A zucchini yellow mosaic virus coat protein gene mutation restores aphid transmissibility but has no effect on multiplication.

An aphid-transmissible (AT) and two non-aphid-transmissible (NAT) isolates of zucchini yellow mosaic virus (ZYMV) were studied. The predicted amino acid sequences of the coat protein (CP) of the three virus isolates were analysed and compared. The NAT isolates differed from the AT isolate in having a Thr instead of an Ala residue at position 10 in the conserved Asp-Ala-Gly triplet in the N-terminal region of CP. Aphid transmissibility was restored in a progeny virus derived from an infectious clone of the ZYMV-NAT isolate in which Thr was changed back to Ala by site-directed mutagenesis. However this mutation did not have any effect on the multiplication rate in squash, which was significantly higher than that of the AT isolate. The involvement of this mutation in aphid transmission and virus multiplication is discussed.