Genetic transformation of plants cells is generally challenged by low gene transfer frequencies. As a
result, means to select transgenic cells over a majority of non-transgenic cells after transformation are important.
This can be accomplished by employing a suitable selection system. In the past, selection systems used for plants
were predominantly based on antibiotics and herbicides. These conventional selection systems alleviate the toxicity
of the selective agents (such as kanamycin, hygromycin or phosphinothricin) in the transgenic cells while killing the
non-transgenic cells. However, in many cases, it has proven to be a better selection strategy to employ selective
agents (such as mannose) that provide the transgenic cells a metabolic advantage while only starving but not actively
killing the non-transgenic cells. This group, often referred to as positive selection systems, should be termed positive
conditional-promotive selection systems, according to the selection system classification, with a new addendum.
Here, selection systems for the production of transgenic plants are reviewed since the production of the first
transgenic plant in 1983, with emphasis on recent developments.
Moreover, useful visual marker genes such as β–glucuronidase and green fluorescent protein are also described as
they have been of substantial aid for the development of the most valuable selection methods as well as for the
study of transgene expression and regulatory sequences.
