FLG22 and FLS2 As an Activator of Plant Pathogen-Associated Molecular Pattern Immunity

The flagellin peptide flg22 is a potent activator of plant pathogen-associated molecular pattern (PTI)-triggered immunity. In plant cells, flg22 binds the LRR-RLK receptor FLS2, leading to an increase in phosphatidyl serine levels and subsequent PTI responses including stomatal closure. However, the specificity of the flg22-mediated responses remains elusive. Several studies have shown that the flagellin-derived peptides from the rice pathogen Acidovorax avenae (g-Proteobacteria) and the plant commensal Sinorhizobium meliloti (a-Proteobacteria) do not induce PTI responses. Furthermore, transgenic expression of a flagellin-encoding gene from the rice-incompatible strain of Agrobacterium tumefaciens (a-Proteobacteria) does not elicit a strong flg22-dependent response in Arabidopsis thaliana or tomato.

In this study, we show that the flg22-binding region of FLS2 is essential for its ability to recognize the flg22-derived peptides. Moreover, we found that the flg22-binding domain of FLS2 is able to bind the full length flagellin molecule in solution. Using chemical cross-linking and immunoprecipitation, we further demonstrate that the interaction between flg22 and FLS2 in solution results in a pronounced accumulation of the complex in intracellular mobile vesicles. We also provide evidence that a subset of cellular proteins is down-regulated by flg22 treatment, most notably the protein kinases CPD and BR6ox2.

Our data indicate that a conserved core of the 22 amino acid flagellin peptide is sufficient for PTI induction and that flg22 binding to the receptor complex of FLS2 and BAK1 is required for its recognition. This binding is dependent on a steric hindrance caused by the Ser-17 residue of the flg22-derived ligand and on an interplay with other domains within FLS2. In addition, the binding kinetics of flg22-g to FLS2 and BAK1 are slower than for flg22-Pae.


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