An increasing number of studies have demonstrated that arbuscular mycorrhizal fungi can cooperate with other soil microorganisms, e.g., bacteria, which develop near or on the surface of the extraradical hyphae where they perform multiple functions. However, the mechanisms involved in this privileged relationship are still poorly known. In the present study, we investigated how the arbuscular mycorrhizal fungus Rhizophagus irregularis MUCL 43194 influences the three pace-making enzymes (i.e., citrate synthase, isocitrate dehydrogenase, and α-oxoglutarate dehydrogenase) of the tricarboxylic acid (TCA) cycle in the phosphate-solubilizing bacterium Rahnella aquatilis HX2. The study was conducted under strict in vitro culture conditions and analysis made at the transcriptional level. Results showed that R. irregularis induced the expression of the gene-encoding citrate synthase (gltA), the pace-making enzyme involved in the first step of the TCA cycle, in R. aquatilis at all time points of observation (i.e., 1, 6, 12, 24, 48, and 72 h). The expression of the gene-encoding isocitrate dehydrogenase (icd) significantly decreased at 6, 12, 24, 48, and 72 h and the expression of the gene-encoding α-oxoglutarate dehydrogenase E1 component (kgdhc) significantly increased at 1, 6, and 48 h. The above results suggested that R. irregularis may influence the level of adenosine triphosphate production in R. aquatilis and thus the metabolism of the bacterium by stimulating the expression of gltA involved in the TCA cycle. Our results suggest a fine-tuned dialog between R. irregularis MUCL 43194 and R. aquatilis HX2 and emphasize the complexity of the interactions that might take place at the hyphal surface of arbuscular mycorrhizal fungi hosting communities of microbes.
Keywords: Citrate synthase; In vitro culture; Isocitrate dehydrogenase; Tricarboxylic acid cycle; α-Oxoglutarate dehydrogenase.