Regulation of aplanospore germination in Vaucheria : Time-dependent responses to calcium modulators.

Germination of aplanospores in Vaucheria longicaulis Hoppaugh var. macounii Blum proceeds through three stages of development. Stage I begins with the initiation of germination and lasts approx. 2 h. During this stage germinating filaments grow at an accelerated rate (266 ± 12 μm · h(-1)). Stage II is characterized by a sharp decline in the growth rate of germinating filaments (96 ± 4 μm · h(-1)) and lasts 4 h. This is followed, during the next 4 h, by a recovery in the growth rate (168 ± 8 μm · h(-1)) of germinating filaments, stage III. Growth rates stabilize and remain unchanged during subsequent development (Oliveira and Fitch, 1988, J. Submicrosc. Cytol. Pathol. 20, 397-406). The Ca(2+)-influx modulators LaCl3, nifedipine and methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4 (2-trifluoromethylphenyl)-pyridine-5-carboxylate (Bay K-8644), the ionophore calcimycin (A23187), the intracellular Ca(2+)-release antagonist 8-N-N'-(diethylamino)-octyl-3,4, 5-trimethoxybenzoate (TMB-8), the Ca(2+)-uptake inhibitor ruthenium red and the phosphoinositide-cycle modulators LiCl and myo-inositol show that the events required for the initiation are distinct from those required for the completion of each stage of germination. These studies in conjunction with microinjection of germinating filaments with inositol 1,4,5-trisphosphate, the natural ligand for Ca(2+) release from Ca-storing organelles (endoplasmic reticulum, vacuole), and treatment with chlorotetracycline (CTC), to visualize the distribution of membrane-bound Ca(2+) reveal that both the initiation and completion of each stage of germination are controlled by Ca(2+) signals which are restricted to well-defined time intervals and are modulated by the origin (source) of Ca(2+).