Transmembrane electric potential difference of germ tubes of arbuscular mycorrhizal fungi responds to external stimuli

Abstract

Measurements of the electric potential difference across the hyphal wall and the cell membrane were made on external hyphae of three species of arbuscular mycorrhizal fungus Gigaspora margarita, Scutellospora calospora and Glomus coronatum and on germ tubes of Gi. margarita. The values of transmembrane electric potential difference recorded ("k40 mV) are less negative than those previously reported from hyphae of arbuscular mycorrhizal fungi closely associated with roots and from filamentous fungi. The external hyphae of arbuscular mycorrhizal fungi grown in soil had similar values of electric potential difference to those grown in soil-less culture, and to germ tubes. Thermodynamic calculations showed that despite these low values of electric potential difference, efficient high-affinity uptake of phosphate is possible. The transmembrane electric potential difference of germ tubes of Gi. margarita became more negative when plant root extract was added to the medium, showing for the first time that the early stages of interaction between plant and fungus occur via direct effects on the plasma membrane rather than via effects on gene expression. Addition of K + reversibly depolarized the transmembrane electric potential difference of germ tubes of Gi. margarita, indicating that despite the low electric potential difference the fungus has control over the permeability of the plasmamembrane to K + .

Ion Dynamics During the Polarized Growth of Arbuscular Mycorrhizal Fungi: From Presymbiosis to Symbiosis

“…These fluxes are correlated with the direction and magnitude of electrical currents described previously for each germination stage of AM spores (Berbara et al 1995). In addition, the electrical membrane potential (Em) across the spore wall from non-germinated spores was found to be more negative than in germinated spores (Ayling et al 2000). On the other hand, Chloride (Cl − ) displays the inverse direction of H + , going out in regions where H + goes in, and vice versa (Fig.…”

“…For instance, measurements of total membrane electric potential carried out by Ayling et al (2000) revealed Em values of germ tubes of G. margarita around −40 mV, a value much lower than −200 mV in Neurospora crassa (Miller et al 1990), −160 mV in Achlya bisexualis (Kropf et al 1984) and −132 mV in pollen tubes (Weisenseel et al 1975). Likewise, Berbara et al (1995), using a one-dimension vibrating voltage probe, measured small total currents (~20 μA cm -2 ) at the tip of germ tubes of G. margarita, which were also much weaker than that found in clover roots and in other fungi (Ayling et al 2000). Such characteristics could be related to the AM fugal H + pumping capacity, since the magnitude of the H + fluxes detected in the AM hyphae was also much lower than that observed in polarized plant cells, such as pollen tubes .…”

“…Electrophysiological studies during the presymbiosis of G. margarita demonstrated that electrical membrane potential (Em) becomes hyperpolarized when plant root extracts were added to the medium (Ayling et al 2000). Actually, root exudates and CO 2 derived from host roots in the vicinity of the AM hyphae have been shown to stimulate the AM hyphal growth at the early stages of fungal development (Bécard and Piché 1989;Nair et al 1991;Bécard et al 1992;Chabot et al 1992).…”

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“…Additional information on the Pi uptake system can be derived from measurements of the transmembrane potential difference (PD) of germ tubes and extraradical hyphae. Values are rather low [around -47 mV, (Ayling et al, 2000)], compared with other organisms, but using this value and an external Pi concentration of 35 µM gives an H + /H 2 PO 4 -stoichiometry of 2.33. This is higher than the most generally accepted ratio of 2.0, but within the range of published values.…”

Root Factors Induce Mitochondrial-Related Gene Expression and Fungal Respiration during the Developmental Switch from Asymbiosis to Presymbiosis in the Arbuscular Mycorrhizal FungusGigaspora rosea

“…In addition, the cytosolic pH was more alkaline when G. rosea was growing in the vicinity of a host root (Jolicoeur et al, 1998). In Gigaspora margarita, transmembrane electric potential difference became more negative when plant root extracts were added to the medium (Ayling et al, 2000).…”