Abstract
Tip-growing organisms maintain an apparently essential tip-high gradient of cytoplasmic Ca2+. In the oomycete Saprolegnia ferax, in pollen tubes and root hairs, the gradient is produced by a tip-localized Ca2+ influx from the external medium. Such a gradient is normally dispensable for Neurospora crassa hyphae, which may maintain their Ca2+ gradient by some form of internal recycling. We localized Ca2+ in N. crassa hyphae at the ultrastructural level using two techniques (a) electron spectroscopic imaging of freeze-dried hyphae and (b) pyroantimoniate precipitation. The results of both methods support the presence of Ca2+ in the wall vesicles and Golgi body equivalents, providing a plausible mechanism for the generation and maintenance of the gradient by Ca2+ shuttling in vesicles to the apex, without exogenous Ca2+ influx. Ca2+ sequestration into the vesicles seems to be dependent on Ca2+–ATPases since cyclopiazonic acid, a specific inhibitor of Ca2+ pumps, eliminated all Ca2+ deposits from the vesicles of N. crassa.
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