N-Glycosylation-Mediated Regulation Of Pathogen Recognition

Eukaryotes use an array of cell surface-localized pattern recognition receptors (PRRs) to recognize conserved microbial signatures, known as microbe-/pathogen-associated molecular patterns, or MAMPs/PAMPs. These receptor signals are then integrated and translated into appropriate innate immune responses that provide the first line of inducible defenses and that are fast-acting and effective against the perceived microbe. In addition to their ligand-binding role, plant cell-surface PRR receptor proteins are the first kinases in a kinase signaling pathway involving mitogen-activeted protein kinases (MAPKs) and calcium-dependent protein kinases (CDPKs), and post-translational modifications of the PRR receptor proteins themselves may provide some specificity and quality control to the pathogen recognition process. All secreted, plasma membrane-bound proteins transit through the secretory pathway prior to trafficking to the cell surface, and acquire N-linked glycans en route via a canonical N-glycosylation pathway. PRRs have been shown to contain many N-linked glycans, and N-glycosylation has been shown to be required for the biogenesis, membrane trafficking and ligand recognition of an evolutionarily young PRR receptor protein involved in the perception of bacterial elongation factor Tu. However, the canonical N-glycosylation pathway is not required for the biogenesis, membrane trafficking and ligand recognition of PRRs involved in the perception of bacterial flagellin and fungal/insect chitin, both ancient perception systems generally present in the plant kingdom. Using a functional genomics approach, we identified a subfamily of glycosyltransferase genes that are required for the perception of bacterial flagellin and fungal/insect chitin. We propose that there may be an alternative N-glycosylation pathway in plants specific for a subclass of immune sensors that are on a fast-track through the secretory pathway for pathogen recognition.