The 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 , 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 106to 107 cells/cm2 (up to 108 cells/g) of leaf . 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 × 108 km2 . 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 1026 cells . 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.