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Universität Hannover (1999)

Interaktionen zwischen assoziativen stickstofffixierenden Bakterien und Mykorrhizapilzen

Selle, Tina

Titre : Interaktionen zwischen assoziativen stickstofffixierenden Bakterien und Mykorrhizapilzen mit Nutzpflanzen

Auteur : Selle, Tina

Université de soutenance : Universität Hannover

Grade : Doktor der Naturwissenschaften (Dr. rer. nat.) 1999

Résumé
Using in vitro-cultivation systems the endophytic diazotrophic bacterium Azoarcus sp. BH72 colonized the outer rhizosphere as well as the interior of roots and shoots of the C4 plants sorghum (Sorghum bicolor (L.) Moench) and rice (Oryza sativa L.) and to a lesser extend of the C3 plant wheat. Wheat plants seemingly did not profit from bacterial nitrogen fixation, showing less growth. The association between plant and bacterium was decisively influenced by root exudates which are significantly different in C3 and C4 plants. The chemotactically driven colonization of plant roots was disturbed by carbon supplements to the medium. Obligate endophytic diazotrophic bacteria (Azoarcus sp. BH72 and Herbaspirillum seropedicae) exhibited a larger potential for colonizing the upper parts of sorghum than facultative endophytic organisms (Azospirillum brasilense Sp245). The most extensive colonization was observed with organisms which are already naturally associated. Thus it should be distinguished between “primary“ and “secondary“ host plants. Both obligate endophytes displayed synergistic effects when inoculated together with sorghum seedlings. In monoxenic culture Azoarcus sp. BH72 promoted plant growth stronger than H. seropedicae, but the plant gained the most profit when both bacteria were added to the hydroponic cultures. Specific antibodies allowed an unambiguous identification of Azoarcus sp. BH72. Using antibody-linked markers, colonization of sorghum was veryfied with microscopical techniques. Azoarcus sp. BH72 penetrated into the root interior at the zones of elongation and differentiation as well as at emergence sites of lateral roots. Bacteria were detected in intercellular gaps between rhizodermic cells, in intercellular spaces and inside cells of root and shoot. By means of acetylene reduction assay and staining techniques activity of nitrogenase was proven in the cultivation system. Active cells of Azoarcus sp. BH72 were located in the zones of elongation and root hairs of sorghum. The enzyme showed a maximum activity with 5 % O2 in the gas phase. A new hydroponic culture system was used for mycorrhization of sorghum roots with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices. A specific cultivation system was developed that allowed the establishment of an association with sorghum, Azoarcus sp. BH72 and G. intraradices, which was stable for months. Root development was diminished in favor of shoot growth by mycorrhization with G. intraradices. Azoarcus sp. BH72 not only increased the extend of mycorrhization and contributed to the plant´s nitrogen supply but caused a decrease of root mass in monoxenic culture as well. The VAM mobilized nutrients that otherwise were not accessible for the plants. Regarding the nitrogen content, the plant got the most benefit from a joint cultivation with both microorganisms. As a mediator the fungus facilitated the translocation of nitrogen fixed by the bacteria towards the plant. Such a translocation was also observed with bacteria alone and without VAM. Nonetheless, in this case, with sufficient nitrogen in the medium, the total amount of nitrogen in the plant was not significantly increased as compared with control plants. On the other hand, when nitrogen was depleted (in hydroponic culture), the nitrogen content of the plant increased. Azoarcus sp. BH72 and G. intraradices showed competitive behaviour in colonizing sorghum. It is hypothesized that the length of day and night influenced the distribution of translocated nitrogen between root and shoot of the short-day plant sorghum. In the culture of the three organisms synergistic effects occured, but all interactions in total may not be considered as three-sided mutualistic. While plants had a benefit from the mixed culture, no obvious advantage was detected for Azoarcus sp. BH72 in a system with sufficient nitrogen supply. Key words : biological nitrogen fixation, Azoarcus sp. BH72, Sorgh

Mots clés : Azoarcus ; Stickstofffixierung ; Durrhahirse ; Glomus intraradices ; Wechselwirkung

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Page publiée le 11 juin 2008, mise à jour le 3 janvier 2019