Colonization pattern of the biocontrol strain Pseudomonas chlororaphis MA 342 on barley seeds visualized by using green fluorescent protein

Pseudomonas chlororaphis MA 342 is a potent biocontrol agent that can be us ed against several seed-borne diseases of cereal crops, including net blotc h of barley caused by the fungus Drechslera ter es. In this study, strain M A 342 was tagged with the gfp gene (encoding the green fluorescent protein) in order to study the fate of cells after seed inoculation. The gfp-tagged strain, MA 342G2, had the same biocontrol efficacy as the wild type when i t was applied at high cell concentrations to seeds but was less effective a t lower cell concentrations. By comparing cell counts determined by microsc opy to the number of CFU, we found that the number of culturable cells was significantly lower than the total number of bacteria on seeds which were i noculated and dried for 20 h. Confocal microscopy and epifluorescence stere omicroscopy were used to determine the pattern of MA 342G2 colonization and cell aggregation on barley seeds. Immediately after inoculation of seeds, bacteria were found mainly under the seed glume, and there was no particula r aggregation pattern. However, after the seeds were sown, irregularly dist ributed areas of bacterial aggregation were found, which reflected epiphyti c colonization of glume cells. There was a trend towards bacterial aggregat ion near the embryo but never within the embryo. Bacterial aggregates were regularly found in the groove of each seed formed by the base of the coleop tile and the scutellum. Based on these results, we suggest that h ZA 342 co localizes with the pathogen D. teres, which facilitates the action of the f ungistatic compound(s) produced by this strain.