SIGNAL FOR POTYVIRUS-DEPENDENT APHID TRANSMISSION OF POTATO AUCUBA MOSAIC-VIRUS AND THE EFFECT OF ITS TRANSFER TO POTATO VIRUS-X

A British isolate of potato aucuba mosaic potexvirus (PAMV) was transm itted by aphids (Myzus persicae) which had fed previously on a source of potato Y potyvirus (PVY). Nucleotide sequence analysis of the PAMV coat protein gene indicated that amino acid residues 14 to 16 from the N terminus of the coat protein have the sequence DAG, which is also f ound in the coat proteins of potyviruses and is required for their aph id transmissibility. A recombinant virus isolate (TXPA7) was produced in which a segment of the coat protein gene of PAMV encoding the 40 N- terminal amino acids was inserted in the genome of potato X potexvirus (PVX) in place of the segment encoding the 28 N-terminal amino acids of PVX coat protein. This isolate, and a second similar recombinant (T XPA5) in which the DAG motif was changed to YTS, were mechanically tra nsmissible to intact plants, in which they caused slightly milder symp toms than PVX. Particles of TXPA7 reacted in immunosorbent electron mi croscopy with PVX- and PAMV-specific antibodies and so were antigenica lly distinguishable from PAMV and PVX particles, which reacted only wi th their homologous antibody, and from TXPA5 particles, which reacted only with the PVX antibody. Recombinant TXPA7 was transmitted by aphid s that had already fed on a source of PVY whereas TXPA5 and PVX were n ot. TXPA7 was not transmitted by aphids that had not fed on a PVY sour ce. It is concluded that (i) the potyvirus-dependent aphid transmissib ility of PAMV results from possession of a domain which includes the D AG motif and is located near the N terminus of the virus coat protein, and (ii) potyvirus-dependent aphid transmissibility can be conferred on PVX, a non-aphid-borne potexvirus, by substituting this domain for the N-terminal part of its coat protein.