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Aragno, Michel

Nom
Aragno, Michel
Affiliation principale
Email
Michel.Aragno@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/753

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  • Publication
    Accès libre
    Plant growth stage, fertiliser management and bio-inoculation of arbuscular mycorrhizal fungi and plant growth promoting rhizobacteria affect the rhizobacterial community structure in rain-fed wheat fields
    (2006)
    Roesti, David
    ;
    Gaur, Rachna
    ;
    Johri, B. N.
    ;
    Imfeld, G.
    ;
    Sharma, S.
    ;
    Kawaljeet, K.
    ;
    Aragno, Michel 
    The goal of this study was first to assess the dynamics of the bacterial community during a growing season in three Indian rain-fed wheat fields which differ mainly through their fertilizer management and yield and then to study the effects of PGPR/AMF bio-inoculations on the bacterial community structure and wheat growth. The bacterial community structure of the rhizosphere soil (RS) and the rhizoplane/endorhizosphere (RE) was determined by PCR-denaturing gradient gel electrophoresis. Seed treatments consisted of consortia of two PGPR strains alone or combined with AMF or AMF alone. The PGPR strains were Pseudomonas spp. which included some or all of the following plant growth promoting properties: phosphate solubilisation and production of indole-3-acetic acid, siderophores, 1-aminocyclopropane-1-carboxylate deaminase and diacetyl-phloroglucinol. The mycorrhizal inoculum was an indigenous AMF consortium isolated from the field with the lowest level of fertilization and yield. Variation partitioning analysis of the DGGE data indicated a predominant effect of the wheat growth stage (30.4% of the variance, P=0.001) over the type of field (9.0%, P=0.027) on the bacterial community structure in the RE. The impact of plant age in the RS was less than in the RE and the bacterial community structure of the field with the highest input of fertilization was very different from the low input fields. The bio-inoculants induced a significant modification in the bacterial community structure. In the RS, the bacterial consortia explained 28.3% (P=0.001) and the presence of AMF 10.6% (P=0.02) of the variance and the same trend was observed in the RE. Plant yield or grain quality was either increased or remained unaffected. For example, protein content was significantly higher in the treated plants' grain compared to the control plants; maximum values were obtained when the PGPR were co-inoculated with the AMF. The percentage of root colonization by AMF was significantly higher in the treatments containing a mycorrhizal inoculum than in the untreated control and remained unaffected by the PGPR treatments. In conclusion, the wheat rhizobacterial community structure is highly dynamic and influenced by different factors such as the plant's age, the fertilizer input and the type of bio-inoculant. In addition, there is a distance-related effect of the root on the bacterial community. Finally, a combined bio-inoculation of diacetyl-phloroglucinol producing PGPR strains and AMF can synergistically improve the nutritional quality of the grain without negatively affecting mycorrhizal growth.
  • Publication
    Accès libre
    Bacterial community associated with the rhizosphere of wheat: interactions with arbuscular mycorrhizal fungi and selection of plant growth promoting rhizobacteria for the increase of wheat growth and soil health in Indian marginal rainfed fields
    (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.