Agriculture Reference
In-Depth Information
to a twofold increase of the phytase activity
in transgenic soybean seeds (Gao
et al
.,
2007). However, the thermostability of the
phytase expressed was not sui cient to
make it eligible for commercial use, since the
pelleting process of soybean requires a high
temperature in order to inactivate some
anti-nutrient compounds of the seeds.
Other research conducted in China follows
the same approach (Yang
et al
., 2011), and
the same team has also developed a
transgenic soybean expressing phytase in its
roots and excreting it into the surrounding
soil in order to make phytate available for
plant uptake (Li
et al
., 2009).
Besides these, two promising events are
under development in the USA, both of
them having been tested in the i eld. h e
i rst relies on a technique that has also been
used in maize: the silencing of transporter
gene expression in an organ-specii c manner
led to a 15- to 30-fold increase of inorganic P
concentration and to a dramatic reduction
of the phytic acid content of soybean seeds
(Shi
et al
., 2007). h e second event, based on
the expression of an
E. coli
phytase, exhibits
a nearly total conversion of phytate into
inorganic P together with a very high level of
phytase expression, in addition to a rather
high thermostability of the enzyme (Bilyeu
et al
., 2008). Seeds of this event used as an
additive to feed were as ef ective as
commercial phytase in reducing the phytate
content of soybean meal and maize meal,
paving the way to promising commercial
applications. Up to now, though, none of
these soybean events have entered the
regulatory pipeline, suggesting that the
conditions for a successful commercial
release might not yet be satisi ed.
In addition to maize and soybean, two
GM phytase-rich barley events are currently
in an advanced development stage with i eld
trials in the EU (see Table 12.1). h e i rst
event was generated thanks to a cisgenic
transformation - i.e. the phytase gene
inserted derives from the same plant species
- and showed a stable 2.8-fold increase in
the phytase activity of the grain (Holme
et
al.
, 2012). h is level of activity is higher than
that of the microbial phytase used as an
additive in feed to make P available from
phytate, which might be of great interest for
those farmers who process their own feed
from home-grown cereals. h e second GM
barley under development relies on the
insertion of a fungal phytase gene in a spring
barley line, and it also displays an increased
phytase activity (Ohnoutkova
et al.
, 2010).
h is last approach has also been applied
to canola in order to raise its phytase
concentration. In contrast to soybean,
canola grains do not have to be toasted prior
to their processing into feed, avoiding the
requirement regarding the thermostability
of the phytase protein. One GM canola event
went through many i eld and feeding trials
in the USA and displayed a high level of
phytase in seeds (Ponstein
et al
., 2002).
However, this research seems to have been
discontinued. Another event under develop-
ment in China has proved to be as ef ective
as microbial phytase in releasing P when
mixed with feed (Peng
et al
., 2006).
In recent years, Danish researchers have
been working on improving a wheat line with
a rationally designed thermostable phytase.
h eir results show that it is possible to
accumulate heat-stable phytase in wheat
that is still ei cient in hydrolysing phytate
and improving zinc and iron availability,
even after a prolonged boiling process
(Brinch-Pedersen
et al
., 2006). h is important
outcome might pave the way for an ef ective
improvement of P and mineral uptake in
cereal food and feed nutrition. Ef orts to
improve the nutritive value of rice are also
facing the thermostability of phytase issue
(Liu
et al
., 2006). It has been suggested that
expressing a bacterial phytase in rice seeds
would allow the use of rice as a feed additive
(Hong
et al
., 2004), but whether this
approach is compatible with high-
temperature processing requires further
investigation. By contrast, a rice plant
transformed with a yeast gene has proved to
produce an elevated level of phytase with a
relatively good stability at temperatures as
high as 70°C (Hamada
et al
., 2005). h e
authors of this study advocate the use of the
whole rice plant as a silage crop or as a feed
additive for monogastric animals.
Finally, lucerne (
Medicago sativa
) has been
modii ed to produce phytase and has been
