Agriculture Reference
In-Depth Information
bean (
Psophocarpus tetragonolobus
) high Lys
protein gene resulted in a 16.7% increase of
the Lys content of rapeseed seeds compared
to non-transgenic lines (Wang
et al
., 2011).
soybean remaining after oil extraction, is
not limited in Trp or Lys content. However,
since soybean meal is used extensively to
supplement cereal grains like maize in
animal rations, its enrichment in EAAs
would still be benei cial and explains the
research ef orts in this direction.
Research is also being conducted to
enhance the Trp content of maize, with at
least two events under development involv-
ing numerous i eld trials in the USA. One of
these has proved to have superior nutritional
quality than the conventional QPM that has
brought considerable improvements for
human consumption in developing countries
in the last decades (Huang
et al
., 2006).
The pipeline for tryptophan-enriched plants
Tryptophan is generally considered the
second most important EAA, since its
dei ciency in certain crops leads to
nutritional and clinical disorders when used
to feed animals (Henry
et al
., 1992). h us,
crop enrichment in Trp would be benei cial
for animal nutrition. h e i rst high-Trp GM
events were obtained through a rice enzyme
(
OASA1D
) involved in Trp catabolism, which
was modii ed in order to become Trp
feedback-insensitive, leading to free Trp
accumulation in rice (Wakasa
et al
., 2006).
Although rice is cultivated primarily for
human consumption, its use as a feed
product is growing. Five dif erent Trp-rich
GM rice events are currently in advanced
development in Japan (see Table 12.2), and
many i eld trials have been conducted that
have coni rmed a 193- to 311-fold increase
of Trp accumulation in modii ed rice seeds
(Wakasa
et al
., 2006). Nutritional experi-
ments with chickens have shown that the
nutritive value of high-Trp GM event HW-1
is similar to that of non-GM rice
supplemented with synthetic Trp (Takada
and Otsuka, 2007).
h e same gene has recently been used for
the development of high-Trp soybean GM
events, resulting in an elevation by at least a
factor of 20 of the free Trp content of soy
seeds. Transgenic seeds obtained, tested in a
trout feeding trial, were at least as ei cient
as Trp-supplemented feed in increasing the
body weight of i sh (Ishimoto
et al
., 2010).
Modii ed high-Trp lines are also richer in
other EAAs such as histidine (Kita
et al
.,
2010). Another GM soybean carrying a
tobacco gene has been proved to display an
increased Trp content (Inaba
et al
., 2007).
Non-GM soybean seeds already feature a
high protein content (about 40%), making
this crop an important staple source of
vegetable protein for both humans and
animals. Soybean meal, the coproduct of
The pipeline for methionine- and cysteine-
enriched plants
Legume crops, such as clover, lucerne or
soybean, are generally poor in sulfur amino
acids, Met and Cys. Besides being an
important component of proteins, Met is
involved in a wide range of biological
processes in plants, including biosynthesis
of the ethylene hormone, replication of
DNA, development of the cell wall and
production of secondary metabolites. h us,
attempts to elevate Met content in plants by
manipulation of the genes involved in its
synthesis are generally constrained by its
extensive catabolism or because they result
in abnormal phenotypes (Amir and Tabe,
2006). Nevertheless, some promising results
have been obtained in
Arabidopsis
by
eliminating the activity of an enzyme
(HMT2) involved in Met metabolism and
transport (Lee
et al
., 2008).
Another approach to increase the
nutritional value of legume plants is to
express heterologous sulfur proteins that
are naturally Met-rich in seeds. A sunl ower
protein gene was introduced in lupin
(
Lupinus angustifolius
) and the nutritive
value of the modii ed seeds was tested in
various animal feed experiments (Molvig
et
al
., 1997). h ey coni rmed the increased
availability of Met and showed that the
transgenic seeds were better than the
control seeds for rats, poultry and sheep
