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

Abstract

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 ascaroside#18 (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 leaves compared to untreated leaves. Dose-response analysis over the nano- and micromolar range revealed a broad optimum concentration for the control of Pt. Microscopic analysis showed very early partial arrest of the fungus at the pre-appressorial stage, with associated enhanced local accumulation of H2O2 and abortive stoma penetration. Very few fungal structures grew in the area between stomata, and the cells surrounding them did not appear to undergo hypersensitive cell death. The results of this study are consistent with and expand upon earlier research that showed ascr#18 activates plant immunity, which protects plants against pathogens. These findings indicate that ascr#18 has a wide-ranging protective effect even at very low concentrations, and could be developed further as a valuable active ingredient for controlling agronomically important plant diseases and pests.