Proton (H⁺) flux signature for the presymbiotic development of the arbuscular mycorrhizal fungi

Ion dynamics are important for cell nutrition and growth in fungi and plants. Here, the focus is on the relationship between the hyphal H⁺ fluxes and the control of presymbiotic growth and host recognition by arbuscular mycorrhizal (AM) fungi. Fluxes of H⁺ around azygopores and along lateral hyphae of Gigaspora margarita during presymbiotic growth, and their regulation by phosphate (P) and sucrose (Suc), were analyzed with an H⁺-specific vibrating probe. Changes in hyphal H⁺ fluxes were followed after induction by root exudates (RE) or by the presence Trifolium repens roots. Differential sensitivity to P-type ATPase inhibitors (orthovanadate or erythrosin B) suggests an asymmetric distribution or activation of H⁺-pump isoforms along the hyphae of the AM fungi. Concentration of P and Suc affected the hyphal H⁺ fluxes and growth rate. However, further increases in H+ efflux and growth rate were observed when the fungus was growing close to clover roots or pretreated with RE. The H⁺ flux data correlate with those from polarized hyphal growth analyses, suggesting that spatial and temporal alterations of the hyphal H⁺fluxes are regulated by nutrient availability and might underlie a pH signaling elicitation by host RE during the early events of the AM symbiosis.