Potato spindle tuber viroid (PSTVd) is the most important subviral pathogen in potato (Solanum tuberosum L.), which causes stunting of the plant and malformation and cracking of tubers. Although potato spindle tuber disease and its causal agent PSTVd have been well characterized, physiological and molecular aspects related to symptom expression in host plants remain largely unknown. In this study, NahG plants and exogenous treatment with a SA homologue, will be used to investigate the role of salicylic acid (SA) and other metabolites in defense responses against PSTVd infection of potato (Solanum tuberosum L.). Metabolomic and gene expression analyses will be complemented by the measurements of viroid accumulation, callose deposition and symptom appearance.