Genomics and Applied Biology 2019, Vol.10, No.2, 10-19
13
of any food and feed. Several mutants are now available with altered amino acid profile of endosperm protein.
These include
opaque-2
(
o
2
),
floury 2
(
fl
2
), Mucronate (Mc) and Defective endosperm B30 (DEB30). The opaque
mutants are recessive (
o
1
,
o
2
,
o
5
,
o
9-11
,
o
13
,
o
16
,
o
17
) and the floury mutation is semi-dominant (fl-1, fl-2 and fl-3),
but ‘Mucronate’ and ‘Defective endosperm’ are dominant mutations. The
opaque
mutations affect the regulatory
network (Mertz et al., 1964; Nelson et al., 1965; Krivanek et al., 2007) whereas floury, Mucronate and defective
endosperm affect the storage proteins (Gibbon and Larkin, 2005).
The
opaque 2
(
o
2
) down regulates the zein protein expression leading to increase in the other seed protein bound
lysine and tryptophan (Henry et al., 2005). This seems to be a compensatory mechanism that activates translation
of other mRNAs instead of zein mRNAs. Besides, deletion mutagenesis conditioning zein protein expression can
alter amino acid composition (Holding, 2014). Down regulation of 22 kD zein protein mediated by RNA
interference is reported to cause
opaque
phenotype more profoundly as compared to 19 kD component possibly
due to greater interaction of 22 KD components with β and γ-zeins resulting in disruption in protein body
formation (Segal et al., 2003; Huang et al., 2004). Besides, Zhang et al. (2010) revealed increase in synthesis of
lysine and tryptophan content in maize carrying
opaque-16
mutant allele. Transfer of
o
16
allele into
opaque 2
genetic background can further increase the lysine content in maize.
The details of proteome modulation that operates to alter amino acid composition is not clear. As per microarray
analysis, 60 genes out of 1,400 genes are reported to be three times up-regulated in the
o
2
mutant and these are
related to stress responses, molecular chaperones and protein turn over (Prioul et al., 2008). Sixty six genes are
down-regulated which are involved in carbon, carbohydrate metabolism and branched chain amino acids (Hunter
et al., 2002).
o
2
mutants revealed significant decrease in LKR/SDH (lysine-ketoglutarate reductase/saccharopine
dehydrogenase)-the first enzyme of lysine catabolism, as compared to wild type (Brochetto-Braga et al., 1992;
Azevedo et al., 2003). Segmental duplication in the Teosinte genome is reported to result diverged copies of the
regulatory
opaque-2
gene (Xu and Messing, 2008) during evolution. Besides, a 15.26kb duplication at the 27-kDa
γ-zein locus is shown to be a major modifier QTL leading to its enhanced expression and endosperm hardness
(Liu et al., 2016). A major QTL has been identified between the marker 0916-2 and Ch7-120.35 within a narrow
interval of 100kb using fine mapping. Further, SDS-PAGE of seed proteins revealed that
o
2
introgression
decreased the accumulation of most of the zein proteins except for 27-kDa γ-zein (Zhang et al., 2015; Zhou et al.,
2016).
Pleiotropic effects of
opaque-2
gene has been recognized. In addition to alteration in amino acid composition, it
affects starch organization making the kernel more soft, opaque in appearance and unpleasant taste. A set of
modifier genes (QTLs) in the
opaque 2
genetic background are known to improve hard and vitreous kernels
(Bjarnason et al., 1976; Ortega and Bates, 1983; Burnett and Larkins, 1999) and accumulation of smaller size
protein bodies (Zhou et al., 2016). Inheritance of
o
2
modifiers is complex (Vasal et al., 1980). The modified
opaque 2
mutants reduced the levels of 22 KD α-zeins and bring about 2-3 times higher levels of 27 KD-γ zeins
(Geetha et al., 1991) along with increased hardness of the kernel (Moro et al., 1995). Besides, Krivanek et al.
(2007) reported a series of amino acid modifier genes for improvement of lysine and tryptophan. Subsequently,
several favourable modifiers have been accumulated in
o
2
genetic background to generate modified QPM genetic
stocks for their use in QPM breeding (Sofi et al., 2009).
7 Mapping of
Opaque-2
Gene and Modifier Complexes
A genotype with both
opaque 2
and modifier complexes is needed for biofortification. The
opaque 2
gene has
been identified in the short arm of Chromosome 7 and it is mapped near to the defective endosperm gene ‘DEB
30’ (Holding and Larkins, 2008; Sofi et al., 2009). While, RFLP analysis in an F
2
population indicated association
of two major QTLs with endosperm modification. One of these QTL is located near to the centromere and another
at near to the telomere on the long arm of above seventh chromosome (Lopes et al., 1995). Mapping of
opaque 2
and QTLs conditioning endosperm modification in the same linkage group envisaged considerable
inter se
association which can be harnessed for QPM breeding. In fact, there exist varying degrees of endosperm
