Alterations in the actin cytoskeleton of pollen tubes are induced by the self-incompatibility reaction in Papaver rhoeas

Self-incompatibility (SI) is a genetically controlled process used to preve nt self-pollination. In Papaver rhoeas, the induction of SI is triggered by a Ca2+-dependent signaling pathway that results in the rapid and S allele- specific inhibition of pollen tube tip growth. Tip growth of cells is depen dent on a functioning actin cytoskeleton. We have investigated the effect o f self-incompatibility (S) proteins on the actin cytoskeleton in poppy poll en tubes. Here, we report that the actin cytoskeleton of incompatible polle n tubes is rapidly and dramatically rearranged during the SI response, not only in our in vitro SI system but also in vivo. We demonstrate that nonspe cific inhibition of growth does not result in similar actin rearrangements. Because the SI-induced alterations are not observed if growth stops, this clearly demonstrates that these alterations are triggered by the SI signali ng cascade rather than merely resulting from the consequent inhibition of g rowth. We establish a detailed time course of events and discuss the mechan isms that might be involved. Our data strongly implicate a role for the act in cytoskeleton as a target for signaling pathways involved in the SI respo nse of P. rhoeas.