Receptor-like kinases (RLKs) that function as pattern-recognition receptors (PRRs) play a key role in plant immune responses. The receptor recognizing flagellin in Arabidopsis, FLS2, is encoded by a membrane resident RLK. FLS2 is involved in preinvasive immunity against bacterial infection. Recent observations revealed that upon flagellin perception FLS2 accumulates in intracellular mobile vesicles and is then degraded. Reminiscent of ligand-induced receptor endocytosis in animals, FLS2 internalization is Wortmannin-sensitive. Mutation of the potentially phosphorylated residue threonine-867 impaired FLS2 endocytosis and flagellin-triggered responses. Furthermore, mutation of a PEST-motif abolished FLS2 endocytosis and downstream flagellin-elicited responses were affected. Thus, FLS2 endocytosis likely involves phosphorylation and ubiquitination events and appears to be interconnected with flagellin signaling. Similarly, TLR4, the mammalian PRR recognizing bacterial lipopolysaccharides (LPS) is internalized in a ligand specific manner. In this addendum, we discuss endocytic processes of plant RLKs focussing on FLS2 and provide a brief comparison with TLR4 endocytosis.
Keywords: Endocytosis; FLS2; LPS; RLK; TLR4; flagellin.