Preferential colonization of internal versus surface sites of leaves by two bacterial species.
Siva Sabaratnam and Gwyn A. Beattie. Iowa State University, Ames, Iowa.

A leaf inhabitant, Pantoea agglomerans, and a phytopathogen, Pseudomonas syringae pv. syringae (Pss), were compared for their population dynamics and their ability to colonize internal sites of bean and maize leaves. Plants were spray-inoculated with GFP-labeled strains, and population sizes were enumerated by plate count and visualized in situ by fluorescence microscopy. The production of extracelluar polysaccharides (EPS) by Pss on leaves was also investigated using immunofluorescence techniques. Populations of P. agglomerans strain BRT98 decreased considerably on beans grown at 45% and 95% relative humidity (RH) and on maize at 45% RH, but increased slightly on maize at 95% RH. In situ visualization and cells enumerated from surface-sterilized leaves revealed that BRT98 remained on leaf surfaces exclusively and showed no evidence of sub-epidermal colonization. In contrast, Pss strain B728a populations increased considerably on bean under low and high humidity, but declined to undetectable levels on maize within 72 h. Interestingly, on bean leaves, B728a colonized sub-epidermal sites exclusively and showed little to no evidence of leaf surface colonization. B728a cells that were present on leaf surfaces in 48 h following inoculation did not react with alginate or levan specific antibodies. In contrast, B728a from the sub-epidermal sites were intensively labeled with antibody specific to alginate, but not with antibody specific to levan. These studies illustrate that Pss and P. agglomerans exhibit distinct leaf colonization patterns, and provide evidence that Pss produces alginate but not levan during its colonization of strictly sub-epidermal sites.