Mitogen-activated protein kinases (MAPKs) are activated when plants respond to infectious agents such as bacterial flagellin. However, the molecular links between the flagellin-activated receptor kinase, called called FLS2, and downstream MAPK activation have remained unknown. Asai et al. developed an Arabidopsis protoplast transient expression system to identify signaling components of this MAPK cascade. By screening candidate molecules for induction of flagellin target gene expression, the authors identified specific MAPKs, MAPK kinases (MAPKKs), and MAPKK kinases (MAPKKKs) that function downstream of the flagellin-activated receptor. Arabidopsis leaves overexpressing components of this MAPK cascade displayed enhanced resistance to bacterial or fungal infection. The authors propose that this signaling pathway is similar to those regulated by Toll receptors in insect and mammalian immune defense systems.
T. Asai, G. Tena, J. Plotnikova, M. R. Willmann, W.-L. Chiu, L. Gomez-Gomez, T. Boller, F. M. Ausubel, J. Sheen, MAP kinase signalling cascade in Arabidopsis innate immunity. Nature 415, 977-983 (2002). [Online Journal]