Ascaroside#18 Promotes Plant Defence by Repressing Auxin Signalling.

Plant immunity against pathogens is primarily triggered by the perception of pathogen-associated molecular patterns (PAMPs). Ascaroside#18, a nematode-derived pheromone, is the first identified nematode-associated molecular pattern conferring broad-spectrum pathogen resistance. Recently, ascr#18 was shown to be recognised by the leucine-rich repeat receptor NILR1, linked to pattern-triggered immunity (PTI) against nematodes. However, the molecular mechanisms downstream of ascr#18 perception remain largely unknown. Here, we show that ascr#18 triggers an immune response that differs from the typical PTI features, with no reactive oxygen species burst or defence-related growth inhibition. Further analysis indicates that the ascr#18-associated resistance mechanism against cyst nematodes (CN) operates independently of the peroxisomal β-oxidation pathway. Transcriptome profiling of Arabidopsis roots treated with ascr#18 revealed strong effects on the regulation of auxin transport and signalling genes, while classical defence genes remained unchanged. These changes, particularly the downregulation of auxin-related genes, occur independently of NILR1. Analysis of CN feeding sites revealed that ascr#18 pretreatment reduced expression of the auxin influx carrier AUX1 and the auxin-responsive genes SAUR69 and IAA27. Promoter-reporter analysis confirmed reduced AUX1 expression in both nematode-infected and non-infected roots treated with ascr#18. Since nematode establishment and the associated feeding cell development are heavily dependent on the modulation of auxin signalling, our results suggest a novel defence mechanism based on its suppression. This mechanism reduces nematode susceptibility without activating classical PTI responses. Our results provide new insights into how plants fend off biotrophic pathogens and point to ways of developing novel strategies for controlling nematodes and other biotrophic pathogens.