Surface tip-to-base Ca2+ and H+ ionic fluxes are involved in apical growth and graviperception of the Phycomyces stage I sporangiophore

Abstract

Net fluxes of Ca(2+) and H(+) ions were measured non-invasively close to the surface of Phycomyces blakesleeanus sporangiophores stage I using ion-selective vibrating microelectrodes. The measurements were performed on a wild type (Wt) and a gravitropic mutant A909 kept in either vertical or tilted orientation. Microelectrodes were positioned 4 μm from the surface of sporangiophore, and ion fluxes were recorded from the apical (0-20 μm) and subapical (50-100 μm) regions. The magnitude and direction of ionic fluxes measured were dependent on the distance from the tip along the growing zone of sporangiophore. Vertically oriented sporangiophores displayed characteristic tip-to-base ion fluxes patterns. Ca(2+) and H(+) fluxes recorded from apical region of Wt sporangiophores were inward-directed, while ion fluxes from subapical locations occurred in both directions. In contrast to Wt, mutant A909 showed opposite (outward) direction of Ca(2+) fluxes and reduced H(+) influxes in the apical region. Following gravistimulation, the magnitude and direction of ionic fluxes were altered. Wt sporangiophore exhibited oppositely directed fluxes on the lower (influx) and the upper (efflux) sides of the cell, while mutant A909 did not show such patterns. A variable elongation growth in vertical position and reduced growth rate upon gravistimulation were observed in both strains. The data show that tip-growing sporangiophores exhibit a tip-to-base ion flux pattern which changes characteristically upon gravistimulation in Wt in contrast to the mutant A909 with a strongly reduced gravitropic response.

Fungal evolution: cellular, genomic and metabolic complexity

“…Gravity perception in fungi has been studied extensively in Phycomyces blakeensianus (Mucorales), where it is mediated by a combination of statoliths made of oxalate crystals and buoyant lipid structures (Schimek et al ., ; Eibel et al ., ; Göttig & Galland, ). Gravitropism in Phycomyces is apparently mediated by a differential flux of H + and Ca 2+ in the mycelium (Živanović, , , ; Göttig & Galland, ). However, statoliths in this fungus seem to have originated from a recent bacterial gene transfer (Nguyen et al ., ).…”

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses

“…). Such considerations appear relevant, particularly in view of the observation that gravireception of the sporangiophore correlates with specific ion fluxes in the growing zone (Živanović , ).…”

“…The ion fluxes correlate with gravitropic bending because the flux directions are altered after gravitropic stimulation. The observed reactions must be specific for gravitropism because they were altered in a gravitropism mutant with a defective madJ gene (Živanović ). The combined results suggest that growth and gravitropic curvature require participation of the cytoskeleton, and entail a redistribution of Ca 2+ and calmodulin as well as protons.…”

“…Energy of about 10 À16 J could be sufficient to open 10 6 mechanosensitive Ca 2+ channels (Howard et al 1988). Such considerations appear relevant, particularly in view of the observation that gravireception of the sporangiophore correlates with specific ion fluxes in the growing zone ( Zivanovi c 2005( Zivanovi c , 2012.…”

Actin cytoskeleton and organelle movement in the sporangiophore of the zygomycete Phycomyces blakesleeanus

“…In order to test the effects of various cytoskeletal inhibitors on organelle transport and positioning, S1 sporangiophores were picked from the mycelium with forceps and were incubated in the respective inhibitors prepared by diluting stock solutions in weakly buffered artificial pond water (1 mM (2-(N-morpholino) ethanesulfonic acid (MES), 0.1 mM KCl, 1 mM NaCl, 0.1 mM CaCl 2 , pH 6.0 ( Zivanovi c 2005( Zivanovi c , 2012. The final inhibitor concentrations were: 40 lM cytochalasin D for 20 min (CD; Schliwa 1982; 4 mM stock in DMSO); 2.5 lM Latrunculin B for 20 min (Lat B; Spector et al 1983; 250 lM stock in DMSO); 40 lM nocodazole for up to 1 h (Raudaskoski et al 1994; 4 mM stock in DMSO); 2 mM 2,3-butanedione monoxime for 40 min (BDM; Forer & Fabian 2005; 40 mM stock in H 2 O).…”