INWARDLY DIRECTED IONIC CURRENTS OF ALLOMYCES-MACROGYNUS AND OTHER WATER MOLDS INDICATE SITES OF PROTON-DRIVEN NUTRIENT TRANSPORT BUT ARE INCIDENTAL TO TIP GROWTH

Electrical currents are generated by tip-growing cells and have been proposed to be important in the establishment and maintenance of cell polarity. Unlike the situation in all other fungi thus far examined, the electrical current of Allomyces macrogynus normally flows out of the hyphal tip, not into it. In this organism, as with another chytrid Blastocladiella emersonii, the electrical current enters the rhizoids. Extracellular ion-selective microelectrodes detected-local acidification of the medium around hyphal apices, but no local accumulation of K+ or Ca2+ at this site. These results suggest that the outward current is generated by H+ efflux, not K+ or Ca2+ efflux. The hyphae of this fungus were found to be weakly chemotropic, in contrast to the rhizoids which were strongly chemotropic. Rhizoid growth was also directed strongly towards the anode of an applied electrical field; the hyphae were directed to the cathode. Chemotropism and galvanotropism are therefore unrelated. We suggest that, as with other chytridomycetes, the rhizoid of A. macrogynus is an organ specializing in nutrient transport. Since transport is often coupled to proton symport the inward electrical current at rhizoids may be the consequence of localized proton-driven solute transport. In hyphae of the oomycete Achlya bisexualis inward current, localized nutrient transport and chemotropic behaviour again are correlated. We conclude that for water moulds, inward currents indicate zones of localized nutrient transport. The current is therefore incidental to the process of tip growth.