Umbravirus gene expression helps potato leafroll virus to invade mesophyll tissues and to be transmitted mechanically between plants.

Potato leafroll virus (PLRV) was mechanically transmissible when inocula also contained the umbravirus Pea enation mosaic virus-2 (PEMV-2). In plants infected with PLRV and PEMV-2, PLRV accumulated in clusters of mesophyll cells in both inoculated and systemically infected leaves. No transmissions were obtained by coinoculation with Potato virus Y, Potato virus X (PVX), Tobacco mosaic virus, or Cucumber mosaic virus (CMV), although PLRV was transmissible from mixtures with CMV(ORF4) (a recombinant that contained the movement protein (MP) gene of the umbravirus Groundnut rosette virus (GRV) in place of the CMV MP gene). In contrast, neither a recombinant PVX that expressed GRV MP nor a mutant of CMV(ORF4), in which the CMV 2b gene was untranslatable, was able to help PLRV transmission. Possibly both a cell-to-cell movement function and counterdefense mechanisms such as those that block posttranscriptional gene silencing are involved in movement of PLRV within plants and its mechanical transmission between plants. Copyright 2001 Academic Press.