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

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 H ₂ O ₂ 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.