Biology Reference
In-Depth Information
in white maize kernels, yellow maize is known
to accumulate carotenoids in the endosperm, and
is thus a good target for biofortification efforts.
Maize exhibits considerable natural variation for
ratios and concentrations of kernel carotenoids,
with some genotypes accumulating as much as
80 μg/g of total carotenoids in grain on a dry-
weight basis. Several of the carotenoids present
in maize have important roles in human health.
Provitamin A carotenoids (
phate and is primarily responsible for the shift
from white to yellow maize. Once the carotenoid
pathway is activated, two other genes,
LcyE
and
CrtRB1,
have been shown to regulate the accu-
mulation of provitamin A-related compounds.
Lycopene epsilon cyclase converts lycopene into
zeta-carotene and eventually to alpha-carotene
through the action of other associated genes.
Naturally existing mutant alleles of
LcyE
with
reduced functionality have been identified that
proportions more lycopene into the beta-carotene
branch of the pathway, thereby enhancing the
flux towards provitamin A-related compounds
(Harjes et al. 2008).
CrtRB1
is a hydroxy-
lase gene that converts beta-carotene into beta-
cryptoxanthin, whose provitamin A activity is
only half that of beta-carotene. Natural genetic
variation for
CrtRB1
has recently been discov-
ered that results in the retention of more beta-
carotene in the maize endosperm (Yan et al.
2010). Molecular markers have been developed
based on the functional polymorphisms within
the abovementioned three genes, which hold
great potential for accelerated development of
high carotenoid lines in a time- and resource-
efficient manner. As these molecular markers
are located within the target genes and are
truly diagnostic of the allelic constitution, they
offer an efficient means of tracking the favor-
able alleles in backcross or pedigree breeding
programs.
Carotenoid degradation also plays an equally
important role in determining the total carotenoid
accumulation as well as its composition. A num-
ber of maize carotenoid cleavage genes have
now been identified (Vallabhaneni et al. 2010).
ZmCCD1
has been found on chr. 9 (bin 9.07),
which effectively cleaves carotenoids, thereby
depleting the pool.
ZmCCD1
is linked to domi-
nant
white cap1
(
wc1
) locus. Dominant
wc1
alle-
les and a higher copy number of ZmCCD1 result
in low endosperm carotenoid content (Vallab-
haneni et al. 2010). Identification of favorable
alleles of CCD genes will likely add signifi-
cantly to the enhanced retention of endosperm
carotenoids.
β
-cryptoxanthin, and
α
β
-carotene) are the precursors of vita-
min A, which is essential in different systems in
the human body and for the prevention of diet-
related chronic diseases. Lutein and zeaxanthin,
on the other hand, have been associated with low-
ering the risk of cataracts, age-related macular
degeneration, and other degenerative diseases.
The fraction of provitamin A carotenoids is typ-
ically only 10-20%, whereas zeaxanthin and
lutein each commonly represent 30-50% of total
carotenoids in maize (Ortiz-Monasterio et al.
2007). Most yellow maize grown and consumed
throughout the world, however, has only 2 μg/g
or less of provitamin A carotenoids. Based on the
analysis of a wide range of temperate and lim-
ited tropical germplam, it appears that tropical
maize contains more
- and
β
-cryptoxanthin and less
β
-carotene than temperate maize, and because
the emphasis was on enhancing
-carotene con-
centration, most of the initial breeding sources
of high provitamin A germplasm were selected
from temperate regions in the CIMMYT provita-
min A breeding program. However, evaluation of
a wider range of tropical germplasm in the future
may identify potential donors for enhanced
β
β
-
carotene content.
The carotenoid metabolic pathway has been
well researched in model species, and key
genes governing critical steps have been iden-
tified. In maize, of the many genes implicated
in carotenoid metabolism, roles of the follow-
ing three in the final accumulation of useful
carotenoids in the grain are worth mentioning.
Phytoene synthase1 (
Y1/Psy1
) catalyses the first
committed step in the pathway leading to for-
mation of phytoene from geranylgeranyl diphos-
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