Screenable Marker Genes
Screenable markers encode a protein which is detectable because it produces a visible pigment or because it fluoresces or modifies the phenotype elsewhere under appropriate conditions. Screenable markers include galactosidase (lacZ; Herrera-Estrella et al. 1983), ft-glucuronidase (GusA; Jefferson 1987), luciferase (luc; Ow et al. 1986), green fluorescent protein (gfp; Haseloff and Amos 1995), red fluorescent protein (rfp; Campbell et al. 2002) and isopentenyl transferase (ipt; Ebinuma et al. 1997). Screenable marker genes can be used as independent genes or as fusion constructs. They cannot be used for positive selection but they can help to improve transformation efficiency, they can be used as visual marker of transformation and they allow the enrichment of transformed tissue and therefore speed up the whole transformation process. Screenable markers are usable within the T-DNA, outside T-DNA on the backbone or as part of the co-transformation vector. In the meanwhile the integration of screenable marker genes outside the T-DNA borders is an often used strategy to identify vector backbone sequences in order to limit the production of events with superfluous DNA. In a similar manner the usage of ipt can serve as a visual screenable marker of backbone integration within the plant genome (Bukovinszki et al. 2006).
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