Mild to Virulent: Coat Protein Mutations Restore Mosaic Symptom Induction in a Korean PepMV Isolate.

Pepino mosaic virus (PepMV) is a significant threat to global tomato production, with symptom severity varying widely among strains and often leading to significant economic losses. Despite extensive studies on aggressive variants, the molecular determinants of mild symptomatology in field isolates, particularly from Korea, remain underexplored. In this study, we characterized a mildly infecting PepMV isolate from asymptomatic tomato plants during a field survey in Jeonju, South Korea. The full-length genome sequence and phylogenetic analysis classified it as a CH2 strain. A full-length cDNA infectious clone of this isolate was constructed and confirmed to induce no mosaic symptoms in tomato plants. To identify symptom determinants, targeted mutagenesis was performed in the coat protein (CP) open reading frame. Substitution mutations at CP position 236 or combined 6/155 substitutions converted the mild isolate into a severe variant, inducing strong mosaic symptoms and significantly higher viral accumulation (up to tenfold). These results demonstrated that specific CP residues act as key regulators of symptom severity in PepMV CH2 strains and provide defined severe mutants as useful tools for screening resistance in tomatoes. Although the mechanism underlying symptom modulation remains unclear, this work advanced our understanding of molecular differences between mild and severe strains and supported targeted strategies for managing this economically important virus.