Activation of a putative MAP kinase in pollen is stimulated by the self-incompatibility (SI) response

Mitogen-activated protein kinases (MAPKs) operate downstream of receptor-ligand interactions, playing a pivotal role in responses to extracellular signals. The self-incompatibility (SI) response in Papaver rhoeas L. triggers a Ca2+-dependent signalling cascade resulting in inhibition of incompatible pollen. We have investigated the possible involvement of MAPKs in SI. We report the enhanced activation of a 56 kDa protein kinase (p56) in SI-induced pollen and provide evidence that p56 has MAPK activity. This provides an important advance in our understanding of the SI response. We believe this is the first direct biochemical demonstration of activation of a MAPK during SI.