Genomics and Applied Biology 2019, Vol.10, No.2, 10-19
12
genetic modifiers in
opaque 2
backgrounds through intrapopulation selection of QPM types exhibiting a higher
frequency of modified
o2
kernels. Subsequently, combining ability studies for QPM status (Vasal et al., 1993a;
1993b) assisted QPM hybrid development under varying climatic and growing conditions. Now, QPM are being
grown on roughly 9 million acres worldwide (Kataki and Babu, 2003).
4 Nutritive Value of Normal Maize and QPM
In most of the developing countries with high population density, 70 percent of protein is provided by cereals
eaten directly. Although, maize kernels harboured more protein than rice, but quality-wise it is poor due to low
status of tryptophan and lysine (which our body cannot synthesize), and high leucine and glutamine content,
causing imbalance of amino acids and malnutrition. Maize endosperm contains about 2 percent lysine and 0.5
percent tryptophan; but there is a need of approximately double of this amount for our normal growth and
development. Surprisingly, the isolation of the
opaque 2
(
o
2
) mutants seemed to be the befitting answer to solve
such nutritional starvation. Ihe lysine and tryptophan content was almost doubled in maize endosperm (Vasal,
1994; Krivanek et al., 2007) by decreasing the synthesis of zein proteins (deficient in lysine and tryptophan) and
increase in the other seed protein bound lysine and tryptophan. QPM was proved to be nutritionally rich compared
to common maize and almost equivalent though not equal to the growth response observed when milk protein
(casein) was fed (Amorin, 1972; Valverde et al., 1981). Digestibility of seed protein of QPM is significantly
higher and is better utilized (due to its better essential amino acid balance) than normal maize. Protein digestibility
of processed
opaque 2
maize is estimated to be 76.5% as against its true digestibility value 82%; whereas it is just
46.5% in case of non-QPM counterpart (Kies et al., 1965; Clark et al., 1967). This proves to be a commercially
viable practical proposition as the daily recommended ration may be cut short to just half by using QPM. In
poultry, it boosts 50% increase in body weight compared to normal maize over a 9 week-period (Mbuya et al.,
2011). Hence, the merit of QPM diversified value added products for children and adults cannot be undermined to
meet food and nutritional security (Atlin et al., 2011). In addition, tryptophan is readily bioavailable by way of its
conversion to Niacin in our body, which theoretically reduces the incidence of Pellagra.
5 Genetic Basis of QPM
In maize, the
Opaque 2
gene codes a transcriptional activator that regulates the most abundant endosperm storage
protein genes. The
o
2
gene is reported to have large effect on lysine and protein content while it has minor effect
on oil content (Lou et al., 2005). Sequencing of the
o
2
gene in a set of cultivated and teosinte (wild progenitor of
cultivated maize) accessions revealed 5.4% polymorphic sites and 72 insertions/deletions, located mostly in
noncoding regions (Henry et al., 2005). Besides, the
o
2
transcriptional activator revealed quite high molecular
diversity compared other transcription factors in maize (Henry et al., 2005).
Several spontaneous (
o
1
,
o
2
,
o
5
,
o
9-11
,
o
13
,
o
16
,
o
17
) and induced mutations affect amino acid composition in maize
seed proteins. Maize carrying
opaque 2
mutation can have nearly double lysine and tryptophan content compared
to normal maize. A
opaque 6
mutant allele also increased lysine and tryptophan content in endosperm. Recently, a
transposable element ‘rbg’ brings about differential expression of
opaque 2
mutant gene in two
opaque 2
NILs
derived from the same inbred line (Chen et al., 2014).
Expression of the
o
2
gene is largely influenced by GxE interaction. Besides, background polygenes accounts for
appreciable variation in amino acid composition and opaque phenotypes (Lou et al., 2005). The discovery of
numerous modifiers associated with
opaque 2
mutation made it possible for kernel vitreousness and increased
seed yield with tolerance to abiotic stresses. Two major QTLs associated with endosperm modification have been
identified. Vasal et al. (1980) produced elite germplasm with hard kernel and much higher quantity of lysine and
tryptophan by combining the
opaque-2
allele with these QTLs.
6 Modulation of Amino Acid Composition and Endosperm Characteristics
A protein with balanced amino acid composition is
in vogue
assist body building process. Ten amino acids
including lysine and tryptophan are essential amino acids (which our body cannot synthesize). These allow the
body to synthesize complete proteins. Therefore, amino acid balance seems to be a determining factor for quality
