MOLECULAR AND GENETIC-ANALYSIS OF NITRITE REDUCTASE CO-SUPPRESSION INTRANSGENIC TOBACCO PLANTS

Silencing of Nia host genes and transgenes (encoding nitrate reductase ) was previously achieved by introducing into tobacco plants the tobac co Nia2 cDNA cloned downstream of the cauliflower mosaic virus (CaMV) 35S promoter. To check whether Nii host genes and transgenes (encoding nitrite reductase, the second enzyme of the nitrate assimilation path way) were also susceptible to silencing, a transgene consisting of the tobacco Nii1 gene with two copies of the enhancer of the 35S promoter cloned 1 kb upstream of the Nii promoter region was introduced into t obacco plants. Among nine independent transformants analysed, two show ed silencing of Nii host genes and transgenes in some descendants afte r selfing, but never after back-crossing with wild-type plants, sugges ting that silencing depends on the number of transgene loci and/or on certain allelic or ectopic combinations of transgene loci. In one tran sformant carrying a single transgene locus in a homozygous state, sile ncing was triggered in all progeny plants of each generation, 20 to 50 days after germination. Field trial analysis confirmed that silencing was not triggered when the transgene locus of this latter line was pr esent in a hemizygous state. In addition, it was revealed that silenci ng can be triggered, albeit at low frequency and later during the deve lopment, when this transgene locus is brought into the presence of a n on-allelic transgene locus by crossing, suggesting that a homozygous s tate is not absolutely required.