Perception of PAMPs: A play of hide and seek in plant-microbe interaction

G. Felix
Friedrich Miescher Institut - Maulbeerstrasse 66 - CH-4058 Basel - Suisse

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.
These flagellins are inactive as elicitors in plants and some even act as antagonists of flg22. Whether these changes in sequence are indeed the result of selective pressure to hide from the recogni­tion system of host plants can now be addressed by swapping elicitor-inactive and elicitor-active flagellins and study the effect on symbiosis and pathogenicity of the corresponding bacteria.

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