Elicitor-induced resistance. A brief history The discovery of plant defence stimulators (PDS), also known as natural defence stimulators (NDS), or elicitors, is linked to research on plant defence mechanisms against pests, now jointly understood as components of plant immune systems. The existence of plant immunity was hypothesized in the early 20th century by researchers studying the phenomenon of plant resistance to diseases. In the 1970s, the study of these mechanisms using biochemical methods made it possible to demonstrate that molecules of microbial origin or from phytopathogenic fungi were capable of triggering the production of antimicrobial metabolites in plants. These molecules were thus termed “elicitors.” Today, the term “elicitor” refers more generally to any product that triggers defence reactions. It is synonymous with the term “resistance inducer,” often used in English. In France, the acronym “SDP” (stimulateur des défenses des plantes) has been adopted, notably among crop protection specialists. Using elicitors for plant protection was first proposed in the 1980s. The first PDS product (Bion®, or synthetic origin, released by Ciba-Geigy) was approved for crop protection purposes in the 1990s. In more recent years, numerous publications have reviewed elicitors of various chemical natures (saccharides, proteins, etc.) origins (microbial or plant cell wall materials, phytopathogenic oomycetes, etc.), and mode of action within the plant. The late 1990s and early 2000s saw further research seeking to increase PDS efficacy under production conditions, as well as the commercialization of several natural PDS (using laminarin, chitosans, xxxxxx, or plant extracts), with varying effectiveness. Scientific studies of PDS and their effects continue to be published regularly; but the range of PDS on the market remains limited, as do their practical applications. Plant defences and PDS action The application of external elicitors seeks to imitate a pest or pathogen attack and thus to trigger plant defences artificially, as a preventive measure. Most non-hormonal elicitors are considered to be PAMP and thus should elicit a PTI. It is believed that PAMP are recognized by most plant species, although there are many exceptions. Hormonal elicitors can activate a powerful response similar to that mobilized by an ETI, but this has only been verified in a few cases. Note that microorganisms used in the biological control of diseases often have a mode of action that involves elicitor-induce...
