Structural insights into glycan recognition by plant immune receptors: a comparative perspective with animal innate immune systems

Pattern recognition receptors (PRRs) function across diverse eukaryotic organisms as a powerful surveillance system to perceive danger signals and to trigger specific adaptive responses. This complex receptor network constitutes the first layer of perception of the innate immune system of plants and mammals. PRRs specifically detect conserved "non-self" microbe-associated molecular patterns (MAMPs) derived from pathogens, or "self"-derived damage-associated molecular patterns (DAMPs) released or synthesized by the host upon tissue damage or infection. MAMP/DAMP recognition by their specific PRRs thereby activate complex immune signaling pathways. In organisms lacking adaptive immunity, that is, those without specialized immune cells, innate immunity plays a central role, and PRRs are often overrepresented in their genomes. In recent years, many carbohydrate-based MAMPs from microbial outer layers and DAMPs from plant cell walls have been characterized, and their putative plant receptors identified. However, the structural basis of the recognition of these glycans by the extracellular ectodomains (ECDs) of plant PRRs remains poorly characterized, especially when compared with the broader knowledge available for glycan perception by carbohydrate recognition domains (CRDs) of mammalians receptors. In this review, we focus on the crystallized PRR glycan-ligand pairs both in plants and animals and explore whether these binding mechanisms might be conserved across kingdoms. Given the significant potential of glycan-derived DAMPs/MAMPs as elicitors of disease resistance activation, we emphasize the need for further mechanistic and structural studies to clarify how ECD-PRRs engage carbohydrate ligands. We also highlight the importance of such structural knowledge to guide the use of PRRs/oligosaccharide pairs as sustainable alternatives for crop protection.