The frequency of silencing in Arabidopsis thaliana varies highly between progeny of siblings and can be influenced by environmental factors

In a collection of 111 transgenic Arabidopsis thaliana lines, silencing of the nptII gene was observed in 62 (56%) of the lines and three distinct npt II-silencing phenotypes were identified. Two T-DNA constructs were used, wh ich differed in distance and orientation of the marker gene relative to the border sequences. Comparison of the sets of lines generated with each vect or, indicate that the T-DNA construct configuration influence the incidence of lines displaying silencing, as well as the distribution of silencing ph enotypes. Twenty lines were investigated more thoroughly. The frequency of silencing varied between siblings in 19 lines, including three lines contai ning a single T-DNA copy. The last line showed 100% silencing. The gus gene present in both constructs could be expressed in the presence of a silence d nptII gene. Investigation of methylation at a single site in the pnos pro moter revealed partial methylation in multi-copy lines, but no methylation in single-copy lines. For 16 lines, the overall frequencies of silencing di ffered significantly between control plants and plants exposed to temperatu re stress; in 11 of these lines at the 0.1% level. In several cases, the fr equency of silencing in progeny of stress-treated plants was higher than fo r the control group, while other lines showed higher frequencies of kanamyc in-resistant progeny for the stress-treated sibling plants.