and mosaic chlorosis appear, leading to a palm tree appearance (leaf drop). The viral defense mechanisms of plants will primarily try to restrict the movement of the virus. In failing this, it may attempt to induce cell death in infected tissue, thereby preventing the spread of virions. Although the precise mechanism of disease induction by potyviruses in plants is unknown, it is known that these viruses cause a significant shutdown of host gene expression during viral replication. Physiological changes in potato plants as a response to PVYNTN infection were intensively studied. At early stages of infection, meaning first 12 hours, photosynthesis related genes, genes involved in perception, signalling and defence response were shown to be differentially expressed. 24 h after inoculation the amount of salicylic acid increased. A disruption in gene expression disrupts the normal cellular function of cells which could be the cause of the physical symptoms that the plant demonstrates. At the time of symptoms development, research on interaction between susceptible potato cultivar and PVYNTN showed changes in cytokinin level. In inoculated leaves showing symptoms modifications in chloroplast structure and size, lower chlorophyll levels and differential activity of soluble and ionically-bound peroxidases were detected. At later stages of PVYNTN infection total protein concentration increased in sensitive potato cultivar while no such pronounced changes were observed in tolerant and moderately tolerant potato cultivars. Gene expression studies revealed changes in expression of genes for heat-shock proteins, catalase, β-1,3-glucanase and genes involved in photosynthesis. Molecular description of Potato virus Y Potyvirus virions consist of non-enveloped filamentous structures that are 680 – 900 nm in length and 11 to 15 nm in width. Morphologically the potyvirus capsid consists of approximately 2 000 copies of coat protein (CP). The capsid encapsulates a single strand of positive sense RNA which is in the order of 10 kb in length and has a non-translated 5’-terminal region (5’-NTR) as well as a 3’-poly-A tail. The positive sense genome contains a single extended open reading frame and acts directly as mRNA. The 144 nucleotide 5’-NTR is particularly rich in adenine residues and has very few guanine residues. Rather than a conventional cap structure, the 5’NTR is associated with a Viral genome linked protein (VPg) which is said to act as an enhancer of transcription. The 5’-leader sequence has an internal ribosome entry site (IRES) and cap-independent translation regulatory elements (CIREs). The IRES directs cap-independent translation through a