Biology Reference
In-Depth Information
2011) have highlighted the complex challenge
of demonstrating the potential of biofortified
crops to improve consumer health. Neverthe-
less, several initiatives to take advantage of the
natural diversity of nutrients and micronutri-
ents found in staple food crops are under way
(
www.harvestplus.org)
. Some of the important
biofortification targets in maize are increased
amino acid content, enhanced mineral and vita-
min content, and/or bioavailability, and reduced
antinutrient content such as low phytic acid.
in maize endosperm is low in lysine content (0.1
g/100 g of protein), which negatively affects the
growth of animals (Osborne and Mendel 1914).
In the maize kernel, the endosperm and the
germ (embryo) constitute approximately 80%
and 10% of the mature kernel dry weight, respec-
tively. In the 1920s, in a Connecticut (USA)
maize field, a natural spontaneous mutation of
maize with soft, opaque grains was discovered,
which was eventually named as
opaque2 (o2)
(Singleton 1939). In 1964, Dr. Oliver Nelson's
team at Purdue University, also in the USA,
discovered that the homozygous recessive
o2
allele had substantially higher lysine (
Enhanced and Balanced Amino
Acid Content
69%)
in grain endosperm compared to normal maize
(Mertz et al. 1964). In
o2
maize, the zein frac-
tion is markedly reduced by roughly 50% with a
concomitant increase in the relative amounts of
nutritionally superior fractions such as albumins,
globulins, and glutelins. The lysine value of
o2
maize is 3.3 to 4.0 g/100 g of protein, which
is more than twice that of endosperm from the
normal maize (1.3 g lysine/100 g protein). The
protein quality of
o2
maize is 43% higher than
that of common maize and 95% of the value of
casein. The decreased level of zein (5-27%) in
o2
maize along with reduced leucine, leads to more
tryptophan for niacin synthesis and thus helps
to combat pellagra and significantly improves its
nutritional quality.
Genes and gene combinations that bring
about drastic alterations in either plant or kernel
characteristics also produce several secondary
or undesirable effects. The low prolamine or
high lysine mutants are no exception. In addi-
tion to influencing several biochemical traits,
they adversely affected a whole array of agro-
nomic and kernel characteristics. The
o2
and
other mutants adversely affect dry matter accu-
mulation, resulting in lower grain yield due
to increased endosperm size. The kernels dry
slowly following physiological maturity of the
grain and have a higher incidence of ear rot.
Other changes generally associated with high
lysine mutants include thicker pericarp, larger
germ size, reduced cob weight, increased color
+
Human beings as well as a number of farm
animals are incapable of synthesizing certain
amino acids; this fact has stimulated research on
improving the levels of some “essential amino
acids” in staple food crops. While cereals are
primarily deficient in lysine (Lys) and trypto-
phan (Trp), legumes are found to be significantly
short of methionine (Met). Consequently, these
three essential amino acids have frequently been
the targets of manipulation in maize as well as
other food crops. In developed countries, the
focus is generally on feed quality, as meat con-
sumption provides a sufficient supply of essential
amino acids for humans. In contrast, in develop-
ing countries where maize is directly consumed
as food, both humanitarian and economic inter-
ests prevail (Ufaz and Galili 2008; Atlin et al.
2011). Here, we highlight two specific cases of
genetic improvement in maize that resulted in a
high nutritional value addition, particularly with
respect to essential amino acid content in the
endosperm.
Quality Protein Maize (QPM)
In normal maize endosperm, the average pro-
portions of various fractions of protein are
albumins 3%, globulin 3%, zein (prolamine)
60%, and glutelin 34%. While the embryo pro-
tein is dominated by albumins (
60%), which is
superior in terms of nutritional quality, the zein
+
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