Ion Currents in Microbial Growth and Development

真菌菌丝，像许多其他顶端生长的生物一样，通过自身驱动电流，使正电荷进入顶端区并离开顶端。哈罗德博士的目标是发现这些电流是如何产生的，以及它们所起的作用，特别是它们在生长和发展的两极分化中所起的作用。水霉菌Achlya的菌丝产生质子流：质子显然被H+- atp酶排出，并通过与氨基酸的同调返回到顶端区。最近的观察表明，电荷的流动对菌丝的延伸并不重要。然而，氨基酸必须持续存在才能使尖端延伸，而且只要菌丝生长，质子就会流入其尖端。哈罗德博士目前正试图解开氨基酸在尖端延伸、产生跨细胞质子电流和趋化生长中的相互交织的作用。这可能有助于了解质子的顶流是否在尖端延伸中起因果作用或指导其进展。神经孢子菌，一种标准的实验室真菌，也能驱动相同极性的跨细胞电流，但它的离子组成似乎与Achlya不同。初步证据表明，有几种离子可能通过与质子的同向作用流入菌丝主干。然而，根尖延伸似乎需要钙离子，并可能与钙电流有关。一种可能的解释是钙离子调节根尖囊泡的胞吐，从而控制根尖的伸长。活细胞通过离子在质膜上的主动运输产生电流和电位，并利用由此产生的离子循环来完成各种有用的工作。该建议提出了一个假设，即由运输系统的空间隔离产生的跨细胞离子流在复杂的、综合的生物过程（如生长和发育）中起着重要的因果作用。

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.