Involvement of Ca2+ in 1-aminocyclopropane-1-carboxylic acid-stimulated pollen germination

The role of 1-aminocyclopropane-1-carboxylic acid (ACC) in pollen germination was investigated in several plant species. It was found that ACC stimulated in vitro pollen germination in all five species of plants tested. EGTA and phenothiazine inhibited the increase in the germination rate induced by ACC. Free Ca2+ levels in the cytosol ([Ca2+](cyt)) in ungerminated and germinated pollen were 136 and 287 nM, respectively. Adding 0.25 mM ACC to the germination medium increased the [Ca2+](cyt) in germinated pollen up to 450 nM. When pollen was treated with both 0.25 mM ACC and 3.6 mu M inositol 1,4,5-trisphosphate, the [Ca2+](cyt) increased to 850 nM, and pollen germination was also stimulated. In the presence of Li+, an inhibitor of inositol monophosphatase, the [Ca2+](cyt) was reduced to 155 nM, and the ACC-stimulated pollen germination was inhibited. The data provided evidence for the involvement of Ca2+ as a messenger in the stimulative effect of ACC on pollen germination.