Dual roles for the variable domain in protein trafficking and host-specific recognition of Heterodera glycines CLE effector proteins

Summary • Soybean cyst nematodes (Heterodera glycines) produce secreted effector proteins that function as peptide mimics of plant CLAVATA3 ⁄ESR (CLE)-like peptides probably involved in the developmental reprogramming of root cells to form specialized feeding cells called syncytia. • The site of action and mechanism of delivery of CLE effectors to host plant cells by the nematode, however, have not been established. In this study, immunologic, genetic and biochemical approaches were used to reveal the localization and site of action of H. glycines-secreted CLE proteins in planta. • We present evidence indicating that the nematode CLE propeptides are delivered to the cytoplasm of syncytial cells, but ultimately function in the apoplast, consistent with their proposed role as ligand mimics of plant CLE peptides. We determined that the nematode 12-amino-acid CLE motif peptide is not sufficient for biological activity in vivo, pointing to an important role for sequences upstream of the CLE motif in function. • Genetic and biochemical analysis confirmed the requirement of the variable domain in planta for host-specific recognition and revealed a novel role in trafficking cytoplasmically delivered CLEs to the apoplast in order to function as ligand mimics.