Early colonization of barley roots by Pseudomonas fluorescens studied by immunofluorescence technique and confocal laser scanning microscopy

Abstract

Highly specific polyclonal antibodies against two Pseudomonas fluorescens strains (DF57 and Ag1), which differed by approximately 10% of their utilizable substrates as tested in Biolog GN plates, were used for in situ labelling of bacterial cells colonizing barley roots grown in sterile soil. By using a confocal laser scanning microscope single bacteria of both strains could be detected on the roots. Seed-inoculated bacteria rapidly colonized the root surface (rhizoplane) by active migration; after 1 day the anterior part of the root was densely covered by bacteria occupying the crevices between epidermal root cells. As the roots became longer, this bacterial population in the rhizoplane formed long strings of closely associated cells. After 7 days, however, the rhizoplane population of string-forming cells was partially detached developing a patchy distribution along the root; a separate population of cells localized in the slime matrix (mucigel) surrounding the root was well developed at this time. Using two different fluorochromes attached to the antibodies, the two strains could be detected simultaneously in co-inoculation experiments. The recordings, however, gave no indications of competition between the two strains during root colonization.