The influence of Glomus intraradices (BEG87) on Pseudomonas fluorescens DF57 in hyphosphere and rhizosphere
soil was examined. Cucumis sativus (Aminex, F1 hybrid) was grown in symbiosis with the arbuscular mycorrhizal
fungus G. intraradices in PVC tubes, consisting of a central root compartment and two lateral root-free
compartments. Two Tn5-luxAB-marked strains of P. fluorescens DF57 were used. Strain DF57-P2, which has an
insertion of Tn5[ratio ][ratio ]luxAB in a phosphate starvation-inducible locus, was used as a phosphate starvation reporter.
Another lux-tagged strain DF57-40E7, which carries a constitutively expressed luxAB fusion, was used as control
for strain DF57-P2 and for measuring the metabolic activity of P. fluorescens DF57. A strain of P. fluorescens
DF57, which carries a constitutively expressed gfp gene, was used in studies of attachment between the bacteria
and the hyphae. G. intraradices decreased the culturability of P. fluorescens DF57 significantly, both in rhizosphere
and hyphosphere soil, whereas the total number of P. fluorescens DF57 measured by immunofluorescence
microscopy was decreased in hyphosphere soil only. G. intraradices did not induce a phosphorus starvation
response in P. fluorescens DF57, and the metabolic activity of the bacteria was not affected by the fungus after 48
h. P. fluorescens DF57 did not attach to G. intraradices hyphae and was not able to use the hyphae as carbon
substrate. The negative effect of G. intraradices on culturability and on number of P. fluorescens DF57 in
hyphosphere soil is discussed.