ABSTRACT One striking feature of viruses with RNA genomes is the modification of the host membrane structure during early infection. This process requires both virus- and host-encoded proteins; however, the host factors involved and their role in this process remain largely unknown. On infection with Tobacco mosaic virus (TMV), a positive-strand RNA virus, the filamentous and tubular endoplasmic reticulum (ER) converts to aggregations at the early stage and returns to filamentous at the late infectious stage, termed the ER transition. Also, membrane- or vesicle-packaged viral replication complexes (VRCs) are induced early during infection. We used microarray assays to screen the Arabidopsis thaliana gene(s) responding to infection with TMV in the initial infection stage and identified an Arabidopsis gene, PAP85 (annotated as a vicilin-like seed storage protein), with upregulated expression during 0.5 to 6 h of TMV infection. TMV accumulation was reduced in pap85 -RNA interference (RNAi) Arabidopsis and restored to wild-type levels when PAP85 was overexpressed in pap85 -RNAi Arabidopsis . We did not observe the ER transition in TMV-infected PAP85 -knockdown Arabidopsis protoplasts. In addition, TMV accumulation was reduced in PAP85 -knockdown protoplasts. VRC accumulation was reduced, but not significantly ( P = 0.06), in PAP85 -knockdown protoplasts. Coexpression of PAP85 and the TMV main replicase (P126), but not their expression alone in Arabidopsis protoplasts, could induce ER aggregations.