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inexpensive synthetic Lys and Trp additives. In
contrast, methionine (Met) is the first limiting
amino acid in maize-based poultry feed that is
available only in synthetic form and at expen-
sive rates. The poultry industry, especially in
Asia, where the demand is significantly rising, is
keenly looking for new maize types - maize for
poultry feed - with an improved level of methio-
nine and high oil content (Hellin and Eren-
stein 2009). Efforts are being made by several
institutions, including CIMMYT, to characterize
maize germplasm for Met content (e.g., Scott and
Blanco 2009) as well as for enhancing Met con-
tent in elite maize germplasm through recurrent
selection (Scott et al. 2008).
ing the chances of foreign pollen landing on
QPM silks (Burris 2001). Using geostatistical
models (“kriging”) under different environmen-
tal conditions, Machida and colleagues (2012)
recently demonstrated that the weighted average
levels of outcrossing (less than 20%) is far less
than previously thought; hence, a greater part
of the QPM grain will not be outcrossed when
grown near a NM crop. Also this study suggested
that the nutritional value of the QPM trait can
be optimally sustained if farmers plant relatively
large areas in a square rather than a rectangular
field. Where absolute or near-absolute isolation
from NM is difficult to achieve, the proponents
of QPM varieties can confidently advocate for
the coexistence of QPM and NM crops.
ImpactandReachofQPMintheDeveloping
World
Possibilities for Genetically Engineered
High-Lysine Maize
In Ethiopia, Tanzania, and Uganda, randomized
trials have shown significantly improved height
and weight of children consuming such varieties,
particularly in Southern Ethiopia, where the pop-
ulation relies heavily on maize (Gunaratna et al.
2008). Despite demonstrated significant food
and feed benefits of QPM, large-scale cultivation
in farmers' fields is yet to be realized in many
developing countries that have released QPM
cultivars. There is an array of reasons for this.
As with most nutritional traits, enhanced amino
acid content is a “hidden trait,” which generally
does not command a premium price in the mar-
ket; hence, any QPM cultivar recommended for
commercial cultivation should be as high yield-
ing or better than the normal cultivars already
present in the market.
An additional challenge with respect to
widespread adoption of QPM is the recessive
nature of the
o2
gene. If the QPM cultivar is
pollinated by normal maize (NM) pollen, there
may be a loss of high protein quality resulting in
erosion of the trait in farmer-saved seed systems.
However, one of the factors contributing to low
levels of outcrossing is the copious amounts of
desirable pollen from the QPM that compete for
the silks with the foreign pollen, thereby reduc-
Genetic engineering efforts targeted towards
enhancement of lysine content in maize kernels
can be broadly classified into two categories: (1)
anti-sense suppression (RNAi) of alpha-zein pro-
duction in transgenic maize, leading to at least
twice the amount of lysine; and (2) deregula-
tion of the aspartate metabolic pathway through
either feedback-insensitive mutant enzymes or
suppressing the catabolic reactions downstream
to lysine synthesis.
RNA interference (RNAi) technology is par-
ticularly aimed at developing a dominant
o2
trait
in maize and has been used specifically to reduce
22-kDa (Segal et al. 2003) and 19-kDa alpha
zeins (Huang et al. 2004 and Huang et al. 2005),
which resulted in moderate increases (15-20%)
in lysine content. In a subsequent study, using
an improved double strand RNA (dsRNA) sup-
pression construct, Huang and colleagues (2006)
reported lysine and tryptophan levels similar to
conventionally bred QPM genotypes. While the
dominant nature of the anti-sense transgene is
a definite advantage as compared to the reces-
sive allele of
o2
, the opaque endosperm still
needs to be modified by endosperm modifier
genes whose epistasis with the transgene has not
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