Biofortified Maize – A Genetic Avenue for Nutritional Security - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 165

O2 gene was cloned using a transposon tagging
strategy with the maize mobile genetic elements,
Spm (Schmidt et al. 1987) and Ac (Motto et al.
1988). The O2 gene encodes a leucine-zipper
class transcription factor required mainly for the
expression of 22 kDa

and 2.7-4.5% in QPM. Three genes associated
with lysine level have been mapped to locations
on chromosomes 2, 4, and 7, besides several
major o2 modifier-QTLs on chromosomes 1, 7,
and 9 (Gibbon and Larkins 2005). Therefore, it
is possible to get favorable responses to selec-
tion for endosperm texture modification as well
as relative content of the essential amino acids, if
they are monitored efficiently, during the QPM
breeding programs.

-zein-coding genes and
a gene encoding a ribosomal inactivating pro-
tein (Lohmer et al. 1991; Bass et al. 1992).
Genotypes with the homozygous recessive allele
( o2/o2 ) have a significant decrease in produc-
tion of

α

α

-zeins and a corresponding increase
in non-zein proteins that are rich in lysine and
tryptophan (Gibbon and Larkins 2005). Addi-
tionally, the recessive allele of the O2 transcrip-
tion factor also reduces the production of the
enzyme lysine keto-glutarate reductase, involved
in free lysine degradation, resulting in enhanced
free lysine in the endosperm of o2 maize. In
the segregating generations, this recessive allele
is selected either visually (identifying mosaic
ears on F 2 harvests) or using molecular mark-
ers. The endosperm hardness modifier genes,
which convert the soft/opaque endosperm to
a hard/vitreous endosperm without much loss
of protein quality, are selected through a low
cost but effective method of light-box screen-
ing, where light is projected through the vit-
reous grains or blocked by the opaque grains.
Endosperm modification is polygenically con-
trolled. However, genetic and molecular anal-
yses revealed some major loci involved in o2
modification; for example, one locus maps near
the centromere of chromosome 7 and the sec-
ond maps near the telomere on the long arm of
chromosome 7 (Lopes et al. 1995). Despite the
presence of o2 and associated endosperm hard-
ness modifier genes, the lysine and tryptophan
levels in segregating families vary widely, indi-
cating the existence of third set of genes that
modify the amino acid content, which neces-
sitates systematic biochemical evaluation of
lysine and/or tryptophan levels in each breeding
generation (Nurit et al. 2007). The lysine con-
tent of normal maize is around 2%, whereas it
is approximately 4% (of the total protein) in
QPM, with a range 1.6-2.6% in normal maize

MolecularMarker-AssistedQPMBreeding

The transfer of the QPM trait into elite maize
lines is not straightforward, in that the o2 allele
has to be in homozygous recessive state along
with the polygenic endosperm modifiers. There-
fore, this process is influenced by three major
factors: (1) each conventional backcross gen-
eration needs to be selfed to identify the o2
recessive gene and a minimum of six back-
cross generations are required to recover satis-
factory levels of the recurrent parent genome,
(2) in addition to maintaining the homozygous
o2 gene, multiple endosperm modifiers must
also be selected, and (3) rigorous biochemical
tests to ensure enhanced lysine and tryptophan
levels in the selected materials in each breed-
ing generation require enormous labor, time,
and financial resources. Although conventional
breeding procedures have been used to convert
commercial lines to QPM forms, these proce-
dures are tedious and time-consuming. Rapid
advances in genomics research and technolo-
gies have led to the use of marker-assisted selec-
tion (MAS), which holds promise in enhancing
selection efficiency and expediting the develop-
ment of new cultivars with higher yield potential
(Ribaut and Hoisington 1998; Xu and Crouch
2008). While marker-assisted foreground selec-
tion (Tanksley 1983; Melchinger 1990) helps in
identifying the gene of interest without exten-
sive phenotypic assays, marker-assisted back-
ground selection (Young and Tanksley 1989;
Frisch et al. 1999a, Frisch et al. 1999b) signifi-
cantly expedites the rate of genetic gain/recovery