Aragno, Michel

2005, Roesti, David, Aragno, Michel

The objective of this thesis was to improve our knowledge on the interactions between wheat, rhizobacteria and arbuscular mycorrhizal fungi (AMF) in the mycorrhizosphere in order to define criteria for the selection of plant growth promoting bacteria (PGPR) strains in view of a PGPR/AMF dual inoculation in Indian wheat fields. First, microcosm systems were set-up to obtain mycorrhizosphere, AMF-free rhizosphere and root-free hyphosphere zones in order to examine the effects of AMF on the rhizobacterial community in the wheat mycorrhizosphere. The results showed that the bacterial community structure was more influenced by the type of rhizospheric fraction, the plant age and the plant specie than by the presence of AMF. However, the bacterial community was affected indirectly by AMF via a modification in the soil pH. In addition, there was a strong increase in the proportion of phosphate solubilizing bacteria in AMF related zones probably resulting from soluble phosphorus depletion in consequence to AMF phosphorus uptake. Secondly, spores of the arbuscular mycorrhizal fungi Glomus geosporum and G. constrictum were harvested from single spore derived pot cultures with either Plantago lanceolata or Hieracium pilosella as host plants to determine if specific bacterial populations were associated with AMF spores. The bacterial communities associated with the spores were more influenced by the AMF than by the host plant. The majority of the bacterial sequences that were common to both G. geosporum and G. constrictum spores were affiliated to taxonomic groups known to degrade biopolymers. These bacteria were probably feeding on the spore's outer hyaline layer. The third part of the study examined how PGPR strains directly affected AMF growth in the hyphosphere. An in vitro device, consisting of a two-compartmental Petri plate system using Ri T-DNA transformed clover roots permitting the separation of the hyphosphere from the mycorrhizosphere, was designed and tested. Even though the PGPR strains tested were all DAPG producers, their effects on the AMF development varied from inhibition to improvement of the hyphal biomass or spore production. For the fourth part of the study, we had to ensure that before applying the selected PGPR strains P. jessenii R62 and P. synxantha R81 in the fields, they were able to colonize the rhizosphere in situ. They were marked with the green fluorescent protein before testing them in greenhouse pot experiments. R62gfp had colonized the root at a later stage than R81gfp, explaining why the PGP effect of R62gfp was delayed. Both gfp strains were located in the upper part of the root but R81gfp was also detected near the root elongation zone. The fifth part was undertaken in the fields to confirm the positive interactions between AMF and the PGPR strains R62 and R81 and to assess the changes in the wheat bacterial rhizospheric community with respect to field conditions, plant age and PGPR/AMF bio-inoculation. As compared to the bacterial community of the rhizoplane/endorrhizosphere, the bacterial community of the root-adhering rhizospheric soil was more influenced by the field conditions such as an increase in fertilizer input. The bacterial community structure was also dependent on the plant's growth stage. In addition, the type of PGPR consortium had a greater impact on the bacterial community structure than the mycorrhizal colonization. The treatment composed of R62/R81 and an indigenous AMF consortium had not only a positive but also in some aspects, a synergistic effect on plant development.