Different helper component mutations associated with lack of aphid transmissibility in two isolates of potato virus Y

Two potato virus Y (PVY) isolates, PVY-0 NAT and PVY-1, were characterized as aphid-nontransmissible in plant-to-plant transmission tests. Membrane and sequential transmission tests determined that their lack of transmissibility was due to a defect in their helper component (HC) proteins. A comparative analysis of the HC genes of the nontransmissible isolates PVY-0 NAT and PVY-1 with a transmissible isolate, PVY-0, showed that a single amino acid substitution differentiated the HC of isolate PVY-0 NAT from that of isolate PVY-0. The substitution was located within a cysteine-rich region at the amino-terminal region of the protein, in the same position as mutations found in other HC-deficient potyviruses. However, the sort of exchange differed (Lys to Glu for potato virus C and for isolates R5A and Connecticut of zucchini yellow mosaic virus; Lys to Asn for isolate PVY-0 NAT). On the other hand, the HC of isolate PVY-1 had two amino acid substitutions; one of them (Gly to Asp) was also within the cysteine-rich region, and the other (Ser to Gly) was in the carboxy-terminal region of the protein. These aphid-nontransmissible isolates presented different HC-related properties. While the HC protein was detected in partially purified preparations obtained from PVY-0 NAT-infected plants, it could not be detected in equivalent preparations obtained from PVY-1-infected plants, even though the presence of an HC in these plants was demonstrated by serological means. In addition, the HC of this viral isolate showed an electrophoretic behavior distinct from those shown by the HCs of other PVY isolates. The relationships between the nucleotide sequence data and these HC-related properties are discussed.