Induced resistance in cucumber by oleamide from Streptomyces lydicus JCK-6019 against cucumber Fusarium wilt and damping-off

Oleamide (cis-9-octadecenamide), a primary fatty acid amide derived from oleic acid, is well known in mammalian systems for its roles in neural and immune regulation. However, its function in plant immunity has remained unexplored. In this study, oleamide was identified as the major bioactive metabolite of Streptomyces lydicus JCK-6019 and was characterized as a novel plant resistance inducer (PRI) in cucumber. Although oleamide exhibited no direct antifungal activity against Fusarium oxysporum f. sp. cucumerinum and Rhizoctonia solani AG-4 in vitro, in vivo assays demonstrated strong disease suppression. Preventive application of oleamide achieved 66.28 % control of Fusarium wilt at 0.1 ng/mL, comparable to the commercial PRI Bion (69.77 %), and 80 % control of damping-off at 0.01 ng/mL, whereas Bion was ineffective. Physiological assays revealed that oleamide reduced malondialdehyde accumulation while enhancing peroxidase and polyphenol oxidase activities, indicating activation of antioxidant defense. Gene expression analysis further revealed upregulation of key defense-related genes (PR1, PR2, PR4, WRKY30, WRKY67, ERF003, peroxidase, and cellulose synthase), which are involved in salicylic acid and jasmonic acid/ethylene signaling, transcriptional regulation, reactive oxygen species metabolism, and cell wall strengthening. Collectively, these results indicate that oleamide primes cucumber defense responses rather than acting as a fungitoxic compound. To our knowledge, this is the first report identifying oleamide as a PRI effective against two major soil-borne cucumber diseases. Its nanogram-level efficacy, non-toxic nature, and capacity to activate plant immunity highlight its potential as an eco-friendly alternative to synthetic fungicides for sustainable crop protection.