2The above-ground parts of plants are normally colonized by a variety of bacteria, yeasts, and fungi. While a few microbial species can be isolated from within plant tissues, many more are recovered from the surfaces of healthy plants. The aerial habitat colonized by these microbes is termed the phyllosphere, and the inhabitants are called epiphytes. While there has been some investigation of the colonists of buds and flowers (1, 48), most work on phyllosphere microbiology has focused on leaves, a more dominant aerial plant structure. Bacteria are by far the most numerous colonists of leaves, often being found in numbers averaging 10 6 to 10 7 cells/cm 2 (up to 10 8 cells/g) of leaf (1,7,41). Because of their numerical dominance on leaves, and because more information is available on the process of bacterial colonization of leaves, we focus on this group of microbes in this review.Compared to most other bacterial habitats, there has been relatively little examination of phyllosphere microbiology. This is somewhat surprising given the abundance of plants in the world and the roles of various phyllosphere bacteria in the important processes discussed below. Leaves constitute a very large microbial habitat. It is estimated that the terrestrial leaf surface area that might be colonized by microbes is about 6.4 ϫ 10 8 km 2 (83). Given the large number of bacteria on leaves in temperate regions of the world and that populations in tropical regions are probably even larger, the planetary phyllosphere bacterial population may be as large as 10 26 cells (83). Clearly, in aggregate, these bacteria are sufficiently numerous to contribute in many processes of importance to global processes, as well as to the behavior of the individual plants on which they live.In the following sections we review available information on the identities and properties of the bacterial colonists of plant surfaces as well as describe the nature of the leaf surface habitat that these bacteria colonize. We emphasize the results of studies using new molecular and microscopic tools that have provided new insights into both the identity and the behavior of epiphytes, as well as into the nature of the plant surfaces that they inhabit. Also, we review recent studies of the interactions of various epiphytic bacteria with plants, which suggest that there is much more interaction between them than in a strict commensalistic relationship, in which they have been traditionally considered to coexist. The interactions that occur between bacteria on leaves have also received considerable attention. We attempt to illustrate how new approaches to defining the nature of leaf surface habitats have helped us to understand the behavior and interactions of epiphytic bacteria as well as why leaves are a particularly suitable habitat for exploring processes in microbial ecology. More comprehensive reviews of phyllosphere microbiology also address other important features of this interesting association (1,7,8,41,47,55,73). MICROBIAL COMMUNITIES ON LEAVESThe microbial ...

We provide here a comparative genome analysis of ten strains within the Pseudomonas fluorescens group including seven new genomic sequences. These strains exhibit a diverse spectrum of traits involved in biological control and other multitrophic interactions with plants, microbes, and insects. Multilocus sequence analysis placed the strains in three sub-clades, which was reinforced by high levels of synteny, size of core genomes, and relatedness of orthologous genes between strains within a sub-clade. The heterogeneity of the P. fluorescens group was reflected in the large size of its pan-genome, which makes up approximately 54% of the pan-genome of the genus as a whole, and a core genome representing only 45–52% of the genome of any individual strain. We discovered genes for traits that were not known previously in the strains, including genes for the biosynthesis of the siderophores achromobactin and pseudomonine and the antibiotic 2-hexyl-5-propyl-alkylresorcinol; novel bacteriocins; type II, III, and VI secretion systems; and insect toxins. Certain gene clusters, such as those for two type III secretion systems, are present only in specific sub-clades, suggesting vertical inheritance. Almost all of the genes associated with multitrophic interactions map to genomic regions present in only a subset of the strains or unique to a specific strain. To explore the evolutionary origin of these genes, we mapped their distributions relative to the locations of mobile genetic elements and repetitive extragenic palindromic (REP) elements in each genome. The mobile genetic elements and many strain-specific genes fall into regions devoid of REP elements (i.e., REP deserts) and regions displaying atypical tri-nucleotide composition, possibly indicating relatively recent acquisition of these loci. Collectively, the results of this study highlight the enormous heterogeneity of the P. fluorescens group and the importance of the variable genome in tailoring individual strains to their specific lifestyles and functional repertoire.

Diseases caused by Xylella fastidiosa have attained great importance worldwide as the pathogen and its insect vectors have been disseminated. Since this is the first plant pathogenic bacterium for which a complete genome sequence was determined, much progress has been made in understanding the process by which it spreads within the xylem vessels of susceptible plants as well as the traits that contribute to its acquisition and transmission by sharpshooter vectors. Although this pathogen shares many similarities with Xanthomonas species, such as its use of a small fatty acid signal molecule to coordinate virulence gene expression, the traits that it utilizes to cause disease and the manner in which they are regulated differ substantially from those of related plant pathogens. Its complex lifestyle as both a plant and insect colonist involves traits that are in conflict with these stages, thus apparently necessitating the use of a gene regulatory scheme that allows cells expressing different traits to co-occur in the plant.

A new set of broad-host-range promoter-probe vectors has been constructed. One subset contains the pVS1 and p15a replicons and confers resistance to either gentamicin or kanamycin. The other set contains the broad-host-range replicon from pBBR1 and confers resistance to kanamycin, tetracycline, ampicillin, or spectinomycin/streptomycin. Both plasmid sets are highly stable and are maintained without selection for more than 30 generations in several bacterial taxa. Each plasmid contains a promoter-probe cassette that consists of a multicloning site, containing several unique restriction sites, and gfp or inaZ as a reporter gene. The cassette is bound by transcriptional terminators to permit the insertion of strong promoters and to insulate the cassette from external transcription enabling the detection of weak or moderate promoters. The vector suite was augmented with derivatives of the kanamycin-resistant gfp promoter-probe plasmids that encode Gfp variants with different half-life times.

The complete genomic sequence of Pseudomonas syringae pv. syringae B728a (Pss B728a) has been determined and is compared with that of P. syringae pv. tomato DC3000 (Pst DC3000). The two pathovars of this economically important species of plant pathogenic bacteria differ in host range and other interactions with plants, with Pss having a more pronounced epiphytic stage of growth and higher abiotic stress tolerance and Pst DC3000 having a more pronounced apoplastic growth habitat. The Pss B728a genome (6.1 Mb) contains a circular chromosome and no plasmid, whereas the Pst DC3000 genome is 6.5 mbp in size, composed of a circular chromosome and two plasmids. Although a high degree of similarity exists between the two sequenced Pseudomonads, 976 protein-encoding genes are unique to Pss B728a when compared with Pst DC3000, including large genomic islands likely to contribute to virulence and host specificity. Over 375 repetitive extragenic palindromic sequences unique to Pss B728a when compared with Pst DC3000 are widely distributed throughout the chromosome except in 14 genomic islands, which generally had lower GC content than the genome as a whole. Content of the genomic islands varies, with one containing a prophage and another the plasmid pKLC102 of Pseudomonas aeruginosa PAO1. Among the 976 genes of Pss B728a with no counterpart in Pst DC3000 are those encoding for syringopeptin, syringomycin, indole acetic acid biosynthesis, arginine degradation, and production of ice nuclei. The genomic comparison suggests that several unique genes for Pss B728a such as ectoine synthase, DNA repair, and antibiotic production may contribute to the epiphytic fitness and stress tolerance of this organism.virulence genes ͉ epiphyte ͉ plant pathogen