Elimination of Selectable Marker Genes from Transgenic Crops

Abstract

The first reports of successful plant transformation appeared in the early 1980s. Since that time there has been a steady increase in the number of plant species that can be transformed and there have been improvements to the transformation efficiencies of many plant species. However, as DNA uptake and its integration into the genome remain at a low frequency, an essential step in the transformation process is the ability to select transformed cells from the majority of non-transformed cells. This is usually achieved by the expression of a selectable marker gene, linked to the transgene, and selection of transformed cells for their ability to proliferate in the presence of the selective agent. Under appropriate conditions transgenic plants can be regenerated from these selected cells and thereafter the selectable marker gene is generally superfluous. Selectable marker genes have traditionally been antibiotic or herbicide resistance genes, the most commonly used being the neomycin phosphotransferase (nptII), hygromycin phosphotransferase (hyg) and phosphinotricin acetyl transferase (bar) genes which confer resistance to the antibiotics kanamycin, hygromycin and the herbicide glufosinate, respectively.