Recombination of strain O segments to HCpro‐encoding sequence of strain N of Potato virus Y modulates necrosis induced in tobacco and in potatoes carrying resistance genes Ny or Nc

Hypersensitive resistance (HR) to strains O and C of Potato virus Y (PVY, genus Potyvirus) is conferred by potato genes Ny_tbr and Nc_tbr, respectively; however, PVY N strains overcome these resistance genes. The viral helper component proteinases (HCpro, 456 amino acids) from PVY^N and PVY^O are distinguished by an eight‐amino‐acid signature sequence, causing HCpro to fold into alternative conformations. Substitution of only two residues (K269R and R270K) of the eight‐amino‐acid signature in PVY^N HCpro was needed to convert the three‐dimensional (3D) model of PVY^N HCpro to a PVY^O‐like conformation and render PVY^N avirulent in the presence of Ny_tbr, whereas four amino acid substitutions were necessary to change PVY^O HCpro to a PVY^N‐like conformation. Hence, the HCpro conformation rather than other features ascribed to the sequence were essential for recognition by Ny_tbr. The 3D model of PVY^C HCpro closely resembled PVY^O, but differed from PVY^N HCpro. HCpro of all strains was structurally similar to β‐catenin. Sixteen PVY^N605‐based chimeras were inoculated to potato cv. Pentland Crown (Ny_tbr), King Edward (Nc_tbr) and Pentland Ivory (Ny_tbr/Nc_tbr). Eleven chimeras induced necrotic local lesions and caused no systemic infection, and thus differed from both parental viruses that infected King Edward systemically, and from PVY^N605 that infected Pentland Crown and Pentland Ivory systemically. These 11 chimeras triggered both Ny_tbr and Nc_tbr and, in addition, six induced veinal necrosis in tobacco. Further, specific amino acid residues were found to have an additive impact on necrosis. These results shed new light on the causes of PVY‐related necrotic symptoms in potato.