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 Lilium longiflorum was controlled from the cell exterior. At [Ca²⁺] higher than 1.0x10⁻⁵M (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 10⁻⁵M 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.