Abstract: Cellular responses rely on signal perception and integration. A nice example of this is self incompatibility (SI), which is an important mechanism to prevent inbreeding. It prevents self-fertilization by using a highly discriminatory cellular recognition and rejection mechanism. Most SI systems are genetically specified by the S-locus, which has a pollen and a pistil S-component. A receptor-ligand interaction is used by Papaver rhoeas to control SI. S proteins encoded by the pistil part of the S-locus interact with incompatible pollen to achieve rapid inhibition of tip growth. The incompatible SI interaction triggers a Ca2+-dependent signaling cascade. A number of SI-specific events are triggered in incompatible pollen, including rapid depolymerization of the actin cytoskeleton; phosphorylation of soluble inorganic pyrophosphatases (SPPases), Prp26.1;activation of a mitogen activated protein kinase, p56; programmed cell death (PCD) involving a caspase-3-like activity. These events contribute to prevent self-fertilization. We are attempting to establish the functional significance of these events, and their possible involvement in integrating a coordinated signaling response. Here we describe the identification of these components shown to be involved in SI, together with recent progress in identifying links between some of them. These data constitute the first steps in elucidating how SI signaling is integrated.

Key words: actin cytoskeleton, mitogen activated protein kinase, pollen tube inhibition, programmed cell death, self-incompatibility, soluble inorganicpyrophosphatases.