Signal-Mediated Depolymerization of Actin in Pollen during the Self-Incompatibility Response

the inhibition of incompatible pollen. SI also stimulates dramatic alterations in the actin cytoskeleton. By measuring the amount of filamentous (F-) actin in pollen before and during the SI response, we demonstrate that SI stimulates a rapid and large reduction in F-actin level that is sustained for at least 1 h. This represents quantitative evidence for stimulus-mediated depolymerization of F-actin in plant cells by a defined biological stimulus. Surprisingly, there are remarkably few examples of sustained reductions in F-actin levels stimulated by a biologically relevant ligand. Actin depolymerization also was achieved in pollen by treatments that increase cytosolic free Ca 2 � artificially, providing evidence that actin is a target for the Ca 2 � signals triggered by the SI response. By determining the cellular concentrations and binding constants for native profilin from poppy pollen, we show that profilin has Ca 2 � -dependent monomeric actinsequestering activity. Although profilin is likely to contribute to stimulus-mediated actin depolymerization, our data suggest a role for additional actin binding proteins. We propose that Ca 2 � -mediated depolymerization of F-actin may