Bacteria in the leaf ecosystem with emphasis on Pseudomonas syringae - a pathogen, ice nucleus, and epiphyte

The extremely large number of leaves produced by terrestrial and aquatic pl ants pl ovine habitats for colonization by a diversity of microorganisms. T his review focuses on the bacterial component of leaf microbial communities , with emphasis on Pseudomonas syringae-a species that participates in leaf ecosystems as a pathogen, ice nucleus, and epiphyte. Among the diversity o f bacteria that colonize leaves, none has received wide, attention than P. syringae, as it gained notoriety for being the first recombinant organism ( Ice(-) P. syringae) to be deliberately introduced into the environment. We focus on P. syringae to illustrate the attractiveness and somewhat unique o pportunities provided by leaf ecosystems for addressing fundamental questio ns of microbial population dynamics and mechanisms of plant-bacterium inter actions. Leaf ecosystems are dynamic and ephemeral. The physical environmen t surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation relative humidity, wind velocity, and leaf wetnes s. Slightly longer-term changes occur as weather systems pass. Seasonal cli matic changes impose still a longer cycle. The physical and physiological c haracteristics of leaves change as they expand, mature, and senesce and as host phenology changes. Many of these factors influence the development of populations of P. syringae upon populations of leaves. P. syringae was firs t studied for its ability to cause disease on plants. However, disease caus ation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generation s on the surfaces of leaves as an epiphyte. A number of genes and traits ha ve been identified that contribute to the fitness of P. syringae in the phy llosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system wit h which to bridge the gap between what is known about the molecular, biolog y of genes linked to pathogenicity and the ecology and epidemiology of asso ciated diseases as they occur in natural settings, the field.