Flg22 and FLS2 in Arabidopsis


Flg22, an extracellular peptide from the Bacillus thuringiensis flagellum, is known to trigger plant-specific immunity. Heterologous expression of Glycine max (soybean) FLS2 in Arabidopsis restores flg22-induced pathogen-specific immune responses, including stomatal closure. The ectodomain of FLS2 binds to a protein complex composed of the cytoplasmic kinases OST1 and BAK1, resulting in their activation. OST1 is a receptor-like kinase (RLK) that phosphorylates and activates anion channels in gaud cells, allowing for ion efflux and stomatal closure. Interestingly, the flg22-derived LRR of FLS2 also binds to the nonimmunogenic flg22 epitope of the phytopathogen Ralstonia solanacearum, suggesting that distinct recognition mechanisms can evolve in plant pathogens.

We used fluorescence resonance energy transfer-fluorescence lifetime imaging to monitor the interaction between GmFLS2 and BAK1 in a coexpression system. Fluorescence signals from donor proteins fused to GFP and acceptor protein fused to RFP were detected by a Leica TCS SMD FLCS confocal microscope. The binding free energies of each residue were calculated using the BeAtMuSiC program. Single substitutions of flg22Pae/flg22Rso residues were simulated, and the changes in binding free energy upon these mutations were calculated. A possible hydrophobic patch was modelled in the region around I21.

Interestingly, substitution of the GmFLS2 residue associated with flg22Rso AYA region, Q368, to F significantly reduced the responsiveness to flg22Rso (Fig. 3k and Supplementary Fig. 9). Moreover, this substitution significantly decreased oxidative burst triggered by 100 nM flg22-a in A. thaliana Col-0 leaves (Fig. 4E). In seedling growth inhibition assays, 2 and 5 uM flg22-a induced similar reductions in plant growth, indicating that GmFLS2 is an effective inhibitor of the flg22-triggered immune response.


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