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 CaCl 2 solution into which they are initially released, whereas dilution into a solution containing KCl and MgCl 2 in addition to CaCl 2 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 CaCl 2 is as effective as continuous exposure in eliciting rapid germination of the entire zoospore population. ( b ) The effective component of GS is KCl: GS lacking KCl does not elicit rapid germination; conversely, buffered KCl alone is as effective as complete GS in eliciting germination. ( c ) Zoospore populations are sensitive to KCl concentration; as the KCl concentration is reduced, the proportion of cells which undergo rapid germination is also reduced. ( d ) At optimal concentration (5 x 10 -2 M), the following salts are equally as effective as KCl in eliciting germination: KI, KBr, NaCl, CsCl, RbCl, and choline chloride. ( e ) At high concentrations (2.5-5 x 10 -2 M), CaCl 2 and MgCl 2 elicit semi-synchronous conversion of zoospores to round cells, but only after sizeable delays ( v . KCl). Conversion of round cells to germlings does not occur in MgCl 2 and is enormously delayed in CaCl 2 ; when formed, the germ tubes appear abnormal. ( f ) Monovalent cation salts of complex divalent anions (sulphate, tartrate, molybdate, tungstate) also exhibit decreased effectiveness ( v . KCl) in eliciting germination. ( g ) The monovalent cation salts NH 4 Cl and LiCl, the divalent cation salt MnCl 2 , and the non-ionic compound sucrose are all ineffective in eliciting rapid germination. When in combination with an effective elicitor (KCl), LiCl totally blocks germination, MnCl 2 and sucrose lead to significant delays in zoospore to round cell conversion, while NH 4 Cl 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 KCl. 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 KCl 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.