ABSTRACT
The hypothesis tested in this present study was that the ectomycorrhizosphere
effect on the bacterial community was not root-growth dependent. The impacts of
ectomycorrhizal infection (Pisolithus albus COI007) and a chemical fertilization
in order to reproduce the fungal effect on the root growth were examined on
(i) the structure of bacterial community and (ii) fluorescent pseudomonad and
actinomycete populations in the mycorrhizosphere of Acacia auriculiformis using
both culture-independent and culture-dependent methods. A. auriculiformis plants
were grown in disinfected soil in pots with or without addition of the ectomycorrhizal
fungus and a N/P/K fertilization to reproduce the fungal effect on the root growth
during 4 months and then transferred in 20 l pots filled with a non disinfected
sandy soil. The fungal and fertilizer applications have significantly improved the
plant growth after 4 month culture in the disinfected soil. In the non disinfected
cultural substrate, these positive effects on the plant growth were maintained.
The structures of the total soil microflora were significantly different within
the treatments as revealed from DNA analysis (DGGE). The structure of fluorescent
pseudomonad populations was also affected by fungal and fertilizer applications.
In contrast, no qualitative effect has been recorded for the actinomycete communities
within each treatment but the fungal inoculation signinficantly decreased the number
of actinomycetes compared to the fertilizer application treatment. These results show
that the mycorrhizosphere effect is not root growth dependent but is mainly due to the
presence of the ectomycorrhizal fungus and more particularly to the extramatricial
mycelium. 

Key words: ectomycorrhiza; fluorescent pseudomonad, actinomycete, soil microflora, PCR-DGGE