Potato Virus X (PVX) is the type species of the genus Potexvirus (family Alphaflexiviridae) and is one of the mosaic-causing viruses in almost all varieties of potato. The virus is distributed throughout the world and results in yield reductions of up to 5–20 %. In India, PVX infection may depress yield up to 10–30 % and in the presence of Potato virus A (PVA) or Potato virus Y (PVY) reduces yield up to 40% in potato. It is primarily mechanically transmitted to potato through farm machineries, by contact between potato plants and infected tubers. Infection may be mild in some potato cultivars and is frequentlylatent.PVX virions are flexuous filaments and contain a single positive-sense ssRNA genome of approximately 6435 nts in length in most of the isolates and encode five open reading frames (ORFs). Strains of PVX have been classified into four groups according to their reactions with the dominant resistance genes, Nb and Nx and with the extreme resistance gene, Rx in potato. The study of the interactions with cells, and more generally with the plant, both locally and systemically, significantly contributed to unveil the mechanisms underlying gene silencing, fundamental not only in plant virology but also in the study of gene expression regulation.