Abstract

Roots can release 1-aminocyclopropane-1-carboxylic acid (ACC), which plays a crucial role as a signaling molecule to drive the bacterial community in the rhizosphere. In our frame of fungal highway studies, we hypothesized that ACC, a precursor of the plant hormone ethylene, acts as a key signaling molecule that influences the interaction between Mucor sp. and bacteria, facilitating bacterial dispersal through mycelial networks in the presence of varying ACC concentrations. Here, we evaluated the chemotactic effect of 1-aminocyclopropane-1-carboxylic acid (ACC) on plant-associated bacteria via the mycelium of the fungus Mucor circinelloides 7H. Bacterial strains were subject to chemiotaxis assays via the capillary method and bacterial dispersion systems in combination with flow cytometry. Members of the Pseudomonas sp. and Bacillus sp. 30B strains can be dispersed throughout the mycelia of M. circinelloides 7H. Compared with the positive control Pseudomonas sp. 18E (497.49 +/- 16.9 cell mL-1), Pseudomonas sp. 38E (476.7 +/- 36.7 cell mL-1) presented the greatest degree of cell movement. Chemotaxis assays demonstrated that ACC-degrading Pseudomonas strains exhibited significant movement toward ACC gradients compared with Bacillus sp. 30B. Flow cytometry analysis revealed that the dispersion of Pseudomonas sp. 38E (from 8 x 103 to 4 x 104 cells mL-1) and Pseudomonas sp. 62F (from 5 x 103 to 2 x 104 cells mL-1) increased by one order of magnitude compared with the control without ACC. This result demonstrated that the native fungal strains M. circinelloides 7H can disperse other genera of bacteria different from those originally found in their mycelium. These findings suggest that fungal mycelia use chemical signals to mediate the dispersal of beneficial bacteria in soil.