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Perception of PAMPs: A play of hide and seek in plant-microbe interaction G. Felix Higher plants interact with a multitude of microbes in their environment.
In order to respond with physiological meaningful reactions plants must
have ways to detect and distinguish microorganisms as symbionts or potential
pathogens. As other higher eukaryotes, plants can detect potential microbial
invaders with perception systems sensitive to structures characteristic
for fungi or bacteria in general. These pathogen-associated molecular
patterns (PAMPs) activate inflammatory responses in the innate immune
system of animals and act as general elicitors of defence responses in
plants. Fungal structures for which plants have evolved perception systems
include the fungal-specific glycosylation of proteins, the fungal sterol
ergosterol and the cell wall component chitin. Similar to the innate immune
system of animals, plants can also detect structures characteristic for
bacteria like lipopolysaccharides, peptidoglycan or flagellin, the subunit
building the filament of the bacterial flagellum. In our work on
flagellin perception in dicotyledonous plants we could restrict elicitor-activity
to the most conserved domain in the N-terminus of flagellin. The
peptide flg22 representing this domain is fully active as elicitor and
triggers defence responses in the picomolar range. Using genetic and biochemical
approaches we identified the membrane bound receptor kinase FLS2 as an
essential component of flagellin perception in A. thaliana. Mutants lacking
functional FLS2 also lack specific binding of the flg22 ligand, are insensitive
to flagellin and, as observed in recent experiments, exhibit enhanced
susceptibility to infection with pathogenic bacteria. Interestingly, symbiotic
rhizobial bacteria and several (but not all) bacteria pathogenic to plants
show striking sequence divergence in their flg22-epitopes.
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