Four GM plant species (Arabidopsis thaliana, Brassica napus, Nicotiana benthamiana and N. tabacum), each expressing the gene encoding the
jellyfish green fluorescent protein (GFP) regulated by the cauliflower
mosaic virus (CaMV) 35S RNA promoter, were assessed for the extent of
transgene silencing and viral genome integration following infection by
CaMV. The first three species are systemic hosts of CaMV, while N. tabacum is only a
local host for a few strains of CaMV. A generalized systemic silencing of
the GFP transgene was not observed in a total of 100 plants of each species
infected with CaMV, although some localized loss of GFP was observed in
CaMV-infected N. benthamiana leaves, and some loss of fluorescence was observed in older
leaves of uninfected as well as infected plants. Progeny seedlings obtained
from the above infected plants also did not exhibit transgene silencing
showing that virus infection did not affect the stability of the transgene.
These progeny plants also did not show signs of virus infection, indicating
that the presence of the CaMV 35S RNA promoter sequences in the plant genome
did not potentiate seed transmission of the virus. Integration of infective
CaMV into the CaMV 35S RNA promoter could not be detected in 944 samples
taken from leaves of the above infected plant species or in 2912 samples
taken from progeny seedlings. Based on a detection limit of one copy per
4000 haploid genomes, we conclude that if integration of virus does occur
into the CaMV 35S RNA promoter, then it occurs at such a low frequency as to
be insignificant.