Colonization pattern of primary tomato roots by Pseudomonas fluorescens A6RI characterized by dilution plating, flow cytometry, fluorescence, confocal and scanning electron microscopy.

Early colonization of primary tomato roots, grown in vitro, by Pseudomonas fluorescens A6RI, introduced by seed bacterization, was monitored for 7 days in three different root zones (zone A, apex+elongation+young hairy zone; zone B, hairy zone; zone C, old hairy zone+collar). Bacterial quantification was assessed by enumeration of (i) colony forming units (cfu) after dilution plating and of (ii) total bacterial cells by flow cytometry. Bacterial distribution and organization in the root zones were analyzed by fluorescence, confocal and scanning electron microscopy. For all sampling dates and zones, the densities of total bacterial cells were significantly higher than those of the cfu. The kinetics of cfu densities varied according to the root zone. Their density decreased with time in zone A, while no variation with time was recorded in zones B and C. Densities of total bacterial cells did not show any significant temporal variation for any of the root zones. Microscopic analyses allowed the characterization of the distribution and organizational patterns of the bacterial cells according to time and space. In 3-day-old plants, bacteria were mostly present as single cells and were evenly distributed in the two root zones analyzed (A and B). In 5- and 7-day-old plants, distribution and organization differed according to the root zone. In zone A, only few single cells were observed, whereas zones B and C were mostly covered by cells localized between epidermal root cells and organized in pairs and strings, respectively.