Quantitative determination of mosaic GFP gene expression in tobacco

A specific form of gene silencing that was observed visually as a mosaic distribution of fluorescent and non-fluorescent cells apparently dispersed at random within tissues was found in a few green fluorescent protein (GFP)-transformed tobacco lines. To characterize this event quantitatively, we studied flow cytometric measurements in GFP-expressing and -silenced cells in T-1 and T-2 progeny of four selected plants. The proportion of silenced cells varied considerably among the T-1 lines but with notable genotype differences. Mosaic expression was inherited into the T-2 generation in which the majority of progenies tested exhibited a level of silencing similar to that of their T-1 parental plants. However, in some T-2 progenies segregation, evident as a decrease or increase in the proportion of fluorescent cells, was observed. We discuss several factors, such as copy number, promoter activity or polyploidy, that may be the possible causes of the gene silencing, but none sufficiently explain the appearance of the mosaic distribution.