Plant selection principle based on xylose isomerase

The xylose isomerase genes (xylA) from Thermoanaerobacterium thermosulfurog enes and Streptomyces rubiginosus were introduced and expressed in three pl ant species (potato, tobacco and tomato) and transgenic, plants were select ed on xylose-containing medium. The xylose isomerase genes were transferred to explants of the target plant by Agrobacterium-mediated transformation. The xylose isomerase genes were expressed under the control of the enhanced cauliflower mosaic virus 35S promoter and the Omega' translation enhancer sequence from tobacco mosaic virus. In potato and tomato, xylose isomerase selection was more efficient than the established kanamycin selection. The level of enzyme activity in the regenerated transgenic plants selected on x ylose was 5-25-fold higher than the enzyme activity in control plants selec ted on kanamycin. The xylose isomerase system enables transgenic cells to u tilize xylose as a carbohydrate source. In contrast to antibiotic or herbic ide resistance-based systems where transgenic cells survive on a selective medium but nontransgenic cells are killed, the xylose system is an example of a positive selection system where transgenic cells proliferate while non -transgenic cells are starved but still survive. The results show that a ne w selection method is established. The xylose system is devoid of the disad vantages of antibiotic or herbicide selection, and depends on an enzyme whi ch is already being widely utilized in specific food processes and that is generally recognized as safe for use in the starch industry.