Agriculture Reference
In-Depth Information
stage, primary transformants with a
desired genetic coni guration can be
identii ed. Transgenic plants containing
inadvertent fusions or disruptions of
transgenes as well as of endogenous genes
can be eliminated rapidly as the DNA
sequence of the input DNA fragment is
known. However, there are still limitations
to breeding via transgene technology. For
instance, only traits for which the
corresponding genes are isolated and
characterized can be transferred or
modii ed (see Section 2.2.5). Moreover,
agriculturally important traits, such as
yield, are controlled by a whole set of genes
and pathways, of which the expression is
regulated as a complex network. Modifying
these traits in crop plants by making use of
the transgenic approach will depend largely
on the knowledge gained on how the
expression of these traits is regulated at the
molecular level and on the availability of
tools to regulate transgene expression in
order to drive the desired metabolic path-
ways in the transgenic plant. On the basis
of the current research in the domain of
systems biology, one may also expect new
breakthroughs for these traits soon.
h e i rst barrier that needs to be
circumvented in order to be able to develop a
transgenic plant is the availability of a
transformation procedure for the plant
species to be transformed (see Section
2.2.5). Two ways of gene transfer can be
distinguished: direct gene transfer, which
makes use of physical forces to introduce the
DNA to the nucleus of the accepting plant
cell, or the
Agrobacterium tumefaciens
'transport' system based on the soil bac-
terium gene transfer mechanism.
h e second barrier is the access to genes
coding for traits of interest. h ere are
numerous transgenes developed today that
code for both input and output traits. Input
traits are those that potentially alter crop
production. An example is Bt maize, which
produces an insect toxic protein in the plant
and which, as a result, does not need the
application of a pesticide to control European
corn borer infection. Another example is the
introduction of resistance to the non-
selective herbicide, Roundup, trait in crops,
allowing the spraying of Roundup for weed
control without damaging crop plants.
Output traits, on the other hand, are those
that alter the harvested product. One
example is the increase of the oleic acid
content of soybeans, resulting in an
improved product for food and industrial
use. Another example is transgenic potatoes
with an improved starch extractability. At
the moment, most currently authorized and
commercialized GM crops code for input
traits, but a switch to more output traits is
expected (see Chapter 12).
h e third barrier is the huge costs
required to commercialize a GM plant or
derived product. During the development of
GM crops, dif erent categories of costs can
be distinguished:
1.
h e development of the transgenic plant
expressing the desired trait in a stable way,
leading to the desired ef ect.
2.
h e development of commercial varieties
containing the gene of interest by breeding.
3.
h e authorization dossier in order to
allow the commercialization of the GMO
event and its derived varieties.
h e introduction of a transgenic crop in the
i eld and of a transgenic plant derived
product into the market thus requires huge
investments, mainly for risk assessment and
deregulation. h erefore, companies are
focusing their investments mainly on crops
that are cultivated on a large scale such as
soybean, maize, canola and cotton, such that
it becomes economically feasible. In research
institutes and universities, on the other
hand, tobacco, rice and
Arabidopsis thaliana
are most frequently used for genetic
modii cation, due to well-developed trans-
formation methods, easy propagation and
well-studied genomes. h ere, they serve as
model organisms for other plant species.
Molecular marker technology is also con-
sidered as a plant biotechnological method.
h e development of molecular markers
