Biofortified Maize – A Genetic Avenue for Nutritional Security - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 172

significant problem in the developed and devel-
oping countries.
A number of nutritional approaches could be
explored to deal with the poor availability of phy-
tate P in maize and the resultant potential for P
pollution. Besides adding microbial phytase to
poultry diets to increase phytate P availability,
genetically lowering the phytic acid content of
maize, thereby improving plant P availability,
appears to be a long-term, sustainable biofortifi-
cation approach. In 1996, USDA scientists devel-
oped two non-lethal low-phytate mutants, lpa1-1
and lpa2-1, which are phenotypically identi-
cal to wild-type maize hybrids. These mutants
show 60% (lpa1-1 ) and 50% (lpa2-1) reductions
in phytic acid P, with no reduction in total P
in the seed. Chick studies have demonstrated
that the relative bioavailability of P in lpa1-1
ranged from 45 to 52%, compared with 10% for
wild-type maize. Phosphorus excretion was also
found to be significantly reduced in chicks fed
with the lpa1-1 maize. The lpa1 mutant does
not accumulate myo-inositol monophosphate or
polyphosphate intermediates. It was proposed
that lpa1 is a mutation in myo-inositol supply,
the first part of the phytic acid biosynthesis path-
way (Raboy et al. 2000). However, recent study
by Shi and colleagues (2007) showed that lpa1
mutants are defective in a multidrug resistance-
associated protein (MRP) ATP-binding cassette
(ABC) transporter that is expressed most highly
in embryos, immature endosperm, germinating
seed, and vegetative tissues. The lpa2 mutant
has reduced phytic acid content in seeds and
accumulates myo-inositol phosphate intermedi-
ates. Maize lpa2 gene encodes a myo-inositol
phosphate kinase (ZmIPK) that belongs to the
Ins(1,3,4)P3 5/6-kinase gene family (Shi et al.
2003). In 2005, maize lpa3 mutant was iden-
tified and characterized (Shi et al. 2005). The
lpa3 mutant seeds have reduced phytic acid
content and accumulate myo-inositol, but not
myo-inositol phosphate intermediates. Maize
lpa3 gene was found to encode a myo-inositol
kinase (MIK).
Based on the phytic acid metabolic path-
way,

target intervention points for developing the
low-phytate trait: (1) inositol (Ins) and Ins(3)P1
synthesis, starting with the conversion of glu-
cose 6-P (top left) to Ins(3)P1; (2) phytic acid
synthesis through either a “lipid-independent”
pathway that proceeds via sequential phospho-
rylation of Ins and soluble Ins phosphates, or
a “lipid-dependent” pathway that uses precur-
sors that include phosphatidylinositol (PtdIns)
and PtdIns phosphates; (3) transport and stor-
age of phytin salts in globoids; and (4) phytase-
encoding transgenes.
Although lpa mutations-based approaches
significantly reduce phytic acid levels in maize
seeds, considerable difficulties are often encoun-
tered because of negative unintended effects on
seed and plant performance, such as compro-
mised germination, emergence, stress tolerance,
and grain filling. Although field trials of the first
set of isogenic maize hybrid pairs obtained fol-
lowing four generation of backcrosses to a nor-
mal parent indicated only 6% yield reduction
(Ertl et al. 1998), later observations on fully iso-
genic pairs, derived after six back crosses clearly
indicated large negative effects on flowering
date, germination, and stress tolerance (Raboy
2009). A recent effort, which identified lpa1
locus coding for an ABC transporter gene, engi-
neered embryo-specific suppression of inositol
phosphate metabolism, which effectively avoids
systemic disruption of phytic acid accumulation,
thereby enabling targeted reduction of phytic
acid levels in seeds with no undesirable conse-
quences on whole plant performance (Shi et al.
2007). Another option is to utilize recently iden-
tified mutants such as lpa3 , which encodes an
Ins kinase whose expression is highly embryo-
specific. Although lpa3 may not affect house-
keeping vegetative Ins phosphate metabolism,
seed chemistry changes may still lead to unfore-
seen consequences that impact crop quality or
performance.
Genetic manipulation of maize kernels to
accumulate high levels of “phytase” is an
alternative and potentially very powerful
approach to problems associated with seed phy-
tate. In an attempt to enhance the bioavailability

Raboy (2009) proposed four potential