Zoospore germination in Blastocladiella emersonii. IV. Ion control over cell differentiation.

Zoospore germination in B. emersonii is accompanied by a series of abrupt, dramatic changes in cell structure. Membranes appear to be variously involved in many of these changes. Germination is subject to simple manipulations of the ionic environment: swimming zoospores can be maintained for long periods in the buffered CaCl2 solution into which they are initially released, whereas dilution into a solution containing KC1 and MgCls in addition to CaCl2 results in rapid, semisynchronous germination of entire zoospore populations. The control of germination by ionic means has been characterized in the following ways: (a) Very brief (40 s to 2 min) exposure to GS, followed by replacement with buffered CaCl2, is as effective as continuous exposure in eliciting rapid germination of the entire zoospore population. (b) The effective component of GS is KC1: GS lacking KC1 does not elicit rapid germination; conversely, buffered KC1 alone is as effective as complete GS in eliciting germination. (c) Zoospore populations are sensitive to KC1 concentration; as the KC1 concentration is reduced, the proportion of cells which undergo rapid germination is also reduced. (d) At optimal concentration (5 x io~ M), the following salts are equally as effective as KC1 in eliciting germination: KI, KBr, NaCl, CsCl, RbCl, and choline chloride. (e) At high concentrations (2-5-5 x ~ M ) . CaCla and MgCl, elicit semi-synchronous conversion of zoospores to round cells, but only after sizeable delays (v. KC1). Conversion of round cells to germlings does not occur in MgCl2 and is enormously delayed in CaCl8; when formed, the germ tubes appear abnormal. (/) Monovalent cation salts of complex divalent anions (sulphate, tartrate, molybdate, tungstate) also exhibit decreased effectiveness (v. KC1) in eliciting germination. (g) The monovalent cation salts NH4C1 and LiCl, the divalent cation salt MnCl2, and the non-ionic compound sucrose are all ineffective in eliciting rapid germination. When in combination with an effective elicitor (KC1), LiCl totally blocks germination, MnCl2 and sucrose lead to significant delays in zoospore to round cell conversion, while NH4C1 has no effect on the population kinetics. (h) LiCl can block germination even when added after the completion of the otherwise sufficient early exposure period to GS (see (a) above). The blocking effect of LiCl can be almost completely reversed by replacement with KC1. On the basis of this characterization it is concluded that (1) rapid germination is not elicited simply by osmotic shock; rather, the cells are capable of responding to other (especially ionic) properties of their chemical environment; and (2) while brief exposure to KC1 is sufficient to elicit germination, there are evidently other ion-sensitive steps occurring after the completion of this initial exposure period. Implications of the results in relation to the regular ion selectivity patterns found in other ion-dependent systems, the possible site(s) of action of the eliciting compounds, and the newly discovered 'zoospore maintenance factor' are discussed.