Identification of the Molecular Make-Up of the Potato virus Y Strain PVYZ: Genetic Typing of PVYZ-NTN

Potato virus Y (PVY) strains were originally defined by interactions with different resistance genes in standard potato cultivars. Five distinct strain groups are defined that cause local or systemic hypersensitive responses (HRs) in genetic background with a corresponding N gene: PVYO, PVYN, PVYC, PVYZ, and PVYE. The nucleotide sequences of multiple isolates of PVYO and PVYN differ from each other by approximate to 8% along their genomes. Additionally, complete genome sequences of multiple recombinant isolates are composed of segments of parental PVYO and PVYN sequences. Here, we report that recombinant isolate PVY-L26 induces an HR in potato 'Maris Bard' carrying the putative Nz gene, and is not recognized by two other resistance genes, Nc and Ny(tbr). These genetic responses in potato, combined with the inability of PVY-L26 to induce vein necrosis in tobacco, clearly define it as an isolate from the PVYZ strain group and provide the first information on genome structure and sequence of PVYZ. The genome of PVY-L26 displays typical features of European NTN-type isolates with three recombinant junctions (PVYEU-NTN), and the PVY-L26 is named PVYZ-NTN. Three typical PVYNTN isolates and two PVYN isolates, all inducing vein necrosis in tobacco, were compared with PVY-L26. One PVYNTN isolate elicited HR reactions in Maris Bard, similar to PVY-L26, while two induced a severe systemic HR-like reaction quite different from the quasi-symptomless reaction induced by two PVYN isolates. 'Yukon Gold' potato from North America produced HR against several PVYNTN isolates, including PVY-L26, but only late and limited systemic necrosis against one PVYN isolate. Consequently, according to symptoms in potato indicators, both PVYZ and PVYNTN isolates appeared biologically very close and clearly distinct from PVYO and PVYN strain groups.