Plants, insects and mammals have a conserved innate immune system that is the first inducible defense against infectious disease, but the exact nature of the signaling molecules in this system remains unclear. In February 28 Nature, Tsuneaki Asai and colleagues from Harvard Medical School, show that in plants the MAP kinase signalling cascade is involved in activation of the innate immune system and confers resistance to both bacterial and fungal pathogens.

Asai et al. developed an Arabidopsis thaliana protoplast transient expression system in which transcription of early defense genes was activated by flagellin — a protein found in the bacterial flagellum. In this system they found a complete plant MAP kinase cascade (MEKK1, MKK4/MKK5 and MPK3/MPK6) and WRKY22/WRKY29 transcription factors that function downstream of the flagellin receptor FLS2 (Nature 2002, 415:977-983).

In addition, they showed that the MAP kinase signaling pathway functioned in response to...