Intracellular calcium and calmodulin involvement in protoplast fusion.

(45)Ca(2+) uptake was compared between fusogenic and nonfusogenic Daucus carota L. protoplasts. Fusogenic protoplasts took 10 minutes to reach calcium equilibrium compared to 5 minutes in the nonfusogenic protoplasts. Intracellular stores of calcium were manipulated by isolating protoplasts in different calcium regimes. Lowering of intracellular calcium lowered fusion potential, while raising intracellular stores of calcium enhanced fusion potential. Regardless of the amount of calcium sequestered in a store, mobilization with A23187 increased fusion levels within 10 minutes. Calmodulin antagonists were potent inhibitors of protoplast fusion. This inhibition was obtained by treating cells with the calmodulin antagonists during protoplast isolation. A23187, however, only allowed a partial recovery from this inhibition, indicating that calcium flux alone was not sufficient for maximum fusion potential. On the basis of the evidence presented, we propose that calcium fluxes during protoplast isolation activate a calmodulin-mediated biochemical process that is necessary for the formation or maintenance of a fusion permissive state.