Status of clean gene (selection marker-free) technology

The ever-increasing world population has created two major problems: more mouths to feed and less land to farm. While classical breeding has enormously helped in providing more food, a lot still needs to be done. Transformation biotechnology can also help especially where classical breeding lacks solution (e.g. limited availability of stable and durable genetic source of resistance). However, plant transformation technologies require selectable marker genes to produce transgenic plants but such genes are of no value thereafter; in fact, marker genes in transgenic plants are perceived to pose potential bio-safety problems. In all genetic transformation technologies based on direct gene transfer (electroporation of protoplasts, particle bombardment etc) the selectable marker genes generally cointegrate with the gene of interest(s) in one Mendelian locus in the plant genome; hence, their removal is highly desirable. This may also help in the acceptability of transgenic plants by society. Transgenic plants that contain the desired gene of interest but lack the selection marker gene used in its production are termed “clean” and the methods utilized in their production are referred to as “clean gene” technology. There are several proved methods of eliminating selectable marker genes and these include: (A) Marker gene excision consisting of (1) intra-genomic relocation of transgenes via transposable elements, (2) site-specific recombination systems and (3) intra-chromosomal recombination. (B) Gene replacement or Targeted gene replacement; and (C) transformation with multiple T-DNAs, which could result in linked and/or unlinked co-integration of transgenes. Unlinked transgenes are then segregated out during meiosis.