Agriculture Reference
In-Depth Information
maize (Houmard
et al
., 2007), while the
combination of both previous approaches in
a single event (MON93066, see Table 12.2)
has produced more than 4000 ppm free Lys
in the endosperm of maize kernels, a
100-fold increase compared to wild-type
maize (Frizzi
et al
., 2008). h is last event has
been patented and several i eld trials are
currently being conducted (see also Table
12.2).
Other approaches to enhance the Lys
content of maize are being explored. One of
these, i rst developed for rice (Wu
et al
.,
2003), consists of the introduction of a gene
encoding for an enzyme (Lysyl-tRNA
synthetase - AtKRS) involved in the form-
ation of tRNA (transfer RNA) lysyl (Wu
et
al
., 2007). h is resulted in the incorporation
of Lys into maize zeins - maize proteins that
normally have little or no Lys in the wild
type - and elevated the Lys content of
modii ed maize seed by up to 26%. Another
Lys-rich maize event is under research with
i eld trials in China: the introduction of a
potato gene expressing a protein with high
Lys content (
sb401
) led to a signii cant
elevation (16-55%) of total Lys in the plant
(Yu
et al
., 2004). More recently, a US-based
team developed transgenic maize lines
expressing a porcine milk protein in the
endosperm (Bicar
et al
., 2008). Field
cultivations coni rmed that the resulting
seeds had an improved amino acid balance
- but no increase in total protein content -
leading to an increase in Lys content of
about 29-47%.
Regarding GM rice, two new events with
increased Lys content are currently in the
early development stage. h e i rst is based
on an original approach: the introduction in
a rice plant of a gene encoding for an altered
form of the tRNA specii c to Lys, which is
introduced instead of other amino acids
during the protein biosynthesis process. h e
transformation generates a meaningful
enrichment of Lys in rice prolamin proteins
by 43-75%, resulting in an increase
comprising between 1% and 6.6% of the
overall Lys content of rice seeds (Wu
et al
.,
2003). h e expression of the
sb401
gene in
rice was also demonstrated to be a relevant
approach (Li
et al
., 2008).
Noteworthy are the ef orts to improve the
protein content of sorghum, generally follow-
ing a common approach of downregulating
the expression of sorghum prolamins, or
kai rins, using RNAi silencing techniques
(Zhao
et al
., 2003; Zhao, 2007; Kumar
et al
.,
2012). h ese kai rins tend to be poorly
digestible, whereas decreasing their expres-
sion enhances EAA content in grains. Among
these new events are the high-Lys sorghum
events developed for Africa in the framework
of the African Biofortii ed Sorghum initiatives
(Wambugu, 2007). Wild-type sorghum
features between 35 and 90% lower Lys than
other cereals, but event P898012, a transgenic
biofortii ed sorghum, already contains up to
112% more Lys than the wild type, among
other nutritional improvements (Taylor and
Taylor, 2011; Kruger
et al
., 2012). h ese new
sorghum GM events have considerable
potential to alleviate malnutrition in Africa,
as well as to enhance the ei ciency of
sorghum as feedstuf s.
h e nutritional quality of wheat as a feed
for livestock is also limited by low levels in
certain EAAs - especially Lys and threonine
(h r) - composing its storage proteins.
Following previous attempts to elevate the
content of wheat-specii c gluten proteins by
expressing additional copies of the
corresponding genes (Shewry
et al
., 2006),
an amaranth (
Amaranthus hypochondriacus
)
albumin gene has recently been introduced
in a wheat line. h e results indicate an
increased content in EAAs, about 30% in the
case of Lys (Tamás
et al
., 2009). Another
approach involving the transformation of
wheat seeds with a chilli pepper (
Capsicum
frutescens
) gene led to a 7.4% increase in Lys
content (Sun
et al
., 2010), whereas the
overexpression of a barley sucrose trans-
porter in wheat also showed increased EAA
levels in Lys, leucine (Leu) and Trp, among
others (Weichert
et al
., 2010). h e feasibility
of this last approach to overcome the
negative correlation between yield and
grain protein content observed in con-
ventional breeding of wheat has been
coni rmed by numerous greenhouse and
i eld trials.
Recently, a new crop species has been
enriched in Lys: the introduction of a winged
