Stability of the transgenes

Desirable new phenotypes created by genetic engineering of plants are frequently unstable following propagation, leading to a loss of the newly acquired traits. [68] This genetic instability is due not to mutation or loss of the transgene but rather to its inactivation. A widely accepted factor causing the variation in transgene expression is the difference in genomic integration sites (position effects). Chromosomal regions with distinct levels of transcriptional activity, adjacent enhancers, or silencing elements may differentially influence the expression of the transgene. Besides the integration site, the copy number of the transgene [69–70] and its configuration [71] may induce gene silencing. As proposed by Finnegan and McElroy, [68] transgene inactivation is a consequence of events including chromatin restructuring, DNA methylation and the inhibition of mRNA processing, transport, export or translation. Silencing phenoma may also result from the introduction of transgenes expressed under the control of strong promoters. It may affect the expression of the transgene alone, leading to a plant devoid of its original interest. Silencing may also affect the expression of homologous host genes, a phenomenon referred to as co-supression that can have dramatic consequences for the survival of the plant if it involves a housekeeping gene or a defence-related gene. Therefore, the limiting process in the application of plant transformation to biotechnology is generally not the production of transformants but the screening required to eliminate transformants with collateral genetic damage or silenced transgene expression that would interfere with meaningful physiological analysis or commercial use.