Selectable Marker and Reporter Genes

The neomycin phosphotransferase (nptll) gene is still the most widely used selectable marker gene and has been used extensively to generate many transgenic dicotyledonous plants and also some monocotyledonous plants. It confers tolerance to aminoglycoside antibiotics (kanamycin, geneticin, puromycin). For various reasons (including intellectual property considerations, fear of transfer of kanamycin tolerance to gut bacteria, etc.), it would be useful to have alternative selectable marker genes. Some alternatives include hpt (hygromycin tolerance), bar or pat (phosphinothricin/Basta tolerance),aroA (EPSP synthase/glyphosate tolerance),14 modified als (chlorsulfuron tolerance), bromoxynil tolerance and carbon utilising genes mannose phosphate isomerase (mannose utilisation) and xylose isomerase (xylose utilisation).
To study specificity of expression, widely used reporter genes include those encoding b-glucuronidase (gus), luciferase (luc) and more recently green fluorescent protein (gfp) from the jellyfish Aequoria victoriae. There are now engineered variants of the latter for improved plant
expression (cryptic intron removed) or altered colours of fluorescence (e.g. blue, red, yellow). An advantage of gfp is that no substrate is required and so in vitro expression can be followed in the same cells or tissues over long periods (weeks/months) and high-resolution studies in live cells can be undertaken using confocal scanning laser microscopy.1