Principles of Plant Genetics and Breeding

382 CHAPTER 20
Bt cotton is another widely grown bioengineered
crop. The pest resistance conferred by the Bt gene has
led to a dramatic reduction in pesticide use and has consequently
reduced adverse impact on the environment
from agropesticides. As indicated previously, Bt sprays
are widely used in organic farming for pest control.
However, such application is ineffective if the insect
bores into the plant. Further, Bt sprays have short duration
activity.
Engineering viral resistance
Even though viruses may utilize DNA or RNA as hereditary
material, most of the viruses that infect plants are
RNA viruses. One of the most important plant viruses in
biotechnology is the cauliflower mosaic virus (CaMV)
from which the widely used 35S promoter was derived
(CaMV 35S promoter). A virus is essentially nucleic acid
encased in a protein coat. The primary method of control
of viral infections is through the breeding of resistance
cultivars. Also, plants can be protected against
viral infection by a strategy that works like inoculation in
animals. Plants may be protected against certain viral
infections upon being infected with a mild strain of that
virus. This strategy, called cross-protection, provides
protection to the plant against future, more severe,
infections.
Engineering transgenic plants with resistance to viral
pathogens is accomplished by the method called coat
protein-mediated resistance. First, the viral gene is
reverse transcribed (being RNA) into DNA from which
a double-stranded DNA is then produced. The product
is cloned into a plasmid and sequenced to identify
the genes in the viral genome. A chimeric gene is
constructed to consist of the open reading frame for
the coat protein to which a strong promoter is attached
for high level of expression in the host. This gene
construct is transferred into plants to produce transgenic
plants.
Successes with this strategy have been reported in
summer squash (the first product developed by this
approach), and for resistance to papaya ring spot virus (a
lethal disease of papaya), among others.
Engineering herbicide resistance
Herbicides constitute one of the most widely used
agrochemicals in crop production. Organisms genetically
engineered for herbicide resistance are among
the major applications of biotechnology in plant food
biotechnology.
Why engineer herbicide-resistant crops?
A successful herbicide should destroy weeds only,
leaving the economic plant unharmed. Broad-spectrum
herbicides (non-selective) are attractive but their use in
crop production can be problematic, especially in the
production of broadleaf crops such as soybean and
cotton. There is a general lack of herbicides that will
discriminate between dicot weeds and crop plants.
Preplant applications may be practical to implement.
However, once the crop is established and too tall for
the safe use of machinery, chemical pest management
becomes impractical. Grass crops (e.g., wheat, corn)
may tolerate broadleaf herbicides better that the reverse
situation. Consequently, when cereal crops and broadleaf
crops are grown in rotation or adjacent fields, the
broadleaf plants are prone to damage from residual
herbicides in the soil, or drift from herbicides applied
to grasses. When a crop field is infested by weed species
that are closely related to the crop (e.g., red rice in a rice
crop or nightshade in a potato crop), herbicides lack
enough sensitivity to distinguish between the plants.
To address these problems, one of two approaches may
be pursued: (i) the development of new selective postemergent
herbicides; or (ii) the genetic development of
herbicide resistance in crops to existing broad-spectrum
herbicides. The latter strategy would be advantageous
to the agrochemical industry (increased market) and
farmers (safer alternative to pesticides that are already
in use). New herbicides are expensive to develop and
take time.
Modes of action and herbicide-resistance mechanisms
Most herbicides are designed to kill target plants
by interrupting a metabolic stage in photosynthesis.
Because all higher plants photosynthesize, most herbicides
will kill both weeds and desirable plants.
Plants resist phytotoxic compounds via one of several
mechanisms.
1 The plant or cell does not take up toxic molecules
because of external barriers such as cuticles.
2 Toxic molecules are taken up but sequestered in
a subcellular compartment away from the target
(e.g., protein) compounds the toxin was designed to
attack.
3 The plant or cell detoxifies the toxic compound by
enzymatic processes, into harmless compounds.
4 The plant or cell equipped with resistance genes
against the toxin may produce a modified target compound
that is insensitive to the herbicide.