Streptomyces fungicidicus-derived secondary metabolites as an antiviral agent to alleviate zucchini yellow mosaic virus in squash.

Zucchini yellow mosaic virus (ZYMV) poses a significant threat to squash crops, causing severe symptoms and substantial yield losses. This study investigates the potential of Streptomyces fungicidicus as a biocontrol agent for managing ZYMV in squash plants by inducing systemic resistance. Approximately 95% of field-collected squash samples were positive for ZYMV, exhibiting chlorotic mottling, vein banding, and leaf distortion. The virus was isolated, purified, and confirmed through RT-PCR (Accession no. PV131044) and transmission electron microscopy (TEM), which displayed flexuous, filamentous particles typical of ZYMV. Among the isolated and tested Streptomyces spp., Streptomyces fungicidicus SF1-RSI2 (Accession no. PV489988) exhibited promising antiviral activity. GC-MS analysis of the SF1-RSI2 culture filtrate identified 35 bioactive compounds, with (-)-spathulenol being the most abundant (13.1%), followed by 9-octadecenoic acid methyl ester (9.24%) and triacetin (8.88%), suggesting a complex mixture of metabolites potentially contributing to the observed antiviral effects. Under greenhouse conditions, the foliar application of SF1-RSI2 culture filtrate, either as a pre-ZYMV-inoculation (protective) or post-ZYMV-inoculation (curative) treatment, enhanced plant growth, delayed symptom onset by up to 3 days, and reduced viral accumulation by up to 49.7% at 5 days post-inoculation (dpi) compared to untreated infected plants. The SF1-RSI2 applications significantly mitigated the detrimental effects of ZYMV on plant growth, chlorophyll content, and oxidative stress markers, such as H₂O₂ and malondialdehyde (MDA), while also notably increasing peroxidase enzyme activity and total phenolic content, indicating enhanced systemic resistance. Additionally, it influenced the transcriptional levels of defense-related genes, with protective treatment resulting in the highest expression levels of Cinnamate-4-hydroxylase (C4H), Cinnamate-3-hydroxylase (C3H), and Chalcone synthase (CHS), which are essential for plant defense mechanisms. HPLC analysis revealed a substantial increase in polyphenolic compounds, particularly chlorogenic acid, in plants treated with SF1-RSI2, highlighting its role in strengthening plant defenses. The findings suggest that S. fungicidicus promotes plant growth and enhances defense mechanisms, presenting a viable biocontrol strategy for managing viral infections in squash.