Somatic variation during long term subculturing of plant cells caused by insertion of a transposable element in a phenylalanine ammonia-lyase (PAL) gene

We have identified a new En/Spm-like transposable element, Tdc1, in the 5′ flanking region of a phenylalanine ammonia-lyase gene (gDcPAL1) that is normally induced by transferring cells of carrot suspension cultures to fresh liquid medium (transfer or dilution effect). The initial integration into gDcPAL1 occurred more than 4 years after culture initiation. Tdc1 was first detected in gDcPAL1 genomic clones of a genomic library made from cells of the same cultured cell line 7 years after its initiation and thus following repeated subculturing. Twelve years after initiation, about 5–10% of the cells had Tdc1 inserted into the gDcPAL1 gene, indicating that Tdc1 insertion into gDcPAL1 occurred in one (or more) cell(s) during the first 4–7 years of subculturing. These mutant cells did not disappear during numerous passages; instead the proportion of cells having this Tdc1 inserted into gDcPAL1 has been increasing over the last 5 years. The promoter activity and the inducibility by transfer/dilution of the gDcPAL1 gene harboring Tdc1 is reduced relative to wild type. Finally, we show that insertion of a transposable element is one of the mechanisms that can cause variation of plant cell cultures during repeated subculture.