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
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