CELLULAR INTERACTIONS BETWEEN ARBUSCULAR MYCORRHIZAL FUNGI AND RHIZOSPHERE BACTERIA

We have investigated whether direct physical interactions occur betwee n arbuscular mycorrhizal (AM) fungi and plant growth promoting rhizoba cteria (PGPRs), some of which are used as biocontrol agents. Attachmen t of rhizobia and pseudomonads to the spores and fungal mycelium of Gi gaspora margarita has been assessed in vitro and visualized by a combi nation of electron and confocal microscopy. The results showed that bo th rhizobia and pseudomonads adhere to spores and hyphae of AM fungi g erminated under sterile conditions, although the degree of attachment depended upon the strain. Pseudomonas fluorescens strain WCS 365 and R hizobium leguminosarum strains B556 and 3841 were the most effective c olonizers. Extracellular material of bacterial origin containing cellu lose produced around the attached bacteria may mediate fungal/bacteria l interactions. These results suggest that antagonistic and synergisti c interactions between AM fungi and rhizosphere bacteria may be mediat ed by soluble factors or physical contact. They also support the view that AM fungi are a vehicle for the colonization of plant roots by soi l rhizobacteria.