Umbravirus gene expression helps Potato leafroll virus to invade mesophylltissues and to be transmitted mechanically between plants

Potato leafroll virus (PLRV) was mechanically transmissible when inocula al so contained the umbravirus Pea enation mosaic virus-2 (PEMV-2). In plants infected with PLRV and PEMV-2, PLRV accumulated in clusters of mesophyll ce lls in both inoculated and systemically infected leaves. No transmissions w ere obtained by coinoculation with Potato virus Y, Potato virus X (PVX), To bacco mosaic virus, or Cucumber mosaic virus (CMV), although PLRV was trans missible from mixtures with CMV(ORF4) (a recombinant that contained the mov ement protein (MP) gene of the umbravirus Groundnut rosette virus (GRV) in place of the CMV IMP gene). In contrast, neither a recombinant PVX that exp ressed GRV MP nor a mutant of CMV(ORF4), in which the CMV 2b gene was untra nslatable, was able to help PLRV transmission. Possibly both a cell-to-cell movement function and counterdefense mechanisms such as those that block p osttranscriptional gene silencing are involved in movement of PLRV within p lants and its mechanical transmission between plants. (C) 2001 Academic Pre ss.