Applied and Environmental Microbiology, August 1999, p. 3674-3680, Vol. 65, No. 8
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Department of Biochemistry,
Received 25 January 1999/Accepted 25 April 1999
Pseudomonas chlororaphis MA 342 is a potent biocontrol
agent that can be used against several seed-borne diseases of cereal crops, including net blotch of barley caused by the fungus
Drechslera teres. In this study, strain MA 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 it was applied at high cell
concentrations to seeds but was less effective at lower cell
concentrations. By comparing cell counts determined by microscopy 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 inoculated and dried for 20 h. Confocal microscopy and
epifluorescence stereomicroscopy 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 particular aggregation pattern. However, after
the seeds were sown, irregularly distributed areas of bacterial
aggregation were found, which reflected epiphytic colonization of glume
cells. There was a trend towards bacterial aggregation 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 coleoptile and the
scutellum. Based on these results, we suggest that MA 342 colocalizes
with the pathogen D. teres, which facilitates the action of
the fungistatic compound(s) produced by this strain.
*
Corresponding author. Mailing address: Department of
Biochemistry, Arrhenius Laboratories for Natural Sciences, Stockholm University, S-10691 Stockholm, Sweden. Phone: 46 8 162469. Fax: 46 8 153679. E-mail: janet{at}biokemi.su.se.
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