Ion Currents in Microbial Growth and Development

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Fungal hyphae, like many other tip-growing organisms, drive electric currents through themselves such that positive charges enter the apical zone and leave distally. Dr. Harold's aim is to discover how these currents arise and what functions they serve, with particular reference to their role in the polarization of growth and development. Hyphae of the water mold Achlya generate a proton current: protons are apparently expelled by a H+-ATPase and return into the apical zone by symport with amino acids. Recent observations suggest that the flow of electric charge is not critical for hyphal extension. However, amino acids must be continuously present for a tip to extend, and so long as a hypha grows protons flow into its tip. Dr. Harold is presently trying to disentangle the intertwined roles of amino acids in tip extension, in generating the transcellular proton current and also in chemotropic growth. This may help in understanding whether the apical influx of protons plays a causal role in tip extension or in guiding its progress. Neurospora, the standard laboratory fungus, also drives a transcellular electric current of the same polarity, but its ionic composition appears to differ from that in Achlya. Tentative evidence suggests that several ionic species flow into the hyphal trunk, possibly by symport with protons. Apical extension, however, seems to require calcium ions and may be linked to a calcium current. One possible interpretation is that calcium ions regulate the exocytosis of apical vesicles and thereby control tip extension. Living cells generate electrical currents and potentials by the active transport of ions across the plasma membrane, and they draw upon the resulting ion circulations to perform diverse kinds of useful work. This proposal addresses the hypothesis that transcellular ion currents, which arise from the spatial segregation of transport systems, play a significant causal role in complex, intergrated biological processes such as growth and development.