Transformation of tobacco and potato with cDNA encoding the full-length genome of Potato leafroll virus: evidence for a novel virus distribution and host effects on virus multiplication

A full-length cDNA copy of the genome of Potato leafroll virus (PLRV) was introduced into the genome of tobacco and potato plants by Agrobacterium tumefaciens-mediated transformation. Transgenic lines were obtained in which the transgene was readily detected by PCR with DNA extracted from T1 tobacco seedlings and clonally multiplied potato plants. PLRV-specific genomic and sub-genomic RNAs, coat protein antigen and virus particles were detected in transgenic plants. Aphids fed on the transgenic tobacco plants readily transmitted PLRV to test plants. Infected transgenic tobacco plants, like non-transgenic (WT) PLRV-infected plants, displayed no symptoms of the infection but transgenic plants of potato were severely stunted. In parallel tests, the mean PLRV titres in WT tobacco plants and transgenic tobacco plants were 600 and 630 ng virus/g leaf, respectively, although differences in PLRV titres among transgenic plants were much greater than those among infected WT plants. In similar tests with potato, the mean PLRV titre of WT plants was 50 ng virus/g leaf whereas higher concentrations (up to 3400 ng virus/g leaf) accumulated in transgenic potato plants. In tissue prints of stems, PLRV was detected in similar proportions of phloem cells in transgenic and infected WT plants. In transgenic tobacco and potato plants, but not in infected WT plants, a few stem epidermal cells also contained virus. From tissue prints of transgenic tobacco leaves, it was estimated that about one in 40000 mesophyll cells contained virus, but in transgenic potato, a greater proportion of mesophyll cells was infected.