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 exp anding use of transgenic plants in scientific research has given hints for transgene inactivation caused by transinactivation, the first ina ctivation of transgene-encoded proteins caused by environmental change s was observed not before 1990. Transgenic Petunia hybrida plants, car rying the maize Al cDNA under the control of the CaMV35S RNA promoter causing a salmon red flower phenotype,were released to the field. Envi ronmental factors, possibly including heat, lead to a stable loss of f lower pigmentation, accompanied by the methylation of the viral promot er. In the same year, the inactivation of the phosphinothricin resista nce 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 treatme nt. In transgenic tobacco, different transgenes could be inactivated b y a heat treatment (37 degrees C), which did not lead to chances in th e growth of the plant. These inactivations seem to be not correlated w ith 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 inacti vation has not been reported in the field, but it may give rise to pro blems if it does occur. Therefore, strategies to analyse the causes of heat-induced transgene inactivation and to circumvent the problem hav e to be developed.