Mechanism of ca2+ inhibition of cytoplasmic streaming in lily pollen tubes

Abstract

Using a Ca2+ ionophore, A23187, the free Ca2+ concentration ([Ca2+]) in the cytoplasm of pollen tubes of Liliutn longiflorum was controlled from the cell exterior. At [Ca2+] higher than 1·0×10−5M (pCa5-0), cytoplasmic streaming was inhibited, and the inhibition was irreversible. The ATP content did not change, but actin filaments were fragmented and formed aggregates. A subsequent decrease in [Ca2+] almost stopped the progress of the actin filament fragmentation, but filamentous actin did not re-form from the fragmented actin. In a previous paper, we reported that pollen tube organelle movement along characean actin bundles was inhibited by Ca2+ at 10−sM levels and the inhibition was reversible. In the present study, the reversibility was also demonstrated using an in situ Ca2+ treatment. Organelles were isolated from pollen tubes that had been treated with high [Ca2+] and A23187. They moved along characean actin bundles in Ca2+-free medium.It is concluded that Ca2+ inhibition of cytoplasmic streaming can be attributed to both inactivation of myosin and fragmentation of actin. The irreversibility of Ca2+ inhibition in situ is attributed to the irreversible fragmentation of actin filaments.