A single conserved amino acid in the coat protein gene of pea seed-borne mosaic potyvirus modulates the ability of the virus to move systemically in Chenopodium quinoa.

Two isolates of pea seed-borne mosaic potyvirus, DPD1 and NY, which both infect pea (Pisum sativum) systemically, differ in their ability to move long distance in Chenopodium quinoa. DPD1 spreads to uninoculated leaves, whereas NY is restricted to the inoculated leaves. The NY isolate was found to move from cell to cell infecting all parts of the inoculated leaves, including the petiole. The coat protein (CP) coding region was identified as the determinant of long-distance movement. Virus chimeras containing the CP coding sequence of NY were restricted to inoculated leaves, whereas chimeras containing the CP coding sequence of DPD1 infected C. quinoa systemically. Mutational analysis of the CP demonstrated that changing the serine at position 47 of the NY CP to proline was sufficient to permit systemic spread of the NY(S47P) mutant. The reverse mutant, DPD1(P47S), in which the proline at position 47 of the CP was changed to serine, was restricted to inoculated leaves. The movement characteristics and CP sequences of 10 additional PSbMV isolates were determined. All isolates caused systemic infection in pea. In C. quinoa 6 of the isolates that were restricted to inoculated leaves had a serine at position 47. Two isolates that infected C. quinoa systemically had a proline at position 47. Two isolates, S6 and NEP-1, infected C. quinoa systemically, but had a serine at position 47 of the CP. This shows that although a proline/serine difference at position 47 of the CP determined systemic spread of the isolates DPD1 and NY, this amino acid alone does not govern the spread of PSbMV in C. quinoa.