ER quality control of immune receptors and regulators in plants

Like in animals, cell surface and intracellular receptors mediate immune recognition of potential microbial intruders in plants. Membrane-localized pattern recognition receptors (PRRs) initiate immune responses upon perception of cognate microbe-associated molecular patterns (MAMPs). MAMP-triggered immunity provides a first line of defence that restricts the invasion and propagation of both adapted and non-adapted pathogens. The Leu-rich repeat (LRR) receptor protein kinases (RKs) define a major class of trans-membrane receptors in plants, of which some members are engaged in MAMP recognition and/or defence signalling. The endoplasmic reticulum (ER) quality control (QC) systems monitor N-glycosylation and folding states of the extracellular, ligand-binding LRR domains of LRR-RKs. Recent progress reveals a critical role of evolutionarily conserved ERQC components for different layers of plant immunity. N-glycosylation appears to play a role in ERQC fidelity rather than in ligand binding of LRR-RKs. Moreover, even closely related PRRs show receptor-specific requirements for N-glycosylation. These findings are reminiscent of the earlier defined function of the cytosolic chaperon complex for LRR domain-containing intracellular immune receptors. QC of the LRR domains might provide a basis not only for the maintenance but also for diversification of recognition specificities for immune receptors in plants.