Stress inactivation of foreign genes in transgenic plants

The inactivation of endogenous plant genes during periods of stress or environmental changes has already been demonstrated. Although the expanding use of transgenic plants in scientific research has given hints for transgene inactivation caused by transinactivation, the first inactivation of transgene-encoded proteins caused by environmental changes was observed not before 1990. Transgenic Petunia hybrida plants, carrying the maize Al cDNA under the control of the CaMV35S RNA promoter causing a salmon red flower phenotype,were released to the field. Environmental factors, possibly including heat, lead to a stable loss of flower pigmentation, accompanied by the methylation of the viral promoter. In the same year, the inactivation of the phosphinothricin resistance gene in single-cell cultures of a transgenic Medicago sativa line was observed in more than 90% of the cells after a 10-day heat treatment. In transgenic tobacco, different transgenes could be inactivated by a heat treatment (37 degrees C), which did not lead to chances in the growth of the plant. These inactivations seem to be not correlated with methylation. The heat-induced inactivation of herbicide resistance genes led to sensitivity to the herbicide during the heat treatment. The transgene activity was regained after the temperature decreased to normal cultivation conditions. To date, heat-induced transgene inactivation has not been reported in the field, but it may give rise to problems if it does occur. Therefore, strategies to analyse the causes of heat-induced transgene inactivation and to circumvent the problem have to be developed.