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Triticeae Genomics and Genetics
TGG 2010, Vol.1, No.2
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There are three commonly used herbicide markers
in plant transformation. The first one
pat/ba
,
(phosphinothricin acetyltransferase) gene isolated from
Streptomyces hygroscopicus
has been widely used as
an effective selectable marker in the presence of
phosphinothricin (PPT) based formulations such as
gluphosinate, bialaphos, basta, etc. by detoxifying
these compounds for the selection of transformed
tissues. It inhibits glutamine synthase. The enzyme
inactivates phosphinothricin by the addition of an
acetyl group from acetyl coenzyme A. This gene is
freely available for research purposes and has proved
particularly useful in cereals and grasses. PPT based
selection has been the most common in cereals
transformation (Kim et al., 1999).
EPSPS
(5-enolpyruvy-
lshikimate, 3-phosphate synthase) oxidoreductase that
inhibits aromatic acid biosynthesis is used as selectable
marker with glyphosate as selection agent.
GOX
(glyphosate oxidoreductase) is another herbicide resistant
marker that degrades glyphosate herbicides used in the
transformation process of wheat (Zhou et al., 1995).
3.2.3 Metabolic/auxotrophic marker genes
This is a positive selection system that provides
metabolic advantage to transformed cells and is
environmentally safe. This kind of selectable markers
enable transformed cells to synthesize an essential
compound that the cells otherwise cannot synthesize.
The medium is made intentionally to lack the essential
component that is required for the cells to grow. So
only successfully transformed cells with the selectable
marker gene and the gene of interest will synthesize
this compound and survive in the medium. The
example of such markers is
manA
or
pmi
(mannose-6-phosphate isomerase). This is the most
advanced positive selection system employed to date
for wheat transformation (Negretto et al., 2000).
Mannose is provided as a source of carbon and the
medium lacks sucrose. So the transformed cells can
use mannose for their carbon requirement whereas
non-transformed cells will not survive. Cyanamide
hydratase gene, also used for wheat transformation
(Weeks, 2000), when inserted in the cells/tissues will
make them able to grow on the medium containing
cynamide whereas the non-transformed cells will die.
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