Streptomyces violaceusniger WZS5–6 suppresses Fusarium oxysporum f. sp. cubense tropical race 4 via antifungal metabolites and host defense induction
Introduction Fusarium wilt of banana (FWB), caused by Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4), poses a serious threat to the safety and sustainable development of the banana industry. Biological control represents one of the most environmentally friendly approaches for managing this disease. Methods In this study, Streptomyces violaceusniger WZS5–6 antifungal activity against Foc TR4 has been investigated through an integrated approach combining antifungal assays, genome analysis, and metabolomic profiling. For the purpose, the effects of the bacterial strain and its cell-free extract on morphological and ultrastructural changes on pathogenic fungal hyphae and spores were assessed using scanning and transmission electron microscopy. LC–MS analysis was used to identify the metabolites responsible for antifungal activity. We further explored the potential of S. violaceusniger WZS5–6 against Foc TR4 through in planta validation. Results Streptomyces violaceusniger WZS5–6 exhibited a strong inhibition rate of 91.57% on Foc TR4. The cell-free extract obtained from S. violaceusniger WZS5–6 strongly inhibited Foc TR4 with an EC50 value of 91.62 µg·mL-1, indicating the presence of antifungal bioactive metabolites. The results showed that S. violaceusniger WZS5–6 significantly inhibited the mycelial growth of Foc TR4 and induced alterations in spore morphology, mycelial ultrastructure, and cell membrane leakage. Metabolomic profiling of the S. violaceusniger WZS5–6 extracts revealed numerous antifungal metabolites, among which the key metabolites, viz., citronellic acid and furanodienone, exhibited strong inhibitory effects on Foc TR4, with antifungal activity of 61.13% and 57.44%, respectively. Moreover, strain WZS5–6 not only demonstrated 61.54% control efficacy against FWB in a pot experiment but also showed promising growth-promoting effects on banana plants. Discussion This study demonstrates that S. violaceusniger WZS5–6 inhibits Foc TR4 through a multi-level mechanism involving cellular disruption, metabolic adaptation, and activation of host defense responses. These findings highlight the potential of S. violaceusniger WZS5–6 as a promising novel candidate strain to be employed as a biological control agent of FWB.