title: Comparative analysis of Ca2+ and H+ flux magnitude and location along growing hyphae of Saprolegnia ferax and Neurospora crassa.
abstract: Calcium and proton ion fluxes were mapped at the growing apices of two hyphal organisms, the oomycete Saprolegnia ferax and the ascomycete Neurospora crassa and pseudohyphal Saccharomyces cerevisiae using self-referencing ion-selective probes. S. ferax exhibited well-defined transport zones absent in N. crassa. Ca2+ fluxes were located within 8 microm of the growing hyphal tip; the net Ca2+ flux was either inward (75% of all experiments) or outward. The inward component of the net flux was inhibited by Gd3+, known to inhibit Ca2+ permeable stretch-activated channels. Because the Ca2+ flux is located at the region of maximal hyphal expansion, exocytosis may contribute to Ca2+ efflux, in addition to the stretch-activated channel mediated influx. Maximal inward H+ flux was observed 10-30 microm behind the hyphal tip where peak mitochondria densities taper off at the onset of a vacuolation zone, presumably due to highly localized H+ cotransporter activity. By contrast, N. crassa exhibited no net Ca2+ flux and a consistently inward H+ flux (93% of all experiments) that was homogeneously distributed up to 60 microm behind the hyphal apex. Both hyphal organisms have similar tip morphology and growth rates, and are reported to have tip-high cytosolic Ca2+ gradients associated with growth. Only S. ferax exhibited tip-localized Ca2+ fluxes and a well defined H+ influx zone just behind the tip. Differences in ecological habitats and cytology--S. ferax is an aquatic organism that grows as a migrating plug of cytoplasm while N. crassa is normally terrestrial with a cytoplasm-rich mycelium and highly active cytoplasmic streaming behind the growing margin--may account for the differences in the 'architecture' of ion transport occurring during the process of tip growth. Net Ca2+ efflux and H+ influx of growing S. cerevisiae pseudohyphae were also measured but localization was not possible due to small cell size.
authors.affiliation: Department of Biology, York University, Toronto, Ontario, Canada. planters@yorku.ca
authors.forename: R R
authors.initials: RR
authors.lastname: Lew
mesh_terms: D002118:Calcium / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D017136:Ion Transport; D009492:Neurospora crassa / Q000254:growth & development / Q000378:metabolism*
chemical_list: D006859:Hydrogen; D002118:Calcium