This study investigates the role of the fungal sterol ergosterol as a general elicitor in the triggering of plant innate immunity in sugar beet. Evidence for this specific function of ergosterol is provided by careful comparison with cholesterol and three plant sterols (stigmasterol, campesterol, sitosterol), which do not enable the integrity of responses leading to elicitation. Our results demonstrate the modification of H(+) flux by ergosterol, due to the direct inhibition of the H(+)-ATPase activity on plasma membrane vesicles purified from leaves. The ergosterol-induced oxidative burst is related to enhanced NADPH-oxidase and superoxide dismutase activities. The similar effects obtained with the fungal elicitor chitosan further reinforce the particular role of ergosterol in the induced defences. The involvement of salicylic acid and/or jasmonic acid signalling in the ergosterol-enhanced plant non-host resistance is also studied. The possible link between ergosterol-triggered plant innate immunity and its putative impact on the structural organization of plant plasma membrane are discussed in terms of the ability of this fungal sterol to promote the formation of lipid rafts.

In wheat, little is known about disease resistance inducers and, more specifically, about the biological activities from those derived from endogenous elicitors, such as oligogalacturonides (OGAs). Therefore, we tested the ability of two fractions of OGAs, with polymerization degrees (DPs) of 2-25, to induce resistance to Blumeria graminis f. sp. tritici and defense responses in wheat. One fraction was unacetylated (OGAs-Ac) whereas the second one was 30% chemically acetylated (OGAs+Ac). Infection level was reduced to 57 and 58% relative to controls when OGAs-Ac and OGAs+Ac, respectively, were sprayed 48 h before inoculation. Activities of various defense-related enzymes were then assayed in noninoculated wheat leaves infiltrated with OGAs. Oxalate oxidase, peroxidase, and lipoxygenase were responsive to both OGAs-Ac and OGAs+Ac, which suggests involvement of reactive oxygen species and oxilipins in OGAs-mediated responses in wheat. In inoculated leaves, both fractions induced a similar increase in H₂O₂ accumulation at the site of fungal penetration. However, only OGAs+Ac led to an increase in papilla-associated fluorescence and to a reduction of formed fungal haustoria. Our work provides the first evidence for elicitation and protection effects of preventive treatments with OGAs in wheat and for new properties of acetylated OGAs.

Ergosterol (a fungal membrane component) was shown to induce transient influx of protons and membrane hyperpolarization in cotyledonary cells of Mimosa pudica L. By contrast, chitosan (a fungal wall component with known elicitor properties) triggered membrane depolarization. In the processes induced by ergosterol, a specific desensitization was observed, since cells did not react to a second ergosterol application but did respond to a chitosan treatment. This comparative study correspondingly shows that ergosterol and chitosan were perceived in a distinct manner by plant cells. Generation of O2*-, visualized by infiltration with nitroblue tetrazolium, was displayed in organs treated with ergosterol and chitosan. This AOS production was preceded by an increase in activity of NADPH oxidase measured in protein extracts of treated cotyledons. In all the previously described processes, cholesterol had no effect, thereby indicating that ergosterol specifically induced these physiological changes known to participate in the reaction chain activated by characteristic elicitors. Contrary to chitosan, ergosterol did not greatly activate secondary metabolism as shown by the small change in content of free phenolics and by the low modification in activity of PAL, the key enzyme of this metabolic pathway. Therefore, future studies have to clarify the signalling cascade triggered by ergosterol recognition.