Ionic control of germination of Blastocladiella emersonii zoospores

Encystment and germination of Blastocladiella emersonii zoospores involve a rapid and radical transformation of the motile but nongrowing spore into a sessile, growing germling. Certain inorganic ions, notably 50 mM KCl, are efficient inducers of germination. By use of the carbocyanine dye DiO-C6-(3), we found that KCl depolarizes the plasma membrane of zoospores and noted good correlation between depolarization and subsequent germination. Zoospores avidly accumulated K+ ions from the medium, attaining an internal concentration of over 50 mM and a concentration gradient of 2,500. Sodium ions, by contrast, were expelled. Internal K+ was required for normal germination but its function is not known. Zoospores also took up considerable amounts of calcium; most of this was associated with the external surface and appeared to be necessary for maintenance of zoospore integrity. KCl (50 mM) and other salts displaced surface calcium but this was not in itself sufficient to induce germination. The calcium ionophore A23187, in the presence of external calcium, was an effective inducer of germination, suggesting a possible role for cytosolic calcium in triggering the transformation. We propose that the first step in the induction of germination by salts is depolarization of the plasma membrane; subsequent events require the intervention of cytoplasmic signals.