Infection with Tomato Mosaic Virus in Nicotiana tabacum cv. Samsun-Physiological and Molecular Approach During Early Stage of Infection.

Although tomato mosaic virus (ToMV) is an economically important tobamovirus, the physiological and molecular events occurring during the asymptomatic phase of infection remain poorly understood. In this study, we investigated the early responses of Nicotiana tabacum cv. Samsun to ToMV infection using an integrated physiological, biochemical, photosynthetic, and molecular approach. Viral accumulation was quantified via RT-PCR. Oxidative stress markers, antioxidant systems, photosynthetic performance, and gene expression were analyzed at 7 days post inoculation (DPI), before visible symptoms developed. Although infected plants remained symptomless, ToMV was detected in 80% of inoculated plants. Early infection induced oxidative stress, evidenced by increased malondialdehyde content, reduced free amino acid levels, and enhanced activities of superoxide dismutase and peroxidase. Total glutathione, phenolic compounds, and phenylpropanoids remained unchanged, whereas flavonoid content decreased significantly. ToMV infection also impaired the photosynthetic apparatus, resulting in reduced chlorophyll and carotenoid contents, decreased electron transport efficiency, and increased energy dissipation within photosystem II. Gene expression analysis revealed significant upregulation of defense- and stress-related genes (WRKY1, HSP70, GR, and DHAR), as well as chloroplast-associated genes (psaA and rbcL). Correlation analyses demonstrated coordinated relationships among viral accumulation, oxidative stress, antioxidant responses, and photosynthetic performance. These findings provide new insights into the asymptomatic phase of ToMV infection and identify potential early markers of host responses to viral infection.