Minisymposium 9: Plant Pathogen/Symbiont Interactions

Abs # 21001: Innate immunity in plants: Sensing the flagellin peptide, flg22

Presenter:	Robatzek, Silke
Authors	Zipfel, Cyril  (A)   Molteni, Cristina  (A)   Boller, Thomas  (A) (B)  Robatzek, Silke  (A)
Affiliations:	(A): Friedrich Miescher-Institute for Biomedical Research (B): Botanisches Institut, Universität Basel

After decades of neglect, "innate immunity" in animals has recently moved to center stage, with the discovery that a first line of defense against infection is brought about by Toll-like receptors via the recognition of "pathogen-associated molecular patterns" (PAMPs). Conceptually, the classic model of the plants' defense against infection has a very similar basis: Microbial "elicitors" have long been known to activate the first line of defense in plants, and many fungal elicitors, such as chitin and ergosterol, correspond to PAMPs. Focussing on bacteria, we have discovered that plants have a highly specific and sensitive perception system for flagellin and the corresponding peptide flg22, representing its most conserved domain. The FLS2 gene, which is involved in flagellin perception, encodes a receptor kinase with an extracellular LRR and an intracellular kinase domain resembling Toll-like receptors in animals. Plants mutated in the FLS2 gene exhibit insensitivity and lack of binding to flg22. Similarly to flagellin signalling through the TLR5 receptor in mammals, further transmission of the flg22 signal involves phosphorylation of the receptor itself and downstream activation of a MAP kinase cascade. In order to elucidate the role of flagellin perception in general resistance, we examined changes in gene expression after flg22 stimulation, using microarray chips of Affymetrix. Gene expression is massively changed in wildtype seedlings challenged for 30 min with flg22, whereas fls2 mutants show no affect of flg22 treatment on gene expression. Interestingly, among the flg22-induced genes are many encoding transcription factors and other components of signalling cascades including many other receptor-like kinases, and a considerable number of resistance genes. Thus, the flg22 stimulus may increase the "awareness" of the plant cell for other stimuli such as the avirulence gene products recognized by the resistance genes products. Indeed, although flagellin does not induce a HR-like cell death, wildtype plants treated with flg22 are more resistant to a virulent bacterial pathogen, Pseudomonas syringae pv. tomato DC3000, than are naïve controls. Plants carrying the fls2 mutation remain fully susceptible after flg22 treatment.