Elicitors

Elicitors are molecules that trigger defense responses in plants when detected as potential threats. They can originate from invading microbes (pathogen- or microbe-associated molecular patterns), from damaged plant tissue (damage-associated molecular patterns), or from herbivores. Recognition of elicitors by plant surveillance systems activates immune responses that help restrict pathogen growth and limit damage.

Common classes of elicitors include MAMPs/PAMPs, DAMPs and, in some contexts, HAMPs. MAMPs/PAMPs are conserved microbial signals such as bacterial flagellin-derived peptides (for example, flg22) and elongation factor Tu peptides (elf18), as well as fungal chitin fragments. DAMPs are endogenous signals released or exposed after tissue damage, including oligogalacturonides, extracellular ATP, and certain plant peptides like systemin. HAMPs refer to herbivore-associated cues that can also provoke defense responses in some plants.

Perception typically occurs at the cell surface via pattern recognition receptors (PRRs), leading to PAMP-triggered immunity (PTI). Some elicitors are detected inside the cell by specialized receptors, contributing to effector-triggered immunity (ETI) when pathogens deploy suppressive or avirulence factors. PTI and ETI trigger signaling cascades that include calcium fluxes, production of reactive oxygen species, MAP kinase activation, and transcriptional reprogramming. This results in defense gene expression and often the production of antimicrobial compounds, strengthening of cell walls, and other protective measures.

Elicitors have practical uses in agriculture as immunostimulants to enhance crop resistance. Examples include chitosan, beta-glucans, laminarin, and harpin proteins. Effectiveness varies with plant species, developmental stage, and environmental context, and elicitation can involve trade-offs with growth or yield.