Soils are fascinatingly complex systems. Not only because of the chemical processes but the dynamic interactions between countless microorganisms and their role in the soil nutrient cycle. Arbuscular mycorrhizal fungi are the symbiotic partners of 80% of all vascular plants and help with plant nutrient uptake. They can form a network of hyphae around plant roots and grow towards nutrient rich patches in the soil. Bacteria are also crucial in making nutrients available to plants but they are slightly less mobile in the soil.
Drs Feiyan Jiang, Gu Feng and colleagues from the Agricultural University (Bejing) and the James Hutton Institute investigated if a phosphorus (P)-solubilising bacteria can “travel” on a mycorrhizal fungi’s hyphae. The researchers found that indeed, the bacteria can move towards nutrients along the fungi hyphae in Petri dishes and soil experiments but the bacteria cannot transfer without the fungi providing some source of energy.
Jiang, Feng and colleagues used the P-solubilising bacteria, Rahnella aquatilis and the well-studied mycorrhizal fungi, Rhizophagus irregularis for the experiments. The researchers divided Petri dishes into an inoculation compartment for some carrot roots and fungi and a target compartment with nutrients. The two compartments were divided by an air gap so the fungi needed to grow hyphae through it in order to reach the target compartment.
For an experiment, scientists replaced the fungi hyphae with nylon fibre in the air gap to test if the hyphae only provides a physical bridge for the bacteria. To test if bacteria need a water-film layer to move across the hyphae or fibre, the researchers also added some water and different sources of carbon (e.g. glucose). Whilst Rahnella aquatilis is a mobile bacterium, the researchers also used non-flagellated bacteria (Micrococcus luteus) as a comparison of bacterial mobility. Jiang, Feng and colleagues also investigated the bacteria and fungi interactions in soils by burying hyphae transport columns before growing maize plants and adding the mycorrhizal fungi and P. The researchers also investigated different gene activities to capture the underlying mechanisms of the bacteria and fungi interactions throughout the experiments.
Jiang, Feng and colleagues found that water-film and fungi exudates drove the movement of P-solubilising bacteria. The bacterium could not travel on the fibre nylon in the air gap except when the fibre at the bottom of the Petri dish, coated with a source of carbon. The non-flagellated bacterium did not arrive at the target compartment. When the bacteria was transported by the mycorrhizal fungi, more organic P was mineralised in every experiment.
“In the present study, we found that AM [arbuscular mycorrhizal] fungal hyphae could serve as highways for phosphate solubilizing bacteria, leading to the heterogeneous distribution of bacteria according to organic P availability and facilitation of P mineralization”, Jiang, Feng and colleagues wrote.
The gene expression analysis showed that a glucose transporter gene and a key gene for cell division in the P-solubilising bacteria increased when the bacteria was travelling on the fungi hyphae, compared to travelling on the coated fibre nylon. Overall, this study has shown that arbuscular mycorrhizal fungi actively recruit, transport and stimulate the P-solubilising bacteria to mineralise the organic P with benefit to the fungi.
“Our results could be harnessed to better manage plant-microbe interactions and improve the ability of biological inocula involving AM fungi and bacteria to enhance the sustainability of agricultural crops in P limited conditions.”