N-terminal region of cysteine-rich protein (CRP) in carlaviruses is involved in the determination of symptom types

Plant viruses in the genus Carlavirus include more than 65 members. Plants infected with carlaviruses exhibit various symptoms, including leaf malformation and plant stunting. Cysteine-rich protein (CRP) encoded by carlaviruses has been reported to be a pathogenicity determinant. Carlavirus CRPs contain two motifs in their central part: a nuclear localization signal (NLS) and a zinc finger motif (ZF). In addition to these two conserved motifs, carlavirus CRPs possess highly divergent, N-terminal, 34 amino acid residues with unknown function. In this study, to analyse the role of these distinct domains, we tested six carlavirus CRPs for their RNA silencing suppressor activity, ability to enhance the pathogenicity of a heterologous virus and effects on virus accumulation levels. Although all six tested carlavirus CRPs showed RNA silencing suppressor activity at similar levels, symptoms induced by the Potato virus X (PVX) heterogeneous system exhibited two different patterns: leaf malformation and whole-plant stunting. The expression of each carlavirus CRP enhanced PVX accumulation levels, which were not correlated with symptom patterns. PVX-expressing CRP with mutations in either NLS or ZF did not induce symptoms, suggesting that both motifs play critical roles in symptom expression. Further analysis using chimeric CRPs, in which the N-terminal region was replaced with the corresponding region of another CRP, suggested that the N-terminal region of carlavirus CRPs determined the exhibited symptom types. The up-regulation of a plant gene upp-L, which has been reported in a previous study, was also observed in this study; however, the expression level was not responsible for symptom types.