Why
engineer virus resistance?
Many plants are susceptible to diseases caused by
viruses,
often transmitted by insects (like aphids) from plant to plant
across
a field. The spread of viral diseases can be very difficult to
control,
and damage to a crop can be severe. Insecticides are sometimes
applied
to control populations of aphids, but often have little impact
on
the spread of the disease. Often the most significant weapons against
viral diseases are cultural controls (such as removing diseased plants)
and plant varieties bred to be resistant (or tolerant) to the virus,
but
they may not always be practical or available. Scientists have
discovered
new genetic engineering methods that provide resistance to
viral
disease where options may have been limited before.
How does GE virus resistance work?
Viruses are very primitive organisms composed of
little
more than a protein-based "coat" encasing a short piece of
genetic
material. This short strand of DNA or RNA contains instructions
for
the virus' coat and movement proteins, and a few other genes to
assist
reproduction. After a virus infects a cell, it removes its
coating
of proteins and then reproduces itself by tricking the cells
they
infect into manufacturing copies of their proteins and genetic
material.
New viruses assemble themselves from the newly made parts and
then
escape to infect other cells. Disease symptoms in the plant are the
result of cellular damage caused by the viruses.
Scientists discovered that by genetically
engineering
plant cells to deliberately overproduce one of the genes
important
to the virus (usually the "coat protein" gene), the virus is
not
able to reproduce. This is thought to work by a process called cosuppression--
the plant cell senses that a gene is being overproduced, and responds
by
blocking both the engineered gene and the virus' copy of the
gene.
The disease-resistance strategy of engineering a virus gene into a
plant is often called coat protein-mediated viral resistance.
Which plants have been genetically
engineered to be virus resistant?
Yellow
Squash and Zucchini
Asgrow seed markets
several varieties of squash and
zucchini resistant to three
important viral diseases: Zucchini Yellow
Mosaic Virus (ZYMV),
Watermelon Mottle Virus 2 (WMV-2), and Cucumber
Mosaic Virus (CMV).
These viral diseases can cause devastating losses to
squash-related
crops (Cucurbits: squash, zucchini, pumpkins,
cucumber,
watermelon) due to leaf mottling and yellowing, stunted plant
growth,
and deformed fruit. GE resistance to the viral diseases is much
better
than conventional sources of resistance, but it is still somewhat
susceptible to variations in pathogen strain and environment. The
Asgrow
varieties are marketed with names like Independence II, Liberator
III, Freedom III, and Destiny III.
Papaya
Beginning in 1992, a devastating outbreak of Papaya
Ring
Spot Virus (PRSV) swept through the papaya plantations of Hawaii--
papaya
production dropped 40% in the course of 5 years. Control of the
disease
was limited to careful observation to remove infected plants.
Researchers
in Hawaii and at Cornell University developed two varieties
of GE
papaya resistant to PRSV using coat protein-mediated resistance.
Seed
of the two varieties, named "SunUp" and "Rainbow," have been freely
distributed to papaya growers since May of 1998.
Potato
The "NewLeaf" line of
Bt-potatoes, marketed
by Monsanto's NatureMark to provide
resistance to the Colorado Potato
Beetle, was expanded in the last
few years to include two additional
virus-resistant varieties,
NewLeaf Plus and NewLeaf Y. These include the
Bt gene plus
genetically engineered resistance to potato viral diseases
(caused
by the Potato Leaf Roll Virus and Potato Virus Y,
respectively). The
"Lew Leaf" line, however, was discontinued by the
developer in
early 2001 and has not been marketed since.
منقووول
الموضوع : Virus Resistance المصدر :منتديات تقى الإسلامية الكاتب: El Helalya
