Manila, The Philippines
July 3, 2009

Pocket K No. 36: Marker-Free GM Plants
International Service for the Acquisition fo Agri-biotech Applications (ISAAA)

Introduction

Selectable marker genes are vital to the research and development of genetically modified (GM) crops. The methods used to introduce foreign DNA in a plant cell, either by microinjection, particle gun, electroporation or Agrobacterium, are relatively inefficient. Pinpointing cells that successfully incorporated foreign DNA in an ocean of non-transformed cells is akin to finding a needle in a haystack. To find transgenic cells, a marker gene is co-introduced with the gene of interest. These dominant genes confer resistance to antibiotics, such as hygromycin (hpt) and kanamycin (nptII), and herbicides, such as phosphinothricin (bar) and chlorosulfuron (als), that kill non-transformed cells. However, antibiotic and herbicide resistance marker genes may not be required in mature plants, especially when they are cultivated in fields.

The presence of these marker genes in commercialized transgenic crops has caused considerable public concern about the medical implications of GM food consumption and GM crop cultivation. Herbicide resistance genes might be transferred by outcrossing to weeds and wild crop relatives. There also exists the possibility, albeit extremely rare, of horizontal gene transfer from transgenic plants to soil and intestinal microorganisms, resulting in pathogens against which antibiotics currently being used are rendered ineffective. However, to date, no experiment has provided any evidence that the antibiotic markers presently in use pose risks to human or animal health.

Not all scientists agree with these claims. The difficulty of proving that the marker genes are indeed harmless has significantly limited the public acceptance of agricultural biotechnology.

A lot of research effort has been directed towards the development of marker-free transformation methods and selectable marker elimination strategies. Besides minimizing public concerns, the absence of resistance genes in transgenic plants could also reduce the costs for developing GM products and lessen the need for time-consuming safety evaluations, thereby speeding up the commercial release of new products. Generation of marker free plants likewise supports single line re-transformation, an important approach towards introduction of multiple genes for complex traits such as resistance to several pathogens and tolerance to abiotic stress.

There are several ways to either avoid or get rid of selectable marker genes. Methods that will allow the removal of DNA in plants as efficiently as it is inserted have been developed, such as the use of site-specific recombination, transposition and homologous recombination. Researchers have also described several substitute marker genes that have no harmful biological activities. The presence of these non-bacterial genes allows the plants to metabolize non-toxic agents normally harmful to them.

Complete Pocket K No. 36:
http://www.isaaa.org/kc/inforesources/publications/pocketk/default.html#Pocket_K_No._36.htm

 

Website: http://www.isaaa.org

Published: July 3, 2009