Igneous phosphate rock solubilization by biofilm-forming mycorrhizobacteria and hyphobacteria associated with Rhizoglomus irregulare DAOM 197198

Biofilm formation on abiotic and biotic surfaces was studied with two hyphobacteria, strongly attached to the surface of the arbuscular mycorrhizal fungus (AMF) Rhizoglomus irregulare (Ri) DAOM 197198 and two mycorrhizobacteria, loosely attached to the roots of different mycorrhizal plants. When the sparingly soluble igneous phosphate rock (PR) from Quebec, or when the chemical hydroxyapatite were used as sole phosphorus (P) source, hyphobacteria Rhizobium miluonense Rm3 and Burkholderia anthina Ba8 produced significantly more biofilms than mycorrhizobacteria Rahnella sp. Rs11 and Burkholderiaphenazinium Bph12, as indicated by the crystal violet assay or by quantifying biofilm exopolysaccharides. As previously observed with planktonic bacteria, biofilms mobilized P by lowering the pH and releasing gluconic acid. The high efficiency of P mobilization by the hyphobacteria Ba8 was linked to the presence of more viable cells in its biofilm as revealed by the hydrolysis of fluorescein diacetate. Scanning electron microscopy micrographs showed a high adherence of the best P-solubilizer hyphobacteria Ba8 on the surface of Quebec PR. Hydroxyapatite porous structure did not allow a good adherence of Ba8. Ba8 formed an important biofilm on the hyphae of Ri DAOM 197198 with low reactive Quebec PR while no biofilm was observed with the high reactive hydroxyapatite. Results confirm the possible presence of specificity between the Ri DAOM 197198 and the hyphobacteria and suggest that the interaction would be regulated by the availability of P

Dissolution biologique des phosphates : interaction bactéries - mycorhizes

Une énorme partie du phosphore (P) soluble ajouté sous forme d’engrais chimiques et de fumiers précipite dans le sol et devient non disponible aux plantes. Par ailleurs, l’utilisation excessive des engrais chimiques n’est pas compatible avec l’agriculture moderne qui se veut durable, ni avec l’agriculture biologique. De plus, ces pratiques ont été effectuées sans tenir compte de la microflore présente au niveau de la mycorhizosphère ce qui conduit à des applications pouvant être onéreuses et néfastes. En effet, les microorganismes bénéfiques du sol, notamment les bactéries solubilisant les phosphates (BSP) et les champignons mycorhiziens arbusculaires (CMA), ont une influence importante sur la fertilité des sols et la productivité végétale. Une nouvelle approche basée sur le piégeage des BSP au niveau de l’hyphosphère du CMA Rhizophagus irregularis (Ri) DAOM 197198, préalablement inoculé avec des suspensions de microorganismes telluriques, a permis d’isoler des BSP compétentes capables de dissoudre efficacement le phosphate de roche (PR) d’origine ignée en milieu liquide. Les travaux ont ensuite permis de prouver l’importance du synergisme entre les hyphobactéries (Burkholderia anthina Ba8 et Rhizobium miluonense Rm3) et le mycélium extraracinaire des CMA Ri dans l’amélioration de la solubilisation des phosphates in vitro. L’étude approfondie des mécanismes qui pourraient être impliquées dans cette interaction montre que les hyphobactéries, principalement la souche B. anthina Ba8, adhèrent fortement à la surface des hyphes et aussi à celle du PR. Il est fortement probable que les interactions décrites ainsi que les caractéristiques bénéfiques aux plantes exprimées par les BSP sont responsables de l’amélioration de la croissance, de la nutrition phosphatée et du rendement en matière fraîche et sèche chez le maïs cultivé en serre, coinoculé avec les BSP et les CMA Ri et fertilisé avec le superphosphate ou le PR du Québec.Soluble phosphorus (P) fertilizers added to soil rapidly precipitate, forming sparingly soluble phosphates, not available to plants. Furthermore, the excessive use of chemical fertilizers to compensate soil P deficiency is not considered sustainable and it leads to costly and potentially harmful applications. Many reports confirmed that beneficial soil microorganisms, including phosphate-solubilizing bacteria (PSB), have a significant influence on soil fertility and crop productivity. Indeed, PSB can also improve phosphate rock (PR) efficiency when directly applied to soil. However, most published works on PSB overlooked the possible interaction between PSB and arbuscular mycorrhizal fungi (AMF), which are ubiquitous in cultivated plants. A new approach based on the trapping of PSB strongly attached to the hyphosphere of AMF Rhizophagus irregularis (Ri) DAOM 197198, previously inoculated with microbial soil suspensions was developed to isolate relevant PSB able to mobilize P from a low reactive igneous PR more efficiently than those directly isolated from the same rhizosphere soil samples. An in vitro study demonstrated that the synergism between hyphobacteria (Burkholderia anthina Ba8 and Rhizobium miluonense Rm3) and Ri hyphae highly improved the solubilisation of PR. Our results go beyond the existing studies and showed specific mechanisms involved on PSB-AMF interactions. Indeed, hyphobacteria, mainly B. anthina Ba8, strongly adhere to Ri hyphal surfaces and PR particles forming a structured biofilm. Under greenhouse conditions, the direct application of PSB and AMF Ri as biostimulants for sustainable corn production showed that these beneficial microorganisms improve growth and P uptake of corn fertilized with superphosphate or Quebec PR

Trapping of phosphate solubilizing bacteria on hyphae of the arbuscular mycorrhizal fungus Rhizophagus irregularis DAOM 197198

A simple method is described for trapping phosphate solubilizing bacteria (PSB) strongly attached to the hyphae of the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis (Ri). Bacteria were isolated from the hyphosphere of mycorrhizal leek plants growing on Turface previously inoculated with soil suspensions, obtained from the mycorrhizosphere of mycorrhizal plants growing in agricultural settings or maple forests in Quebec, Canada. Among the best PSB strongly attached to the hyphae of Ri, 26 isolates belonged to Burkholderia spp. and one was identified as Rhizobium miluonense. Four hyphobacteria exhibiting high potential of inorganic and organic P mobilization were further compared with four equivalent mycorrhizobacteria directly isolated from mycorrhizospheric soils sampled. In general, hyphobacteria were superior in mobilizing P from hydroxyapatite and from a low reactivity igneous phosphate rock from Quebec. Release of gluconic acid or the product of its oxidation 2-ketogluconic acid, are the main mechanisms involved in P solubilization. In a two compartments Petri plate system, Ri extraradical hyphal exudates, supported PSB growth and activity. In the absence of PSB Ri showed a negligible P solubilization activity. In the presence of PSB a substantial increase in P mobilization was observed, and the superiority of hyphobacterial activity was also observed under this system. Our results suggest that in developing a bioinoculant based on selected PSB, their interaction with AMF hyphae should not be overlooked