Bacteria in the Leaf Ecosystem with Emphasis onPseudomonas syringae—a Pathogen, Ice Nucleus, and Epiphyte

Abstract

SUMMARY The extremely large number of leaves produced by terrestrial and aquatic plants provide habitats for colonization by a diversity of microorganisms. This 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 of bacteria 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. We focus on P. syringae to illustrate the attractiveness and somewhat unique opportunities provided by leaf ecosystems for addressing fundamental questions of microbial population dynamics and mechanisms of plant-bacterium interactions. Leaf ecosystems are dynamic and ephemeral. The physical environment surrounding phyllosphere microbes changes continuously with daily cycles in temperature, radiation, relative humidity, wind velocity, and leaf wetness. Slightly longer-term changes occur as weather systems pass. Seasonal climatic changes impose still a longer cycle. The physical and physiological characteristics 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 first studied for its ability to cause disease on plants. However, disease causation is but one aspect of its life strategy. The bacterium can be found in association with healthy leaves, growing and surviving for many generations on the surfaces of leaves as an epiphyte. A number of genes and traits have been identified that contribute to the fitness of P. syringae in the phyllosphere. While still in their infancy, such research efforts demonstrate that the P. syringae-leaf ecosystem is a particularly attractive system with which to bridge the gap between what is known about the molecular biology of genes linked to pathogenicity and the ecology and epidemiology of associated diseases as they occur in natural settings, the field.

Distribution and severity of bacterial brown spot on dry beans in South Africa: An update

“…31 However, frost injury can occur because Pss has the ability to catalyse ice formation at temperatures above -5 °C. 32 The results discussed here do not take into account epiphytic cells that could have been present on the leaf surfaces, even on leaves appearing healthy.…”

Pumpkin powdery mildew disease severity influences the fungal diversity of the phyllosphere

“…In past years, most of the researchers focused 65 on screening plant growth-promoting microorganisms from plants which can help us manage 66 diseases (Compant et al, 2005;Everett et al, 2005;Hirano & Upper, 2000; Whipps et al, 67 2008). However, not all the microbes in the natural environment are considered culturable.…”

AlgR functions in algC expression and virulence in Pseudomonas syringae pv. syringae

“…The infection of host plants by P. syringae involves epiphytic (surface) colonization, entry, establishment of infection sites in the intercellular spaces (apoplast), multiplication within host tissue and production of disease symptoms (Alfano & Collmer, 1996;Boch et al, 2002;Hirano & Upper, 2000).…”

“…Strains of the phytopathogen Pseudomonas syringae, which are subdivided into 50 pathovars according to host range, induce a wide variety of symptoms on plants including leaf spots, blights and cankers (Hirano & Upper, 2000). Alginate, a co-polymer of O-acetylated b-1,4-linked D-mannuronic acid and L-guluronic acid, has been reported to function in the virulence of P. syringae by facilitating dissemination of the bacterium in planta and by enhancing epiphytic fitness Yu et al, 1999).…”