HC-Pro and CP genes of a potyvirus facilitate cell-to-cell movement and are involved in the systemic movement of the viruses. The interaction between HC-Pro and CP is mandatory for aphid transmission. Two turnip mosaic virus (TuMV) isolates, RC4 and YC5, were collected from calla lily plants in Taiwan. The virus derived from the infectious clone pYC5 cannot move systemically in Chenopodium quinoa plants and lacks aphid transmissibility in Nicotiana benthamiana plants, like the initially isolated virus. Sequence analysis revealed that two amino acids P5 and A206, of YC5 CP uniquely differ from RC4 and other TuMV strains. Recombination assay and site-directed mutagenesis revealed that the fifth residue of leucine (L) at the N-terminal region of CP (TuMV-RC4), instead of proline (P) (TuMV-YC5), is critical to permit the systemic spread in C. quinoa plants. Moreover, the single substitution mutant YC5-CPP5L became aphid transmissible similar to RC4. Fluorescence microscopy revealed that YC5-GFP was restricted in the petioles of inoculated leaves, while YC5-CPP5L-GFP translocated through the petioles of inoculated leaves, main stem, and the petioles of upper uninoculated leaves of C. quinoa plants. In addition, YC5-GUS was blocked at the basal part of the petiole connecting to the main stem of the inoculated C. quinoa plants, while YC5-CPP5L-GFP translocated to the upper leaves. Thus, a single amino acid, the residue L5 at the N-terminal region right before the 6DAG8 motif, is critical for the systemic translocation ability of TuMV in a local-lesion host and for aphid transmissibility in a systemic host.
Keywords: Virology.