Blue Light-Induced Changes in Membrane Potential and Intracellular pH of Phycomyces Hyphae

Abstract

The electric membrane potential, intracellular pH, and their blue light-induced changes in (+) and (−) hyphae of the wild type and in several phototropic mutants of the fungus Phycomyces blakesleeanus (Burgeff) were measured: The membrane potentials of (+) and (−) hyphae are of similar magnitude and strongly respond to changes in extracellular pH (=pH o ). In (+) hyphae we measured a potential of −132 ± 28 mV at pH o 4.4, −173 ± 25 mV at pH o 6.1 and −187 ± 12 mV at pH o 7.3; in (−) hyphae we found −124 ± 21 mV at pH o 4.5, −170 ± 12 mV at pH o 6.0 and −187 ± 15 mV at pH o 7.4. Increasing the K + concentration of the medium causes only a slight depolarization. Diethylstilbestrol, an inhibitor of the plasmalemma H + -ATPase, rapidly depolarizes the membrane potential by 72±5 mV. No significant differences in membrane potential of mad-mutants and the wild type hyphae were found. Blue light induces a hyperpolarization of the membrane potential. In (+) hyphae 38 ± 4 mV was induced with 15 W/m 2 blue light. With 1 MW/M 2 of blue light 41 ±9 mV was induced. With (−) hyphae, however, only 28 ± 3 mV potential could be produced by 15 W/m 2 . No hyperpolarization was induced in the presence of diethylstilbestrol. Addition of the cationophore monensin caused a hyperpolarization in the dark. The single mutants (madA, madB, madC) hyperpolarize from 22 to 23 mV in blue light of 15 W/m 2 . Mad double and triple mutants showed no response. Hyphae of Phycomyces acidify the medium. The rate of acidification in the medium increases in blue light. The pH-sensitive microelectrodes show an intracellular pH of 7.55 ± 0.22 in (+) hyphae and 7.71 ± 0.21 in (−) hyphae. Exposure to blue light for 30 min decreases the intracellular pH of (+) hyphae to 7.25 ± 0.11. This suggests that a decrease of the intracellular pH most likely is the cause of the blue light-induced hyperpolarization of the membrane potential.