Characterization of position-induced spatial and temporal regulation of transgene promoter activity in plants

Quantitative differences in transgene expression between independent transf ormants are generally ascribed to different integration sites of the transg ene (position effect). The contribution of spatial and temporal changes in transgene promoter activity to these position-induced differences in transg ene expression in planta are characterized, using the firefly luciferase (l uc) reporter system. The activity of three different promoters (Cauliflower Mosaic Virus (CaMV) 35S, modified CaMV 35S and the promoter of an Arabidop sis thaliana Lipid Transfer Protein gene) was shown to vary not only among independent transformants, but also between leaves on the same plant and wi thin a leaf. The differences in local LUC activity between leaves and withi n a leaf correlated with differences in local luc mRNA steady-state levels. Imaging of LUC activity in the same leaves over a 50 d period, shows that individual transformants can show different types of temporal regulation. B oth the spatial and the temporal type of luc transgene expression pattern a re inherited by the next generation. It is concluded that previously report ed position-induced quantitative differences in transgene expression are pr obably an accumulated effect of differences in spatial and temporal regulat ion of transgene promoter activity.