The possibility of generating pathogen-free vines through tissue culture makes it essential to understand the distribution of the pathogen in grapevines. A highly sensitive magnetic capture hybridization procedure along with real-time polymerase chain reaction were used to measure the distribution of tumorigenic A. vitis in dormant canes and green shoots of grapevines. Tumorigenic A. vitis was distributed from the basal to apical nodal and internodal tissues of canes as well as in nonlignified green shoots. In experiments conducted in 2013, A. vitis was detected in up to 17% of shoot tips and 52% of meristems of greenhouse-grown plants initiated from known A. vitis-contaminated cuttings. A lower frequency of detection was observed from surface-disinfected shoot tips (7%) as compared with nondisinfected tips (37%), suggesting epiphytic survival on green tissues. In 2014, vines propagated from cuttings collected from crown gall-infected vines from a different vineyard yielded lower incidences of A. vitis from shoot tips, and the bacterium was not detected in meristems. Tumorigenic A. vitis was also detected in cuttings of wild grapevines (Vitis riparia) that were collected both adjacent to and far removed from commercial vineyards.

Environmental Interactions and Epistasis Are Revealed in the Proteomic Responses to Complex Stimuli

Ultimately, the genotype of a cell and its interaction with the environment determine the cell’s biochemical state. While the cell’s response to a single stimulus has been studied extensively, a conceptual framework to model the effect of multiple environmental stimuli applied concurrently is not as well developed. In this study, we developed the concepts of environmental interactions and epistasis to explain the responses of the S. cerevisiae proteome to simultaneous environmental stimuli. We hypothesize that, as an abstraction, environmental stimuli can be treated as analogous to genetic elements. This would allow modeling of the effects of multiple stimuli using the concepts and tools developed for studying gene interactions. Mirroring gene interactions, our results show that environmental interactions play a critical role in determining the state of the proteome. We show that individual and complex environmental stimuli behave similarly to genetic elements in regulating the cellular responses to stimuli, including the phenomena of dominance and suppression. Interestingly, we observed that the effect of a stimulus on a protein is dominant over other stimuli if the response to the stimulus involves the protein. Using publicly available transcriptomic data, we find that environmental interactions and epistasis regulate transcriptomic responses as well.

Resident bacteria on leaves enhance survival of immigrant cells of Salmonella enterica

Although Salmonella enterica apparently has comparatively low epiphytic fitness on plants, external factors that would influence its ability to survive on plants after contamination would be of significance in the epidemiology of human diseases caused by this human pathogen. Viable population sizes of S. enterica applied to plants preinoculated with Pseudomonas syringae or either of two Erwinia herbicola strains was ≥10-fold higher than that on control plants that were not precolonized by such indigenous bacteria when assessed 24 to 72 h after the imposition of desiccation stress. The protective effect of P. fluorescens, which exhibited antibiosis toward S. enterica in vitro, was only ≈50% that conferred by other bacterial strains. Although S. enterica could produce small cellular aggregates after incubation on wet leaves for several days, and the cells in such aggregates were less susceptible to death upon acute dehydration than solitary cells (as determined by propidium iodide staining), most Salmonella cells were found as isolated cells when it was applied to leaves previously colonized by other bacterial species. The proportion of solitary cells of S. enterica coincident with aggregates of cells of preexisting epiphytic species that subsequently were judged as nonviable by viability staining on dry leaves was as much as 10-fold less than those that had landed on uncolonized portions of the leaf. Thus, survival of immigrant cells of S. enterica on plants appears to be strongly context dependent, and the presence of common epiphytic bacteria on plants can protect such immigrants from at least one key stress (i.e., desiccation) encountered on leaf surfaces.

Assessment of root uptake and systemic vine-transport of Salmonella enterica sv. Typhimurium by melon (Cucumis melo) during field production

Among melons, cantaloupes are most frequently implicated in outbreaks and surveillance-based recalls due to Salmonella enterica. There is limited but compelling evidence that associates irrigation water quality as a significant risk of preharvest contamination of melons. However, the potential for root uptake from water and soil and subsequent systemic transport of Salmonella into melon fruit is uncharacterized. The aim of this work was to determine whether root uptake of S. enterica results in systemic transport to fruit at high doses of applied inoculum through sub-surface drip and furrow irrigation during field production of melons. Cantaloupe and honeydew were grown under field conditions, in a silt clay loam soil using standard agronomic practices for California. An attenuated S. enterica sv. Typhimurium strain was applied during furrow irrigation and, in separate plots, buried drip-emitter lines delivered the inoculum directly into the established root zone. Contamination of the water resulted in soil contamination within furrows however Salmonella was not detected on top of the beds or around melon roots of furrow-irrigated rows demonstrating absence of detectable lateral transfer across the soil profile. In contrast, positive detection of the applied isolate occurred in soil and the rhizosphere in drip injected plots; survival of Salmonella was at least 41 days. Despite high populations of the applied bacteria in the rhizosphere, after surface disinfection, internalized Salmonella was not detected in mature melon fruit (n=485). Contamination of the applied Salmonella was detected on the rind surface of melons if fruit developed in contact with soil on the sides of the inoculated furrows. Following an unusual and heavy rain event during fruit maturation, melons collected from the central area of the beds, were shown to harbor the furrow-applied Salmonella. Delivery of Salmonella directly into the peduncle, after minor puncture wounding, resulted in detection of applied Salmonella in the sub-rind tissue below the fruit abscission zone. Results indicate that Salmonella internalization from soil and vascular systemic transport to fruit is unlikely to occur from irrigation water in CA production regions, even if substantially above normal presumptive levels of contamination. Although contaminated irrigation water and subsequently soil in contact with fruit remains a concern for contamination of the external rind, results suggest an acceptable microbial indicator threshold and critical limit for the presence of Salmonella in applied water may be possible by defining appropriate microbiological standards for melon irrigation in California and regions with similar climate, soil texture, and crop management practices.

Influence of immigration on epiphytic bacterial populations on navel orange leaves

Factors that influenced the increase in epiphytic bacterial population size on navel orange leaves during winter months were investigated to test the assumption that such populations were the result of multiplication on orange leaves. The population sizes of bacteria of different kinds, including ice nucleation-active (Ice(sup+)) bacteria, were from 6- to 30-fold larger on leaves of navel orange trees adjacent to other plant species than on trees growing near other citrus species. Total and Ice(sup+) bacterial population sizes on other plant species growing near navel orange trees were from 18- to 60-fold and 2- to 18,000-fold larger, respectively, than on navel orange trees. About twice the number of bacterial cells of a given type were deposited onto petri dishes opened simultaneously in navel orange orchards with other plant species nearby as in orchards surrounded by citrus trees. Epiphytic bacteria and airborne bacteria were more numerous near the upwind edge of orchards bordering on other plant species, but not in orchards adjacent to other citrus trees, and decreased with distance from other plant species. Navel orange leaves also exhibited progressive increases in the ability to supercool as a function of increasing distance from the upwind edge of orchards adjacent to other plant species but not in orchards adjacent to other citrus trees. While the population size of three different bacterial strains remained nearly constant for 60 days after inoculation, total bacterial populations increased more than 50-fold during this period. These results suggest that immigration of bacteria from plants having high epiphytic bacterial populations could account for most, if not all, of the seasonal increase in bacterial populations on navel orange leaves and have important implications for procedures to modify bacterial communities on leaves.

Raindrop Momentum Triggers Growth of Leaf-Associated Populations of Pseudomonas syringae on Field-Grown Snap Bean Plants

Observational and microclimate modification experiments were conducted under field conditions to determine the role of the physical environment in effecting large increases in phyllosphere population sizes of Pseudomonas syringae pv. syringae, the causal agent of bacterial brown spot disease of snap bean (Phaseolus vulgaris L.). Comparisons of daily changes in population sizes of P. syringae on three plantings of snap bean cultivar Cascade and one of cultivar Eagle with weather conditions indicated a strong association of rainfalls with periods of 1 to 3 days in duration during which increases in bacterial population sizes were greater than 10-fold and up to 1,000-fold. The effects of rain on populations of P. syringae were explored further by modifying the microclimate of bean plants in the field with polyethylene shelters to shield plants from rain and fine-mesh inert screens to modify the momentum of raindrops. After each of three separate intense rains, the greater-than-10-fold increases in population sizes of P. syringae observed on plants exposed to the rains did not occur on plants in the shelters or under the screens. The screens decreased the velocity and, thus, the momentum of raindrops but not the volume or quality of rainwater that fell on plants under the screens. Thus, the absence of increases in population sizes of P. syringae on plants under the screens suggests that raindrop momentum plays a role in the growth-triggering effect of intense rains on populations of P. syringae on bean plants under field conditions.

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.

Inoculum Density-Dependent Mortality and Colonization of the Phyllosphere by Pseudomonas syringae

Pseudomonas syringae inocula containing cell concentrations ranging from 10 to 10 cells per ml were applied to the primary leaves of bean plants. The plants were incubated under conditions of high temperature and illumination and low relative humidity. Bacterial mortality rates and the proportional population decline of the inoculum were lowest at the highest inoculum concentrations. Addition of a high concentration of heat-killed cells to the inoculum containing a low concentration of viable cells significantly reduced both the mortality rate and the proportional population decline of the viable cells. The mechanisms underlying this density-dependent mortality may include cooperative protective effects of extracellular factors, such as bacterial extracellular polysaccharides, and physical protection by neighboring cells. Although epiphytic populations derived from inoculum concentrations of 10 or 10 cells per ml tended toward 10 CFU/g, the presumed carrying capacity of the leaf, populations derived from lower inoculum concentrations never achieved this carrying capacity. Assuming that epiphytic populations of P. syringae reside in discrete protected sites, our results suggest that at low inoculum concentrations, following a period of environmental stress, the number of viable cells may have dropped to zero in some sites; hence, the carrying capacity of the leaf could not be achieved.

Stress tolerance and environmental fitness of Pseudomonas fluorescens A506, which has a mutation in RpoS

Establishment of suppressive populations of bacterial biological control agents on aerial plant surfaces is a critical phase in biologically based management of floral diseases. Periodically, biocontrol agents encounter inhospitable conditions for growth on plants; consequently, tolerance of environmental stresses may contribute to their fitness. In many gram-negative bacteria, including strains of Pseudomonas spp., the capacity to survive environmental stresses is influenced by the stationary phase sigma factor RpoS. This study focused on the role of RpoS in stress response and epiphytic fitness of Pseudomonas fluorescens A506, a well-studied bacterial biological control agent. We detected a frameshift mutation in the rpoS of A506 and demonstrated that the mutation resulted in a truncated, nonfunctional RpoS. Using site-directed mutagenesis, we deleted a nucleotide from rpoS, which then encoded a full-length, functional RpoS. We compared the stress response and epiphytic fitness of A506 with derivative strains having the functional full-length RpoS or a disrupted, nonfunctional RpoS. RpoS had little effect on stress response of A506 and no consistent influence on epiphytic population size of A506 on pear or apple leaves or flowers. Although the capacity of strain A506 to withstand exposure to environmental stresses was similar to that of other fluorescent pseudomonads, this capacity was largely independent of rpoS.

Tobacco nectaries express a novel NADPH oxidase implicated in the defense of floral reproductive tissues against microorganisms

Hydrogen peroxide produced from the nectar redox cycle was shown to be a major factor contributing to inhibition of most microbial growth in floral nectar; however, this obstacle can be overcome by the floral pathogen Erwinia amylovora. To identify the source of superoxide that leads to hydrogen peroxide accumulation in nectary tissues, nectaries were stained with nitroblue tetrazolium. Superoxide production was localized near nectary pores and inhibited by diphenylene iodonium but not by cyanide or azide, suggesting that NAD(P)H oxidase is the source of superoxide. Native PAGE assays demonstrated that NADPH (not NADH) was capable of driving the production of superoxide, diphenyleneiodonium chloride was an efficient inhibitor of this activity, but cyanide and azide did not inhibit. These results confirm that the production of superoxide was due to an NADPH oxidase. The nectary enzyme complex was distinct by migration on gels from the leaf enzyme complex. Temporal expression patterns demonstrated that the superoxide production (NADPH oxidase activity) was coordinated with nectar secretion, the expression of Nectarin I (a superoxide dismutase in nectar), and the expression of NOX1, a putative gene for a nectary NADPH oxidase that was cloned from nectaries and identified as an rbohD-like NADPH oxidase. Further, in situ hybridization studies indicated that the NADPH oxidase was expressed in the early stages of flower development although superoxide was generated at later stages (after Stage 10), implicating posttranslational regulation of the NADPH oxidase in the nectary.

Pseudomonas syringae genes induced during colonization of leaf surfaces

The foliar pathogen and ice nucleator, Pseudomonas syringae pv. syringae B728a, demonstrates a high level of epiphytic fitness on plants. Using a promoter-trapping strategy termed habitat-inducible rescue of survival (HIRS), we identified genes of this organism that are induced during colonization of healthy bean leaf surfaces. These plant-inducible genes (pigs) encode diverse cellular functions including virulence, transcription regulation, transport, nutrient acquisition and other known and unknown loci, some of which may result in antisense transcripts to annotated P. syringae genes. Prominent among the pigs was ssuE, a gene in the sulfate-starvation regulon, indicating that sulfate is not abundant on leaf surfaces. inaZ reporter gene fusion assays of the plant-inducible loci revealed up to 300-fold higher levels of pig transcriptional activity on plant leaves compared with minimal medium. However, the maximum levels of pig transcriptional activity were typically too weak to be measured using a gfp reporter gene. One exception was orf6 in the hrp/hrc pathogenicity island which was highly induced in epiphytic P. syringae cells. Four pigs were disrupted by insertional mutagenesis. While growth of the ssuE mutant was impaired under certain conditions in laboratory medium, the epiphytic and virulence properties of the mutants on bean plants were identical to wild-type P. syringae. Our results demonstrate the utility of HIRS to identify genes expressed on leaves and provide new insight into the leaf surface environment.

Spatial organization of dual-species bacterial aggregates on leaf surfaces

The spatial organization of cells within bacterial aggregates on leaf surfaces was determined for pair-wise mixtures of three different bacterial species commonly found on leaves, Pseudomonas syringae, Pantoea agglomerans, and Pseudomonas fluorescens. Cells were coinoculated onto bean plants and allowed to grow under moist conditions, and the resulting aggregates were examined in situ by epifluorescence microscopy. Each bacterial strain could be localized because it expressed either the green or the cyan fluorescent protein constitutively, and the viability of individual cells was assessed by propidium iodide staining. Each pair of bacterial strains that was coinoculated onto leaves formed mixed aggregates. The degree of segregation of cells in mixed aggregates differed between the different coinoculated pairs of strains and was higher in mixtures of P. fluorescens A506 and P. agglomerans 299R and mixtures of P. syringae B728a and P. agglomerans 299R than in mixtures of two isogenic strains of P. agglomerans 299R. The fractions of the total cell population that were dead in mixed and monospecific aggregates of a gfp-marked strain of P. agglomerans 299R and a cfp-marked strain of P. agglomerans 299R, or of P. fluorescens A506 and P. agglomerans 299R, were similar. However, the proportion of dead cells in mixed aggregates of P. syringae B728a and P. agglomerans 299R was significantly higher (13.2% +/- 8.2%) than that in monospecific aggregates of these two strains (1.6% +/- 0.7%), and it increased over time. While dead cells in such mixed aggregates were preferentially found at the interface between clusters of cells of these strains, cells of these two strains located at the interface did not exhibit equal probabilities of mortality. After 9 days of incubation, about 77% of the P. agglomerans 299R cells located at the interface were dead, while only about 24% of the P. syringae B728a cells were dead. The relevance of our results to understanding bacterial interactions on leaf surfaces and the implications for biological control of pathogenic and other deleterious microorganisms is discussed.

Influence of growth environment on tolerance to UV-B radiation, germination speed, and morphology of Metarhizium anisopliae var. acridum conidia

We previously reported wide variability in UV-B tolerance among different Metarhizium anisopliae isolates [Braga, G.U.L., Flint, S.D., Miller, C.D., Anderson, A.J., Roberts, D.W., 2001a. Variability in response to UV-B among species and strains of Metarhizium isolated from sites at latitudes from 61 degrees N to 54 degrees S. J. Invertebr. Pathol. 78, 98-108] as well as wide phenotypic variability in some of these isolates in response to alterations in their growth environments [Rangel, D.E.N., Braga, G.U.L., Flint, S.D., Anderson, A.J., Roberts, D.W., 2004. Variations in UV-B tolerance and germination speed of M. anisopliae conidia produced on artificial and natural substrates. J. Invertebr. Pathol. 87, 77-83]. Studies on other biological systems have demonstrated that strong selective pressure for tolerance to a stress factor may reduce the phenotypic variability of this trait. In the present study, conidia of the isolate most tolerant to radiation and heat, ARSEF 324, presented very little phenotypic variability in UV-B tolerance in response to production on either artificial culture medium or infected insects. The phenotypic plasticities in two other traits (conidial morphology and germination speed), however, were considerably higher.

Aggregates of resident bacteria facilitate survival of immigrant bacteria on leaf surfaces

The fate of immigrant bacterial cells on leaves under stressful conditions was determined as a function of the anatomical features and the local spatial density of resident cells at their landing site. Pantoea agglomerans 299R was established on bean leaves and the survival of immigrant cells of Pseudomonas fluorescens A506 and Pseudomonas syringae B728a, as well as P. agglomerans itself, was determined by epifluorescence microscopy following subsequent exposure of plants to desiccation stress. Resident and immigrant bacterial strains constitutively expressed the cyan and the green fluorescent protein, respectively, and the viability of individual cells was assessed directly on leaf surfaces following propidium iodide staining. Although only a small fraction of the immigrant cells landed on established bacterial aggregates, their fate was usually strongly influenced by the presence of indigenous bacteria at the site at which they landed. Immigrants of P. agglomerans 299R or P. fluorescens A506 that arrived as solitary cells had about double the probability of survival when landing on aggregates formed by P. agglomerans 299R than when landing on uncolonized areas of the leaf surface. In contrast, the survival of P. syringae B728a was similar irrespective of whether it landed on colonized or uncolonized parts of a leaf. The nature of plant anatomical features at which immigrant bacteria landed also strongly influenced the fate of immigrant bacteria. The fraction of immigrant cells of each species tested that landed on veins, glandular trichomes, or epidermal cells altered by P. agglomerans that died was always less than when they landed on normal epidermal cells or at the base of hooked trichomes. Depending on the process by which immigrants arrive at a leaf, only a small fraction of cells may be deposited on existing bacterial aggregates. Although uncolonized sites differed greatly in their ability to influence the survival of immigrant cells, the fate of an immigrant bacterium will depend on the nature of the leaf structure on which it is deposited, and apparently indirectly on the amount of nutrients and water available at that site to support the development of bacterial aggregates.

Pseudomonas syringae Responds to the Environment on Leaves by Cell Size Reduction

ABSTRACT The length and volume of cells of the plant-pathogenic bacterium Pseudomonas syringae strain B728a were measured in vitro and with time after inoculation on bean leaf surfaces to assess both the effect of nutrient availability on the cell size of P. syringae and, by inference, the variability in nutrient availability in the leaf surface habitat. Cells of P. syringae harboring a green fluorescent protein marker gene were visualized by epifluorescence microscopy after recovery from leaves or culture and their size was estimated by analysis of captured digital images. The average cell length of bacteria grown on leaves was significantly smaller than that of cultured cells, and approached that of cells starved in phosphate buffer for 24 h. The average length of cells originally grown on King's medium B decreased from approximately 2.5 to approximately 1.2 mum by 7 days after inoculation on plants. Some decrease in cell size occurred during growth of cells on leaves and continued for up to 13 days after cell multiplication ceased. Although cultured cells exhibited a normal size distribution, the size of cells recovered from bean plants at various times after inoculation was strongly right-hand skewed and was described by a log-normal distribution. The skewness of the size distribution tended to increase with time after inoculation. The reduced cell size of P. syringae B728a on plants was readily reversible when recovered cells were grown in culture. Direct in situ measurements of cell sizes on leaves confirmed that most cells of P. syringae respond to the leaf environment by reducing their size. The spatial heterogeneity of cell sizes observed on leaves suggest that nutrient availability is quite variable on the leaf surface environment.

Construction and characterization of a proU-gfp transcriptional fusion that measures water availability in a microbial habitat

We constructed and characterized a transcriptional fusion that measures the availability of water to a bacterial cell. This fusion between the proU promoter from Escherichia coli and the reporter gene gfp was introduced into strains of E. coli, Pantoea agglomerans, and Pseudomonas syringae. The proU-gfp fusion in these bacterial biosensor strains responded in a quantitative manner to water deprivation caused by the presence of NaCl, Na(2)SO(4), KCl, or polyethylene glycol (molecular weight, 8000). The fusion was induced to a detectable level by NaCl concentrations of as low as 10 mM in all three bacterial species. Water deprivation induced proU-gfp expression in both planktonic and surface-associated cells; however, it induced a higher level of expression in the surface-associated cells. Following the introduction of P. agglomerans biosensor cells onto bean leaves, the cells detected a significant decrease in water availability within only 5 min. After 30 min, the populations were exposed, on average, to a water potential equivalent to that imposed by approximately 55 mM NaCl. These results demonstrate the effectiveness of a proU-gfp-based biosensor for evaluating water availability on leaves. Furthermore, the inducibility of proU-gfp in multiple bacterial species illustrates the potential for tailoring proU-gfp-based biosensors to specific habitats.

Location and survival of leaf-associated bacteria in relation to pathogenicity and potential for growth within the leaf

The growth and survival of pathogenic and nonpathogenic Pseudomonas syringae strains and of the nonpathogenic species Pantoea agglomerans, Stenotrophomonas maltophilia, and Methylobacterium organophilum were compared in the phyllosphere of bean. In general, the plant pathogens survived better than the nonpathogens on leaves under environmental stress. The sizes of the total leaf-associated populations of the pathogenic P. syringae strains were greater than the sizes of the total leaf-associated populations of the nonpathogens under dry conditions but not under moist conditions. In these studies the surface sterilants hydrogen peroxide and UV irradiation were used to differentiate cells that were fully exposed on the surface from nonexposed cells that were in "protected sites" that were inaccessible to these agents. In general, the population sizes in protected sites increased with time after inoculation of plants. The proportion of bacteria on leaves that were in protected sites was generally greater for pathogens than for nonpathogens and was greater under dry conditions than under moist conditions. When organisms were vacuum infiltrated into leaves, the sizes of the nonexposed "internal" populations were greater for pathogenic P. syringae strains than for nonpathogenic P. syringae strains. The sizes of the populations of the nonpathogenic species failed to increase or even decreased. The sizes of nonexposed populations following spray inoculation were correlated with the sizes of nonexposed, internal populations which developed after vacuum infiltration and incubation. While the sizes of the populations of the pathogenic P. syringae strains increased on leaves under dry conditions, the sizes of the populations of the nonpathogenic strains of P. syringae, P. agglomerans, and S. maltophilia decreased when the organisms were applied to plants. The sizes of the populations on dry leaves were also correlated with the sizes of the nonexposed populations that developed following vacuum infiltration. Although pathogenicity was not required for growth in the phyllosphere under high-relative-humidity conditions, pathogenicity apparently was involved in the ability to access and/or multiply in certain protected sites in the phyllosphere and in growth on dry leaves.

Characterization of type IV pilus genes in Pseudomonas syringae pv. tomato DC3000

Many strains of Pseudomonas syringae produce retractile pili that act as receptors for lytic bacteriophage phi 6. As these are also characteristics of type IV pili, it was postulated that P. syringae may possess genes for type IV pilus biogenesis. A cosmid clone bank of P. syringae pv. tomato DC3000 genomic DNA was used to complement a mutant of Pseudomonas aeruginosa defective in the PilD (XcpA) prepilin peptidase gene by selection for restoration of extracellular protein secretion, a function also known to require PilD. A cosmid able to complement this mutant was also able to complement mutations in the pilB and pilC genes, suggesting that, if the organization of these genes is similar to that of P. aeruginosa, the cosmid may contain the P. syringae pilA. This was confirmed by sequencing a region from this plasmid that was shown to hybridize at low stringency to the P. aeruginosa pilA gene. The deduced P. syringae PilA polypeptide possesses the characteristic properties of the type IV pilins. Heterologous expression of the P. syringae pilA in P. aeruginosa was also shown, conferring not only phi 6 phage sensitivity to P. aeruginosa pilA mutants but also sensitivity to PO4, a lytic bacteriophage specific for the pilus of P. aeruginosa. This suggests that additional components might be present in the mature pilus of P. aeruginosa that are the true receptors for this phage. Chromosomal mutations in P. syringae pv. tomato DC3000 pilA and pilD genes were shown to abolish its sensitivity to bacteriophage phi 6. To determine the importance of P. syringae pilus in plant leaf interactions, these mutations were tested under laboratory and field conditions. Although little effect was seen on pathogenicity, culturable leaf-associated population sizes of the pilA mutant were significantly different from those of the wild-type parent. In addition, the expression of the DC3000 pilA gene appears to contribute to the UV tolerance of P. syringae and may play a role in survival on the plant leaf surface.

Establishment of Bacterial Antagonists of Erwinia amylovora on Pear and Apple Blossoms as Influenced by Inoculum Preparation

The influence of inoculum preparation on the establishment of bacterial antagonists that suppress fire blight and Erwinia amylovora on blossoms was evaluated. Aqueous suspensions of Pseudomonas fluorescens A506, E. herbicola C9-1R, or E. amylovora 153N were prepared from cells harvested from the surface of an agar medium or from cells that were lyophilized after culture under similar conditions. Bacterial suspensions (1 x 10(8) CFU/ml) were sprayed on pear and apple trees at 50% bloom near midday. The incidence of recovery (proportion of blossoms containing detectable populations) and the population sizes of the bacteria on individual blossoms with detectable populations were followed over a period of several days. Fluorescent microspheres (1 mum in diameter) were added to sprays at a concentration of 1 x 10(7) microspheres per ml to mark blossoms that were open during application of bacteria. After dilution-plating, the stigmas and styles of each blossom were examined for the presence of microspheres with an epifluorescence microscope. In three of five trials, bacteria applied as suspensions of lyophilized cells were recovered from a greater proportion of blossoms than bacterial cells harvested directly from culture media. Every blossom harvested within 6 days after spraying had microspheres present on the surfaces of the styles and stigmas; thus, lack of establishment of detectable populations, rather than escape of blossoms from spray inoculation, accounted for the differences in proportion of blossoms colonized by the different preparations of bacteria. The use of lyophilized cells in field trials decreased variability in the establishment of bacteria on blossoms.

The acquisition of indigenous plasmids by a genetically marked pseudomonad population colonizing the sugar beet phytosphere is related to local environmental conditions

The transfer of naturally occurring conjugative plasmids from the indigenous microflora to a genetically modified population of bacteria colonizing the phytospheres of plants has been observed. The marked strain (Pseudomonas fluorescens SBW25EeZY6KX) was introduced as a seed dressing to sugar beets (Beta vulgaris var. Amethyst) as part of a field experiment to assess the ecology and genetic stability of deliberately released bacterial inocula. The sustained populations of the introduced strain, which colonized the phytosphere, were assessed throughout the growing season for the acquisition of plasmids conferring mercury resistance (Hg(supr)). Transconjugants were isolated only from root and leaf samples collected within a narrow temporal window coincident with the midseason maturation of the crop. Conjugal-transfer events were recorded during this defined period in two separate field release experiments conducted over consecutive years. On one occasion seven of nine individual plants sampled supported transconjugant P. fluorescens SBW25EeZY6KX, demonstrating that conjugative gene transfer between bacterial populations in the phytosphere may be a common event under specific environmental conditions. The plasmids acquired in situ by the colonizing inocula were identified as natural variants of restriction digest pattern group I, III, or IV plasmids from five genetically distinct groups of large, conjugative mercury resistance plasmids known to persist in the phytospheres of sugar beets at the field site. These data demonstrate not only that gene transfer may be a common event but also that the genetic and phenotypic stability of inocula released into the natural environment cannot be predicted.

Characterization of the fimA gene encoding bundle-forming fimbriae of the plant pathogen Xanthomonas campestris pv. vesicatoria

The fimA gene of Xanthomonas campestris pv. vesicatoria was identified and characterized. A 20-mer degenerate oligonucleotide complementary to the N-terminal amino acid sequence of the purified 15.5-kDa fimbrillin was used to locate fimA on a 2.6-kb SalI fragment of the X. campestris pv. vesicatoria 3240 genome. The nucleotide sequence of a 1.4-kb fragment containing the fimA region revealed two open reading frames predicting highly homologous proteins FimA and FimB. FimA, which was composed of 136 amino acids and had a calculated molecular weight of 14,302, showed high sequence identity to the type IV fimbrillin precursors. fimB predicted a protein product of 135 amino acids and a molecular weight of 13,854. The open reading frame for fimB contained near the 5' end a palindromic sequence with a terminator loop potential, and the expression level of fimB in vitro and in Xanthomonas was considerably lower than that of fimA. We detected an efficiently transcribed fimA-specific mRNA of 600 bases as well as two weakly expressed, longer mRNA species that reacted with both fimA and fimB. A homolog of fimA but not of fimB was detected by Southern hybridization in strains of X. campestris pv. vesicatoria, campestris, begoniae, translucens, and graminis. A fimA::omega mutant of strain 3240 was not significantly reduced in virulence or adhesiveness to tomato leaves. However, the fimA mutant was dramatically reduced in cell aggregation in laboratory cultures and on infected tomato leaves. The fimA mutant strain also exhibited decreased tolerance to UV light.

Microbial population dynamics on leaves

Microbial population dynamics on leaves in time and space are a function of immigration, emigration, growth, and death. Insight into the relative significance of each population process to the generation of specific dynamics for individual microorganisms is necessary to understanding the ecology and life history strategy of the microorganism and to developing effective control strategies. Additionally, information on the significance of within-leaf versus extra-leaf processes to the generation of phyllosphere dynamics is important to determining the range of spatial scales over which a population should be studied. Unfortunately, such information is difficult to obtain due to the lack of effective methodologies for distinguishing these processes within phyllosphere populations. Future research efforts should focus on the quantification of immigration, emigration, growth, and death relative to the population dynamics of phyllosphere microorganisms.

Utility of microcosm studies for predicting phylloplane bacterium population sizes in the field

Population sizes of two ice nucleation-active strains of Pseudomonas syringae were compared on leaves in controlled environments and in the field to determine the ability of microcosm studies to predict plant habitat preferences in the field. The P. syringae strains investigated were the parental strains of recombinant deletion mutant strains deficient in ice nucleation activity that had been field tested for their ability to control plant frost injury. The population size of the P. syringae strains was measured after inoculation at three field locations on up to 40 of the same plant species that were studied in the growth chamber. There was seldom a significant relationship between the mean population size of a given P. syringae strain incubated under either wet or dry conditions in microcosms and the mean population size which could be recovered from the same species when inoculated in the field. Specifically, on some plant species, the population size recovered from leaves in the field was substantially greater than from that species in a controlled environment, while for other plant species field populations were significantly smaller than those observed under controlled conditions. Population sizes of inoculated P. syringae strains, however, were frequently highly positively correlated with the indigenous bacterial population size on the same plant species in the field, suggesting that the ability of a particular plant species to support introduced bacterial strains is correlated with its ability to support large bacterial populations or that indigenous bacteria enhance the survival of introduced strains. Microcosm studies therefore seem most effective at assessing possible differences between parental and recombinant strains under a given environmental regime but are limited in their ability to predict the specific population sizes or plant habitat preferences of bacteria on leaves under field conditions.

Plasmid stability and ecological competence in recombinant cultures

The instability of cell cultures containing plasmid vectors is a major problem in the commercial exploitation of molecular cloning techniques. Plasmid stability is influenced by the nature of the host cell, the type of plasmid and/or environmental conditions. Plasmid encoded properties may confer a selective advantage on the host cell but can be an energy drain due to replication and expression. Stability of recombinant cultures ultimately may be determined by the cost to benefit ratio of plasmid carriage. The relative competition between plasmid containing and plasmid-free or indigenous populations can determine the degree of dominance of recombinant cultures. The use of inocula in biotechnological processes in which dynamic environmental conditions dominate may also result in instabilities resulting from the characteristics of the ecosystem. In such dynamic conditions plasmid stability is just one contribution to culture stability. Strategies to enhance plasmid stability, within such environments, based on manipulation of physiological state of host cells, must consider the responsiveness or plasticity of both cells and populations. The robustness of cells or the responses to stresses or transient environmental conditions can influence the levels of instability detected; for example, instability or mutation in the host genome may lead to enhanced plasmid stability. Competition among subpopulations arising from unstable copy number control may determine the levels of recombinant cells in open versus closed fermenter systems. Thus the ecological competence (ability to survive and compete) of recombinant cells in dynamic or transient environments is fundamental to the understanding of the ultimate dominance or survival of such recombinant cultures and may form the basis of a strategy to enhance or control stability either in fermenter systems or dynamic process environments. The creation of microniches in time and/or space can enhance plasmid stability. Transient operation based on defined environmental stresses or perturbations in fermenter systems or in heterogeneous or dynamic environments found in gel immobilized cultures have resulted in enhanced stability. Spatial organization resulting from immobilization has the additional advantage of regulated cell protection within defined microenvironments and controlled release, depending on the nature of the gel, from these microenvironments or microcosms. This regulation of ecological competence allied to the advantages of microbial cell growth in gel microenvironments combined with the spatial organization (or juxtapositioning of cells, selective agents, nutrients, protectants, etc.) possible through immobilization technology offers new strategies to enhance plasmid and culture stability.