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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

Yanping Tian;Jari P.T.Valkonen

  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 Nytbr,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~N 605 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.……