Abstract
Selectable marker genes (SMGs) and selection agents are useful tools in the production of transgenic plants by selecting transformed cells from a matrix consisting of mostly untransformed cells. Most SMGs express protein products that confer antibiotic- or herbicide resistance traits, and typically reside in the end product of genetically-modified (GM) plants. The presence of these genes in GM plants, and subsequently in food, feed and the environment, are of concern and subject to special government regulation in many countries. The presence of SMGs in GM plants might also, in some cases, result in a metabolic burden for the host plants. Their use also prevents the re-use of the same SMG when a second transformation scheme is needed to be performed on the transgenic host. In recent years, several strategies have been developed to remove SMGs from GM products while retaining the transgenes of interest. This review describes the existing strategies for SMG removal, including the implementation of site specific recombination systems, TALENs and ZFNs. This review discusses the advantages and disadvantages of existing SMG-removal strategies and explores possible future research directions for SMG removal including emerging technologies for increased precision for genome modification.
Highlights
Selectable marker genes (SMGs) and selection agents are useful tools in the production of transgenic plants by selecting transformed cells from a matrix consisting of mostly untransformed cells
Review Since the first recombinant DNA was successfully created through the removal of a specific gene from a bacterium and inserting it into another bacterium by the use of the restriction enzyme in 1973 [1], recombinant DNA technology has emerged as a powerful tool for editing genes and genetic elements in vectors to produce novel recombinant DNA products
Among the most controversial concerns is that a SMG coding for an antibiotic resistance originally from a bacterium might be horizontally transferred from GM plants back to bacteria causing new antibiotic resistance problems, for example, in the human- or animal gut environment
Summary
One of the most unique characteristics of ZFN technology is that a DSB at a specific locus can be induced through ZFN-mediated cleavage in vivo and mutate the allele by NHEJ or replace the original allele with researcher-designed donor DNA sequence by homologous recombination (HR) repairing machinery. An approach of combining ZFNs for initial targeting of the site-specific recombination system to a locus of interest and subsequent use of site-specific recombination system for genome editing and SMG removal should be welcomed by many researchers. After expression of TALEN, DSB will be induced at both TALEN-binding sequences and release the SMG (Figure 9) This strategy can be devised for maker-free transgenic plant production, or native chromosome fragment removal
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