Influence of an arbuscular mycorrhizal fungus on Pseudomonas fluorescens DF57 in rhizosphere and hyphosphere soil

The influence of Glomus intraradices (BEG87) on Pseudomonas fluorescens DF5 7 in hyphosphere and rhizosphere soil was examined. Cucumis sativus (Aminex , F-1 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. flu orescens DF57 were used. Strain DF57-P2, which has an insertion of Tn5:: lu xAB in a phosphate starvation-inducible locus, was used as a phosphate star vation reporter. Another lux-tagged strain DF57-40E7, which carries a const itutively expressed luxAB fusion, was used as control for strain DF57-P2 an d 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. intr aradices decreased the culturability of P. fluorescens DF57 significantly, both in rhizosphere and hyphosphere soil, whereas the total number of P. fl uorescens DF57 measured by immunofluorescence microscopy was decreased in h yphosphere soil only. G. intraradices did not induce a phosphorus starvatio n response in P. fluorescens DF57, and the metabolic activity of the bacter ia was not affected by the fungus after 48 h. P. fluorescens DF57 did not a ttach to G. intraradices hyphae and was not able to use the hyphae as carbo n substrate. The negative effect of G. intraradices on culturability and on number of P. fluorescens DF57 in hyphosphere soil is discussed.