The Nematode Signaling Molecule ascr#18 Induces Prepenetration Defenses in Wheat Against a Leaf Rust Fungus

Abstract Leaf rust, caused by the pathogenic biotrophic rust fungus Puccinia triticina (Pt), is one of the most destructive wheat diseases worldwide; its negative impact on crop yields is exacerbated by increasing temperatures due to climate change. Ascarosides are nematode pheromones that induce resistance to microbial pathogens and pests in a wide range of crops, making them valuable components in biocontrol scenarios. We investigated the effect on infection of various wheat (Triticum aestivum) genotypes with the virulent Pt race 77WxR by ascr#18, the major ascaroside secreted into the rhizosphere by plant-parasitic nematodes. Spraying the leaves with ascr#18 24 hours before inoculation with fungal uredospores slowed disease development and resulted in a reduction of the number of rust pustules on treated compared to untreated leaves. Dose-response analysis over the nano- and micromolar range revealed a broad optimum concentration down to 0.01 nM ascr#18. Microscopic analysis showed very early arrest of the fungus at the appressorial stage, with associated enhanced local accumulation of H2O2 and abortive stoma penetration. The results of this study are consistent with and extend previous research that has shown that ascr#18 activates plant immunity and thus protects plants from pathogens even at very low doses.