Systemic movement of an RNA plant virus determined by a point substitution in a 5' leader sequence

The ability of viruses to move through infected plants is an important determinant of host range and pathogenicity. We have investigated the genetic basis for the inability of the Type strain of barley stripe mosaic hordeivirus to undergo long-range systemic movement in the tobacco Nicotiana benthamiana. We show that, in this model system, a short open reading frame in the 5' leader of the smallest viral genomic RNA prevents long-range vascular movement. As predicted by the ribosome scanning model, the leader open reading frame decreases the efficiency with which the 5'-proximal gene is translated in vitro. Thus, systemic pathogenicity in this system may be determined by the efficiency of translation of a viral gene in vivo and is not determined by the primary sequence of the encoded protein.