6‐Hydroxynicotinic Acid From <i>Cucumis melo</i> Inhibits Prehaustorium Formation in <i>Phelipanche aegyptiaca</i> via Disruption of Auxin Signalling Pathway

Phelipanche aegyptiaca, a root holoparasitic weed, severely threatens agricultural productivity due to its detrimental effects. This species relies on a specialised organ, namely the haustorium, to extract nutrients from its host plants. The germination and haustorium formation of P. aegyptiaca are initiated by sensing host plant root exudates. Differences in exudate composition are crucial markers of host resistance. Host plant root exudates significantly influence the development and survival of P. aegyptiaca. To identify root exudates affecting the parasitic weed's growth, we analysed differential metabolites in resistant and susceptible Cucumis melo varieties. Among these, 6-hydroxynicotinic acid was identified as a key compound. Prehaustorium formation, which is induced by haustorium-inducing factors, such as indole-3-acetic acid (IAA), was suppressed in the presence of 6-hydroxynicotinic acid. This compound exerts an inhibitory effect by reducing the expression of genes related to the auxin signalling pathway of P. aegyptiaca, thus weakening parasitism. Our results support a model in which 6-hydroxynicotinic acid inhibits prehaustorium development in P. aegyptiaca by disrupting gene expression and endogenous metabolism.