Agriculture Reference
In-Depth Information
plants. In fact, some EAAs are found in very
limiting amounts in plants, such as lysine
(Lys) and tryptophan (Trp) in cereals and
methionine (Met) or cysteine (Cys) in
legume crops. h e interest for such an
enrichment varies by country and diet. In
developing countries where plants provide
people with most of their protein, this would
help prevent malnutrition, while in developed
countries the challenge is to improve the
economic ei ciency of the conversion of plant
proteins into animal proteins, the latter
representing the i rst source of EAAs in these
countries (Ufaz and Galili, 2008).
Despite the potential high economic
benei ts, conventional breeding for high
protein and EAA content is very dii cult to
achieve, because of the well-studied negative
correlation between yield and protein
content (Barneix, 2007). Over time, the
ef orts to breed high-yielding maize var-
ieties, for instance, have generated a shift in
grain composition from protein to starch
(Scott
et al
., 2006). h e only success so far is
the quality protein maize (QPM), richer in
Lys and Trp, that was bred at the Maize and
Wheat Improvement Center (CIMMYT) in
the late 1990s and which has produced very
positive results in many regions of the world
(Vasal, 2000). h e dii culties in conventional
breeding arise from the fact that amino acids
such as Lys, Trp or Met play an important
role in plant development, so their high
content is generally associated with
abnormal plant growth and inferior agro-
nomic traits.
GM techniques, in contrast, allow a seed-
specii c expression of traits that has proven
to be a promising approach to overcome
these limitations. GM techniques also allow
the insertion of new quality traits in high
agronomic performing lines of multiple
species, making the breeding process
shorter. None the less, such plant enhance-
ments are especially relevant for feedstuf s
destined for monogastric mammals, since
ruminants need EAAs that in addition are
resistant to rumen proteolysis. We review
here the GM approaches that have proved to
be suitable in elevating the content of Lys,
Trp, Met and Cys in plants (see Table 12.2).
The pipeline for lysine-enriched plants
Lysine is considered to be the most limiting
amino acid in cereals, especially for swine
and poultry nutrition. Although maize is
one of the most productive crops on a per
hectare basis in terms of energy and yield,
its nutritional quality is rather poor when it
comes to amino acid content. h erefore,
maize meal-based rations have to be
supplemented with Lys in poultry or swine
diets, generally from soybean meal or with
synthetic Lys produced by fermentation
(Johnson
et al
., 2001; Huang
et al
., 2008).
Other crops such as rice or rapeseed suf er
the same limitations. Hence, numerous GM
researches have been focusing on the
enhancement of Lys content in seeds used
for livestock nutrition.
In maize, various research teams have
developed Lys-rich GM events, with
promising results. A rather simple approach
is based on the expression in maize seeds of
a bacterial enzyme (CordapA), which is
involved in Lys biosynthesis but which has
been made insensitive to Lys feedback
inhibition (i.e. the mechanism that regulates
Lys production caused by its own
accumulation) by a mutation (Huang
et al
.,
2005). h is modii cation led to the
development of maize event LY038,
authorized for cultivation in the USA in
2005. h is event, as well as its stack with
event MON810 (that makes the maize insect
resistant), was tested in many i eld and
feeding trials and turned out to be superior
in poultry nutrition to its non-GM
counterpart (Lucas
et al
., 2007). However,
these events were never commercialized and
in 2009 the applications for approval
submitted to the European Food Safety
Authority (EFSA) were withdrawn by the
applicant. No oi cial reason was provided,
but it is likely that the cost-benei t ratio of
this innovation was not sui cient to justify
its commercial release.
Another GM event obtained by silencing
through RNAi (RNA interference), the
expression of a gene involved in Lys
catabolism (i.e. degradation), has resulted in
a 30-fold increase of free Lys content in
