INTRODUCTION

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Plants cover a significant proportion of the global land area. Each plant produces many leaves. Each leaf, in turn, may be inhabited by a qualitatively and quantitatively diverse assemblage of microorganisms, including fungi, yeasts, bacteria, and bacteriophages (cf. 158, 229, 249). Colonization of leaves by microbes is not limited to terrestrial plants but includes aquatic plants as well (90). When you view a tree (or any plant), think of it as a support system for 1,000 leaves. Then think of each of these leaves as a habitat for 1 to 10 million bacteria. Then multiply this by the almost infinite number of individual trees and other plants present on our planet, and the diversity of habitats that these leaves provide leads to an appreciation of the enormity and importance of leaf-microbe ecosystems.

In 1961, J. Ruinen published a paper entitled "The Phyllospherean Ecologically Neglected Milieu" (249). Up to that time, interest in leaves and their microbial inhabitants was largely centered on diseased leaves and the microbes that caused the various foliar diseases. Phytopathogenic microbes and the economic impact of diseases on crop production continue to provide a major motivation for research on the phyllosphere as an ecosystem. Interest in leaves as habitats for microorganisms, not merely the causal agents of disease, has grown since the midcentury. Indeed, Ruinen's research focused on the possible colonization of leaves of plants in the tropics (Indonesia and Surinam) by Beijerinckia and other nitrogen-fixing bacteria. Since then, a relatively small but devoted band of scientists has worked diligently to explore the microbiology of leaves. At 5-year intervals, beginning in 1970, international symposia have been organized to provide a forum for discussions on phyllosphere microbiology (1970, Newcastle-upon Tyne, England; 1975, Leeds, England; 1980, Aberdeen, Scotland; 1985, Wageningen, The Netherlands, 1990, Madison, Wisconsin; 1995, Bandol, France; 2000, Berkeley, California). The proceedings of these conferences summarize the progress made in understanding the nature of leaf habitats, the microbes that colonize leaves, and interactions among microbes and between microbes and plants (8, 26, 60, 78, 209, 229). In this review, we focus on the bacterial component of leaf ecosystems.

Although the phyllosphere may no longer be a completely neglected milieu, it continues to be a source of wonderful biological questions and undoubtedly will continue to hold many interesting surprises for future investigators. Not only are these communities of bacteria interesting in and of themselves, they are connected directly through their habitats to the reduction of most of the carbon upon which nearly all other organisms depend. Some of these bacteria are known pathogens, destroying or diminishing the photosynthetic output of their leaf habitats. Others are thought to be beneficial. The nature of the interactions, if any, of the majority of leaf-inhabiting bacteria with the leaves that they inhabit remains unknown, an indication of how little is really known about these bacteria.

Before one can integrate the parts of a puzzle into a whole, one must first find the parts and decide which puzzle they belong to. Our interest is the interactions of populations of bacteria with populations of leaves under "real world" field conditions. Factors that affect these interactions include plant and bacterial genetics, weather and other aspects of the physical environment, and plant phenology. Estimates of sizes of such populations are influenced by sampling and bacteriological and statistical methods. The synthesis summarized at the end of the review includes pieces drawn from these several areas. Although the synthesis is an attempt to create a unified picture of the entire puzzle, it still lacks much more than it contains.

We begin this review with a brief general description of the leaf as a habitat for bacteria and what is known about the types of bacteria that colonize leaves. In the remainder of the review we focus on Pseudomonas syringae, a species that participates in phyllosphere bacterial communities as a pathogen, ice nucleus, and epiphyte. Among the diversity of microbes that colonize leaves, none has received wider attention than P. syringae, as it gained notoriety for being the first recombinant organism (Ice P. syringae) to be deliberately introduced into the environment. Our intent is to use this bacterium to illustrate what has been and can be learned about plant-bacterium interactions by integrating knowledge from population studies in the field and molecular biology studies conducted in the laboratory.

SETTING THE STAGE

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General Characteristics of Leaf Habitats

The first piece of the puzzle is the habitat itself. If we allow a rather gross anatomical simplification, leaves can be viewed as consisting of three general componentsthe epidermis, mesophyll, and veins or vascular bundles (Fig. 1A). The bacteria that are central to this review are found (or thought to be foundmore about that later) on the surface of the epidermis and in the intercellular spaces (apoplast) of the mesophyll but not internal to plant cells. Bacteria pathogenic to leaves are thought to occupy the apoplast more frequently than commensal organisms, while both occupy the surface of the epidermis. Bacteria found within the vascular bundles are said to be endophytic and, regardless of their importance, are ignored in this review. Most of the research and interest in leaves as habitats for bacteria have focused on the nature of the leaf surface and its dwellers. This is reflected in terms such as phylloplane to refer specifically to the surface of leaves and epiphytes or epiphytic bacteria to set apart bacteria that are found on the leaf surface from those that are in the apoplast (Fig. 2). The term phyllosphere has been used to describe the environment of leaf surfaces. We interpret environment as including the zone below the surface as well (i.e., the entire leaf). Where appropriate, we use the term phyllosphere interchangeably with leaf associated to refer to bacterial populations in and on leaves.

View larger version (106K): [in this window] [in a new window]   FIG. 1.   (A) Three-dimensional schematic of a leaf. Phyllosphere bacteria colonize the surface of the epidermis and the intercellular spaces (apoplast). Reprinted from reference 299 with permission. (B) Low-magnification electron micrograph of the adaxial surface of a pea (Pisum sativum) leaf. Reprinted from reference 203 with permission. (C) Electron micrograph of a carbon replica of the adaxial surface of a corn (Zea mays) leaf. Note the waxy protuberances. Reprinted from reference 203 with permission.

View larger version (79K): [in this window] [in a new window]   FIG. 2.   (A) Scanning electron micrograph of bacteria on the surface of a corn (Z. mays) leaf. Photograph taken by J. Lindemann and M. Garment; reprinted from reference 312 with permission. (B) Imprint of a corn leaf. Segments of a field-grown corn leaf were gently pressed onto the surface of King's medium B (146), a nonselective medium.

Certain features of leaf surface topography set these habitats apart from others and evoke intriguing questions as to how bacteria have adapted to life at the interface between the leaf and the atmosphere. The cuticle layer coating the epidermis separates the leaf proper from the surrounding air and functions to protect and waterproof the plant surface (144). The layer, composed of cutin (mainly esterified hydroxy fatty acids) and waxes, forms a lipophilic barrier with low permeability. Dotting the surface landscape are stomates, trichomes, and other leaf surface appendages. All of these structures together with the surface waxes provide a topography that, scaled to the size of a bacterium, is far from smooth and uniform and may be imagined as a jumbled matrix of peaks, valleys, caves, and plains for bacterial colonization (Fig. 1B, 1C, and 2A).

In the past few decades there has been much interest in life in extreme environments, such as hydrothermal vents, acid seepage from mines, and similarly uninviting habitats. That bacteria colonize such environments is not at all surprising to us. After all, they flourish on leaf surfaces! This habitat as well has been frequently regarded as hostile to bacteria. Is the phyllosphere an extreme environment? Compared to a hot spring or hydrothermal vent, the 40 to 55°C sometimes found on leaf surfaces exposed to intense sunlight is relatively benign. Although desiccation of leaf surfaces may provide a stress for bacteria, that stress may be no greater than on exposed skin, very dry soil, or other such habitats. Similarly, the relatively cool 5 to 10°C temperatures and dilute (oligotrophic) substrate concentrations that may be typical of leaves that are wet with dew on a clear night may be relatively benign compared to the cold, dilute conditions of many streams or marine environments. Nor is the UV radiation any more intense on the surface of a leaf than in shallow, clear water. What is unique, or nearly so, to the leaf habitat is that a bacterium on a leaf may be exposed to all of these conditions during the course of a single day and is normally exposed to most of them every day. If the leaf surface is a harsh environment, then the property that makes it harsh is not the extremes to which it is exposed, but the frequent, repeated, rapid alteration between these very different conditions, any one of which may be considered stressful to at least some bacteria.

Thus, an important feature of leaf surfaces that sets the phyllosphere apart from most other types of microbial habitats is that epiphytic bacteria exist in a continuously fluctuating physical environment. Exposed to the atmosphere and the sun, leaf surfaces and, hence, their resident microbes are subjected to changes in aspects of microclimate such as temperature, relative humidity, wind speed, radiation, moisture, and others that occur on time scales of seconds to hours. As moisture levels change, for example, as might occur with the formation of dew during night and its subsequent evaporation during the day, so do substrate concentrations and osmolarity and other measures of water availability and activity. Intense rains may catastrophically perturb the system, as large numbers of bacteria (e.g., 10⁵ CFU per bean leaflet in 15 min [171]) may be removed and deposited on the ground during rainfall.

Leaf habitats are intrinsically ephemeral in nature, surviving only a few weeks for many annual plants in the temperate zone to a few years for tropical or evergreen plants. For plants growing in temperate areas where climate changes are distinctively seasonal, leaves of annual plants emerge, develop, and senesce in a period of several weeks. During this period, the physical and biological nature of the habitat is continually changing as leaves expand, mature, and weather with age. With the onset of winter, these microbial habitats are destroyed. What happens to their resident bacterial communities?

Many pieces of the puzzle come in the forms of the kinds of bacteria that inhabit the phyllosphere. A diversity of bacterial species are known to colonize the phyllosphere (Fig. 2 and 3). The specific types and their relative abundances vary with a number of factors related to the plant (e.g., plant species, phenology, and age) and the environment in which the plants are grown (e.g., geographic area and weather conditions within a geographic area) (12, 13, 45, 59, 63, 68, 71, 86, 158, 159, 213, 248, 270, 271, 276, 285, 317). Additionally, what one finds depends on how one looks, and "looking" for phyllosphere bacteria has relied extensively on cultural methods. The extent to which reliance on cultural methods limits the study of bacterial populations and communities remains to be addressed for the phyllosphere (31). The only study to address this issue dealt with one species of bacteria on leaves of growth chamber-grown plants (308). Furthermore, the utility of various ribosomal DNA-based methods (reviewed in reference 6) for studies of bacterial communities in the phyllosphere remains unexplored.

View larger version (39K): [in this window] [in a new window]   FIG. 3.   Qualitative variability in bacterial populations on individual leaflets of field-grown snap bean (Phaseolus vulgaris) plants. Leaflets, sampled on the same day, were gently pressed onto King's medium B (146).

One characteristic of communities of phyllosphere bacteria that has been frequently noted is the abundance (some have termed it preponderance) of pigmented forms (Fig. 3). Indeed, the abundance of chromogenic species common to the phyllosphere (e.g., Erwinia herbicola [renamed Pantoea agglomerans; however, we will refer to the bacterium by the more familiar name E. herbicola], xanthomonads, fluorescent pseudomonads, and pink-pigmented facultative methylotrophs) has led to the yet unresolved speculation that pigment production may confer a selective advantage to bacteria that colonize a habitat which is exposed to solar radiation daily (58, 182, 251, 272). At present there is some effort under way to determine if UV radiation affects the survival or fitness of bacteria in the phyllosphere (273). Once the importance of solar radiation to survival of bacteria adapted to the phyllosphere is better understood, the importance of pigmentation in this process may become better resolved.

Many phyllosphere bacteria are non-spore-forming, gram-negative or gram-positive heterotrophs (13, 68). Some of the bacterial species or individual strains that are well adapted to life on leaf surfaces may be distinct from those commonly found in the soil or rhizosphere (158, 270, 271).

Several studies have examined the distribution of specific groups or species of bacteria of interest across plant species (e.g., ice nucleation-active bacteria [189] and pink-pigmented facultative methylotrophs [45]). In general, such studies have found the same species or groups of bacteria present on a broad range of plants. The conclusion has been drawn that these groups are therefore well adapted to the phyllosphere. Note, however, that such studies do not separate the possibility that these species (or groups) are themselves highly variable and that very different individuals are each well adapted to a few plants from the possibility that most members of these groups are adapted to the phyllosphere of many plants.

There have been fewer attempts to determine bacterial community structure and diversity on a particular plant species (Table 1). Among these are bacteria from leaves of perennial rye (Lolium perenne) (59), olive (Olea europea) (68, 71), snap bean (Phaseolus vulgaris) (210), and sugar beet (Beta vulgaris) (276). Ercolani made two collections of bacteria from olive leaves, roughly 1,700 strains each, during each of two sampling periods spanning several years in two different decades. These collections, which included bacteria present on leaves of different age groups, were characterized phenetically (68-71) (Table 1). Differences in specific types were associated with leaf age and sampling time. Overall, the communities on olive leaves were dominated by strains of P. syringae (likely P. syringae pv. savastanoi, causal agent of olive knot disease). An interesting array of other gram-negative and gram-positive species were identified at lower frequencies. On leaves of perennial rye, the dominant components were identified as strains of Pseudomonas fluorescens and Xanthomonas campestris, with a number of other minor components (13). On bean, we have observed that the dominant species may change with plant age or with environmental conditions (118). Hence, while the bacterial species composition varies with plant species and other factors, the presence of a few dominant components with a number of species that occur at lower frequencies appears to be characteristic of the few phyllosphere bacterial communities characterized thus far. The distribution of bacterial species numbers among leaves thus fits a common pattern observed for other organisms (235).

What ecological roles do phyllosphere bacteria play? Do they have any effect on their habitat, or are they merely adapted to utilize resources otherwise "wasted" by the plant? What is their importance in the global scheme of things?

Bacteria that alter the leaf habitat in obvious ways. We know most about those bacterial species that include strains that visibly modify their leaf habitats. Among these are some of the major groups of plant-pathogenic bacteria (140), pathovars of P. syringae and X. campestris and Erwinia spp., that to various degrees have been found as epiphytes on leaves of their susceptible host and nonhost plants (cf. 114, 161, 228). Bacterial ice nucleation alters the likelihood of frost injury at temperatures only slightly below 0°C. Some strains of several species that inhabit the phyllosphere are ice nucleation active (INA), including P. syringae, P. fluorescens, E. herbicola, and X. campestris pv. translucens (reviewed in references 121 and 175). Bacteria that produce plant hormones may also alter leaf and fruit habitats. For example, strains of E. herbicola that produce indoleacetic acid cause russetting of pear fruit (194). Habitat modification by pathogenic and ice nucleation-active P. syringae will be discussed further in subsequent sections.

Nitrogen-fixing bacteria. There are a number of reports on the isolation and identification of free-living nitrogen-fixing bacteria from the phyllosphere of various plants, most frequently from the tropics (2, 30, 139, 156, 214, 223, 248-252, 261, 262, 268, 287). The importance of these bacteria, their frequency or population sizes, and the overall role that they may play in the nitrogen cycle are difficult to assess. Most isolations have been by enrichment culture rather than direct plating, making quantitation difficult. Ruinen reported on the presence of large numbers of nitrogen-fixing bacteria on leaves of a range of plants grown in nitrogen-poor soils in Indonesia and Surinam (248-251). However, her quantitation was by direct microscopic observation of bacteria on leaf surfaces, a method that could not distinguish nitrogen-fixing bacteria from the many other genera present on leaves.

2

Pink-pigmented facultative methylotrophs (PPFM). PPFMs in the genus Methylobacterium are a physiologically interesting group of bacteria that preferentially utilize substrates lacking carbon-carbon bonds (e.g., methanol and methyl amine) as sources of energy and carbon (92-94, 225). PPFMs (mainly Methylobacterium mesophilicum, formerly Pseudomonas mesophilica) have been found in large numbers from leaves of a diversity of plant species, including angiosperms (monocots and dicots), gymnosperms, and even lower plants (e.g., bryophytes and ferns) (12, 19, 43-45, 116, 118, 317). In the midwestern and northeastern United States, PPFMs may constitute a significant proportion of the bacterial community on leaves of field-grown plants, depending on plant species and a number of other factors. In other areas (or climates), these bacteria may be less abundant. PPFMs were not detected on leaves of olive plants in southern Italy (68, 71). Although they are not always present in large numbers, PPFMs frequently constituted up to 79% of the heterotrophic bacteria recovered from leaves of white clover (Trifolium repens) (45) or more than 90% of the bacteria culturable from snap bean (Phaseolus vulgaris) (118). We continue to be impressed with the consistent success (as reflected in their great abundance) of the PPFMs as leaf colonizers. Although they grow slowly in culture relative to, say, P. syringae or E. herbicola, by the time snap bean pods are ready for harvest PPFMs predictably establish large population sizes (>10⁷ CFU per leaflet), regardless of weather conditions. (P. syringae, on the other hand, only does so under favorable weather conditions. More to be said about this in a later section.)

PSEUDOMONAS SYRINGAE

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The Species: Pseudomonas syringae van Hall 1902

P. syringae was initially isolated from a diseased lilac (Syringa vulgaris L.) by M. W. Beijerinck in 1899 and subsequently characterized and named by C. J. J. van Hall (318). The species designation is thus linked to the diseased host where the bacterium was first found. Bacteria with similar characteristics have been isolated from diseased tissues of a very large number of other plant species. Although once numbering more than 40 species, all are now classed as the single species P. syringae (61). P. syringae is easily identified as a gram-negative strict aerobe in the  subclass of the Proteobacteria which is rod-shaped, with polar flagella, with few exceptions produces fluorescent pigments, is oxidase and arginine dihydrolase negative (phenotypes that distinguish it from most of the other fluorescent pseudomonads), and does not rot potato (which distinguishes it from Pseudomonas viridiflava) (61).

The infrasubspecific epithet pathovar is used to distinguish among bacteria within the species that exhibit different pathogenic abilities (64, 319, 320). Nutritional, biochemical, physiological, and nucleic acid-based tests (e.g., DNA hybridization, restriction fragment length polymorphism, and repetitive DNA PCR-based genetic fingerprinting) have also been found useful in determining pathovars of P. syringae (49, 54-56, 66, 84, 104, 164, 198, 200, 244, 257, 259, 300, 318, 321). Findings from the various tests are generally in agreement with the groupings made on the basis of host range.

Strains within most of the pathovars exhibit rather narrow host ranges (29). The exception may be pathovar syringae, which includes the strain originally isolated from lilac (i.e., the type strain for the species). More than 80 plant species are listed as hosts for strains of P. syringae pv. syringae (29). From published data, it is not clear whether pv. syringae is a repository for strains that may in actuality have quite limited host ranges (36, 164, 198, 247, 253). A single plant species may serve as host for strains within two (or more) different pathovars, one of which is invariably pv. syringae. For example, P. syringae pv. tomato and P. syringae pv. syringae both cause disease on tomato, bacterial speck and fleck, respectively. Although the symptoms of the diseases are fairly similar (small necrotic lesions), strains within the pathovars are clearly different with respect to a number of phenotypes, including nutritional, biochemical, and serological parameters, phage sensitivity, DNA-based characteristics, and others (48, 54-56, 138).

Thus, the species P. syringae has generally evolved to interact with a wide range of plants in most regions of the world. However, within the species, there is a great deal of specialization with respect to plants with which individual strains are likely to interact. Bacteria that cause disease on a plant are usually much better colonists of that plant than those that do not. Further specialization is indicated by the observation that some strains differ with regard to the mechanisms of their interactions with a single plant, as evidenced by the fact that they cause different diseases of the same host.

Must all strains of P. syringae be pathogenic? One finds what one looks for. Until the 1970s, plant pathologists were looking for pathogens in diseased tissue. Isolates unable to cause the symptoms of interest would have been discarded. Thus, all known strains of P. syringae were pathogens and pathogenicity was one criterion for belonging to the species P. syringae. When the association between frost injury to plants and ice-nucleating bacteria was discovered in the 1970s, searches began for bacteria that were active as ice nuclei (189). The bacteria most frequently found to be ice nucleation active, including the most efficient ice nuclei, had characteristics that placed them in the species P. syringae (Fig. 4). However, for some of these strains, plants could not be found on which they would cause disease. The question of whether it was possible that some strains of P. syringae might not cause disease at all became one of several important issues in the debate about the safety of deliberate environmental introduction of recombinant organisms. Among the many concerns surrounding the field tests of recombinant Ice deletion mutants of P. syringae (the first deliberate release into the environment of a recombinant organism) was the pathogenic potential of these bacteria. However, the parent strains of P. syringae from which the mutants were derived were initially isolated from leaf surfaces of healthy potato (strain TLP2), citrus (Cit7) and strawberry (S203) (169, 177). Pathogenicity tests were conducted on 30 to 40 different plant species or cultivars, none of which could be identified as a susceptible host for either the parent or Ice mutant strains. One could argue that plant species X was not tested. After all, is not P. syringae a plant pathogen? One could additionally argue that injection of plants grown in growth chambers or greenhouses may not always be a good indicator of the pathogenic potential of a strain, as plant reactions may not resemble those encountered in the field and hence be difficult to interpret. To add fuel to the fire, plant pathologists have long accepted the finding of Klement (153) that the rapid necrotic reaction that develops following inoculation of a nonhost (e.g., tobacco) with large doses of a strain of P. syringae, referred to as the hypersensitive reaction (HR), is indicative of its pathogenic potential. Hence, a strain that causes the HR on tobacco must be a pathogen of some other plant species. The parent Ice⁺ and mutant Ice strains caused the HR when infiltrated into tobacco. On the other hand, if we were not so entrenched with the model that P. syringae is a plant pathogen and hence every strain must be pathogenic on some plant species, the results of Lindow (177) and Lindemann and Suslow (169) would not be so difficult to accept. Once again the dogma seemed in conflict with the data. Today many of us accept the concept that P. syringae has evolved to live in association with leaves, and incidentally, some (many?) strains cause lesions on some plants.

View larger version (83K): [in this window] [in a new window]   FIG. 4.   P. syringaepathogen, ice nucleus, and epiphyte. (A) Foliar and (B) pod symptoms of bacterial brown spot disease of snap bean (P. vulgaris) caused by P. syringae pv. syringae. (C) An ice nucleation event occurred in the test tube on the right due to the large numbers of ice nucleation-active P. syringae present on the leaf. (D) Symptoms of frost injury to snap bean plants in the field. (E) Asymptomatic snap bean leaveshabitats for P. syringae. Figures B, C, and E are reprinted from reference 111 with the permission of the publisher.

P. SYRINGAE AND BACTERIAL ICE NUCLEATION

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Discovery

Something associated with dead leavesin one case derived from ground-up dry leaves, in the other from decaying leaf litterbecame the common focal point for two groups, each pursuing completely different lines of investigation in disciplines as widely separated as atmospheric sciences and plant pathology (282). G. Vali, R. Schnell, and colleagues at the University of Wyoming searched for biogenic sources of ice nuclei that play a role in precipitation processes. S. E. Lindow, D. C. Arny, and C. D. Upper at the University of Wisconsin-Madison sought to understand how dried corn leaf powder affected the susceptibility of corn to frost injury. Their paths crossed with the isolation and identification of P. syringae from their respective leaf preparations and the discovery of its ability to nucleate supercooled water to form ice (9, 201). The story of the discovery of bacterial ice nucleation as narrated by Upper and Vali (282) is not only entertaining but informative and insightful in depicting the process of scientific inquirythe delight in finding the unexpected; the frustration in overcoming accepted dogmas; and the skepticism toward accepting the unexpected. The discovery of biological ice nucleation opened new avenues of research ranging from such seemingly unrelated topics as winter survival of insects, snow making, and food processing (163). There are several excellent reviews on bacterial ice nucleation (175, 178, 294, 296, 297, 313, 314, 316). We include here only a few aspects deemed relevant to our overall goal of generating interest in the leaf ecosystem and its bacterial inhabitants.

The ability of bacteria to nucleate supercooled water to form ice is uniquely limited to P. syringae and a handful of other bacterial species, many of which dwell in the phyllosphere. (Of course, ice-nucleating bacteria were first found associated with leaves, and this is where people have looked. Searches may not have been as extensive in other habitats.) Among these are strains of E. herbicola-like, Erwinia ananas, Erwinia uredovora, P. fluorescens, P. viridiflava, and X. campestris pv. translucens (9, 142, 145, 190, 201, 216, 217, 226). Not all strains within these species are ice nucleation active. Indeed, ice nucleation activity has been used as one (among several) trait to distinguish strains among some of the P. syringae pathovars (108, 138, 226). For example, strains within pv. syringae frequently exhibit the ice phenotype, while none of the strains tested thus far within pv. tomato or morsprunorum are ice nucleation active. Among strains that are active in ice nucleation, not every cell is active at a given time and temperature (Fig. 5) (9, 175, 195, 201). The fraction of cells within a population that are active as ice nuclei increases with decreasing temperatures below 1°C. That only a few bacterial species are ice nucleation active; that not all members within such species exhibit the phenotype; and the stochastic nature of the activity within strains that exhibit the phenotype have raised intriguing questions regarding the evolution and molecular basis for bacterial ice nucleation. Sequence analysis and biochemical characterization of ice genes and proteins have suggested some answers to these questions (292, 295, 314, 316).

View larger version (17K): [in this window] [in a new window]   FIG. 5.   Ice nucleation spectrum of a strain of P. syringae. Cells of P. syringae were suspended in phosphate buffer (0.1 M, pH 7.0) to ca. 108 CFU/ml. Tenfold serial dilutions were prepared from this suspension. Droplets from the original and each diluted suspension were placed on an aluminum block. The block was cooled, and the temperatures at which droplets froze were recorded. Each symbol represents determinations from one of the five suspensions. The concentration of ice nuclei N at temperature T [N(T)] in the suspension was calculated by the method of Vali (283). Nucleation frequency is the ratio of the number of ice nuclei to bacterial cell density.

Orser et al. (219, 220) were the first to demonstrate that the ice phenotype could be transferred to Escherichia coli by expression of DNA clones from the genome of P. syringae and E. herbicola. Since then, ice (or ina, for ice nucleation activity) genes have been cloned and sequenced from strains of P. syringae (96), P. fluorescens (291), E. herbicola (290), E. ananas (1), and X. campestris pv. translucens (325). In all cases, the genes contain a single open reading frame (ca. 3,600 to 4,000 bp) with a large central region consisting of a hierarchy of repeated motifs of 24, 48, and 144 nucleotides in length flanked by unique 5' and 3' sequences. The significance of the ordered internal repeats in the function of ice proteins as templates for ice formation has been noted (96, 291, 293). Results of a wide range of experimental and theoretical approaches suggest that ice proteins assemble to form aggregates of various sizes in association with the outer membrane of bacterial cells (32, 42, 52, 74, 91, 95, 141, 155, 196, 212, 267, 280, 290-294, 297, 298, 313-315). The number of protein monomers that are assembled in a proper conformation at any given time may account for the various temperatures at which individual cells are active in ice nucleation. The larger the aggregate, the more efficient the ice nucleus. For an excellent review on ice nucleation theory and measurement, see reference 283. For models of bacterial ice nuclei, see references 141 and 295.

It is curious that only a proportion of the strains within a bacterial species exhibit the ice phenotype. Edwards et al. (65) found a close correlation between the Ice⁺ phenotype and presence of an ice gene as detected by Southern hybridization using an 800-bp fragment from the internal repetitive region of the inaX gene from X. campestris pv. translucens. The probe hybridized to genomic digests from one of five E. herbicola strains, 9 of 26 P. syringae strains, one of two P. fluorescens strains, and 8 of 16 X. campestris strains. With the exception of two strains (one each of P. syringae and X. campestris), the strains that exhibited an Ice phenotype did not hybridize to the probe. The two exceptions apparently contained the ice gene but did not express the Ice phenotype under the conditions tested. Thus, the gene was apparently missing, not just not expressed, in most strains that did not exhibit the Ice phenotype. Comparative analyses of the sequences of ice genes and proteins, including examination of codon usages, have led to the thought that ice genes present today diverged from a common ancestral gene. Wolber (316) suggested that the bacterial species known to contain members that are ice nucleation active acquired the gene via horizontal transfer, perhaps via conjugation between members of "epiphytic communities." It might be interesting to compare the divergence of ice genes with those of other genes within ice-nucleating bacterial species.

Although we do not have clear answers as to what selective advantage(s) is conferred upon bacteria that express the Ice phenotype, it is clear that the presence of these bacteria on leaf surfaces can alter (i.e., destroy) leaf habitats at subzero temperatures (9, 174, 175, 188, 191, 195). Frost-sensitive plants are injured when ice forms within plant tissues. Without ice, such injury does not occur. In the absence of heterogeneous ice nuclei, water associated with leaves will supercool. Supercooling in the temperature range of 0 to roughly 5°C is primarily limited by the presence of INA bacteria. Below 5°C, other heterogeneous ice nuclei, including those produced by plants themselves, probably also limit supercooling. Thus, INA bacteria are responsible for ice formation, and hence injury to plants, mainly in the range from 0 to 5°C. Because most cells within a population of INA bacteria are not active at a given temperature and time, the larger the bacterial population, the greater the likelihood that one or more of the cells will be active at relatively warm temperatures (e.g., 2°C). Hence, the amount of frost injury sustained at a given temperature is a function of the population sizes of INA bacteria in the phyllosphere. Once the quantitative relationship between numbers of INA bacteria and frost injury was known, it was apparent that the hazard of frost injury to sensitive crops could be diminished by decreasing population sizes of INA bacteria (174-176).

S. E. Lindow and colleagues demonstrated that population sizes of INA bacteria could be reduced sufficiently to achieve measurable decreases in frost injury by application of naturally occurring and chemically induced Ice strains of P. syringae, P. fluorescens, and E. herbicola (174, 176, 178, 184, 192, 193). Ice strains were effective in preventing or minimizing colonization of plants by INA bacteria but not in eliminating established populations of the target microbes. Establishment of relatively large population sizes of the antagonists on leaf surfaces was required for effective exclusion of INA bacteria from leaf surfaces. Hence, manipulation of INA bacterial population sizes by application of non-INA strains is likely mediated by preemptive or competitive exclusion rather than displacement mechanisms (170, 178-180, 184). Competition for limiting resources on leaf surfaces (e.g., nutrients and space) was suggested as a possible mechanism by which established Ice bacteria may prevent buildup of incoming Ice⁺ strains (309, 310). Because very similar strains of bacteria are involved in such exclusion, any density-dependent mechanism regulating population size is consistent with the available data. Thus, the term competition, in this context, appears to include a broad range of mechanisms, some of which may not normally be thought of as competition. Such a mechanism would imply that antagonist strains with ecological habitat requirements similar or identical to that of the target strains may be more effective in excluding the latter than antagonist strains that are not as closely related. This line of reasoning led to construction of recombinant Ice strains of P. syringae and P. fluorescens by deleting a roughly 1- to 1.5-kb fragment (depending on construct) of the ice gene, followed by marker exchange of the mutated gene into the genome of the recipient strain (cf. 184, 296). These strains became the first recombinant microbes deliberately released into the environment (181, 184, 197, 296). With the exception of the Ice phenotype, the mutants were similar to their respective wild-type strains in all tests, including lack of pathogenic ability, relative ability to colonize and survive in association with plants in controlled environments, survival in soil, survival following freezing and thawing, and many other traits. Indeed, because of regulatory requirements for these first releases, no P. syringae strain has been as extensively and intensively characterized as the Ice mutants and their respective wild-type strains (169, 181, 197).

After an arduous path through a maze of regulatory, political, and societal obstacles spanning more than 5 years, approval was granted to Lindow and researchers at DNA Plant Technology (formerly Advanced Genetic Sciences, Oakland, Calif.) to field test the Ice mutants on potato and strawberry plants (181, 184). When the P. syringae Ice mutants and their respective Ice⁺ parental strains were coinoculated onto potato plants in the field, population sizes of the parental strains were reduced over 300-fold (181, 184). Population sizes of naturally occurring (i.e., indigenous) Ice⁺ P. syringae were reduced about 50-fold on plants inoculated with the mutants compared to untreated controls. The larger reduction in population size of the Ice⁺ parental strains than of naturally occurring Ice⁺ P. syringae by the Ice mutants is consistent with some degree of specificity in interactions among P. syringae strains on leaves. However, the efficacy of the Ice mutants was similar to that found with some naturally occurring non-INA strains. The reduced population sizes of Ice⁺ bacteria treated with the Ice strains correlated with reduced amounts of frost injury (ca. 70 to 80%) to potato plants during a natural radiative frost event.

None of the environmental disasters that were suggested prior to the deliberate release of recombinant Ice bacteria have become reality. This is not surprising given that the experiments were soundly based on what was known about the ecology of P. syringae and phyllosphere bacterial population dynamics at the time. The monumental efforts of Lindow and scientists at DNA Plant Technology allowed other scientists, including us, to utilize the power of molecular tools to address fundamental ecological questions on interactions of bacteria with plants as they occur in the real world. In addition, a commercial preparation of a mixture of naturally occurring Ice strains is now available for control of frost injury on some crops (184).

EPIPHYTIC POPULATIONS OF P. SYRINGAE

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Discovery

The "birth" of much of our current understanding of the epidemiology of foliar bacterial diseases and interest in leaves as habitats for bacteria can be traced to the seminal findings of J. E. Crosse (East Malling Research Station, U.K.) published in 1959 (46). Among several key findings was the revelation that populations of pathogenic P. syringae pv. morsprunorum, the causal agent of bacterial canker and leaf spot of stonefruit trees, could be recovered in large numbers from the surfaces of asymptomatic cherry leaves. Moreover, Crosse suggested that these surface or epiphytic populations of the pathogen and not populations in lesions on cherry leaves, as thought at the time, were the likely main source of inoculum for the infection of stems and branches leading to the development of canker infection. Crosse's pioneering work was subsequently generalized by C. Leben in the United States into a conceptual framework for the role of epiphytic populations in the epidemiology of diseases caused not only by pathovars of P. syringae but also by other phytopathogenic bacteria, most notably pathovars of X. campestris (161). The careful work of Crosse and advocacy of Leben gradually brought the possibility that these bacteria may not always reside in lesions to the consciousness of phytobacteriologists. Although the dogma that phytopathogenic bacteria are confined to lesions was gradually abandoned, the role of such bacteria growing in association with healthy tissues was not well understood. However, sufficient interest was generated in these so called epiphytic populations of phytopathogenic bacteria that much of the research in phytobacterial epidemiology during the 1960s and 1970s was centered on searching for and successfully finding pathogen populations on healthy leaves of host and nonhost plants (cf. 114). The role of P. syringae in the phyllosphere was extended to that of an epiphyte as well as pathogen. Because old ideas seldom die without a battle, however, there was substantial support for the idea that these bacteria resided in "latent lesions." If the function of pathogens is to cause disease, they must be invading tissues and surviving or growing there until conditions that favor lesion development occur. Note the importance in the above argument of the assumption that the primary role of these bacteria is to cause disease. As we will describe below, we still do not fully understand the importance of the ability to cause disease to the overall fitness of P. syringae.

In his paper describing "epiphytic" populations of P. syringae pv. morsprunorum, Crosse described a method to quantitate leaf surface population sizes of the pathogen (46). Bacteria were removed from cherry leaves by vigorous shaking of leaf samples (2 min every half hour for up to 4 h) in sterile water (with or without a surfactant) followed by dilution plating of the washings. With some modifications, his basic leaf-washing method became a standard way of enumerating leaf surface bacterial population sizes. Others have since used leaf sonication to remove bacteria from leaves (reviewed in reference 136). Thus, measurement of epiphytic bacterial population sizes has been based on the ease with which culturable bacteria can be removed from leaves by either washing or sonication. While we conceptualize epiphytic bacteria as those that colonize the surfaces of leaves, what we actually measure are culturable bacteria that can be removed by methods such as leaf washing or sonication. The possibility that it may be more difficult to remove bacteria from some parts of the surface structures of leaves than from others and the possibility that either washing or sonication may remove some bacteria from the apoplast (i.e., intercellular spaces) demonstrate the practical difficulties encountered in enumerating "epiphytic" bacterial population sizes. Qualitatively, these bacteria clearly reside within and on leaves. This has been established microscopically. To this day, however, satisfactory methods have not been developed that allow facile quantitative separation of "epiphytic" from "apoplastic" populations of these bacteria. Thus, although it is likely that the bulk of bacteria in leaf washings or sonicates were on the "surface" (i.e., epiphytic), it is also possible that some bacteria in "protected sites" on the epidermal layer were not so readily removed or that some bacteria from the apoplast were. An alternative means to measure numbers of leaf-associated bacteria is to homogenize the leaf and plate from the homogenate. This method provides no information regarding where the bacteria were but does allow somewhat better recovery of bacteria from leaves. We have used homogenization as the method of choice for many years and refer to the bacterial populations enumerated as phyllosphere or leaf-associated bacteria.

Attempts have been made to estimate the proportion of bacteria that are internal, as opposed to on the surface, using various sterilants and UV irradiation of leaves (16, 143, 227, 258, 275, 307; unpublished data). In growth chamber assays, more than 99% of the bacteria (P. syringae and PPFMs) that were spray inoculated onto leaves immediately before treatment were killed when hydrogen peroxide was used as the sterilant (unpublished data). Almost none of the bacteria infiltrated into the apoplast before treatment were killed. Thus, this method appears to work quite well for highly artificial controls. When applied to bacteria that have grown in association with leaves in the growth chamber (307) or field (unpublished data), most of the P. syringae (75 to >90%) and >99% of the PPFMs were sensitive to the peroxide treatment. Results such as these are consistent with the view that the majority of P. syringae and virtually all of the PPFMs are "epiphytic" across a population of healthy leaves.

Enumeration of bacteria by cultural means is dependent on the assumption that the cells of interest (or at least a constant proportion thereof) are recoverable by plating. Wilson and Lindow (308) found that at least 25%, and usually more, of the viable P. syringae were recoverable by plating from leaves over a range of conditions in the growth chamber. Thus, estimates of bacterial population size based on culture methods should not be off by more than about a factor of 4. Although fourfold seems like a potentially large error, compared to the normal variability in the field (Fig. 6) and the five to six orders of magnitude across which such population sizes may fluctuate, it is of minor consequence. (The art of measuring population sizes of these bacteria from field samples is such that a factor of 4 is often close to the limit with which differences can be resolved.) Furthermore, the physical environment seemed to have the largest effect on the proportion of cells that were viable but not culturable. In field experiments, samples from all treatments (plots) are normally taken at the same time and hence under the same weather (physical) conditions. Thus, there is some reason to believe that differences in the proportion of viable but nonculturable cells across treatments should be small at any given sampling time. Nonetheless, the possibility that enumeration of phyllosphere bacterial population sizes from field experiments may be influenced by some variable proportion of cells that are viable but do not appear in culture remains an important caveat.

View larger version (42K): [in this window] [in a new window]   FIG. 6.   Quantitative variability in population sizes of P. syringae on individual leaves. (A) Each plate represents an equivalent dilution from washings of different individual rye leaves. (B) Lognormal distribution of population sizes of P. syringae on two sets of individual bean leaflets. The mean population size for both sets of leaflets is approximately 5.0 log CFU/leaflet. The population variances are 2.5 for set A () and 0.23 for set B (). Reprinted from reference 111 with the permission of the publisher.

Crosse found great variability among population sizes of P. syringae pv. morsprunorum associated with individual leaves (46). Although he chose to combine large numbers of leaves that were washed and dilution plated to achieve some measure of the central tendency of population size, it has since been recognized that much of the information regarding interactions of bacteria with individual leaves resides in the variability about this central tendency (107, 109). Bacterial population sizes often follow (or, more correctly, can be modeled by) the lognormal distribution across populations of leaves (Fig. 6) (109). Less frequently, the related Weibull distribution provides a better "fit" to bacterial population data (134). These distributions are skewed. Estimation of means by bulking samples (several leaves combined to form a single sample) from such skewed distributions tends to overestimate the mean and underestimate the variability of epiphytic or leaf-associated bacterial population sizes. Variability in bacterial population sizes is present not only among individual leaves but also at other scales, including sites on and in a single leaf (149).

Why are we so concerned about measuring population sizes of P. syringae in the phyllosphere? In his original paper, Crosse noted that larger numbers of P. syringae pv. morsprunorum were associated with a highly susceptible cultivar of cherry than with a more resistant one (46). Although it is generally accepted that epiphytic populations of P. syringae on asymptomatic leaves of susceptible and resistant host plants and on nonhost plants may serve as sources of inoculum for disease, there are few pathogen-host systems for which a quantitative relationship has been found between epiphytic population sizes and subsequent disease. The P. syringae pv. syringae-snap bean-bacterial brown spot disease is one such system (111, 112, 167, 246). The experimental approaches used by Lindemann et al. (167) and Rouse et al. (246) were similar in the sense that the relationship between epiphytic population sizes of P. syringae pv. syringae (measured by leaf washings) and amounts of brown spot disease were evaluated in what were essentially dose-response experiments conducted in the field. Different methods, however, were used to establish the necessary range of pathogen doses in experimental field plots. In both studies, pathogen population sizes on sets of individual bean leaflets collected at various times during the growing season were estimated, and information from the lognormal distribution of numbers of bacteria (population means and variances) was taken into account. Lindemann et al. (167) found that mean pathogen population sizes were not predictive of brown spot disease. However, the frequencies with which epiphytic population sizes of P. syringae pv. syringae were equal to or greater than approximately 10⁴ CFU per leaflet on asymptomatic individual bean leaflets were predictive of disease a week later. The probit-lognormal model of Rouse et al. (246) generalized the threshold model by incorporating the notion that the probability of disease can be expressed in terms of the probability of a leaflet having a particular number of bacteria multiplied by the probability of disease occurring on that leaflet given that particular number of bacteria summed over populations of leaves. Fifty percent effective dose values of approximately 1 × 10⁵ to 5 × 10⁵ CFU per leaflet were obtained when the probit-lognormal model was applied to field data. Quantitative models relating bacterial population sizes on individual leaves have successfully predicted disease a short time later. Thus, a quantitative relationship was established between epiphytic population sizes of P. syringae pv. syringae and the amount of disease that followed. From this finding, in turn, comes the realization that the amount of disease will be determined by the development of large population sizes of the bacteria in association with leaves.

So we arrive at a rather large, paradoxical piece of the puzzle. P. syringae has two ways to destroy its own habitat, frost injury and lesion formation. To our knowledge, lesions may provide a place for the bacteria to survive during unfavorable weather conditions but little other advantage. Population sizes of the bacteria are highly variable in lesions but may remain quite large, even after several weeks of dry weather, when population sizes associated with asymptomatic leaves have declined markedly. Although several mechanisms have been proposed that may provide a selective advantage to the bacteria for causing frost injury, none has been demonstrated experimentally. Thus, there is no clear selection for destroying the leaf habitat. For most organisms, habitat destruction is regarded as highly unfavorable. Both of these events, lesion formation and frost injury, become highly likely only when the population sizes of the bacteria are relatively large, that is, when the bacteria have been particularly successful on or in their habitat. The bacteria reward themselves for success by destroying their habitat! Is this not paradoxical? To try to explain this phenomenon, we proposed a model in which lesion formation (and, we should add, frost injury) are unfortunate accidents of overpopulation that benefit neither the bacteria nor the plant (117). The real function of P. syringae is to live on healthy leaves. Perhaps it has means to communicate with some plants (we will call them hosts) in ways that improve the habitat for P. syringae in some way without causing (much) damage. Only when conditions become unusually favorable and population sizes of the bacteria become too large does the entire system crash, to the detriment of both host and bacteria.

When we first proposed this model a decade ago (117), we did so with tongue in cheek in an attempt to provoke discussion. To our amazement, nothing we have learned in the ensuing decade has provided strong evidence against this model. We reiterate it here to remind the reader that we tend to assign functions to organisms on the basis of those activities in which we are most interested, in this case causation of disease or frost injury. Perhaps we should look harder for those functions with real importance to the bacteria themselves.

Returning to our anthropomorphic interest, the realization that large bacterial population sizes are necessary for either disease or frost injury led to a search for those processes and factors that affect the relative abundance of P. syringae in the phyllosphere.

In this section, we explore what is known or what we think we know about the dynamics of population sizes of P. syringae across different time scales. In general, population sizes of P. syringae are frequently relatively small on young annual plants or emerging leaves during plant development and may increase to large numbers under suitable conditions (107, 174, 266). On perennials, population sizes of P. syringae are generally higher in spring than in summer (67, 68, 98, 174, 202, 243). The time scale over which such changes occur is remarkable. Increases occasionally occur in bursts lasting a day or more, with doubling times of a few hours (116). Infrequently, decreases can occur even more rapidly. For long periods, however, changes are relatively small (107).

At any given time, the number of P. syringae (and other bacterial components) present on a given leaf is the sum of immigration, emigration, growth, and death since the emergence of that leaf. Factors that affect the relative rates with which these population processes occur will, in turn, affect bacterial population sizes. For strains of P. syringae that are pathogenic and/or ice nucleation active, bacterial population sizes will, in turn, affect the probability of occurrence of disease or frost injury. The challenge is to determine the relative quantitative contributions of immigration, emigration, growth, and death to bacterial population sizes and identify those factors (biotic or abiotic) that influence these processes. This is a daunting task experimentally due to the complexity of interacting factors that operate in nature and difficulties associated with simultaneous measurements of rates at which each of the population processes occur in the field (cf. 147). However, some progress has been made, as described below.

Bacteria may arrive on leaf surfaces in a number of ways (cf. 288). Many phytopathogenic bacteria, including several pathovars of P. syringae and X. campestris, may be seed borne (255). Indeed, production of (relatively) pathogen-free seed is an effective way to reduce the hazard from such diseases. Numerous other nonpathogenic phyllosphere bacteria, such as E. herbicola and the PPFMs, have been found associated with seeds (126, 286). P. syringae grows rapidly on preemergent seedlings and is available to colonize leaves as the plant emerges (80, 110). Presumably, other bacteria behave similarly. Thus, seed-borne pathovars of P. syringae are able to flourish in association with the plant below ground as well as above. Although the spermosphere is quite different from the rhizosphere, perhaps it is time to reexamine the issue of whether the separation between phyllosphere- and rhizosphere-associated bacteria is, indeed, as strong as it is thought to be. In field experiments, marked strains of P. syringae pv. syringae that were inoculated onto bean seeds or foliage were subsequently recovered from the rhizosphere of snap bean and wheat plants (119; unpublished data). The latter was planted as a fall-winter cover crop following a planting of snap beans that was inoculated with a marked strain of P. syringae (119). By growing in the spermosphere and in association with preemergent seedlings, P. syringae is able to make the transition from seed to leaves, its preferred habitat, and bridge the gap between generations of host plants. Although we have found P. syringae in the rhizosphere, we have no evidence that it is able to move from roots to aboveground plant parts. Emigration and immigration of bacteria from one aerial part of a plant to another (such as from buds to leaves) without physical departure of the microbes from plants is also expected but has been less critically demonstrated to occur in the field.

Leaf habitats are unusually open systems. Exposure to the atmosphere provides ample opportunity for aerial immigration and emigration of phyllosphere bacteria. Indeed, plant canopies were found to be a major source of bacteria in the troposphere, at least during the growing season in those parts of the temperate zone that support extensive plant growth (168). In a study on the effect of cropping patterns on population sizes of P. syringae pv. syringae, Lindemann et al. (166) found that numbers of the pathogen were larger on leaves of snap bean plants established in experimental plots within a major snap bean-growing area than in plots planted away from the commercial snap bean production area. The seed with which all of the plots were planted was naturally infested with the pathogen. Presumably, P. syringae that originated in the large commercial plantings of snap bean dispersed (probably aerially) to the experimental plots. These additional immigrants were sufficiently abundant to influence the population sizes in the experimental plots. Regardless of what the mechanism turns out to be, there is clearly a measurable effect of surrounding vegetation on population sizes of P. syringae (166; S. S. Hirano, J. A. Virata, K. A. Lindgren, and C. D. Upper, asbstr., Phytopathology 87:S42, 1997).

Lindemann and Upper (171) found that although viable bacteria were present in the air at all times of day, significant upward fluxes of bacteria (i.e., emigration) occurred only during the warmest part of sunny days, when leaves were dry and wind speeds exceeded about 1 m s¹. Emigration rates were highly variable and lognormally distributed, with a median value on the order of about 100 CFU m² s¹ (Fig. 7A). On average, this corresponds to a loss of roughly 10³ CFU per h per bean leaflet with an area of 50 cm². Immigration rates, as measured by the number of bacteria deposited onto the surface of solid medium in petri dishes exposed at canopy height, were also highly variable (Fig. 7B). Most of the time (67% of 130 measurements made over three growing seasons at various times during the day and night), deposition rates were between 1 and 10 CFU m² s¹. This corresponds to roughly 18 to 180 bacterial immigrants arriving per h on a bean leaflet 50 cm² in area. From the estimates of Lindemann and Upper (171), we are left with the conclusion that at least during dry sunny weather, on average, there is a net loss of total culturable bacteria from bean canopies. The net loss was estimated to be over 10⁴ CFU per bean leaflet per day during those periods when atmospheric conditions were conducive for dispersal.

View larger version (15K): [in this window] [in a new window]   FIG. 7.   Immigration and emigration of bacteria to snap bean canopies. (A) Emigration rates are based on measurements of concentrations of air-borne bacteria at canopy height and 1.5 m above the canopy using six-stage Andersen viable samplers (171). (B) Immigration was measured by the deposition of air-borne bacteria onto petri dishes filled to the rim with King's medium B (146). Measurements of immigration and emigration were made over canopies of snap beans during three growing seasons. Data from J. Lindemann; reprinted from reference 118 with the permission of the publisher.

On plants such as citrus where little growth of P. syringae occurs and population sizes of P. syringae are normally quite small, immigration appears to be an important contributor to the numbers of bacteria found on leaves (185, 187). Both bacterial population sizes and immigration rates, measured as rates of deposition on exposed petri dishes, were highest near the edge and decreased with distance away from the edge of a citrus grove (187). Epiphytic bacterial population sizes were only slightly larger than could be explained by total numbers of immigrants summed over several months. On this basis, it was suggested that there may have been no growth of bacteria on citrus leavesthat all of the bacteria on these leaves had grown on nearby plants, moved to the citrus leaves, and accumulated there by deposition (187).

The estimates of Lindemann and Upper (171) were for total culturable bacteria. Rates of immigration and emigration for specific bacterial components and species over snap bean canopies are expected to be smaller than those noted above. In field experiments in which numbers of P. syringae were estimated on a series of deposition plates exposed during a continuous 11- or 14-day period, the average number of P. syringae arriving on a bean leaf-sized area was about 35 CFU per day (S. S. Hirano, J. A. Virata, K. A. Lindgren, and C. D. Upper, abstr., Phytopathology 87:S42, 1997). Such numbers of immigrants are orders of magnitude too small to account for the population sizes of P. syringae frequently encountered on leaves of snap beans or other plants that carry relatively large numbers of bacteria in the field. So we are faced with another paradox. Immigration rates are too small to affect population sizes of P. syringae, yet strong circumstantial evidence points to the availability of immigrants as very important to relative population sizes of this bacterium on plants such as beans, where the bacteria grow to relatively large population sizes (166).

There is a plausible explanation for how small numbers of immigrants can have a relatively large effect on population sizes of P. syringae. The numbers of bacteria associated with very young leaves, as they first begin to expand, are very small. New leaves provide (nearly) empty habitats for bacterial colonization. If the immigration rate of P. syringae is low relative to that of other bacteria, then the probability that a given new leaf will receive even one immigrant of that species is very low. Immigrants of some other species will probably arrive first and begin the process of colonization of the leaf. Thus, by the time that a P. syringae immigrant arrives, its likelihood of success is smaller than if it had been the first bacterial cell to arrive on the leaf. When the numbers of P. syringae available to immigrate to a new leaf are large, then P. syringae is more likely to arrive early in the process of colonization of the leaf, when there is ample empty habitat. Hence, leaves subjected to large numbers of immigrants of P. syringae are likely to carry larger population sizes of that bacterium.

Immigration and emigration of phyllosphere bacteria also occur during rain. The classical explanation for the air-borne dissemination of foliar bacterial pathogens is that they are carried by the wind in aerosols or ballistic particles generated when raindrops strike diseased plant parts or plants supporting surface bacterial populations (33, 50, 72, 75-77, 288, 289). Thus, outbreaks or spread of bacterial foliar blights have been attributed to dissemination of the pathogen by rainsplash or windblown rain ^</