Abstract: Soybean is a major source of plant protein and oil, yet its production is severely constrained by diverse pathogen infections. Here, we identify eugenol as a pathogen-induced defense metabolite in soybean with broad-spectrum antimicrobial activity against fungal, oomycete, and bacterial pathogens. Pathogen-associated molecular patterns (PAMPs), including Flg22, Chitin, and Elicitin, strongly induced soybean eugenol synthase (GmEGS) genes and promoted eugenol accumulation. Phylogenetic and expression analyses identified five GmEGS genes, among which GmEGS1a, GmEGS1c, and GmEGS2a were strongly induced by PAMPs and pathogen infection. Eugenol inhibited mycelial growth, spore germination, and zoospore release, while transcriptome analyses of Phytophthora sojae and Fusarium graminearum showed disruption of central carbon, amino acid, and membrane-associated metabolic pathways. In soybean, exogenous eugenol induced antioxidant metabolism and protein homeostasis without activating canonical immune responses, consistent with a protective effect associated with its antimicrobial activity rather than canonical immune activation. Overexpression of GmEGS1a or GmEGS1c increased endogenous eugenol accumulation and conferred broad-spectrum disease resistance without detectable growth penalties. Moreover, exogenous eugenol reduced disease severity in soybean, rice, maize, and tomato. Together, our findings establish GmEGS-mediated eugenol biosynthesis as an inducible chemical defense pathway and highlight eugenol as a promising natural antimicrobial compound for sustainable crop protection.

Key words: broad-spectrum antimicrobial, eugenol, GmEGS, PAMPs, soybean