A screening can also be possible by screening or scorable or reporter gene, incorporated into the transformation vectors, which allows for the detection of transformed cells, tissues or plants (Table27.2). The essential features of an ideal reporter gene are:

i.  An efficient and easy detection with high sensitivity

ii.   Lack of endogenous activity in plant cells

iii.   A relatively rapid degradation of the enzyme

The screening markers presently used are mostly derived from bacterial genes coding for an enzyme that is readily detected by the use of chromogenic, fluorigenic, photon emitting or radioactive substrates. A screening marker gene is functional only if an enzyme with comparable activity is not present in non-transformed cells. The utility of any particular gene construct as a transformation marker varies depending on the plant species and the tissue involved. The kanamycin resistance gene is probably the most extensively used selectable marker phenotype and Uid A gene (also referred to as gus), which encodes β-glucuronidase, is the most versatile reporter gene. The screened cells and the plants regenerated from transformation are further subjected to biochemical analyses, such as Southern hybridization, PCR and Northern hybridization. The former determines the presence and the number of copies of the introduced gene while the latter demonstrates the presence of transcripts of the transgene.

Table 27.2 : Screenable marker genes used in plant transformation