Aggregates formed as a result of the expression of yeast Met2 gene in transgenic tobacco plants, stimulate the production of stress-protective metabolites and increased the plants tolerance to heat stress

Methionine biosynthesis has taken different evolutionary pathways in bacteria, fungi and plants. To gain insight into these differences and to search for new ways of manipulating methionine biosynthesis in plants, the yeast (Saccharomyces cerevisiae) Met2 gene and the bacteria (Leptospira meyeri) MetX gene, both encoding homoserine O-acetyltransferase, were expressed in tobacco plants. We found protein aggregates in extracts of these transgenic plants, whose levels were much higher in plants grown at 35 degreesC than at 25 degreesC. It appears that the yeast and the bacterial proteins are heat labile and tend to change their intracellular conformation. These conformational changes of the transgenic proteins were more prominent at high temperature and most probably triggered aggregation of the yeast and the bacterial proteins. Moreover, plants expressing the yeast gene that grew at 35 degreesC over-accumulated stress-associated metabolites, such as phenolic compounds, including tannins, as well as the amino acid arginine. In addition, the transgenic plants expressing high levels of the foreign genes show growth retardation, which further suggests that, these plants suffer from internal stress. The changes in protein conformation and the consequent triggering of stress response may account for the ability of these transgenic plants to tolerate more extreme heat stress (60 degreesC) than the wild-type plants.