For genetic transformation of plants, when transformation frequencies are low, selectable markers are often used. The expression of a selectable marker gene results in a product that allows the survival of the transformed cells in the presence of a selective agent that prevents regeneration of the non-transformed cells.
Selectable markers mainly fall into two categories, according to the mode of action of the selectable gene product: either conferring resistance to transformed cells by enzymatic detoxification of the selective agent, or providing a growth advantage to transformed cells over non-transformed cells. Selection methods based on the former mechanism are generally refered to as “negative selection”, whereby the transformed cells are able to tolerate as substance which inhibits the growth of or even kills non-transformed cells. These include selection based on antibiotics and herbicides, as described in details in the technology landscapes “Antibiotic resistance genes and their uses in genetic transformation, especially in plants” and “Resistance to Phosphinothricin“. Selection methods based on the latter mechanism are referred to as “positive selection”, the subject of this technology landscape.
Rather than conferring resistance to a negative or toxic substance, positive selection involves conferring onto the transformed cell a metabolic advantage such as the capability of sugar consumption, or other competitive advantages for stimulating cell growth over nontransformed cells such as response to hormone and adaptation to extreme temperature. Positive selection systems in the purest sense identify and select genetically transformed cells without damaging or killing the non-transformed cells in the population, and without co-introduction of antibiotic or herbicide resistance genes.