Evaluation of the Antiviral Activity of Nocardiopsis alborubida Actinomycetes against Cucumber Mosaic Virus (CMV) in Squash (Cucurbita pepo L.) Plants

Plant viruses cause significant economic losses and pose a threat to the sustainability of the agricultural sector. Actinomycetes bacteria are a benign, effective, and environmentally friendly method of disease control. An endophytic actinomycete isolate designated as HESN was recovered from Melissa plants (Melissa officinalis L.) and exhibited 99.79% sequence similarity with Nocardiopsis alborubida (Acc no. NR026339.1). In this study, N. alborubida culture filtrate (NCF) was evaluated for its antiviral activity against cucumber mosaic virus (CMV) on squash plants. Treatment of squash plants with NCF under greenhouse conditions, either 24h before or after virus inoculation, effectively controlled CMV infection. Pre-inoculation treatment reduced CMV accumulation by 50.3% (P≤0.01) at 21 days post-inoculation (dpi) with a further increase in suppression reaching 66.9% at 40 dpi compared to the positive control (P≤0.01). Furthermore, foliar treatment of infected squash plants significantly improved total plant chlorophyll and growth parameters compared to the untreated CMV-infected plants. In addition, the oxidative reductive enzymes, peroxidase (POD) and polyphenol oxidase (PPO), exhibited elevated activity at 4 dpi in NCF-treated plants before CMV inoculation. Following 14 days of viral inoculation, PPO activity increased, whereas POD level decreased across all treatments. GC/MS analysis of the NCF revealed that lactic acid 2TMS derivative was the predominant compound, followed by glycerol monostearate, 1-monopalmitin, propanedioic acid, bis (trimethylsilyl) ester, butanedioic acid, glycerol, and 1-hexadecanol. These metabolites produced by N. alborubida demonstrated significant protective and curative effects against CMV infection, providing insights into controlling plant viruses in an environmentally acceptable manner.