Selectable Marker and Reporter Genes

Abstract

In plant transformation systems, the efficiency of stable gene transfer is low. Only a fraction of the cells exposed to the foreign DNA integrate the DNA into the genome, thus providing the basis for regeneration of transgenic plants. Therefore, systems are required which allow selection of such transformed clones. A selective system consists of a selective agent, i.e., a phytotoxic compound interfering with the plant cellular metabolism, and a selectable marker gene, i.e., a DNA sequence coding for a product which allows detoxification or evasion of the selective agent through enzymatic modification of the selective agent or expression of an altered target, respectively. Clones with integrated selectable marker genes can be detected through negative selection of untransformed cells or tissue in vitro or whole plants in vivo. Co-transformation of nonselectable sequences linked or nonlinked with selectable marker genes allows recovery of tissue or plants which contain the nonselectable sequence. A selection system for transformed plant cells must meet several requirements: the selectable marker gene should allow expression in a variety of plant tissues, the background metabolic activity or resistance should be minimal, and a clear change of phenotype should be visible. Over the past decade, a number of such systems have been developed (cf. Table 1). They include the use of antibiotics, herbicides, substrate analogues, or high concentrations of other compounds. Several of them are widely used in plant transformation work.