Author(s): Konsam Sarika 1, Firoz Hossain 1,*, Vignesh Muthusamy 1, Rajkumar U. Zunjare 1, Aanchal Baveja 1, Rajat Goswami 1, Nepolean Thirunavukkarasu 1, Sunil K. Jha 2, Hari S. Gupta 1
Maize is one of the most important food cropsin sub-Saharan African, Latin American and many of the Asian countries.It is also an important source of poultry and livestock feed worldwide. Storage protein of maize, prolamin also known as zein, constitutes about 70% of the total protein. Prolamin is characterized by limiting level of two essential amino acids, lysine and tryptophan[3,4]. Maize, therefore, being poor in nutritional quality does not provide balanced nutrition to human and mono-gastric animals such as poultry and pig. A mutation, opaque2 (o2 ) discovered in 1920s was found to be nutritionally superior in lysine and tryptophan compared to normal maize . However, the improvement in the quality was deterred by the pleiotropic effects of the mutant that causes soft endospermmaking the kernel more prone to insect infestation and pathogen susceptibility with poor processing quality and reduced yield. Several other genetic mutations viz., floury1 (fl1 ),floury2 (fl2 ), floury3 (fl3 ), opaque5 (o5 ), opaque6 (o6 ), opaque7(o7 ), opaque15 (o15 ), Defective endosperm (Def-B30 ), Mucronate (Mc) that affect the lysine content in maize endosperm, have been discovered. Different combinations of these mutants to further increase the lysine and tryptophan were also tried, but could not succeed due to adverse pleiotropic effect that imposed severe constraints in implementing them[8,9].
Researchers found that the opaqueness caused due to o2 can be overcome with the accumulation of o2 -modifiers and led to the development of Quality Protein Maize (QPM) with improved lysine content from 0.15 to 0.37% and tryptophan from 0.04 to 0.08% on average [10,11]. The exact mechanism of the o2 endosperm modification in QPM is not known but a possible role of 27-kDa [gamma]-zein in recovering the vitreous phenotype has been put forward . Genetic mapping of o2 modifiers in QPM was found to be the locus encoding linked with 27-kDa [gamma]-zein storage proteinson chromosome 7. Wu and Messing later demonstrated that silencing of 27- and 16-kDa [gamma]-zein genes resultin clumping of protein bodies and thus opacity of QPM seeds.
Yang et al. discovered a recessive mutant from Robertson's Mutator stocks and named it temporarily as opaque16 (o16 ). The o16 located on chromosome 8 induces higher lysine content compared to normal maize. The locus o16 in o2o2 genetic background increases lysine by ~30% over o2o2 or o16o16 alone. In our earlier studies, genotype with o16o16 possessed nearly on average two-fold more lysine (0.247%) and tryptophan (0.072%) compared to normal maize (0.125% lysine and 0.035% tryptophan). The effect of o16 on higher accumulation of lysine was also reported by Zhang et al.[16,17]. Yang et al.  reported the presence of opaque phenotype in two o16 -based inbreds. However, the effects of o16 on degree of influence on endosperm opaqueness, hardness, zein profile and organization of starch granules with proteinaceous matrix in kernel in...