Maize line DBT418 was genetically modified to contain two novel
genes, cry1Ac and bar, for insect and herbicide tolerance
respectively. Both genes were introduced into a maize line by
particle acceleration (biolistic) transformation.
The transgenic maize line DBT418 was developed to resist ECB by
producing its own insecticide. This event was genetically
engineered by introducing the cry1Ac gene, isolated from the common
soil bacterium Bacillus thuringiensis (Bt), into the maize line
AT824. The cry1Ac gene produces the insect control protein Cry1Ac,
a delta-endotoxin. Cry proteins, of which Cry1Ac is only one, act
by selectively binding to specific sites localized on the lining of
the midgut of susceptible insect species. Following binding, pores
are formed that disrupt midgut ion flow, causing gut paralysis and
eventual death due to bacterial sepsis. Cry1Ac is lethal only when
eaten by the larvae of lepidopteran insects (moths and
butterflies), and its specificity of action is directly
attributable to the presence of specific binding sites in the
target insects. There are no binding sites for the delta-endotoxins
of B. thuringiensis on the surface of mammalian intestinal cells,
therefore, livestock animals and humans are not susceptible to
these proteins.
In addition to the cry1Ac gene, DBT418 was developed to allow for
the use of glufosinate ammonium, the active ingredient in
phosphinothricin herbicides (Basta®, Rely®, Liberty®, and Finale®),
as a weed control option, and as a breeding tool for selecting
plants containing the cry1Ac gene. DBT418 contains the bar gene
isolated from a common soil actinomycete, Streptomyces
hygroscopicus. This gene allows for the production of the enzyme
phosphinothricin N-acetyltransferase (PAT) which confers tolerance
to glufosinate.
The PAT enzyme in maize line DBT418 converts L-phosphinothricin
(PPT), the active ingredient in glufosinate ammonium, to an
inactive form, thereby conferring resistance to the herbicide. In
the absence of PAT, application of glufosinate leads to reduced
production of the amino acid glutamine and increased ammonia levels
in the plant tissues, resulting in the death of the plant. The PAT
enzyme is not known to have any toxic properties.
Expression of the Cry1Ac protein and PAT occurred in most, but not
all, tissues of the maize plant with levels highest in leaves, and
lower levels in roots, prop roots, stalk, tassel, cob, husk, and
kernels. No Cry1Ac or PAT proteins were detected in silk or pollen.
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