Self incompatibility (SI) is one of the most important mechanisms for promoting outbreeding in flowering plants. It prevents self-fertilization through a genetically controlled cell-cell recognition system [1,2]. My lab studies SI in Papaver rhoeas (the field poppy), where SI is mediated by an interaction between the pistil S-gene product when it meets “self” (incompatible) pollen. This triggers a Ca²⁺-dependent signalling cascade in incompatible pollen, resulting in inhibition of pollen tube growth.
The SI-specific signals involve activation of several protein kinases, including a MAP kinase, p56 [3]. Another phosphoprotein, p26, which is hyper-phosphorylated rapidly in response to SI, has been identified as a soluble inorganic pyrophosphatase [4], which we postulate may play an important role in inhibition of incompatible pollen. Another target of the SI response is the pollen actin cytoskeleton, which undergoes rapid reorganization and depolymerization [5,6]. We have recently demonstrated that programmed cell death (PCD) is triggered by the SI response and that a caspase-like/DEVDase activity is activated by SI in incompatible pollen [7]. Compelling evidence suggests that the caspase-like activity is triggered very early in the signalling cascade, and also that it plays an active role in processes leading to inhibition.
SI-induced PCD appears to involve a number of signalling cascades and targets which recent data suggest may be involved in cross-talk. I will discuss progress on identifying the signals that mediate SI, the involvement of the actin cytoskeleton and PCD.

1. Franklin-Tong, V.E., Franklin, F.C.H. (2003) Trends in Plant Sciences 8, 598-605.
2. Franklin-Tong, V.E. and Franklin, F.C.H. (2003) Phil. Trans Roy. Soc. Lond. B, 358, 1025-32..
3. Rudd, J.J., Osman, K., Franklin, F. C. H., Franklin-Tong V. E. (2003) FEBS Letters 547, 223-227.
4. Rudd, J.J. and Franklin-Tong, V.E. (2003) Journal of Experimental Botany 54, 141-148.
5. Geitmann, A., Snowman, B.N., Emons, A.M.C. and Franklin-Tong, V.E. (2000) Plant Cell 12, 1239-1251.
6. Snowman, B.N., Kovar, D.R., Shevchenko, G., Franklin-Tong, V.E., and Staiger, C.J. (2002) Plant Cell 14, 2613-2626.
7. Thomas, S.G., Franklin-Tong, V.E. (2004) Nature 429, 305-309.