Real-time dynamics of peptide ligand–dependent receptor complex formation in planta

In plants, the flagellin and CLAVATA3 signaling pathways act through induced and preassembled receptor complexes, respectively. Monitoring receptor dynamics Plants use structurally related receptor complexes to respond to pathogens and growth signals, for example, using the flagellin (flg) and CLAVATA (CLV) receptors, respectively. Somssich et al. used multiparameter fluorescence imaging spectroscopy (MFIS) to assess the distribution of the receptor proteins and complexes at the membrane and the effect of their respective ligands. MFIS revealed that, before the presence of the bacterial peptide flg22, the receptors were kept apart and that the addition of flg22 triggered first receptor dimerization and then oligomerization of the dimeric complexes. In contrast, the receptors for the meristem-regulating peptide CLV3 existed as complexes before the presence of the ligand, and CLV3 induced their aggregation into membrane subdomains. This study demonstrates the usefulness of MFIS for analyzing receptor dynamics in living plant cells and reveals distinct characteristics of pathogen-sensing and growth-regulating pathways mediated by related receptor complexes. The CLAVATA (CLV) and flagellin (flg) signaling pathways act through peptide ligands and closely related plasma membrane–localized receptor-like kinases (RLKs). The plant peptide CLV3 regulates stem cell homeostasis, whereas the bacterial flg22 peptide elicits defense responses. We applied multiparameter fluorescence imaging spectroscopy (MFIS) to characterize the dynamics of RLK complexes in the presence of ligand in living plant cells expressing receptor proteins fused to fluorescent proteins. We found that the CLV and flg pathways represent two different principles of signal transduction: flg22 first triggered RLK heterodimerization and later assembly into larger complexes through homomerization. In contrast, CLV receptor complexes were preformed, and ligand binding stimulated their clustering. This different behavior likely reflects the nature of these signaling pathways. Pathogen-triggered flg signaling impedes plant growth and development; therefore, receptor complexes are formed only in the presence of ligand. In contrast, CLV3-dependent stem cell homeostasis continuously requires active signaling, and preformation of receptor complexes may facilitate this task.