Agriculture Reference
In-Depth Information
The biosynthetic pathway of vitamin B
3
is
not fully resolved in plants. An anticipated path
runs from asparagine through a-aminosucci-
nate via the quinolinate and pyridine nucleotide
cycles (Katoh
et al
., 2006; Roje, 2007), and nia-
cin biosynthetic activities have been reported re-
cently for potato tuber (Katahira and Ashihara,
2009). Mammals can make vitamin B
3
from
tryptophan in the liver (and some species can
also produce it in the kidney) (Kurnasov
et al
.,
2003); however, humans cannot synthesize tryp-
tophan
de novo
and need to have either the amino
acid or the vitamin supplied with their food.
The RDA values for women and men are
14
and
16
mg, respectively (Otten
et al
., 2006)
(
Table 18.1
). A
100
g boiled or baked potato pro-
vides ~10% of the RDA (
Table 18.2
). However,
French fries appear to be a relatively poor source
(~ 7% RDA
100
g
-1
), while chip snacks have a
comparably high content (~15% RDA
100
g
-
1
;
Table 18.2
). Niacin is highly stable as a nutrient
and comparably resistant to heat, light, air, acid,
and alkali (Richardson, 1990).
Little information is available for niacin
content variation in different potato germplasm;
those that exist focus only on a few cultivars
with no major variation (e.g. Pontiac and Seba-
go with 1.2 and 1.3 mg
100
g
-
1
edible portion;
Wills
et al
., 1984). No transgenic approaches
have been published for generating potatoes
with modified enzyme activities in the B
3
biosyn-
thetic pathway, and only very little knowledge is
established for plants in general. However, the
few existing reports indicate that plants are de-
cidedly sensitive to any deviations from normal
B
3
biosynthetic activities. For instance, loss-of-
function
Arabidopsis thaliana
mutants affected in
the biosynthesis of quinolinate have reduced
NAD
+
contents and senesce early (Schippers
et al
., 2008). Upregulation of NAD
+
synthetase,
a key enzyme in the pyridine nucleotide cycle,
led to increased niacin and nicotinamide levels
in Arabidopsis, but surprisingly not NAD
+
.
Again, however, the transgenic plants showed
early senescence and also early flowering pheno-
types, and were affected negatively in seed devel-
opment (Shinnosuke
et al
., 2010).
The importance of niacin in human nutri-
tion makes increasing vitamin B
3
content in po-
tato a beneficial trait for this crop, especially
since nixtamalization is not required to have
good access to the phytonutrients; however, the
clear negative impacts on plant growth and de-
velopment strongly suggest that it will be diffi-
cult to generate plants with elevated vitamin B
3
levels by transgenic approaches without signifi-
cantly affecting other biological processes. It
might be advantageous to investigate variation
in existing germplasm to establish marker-assisted
programs as an alternative approach.
Vitamin B
5
(pantothenate)
4¢-
phosphopantetheine, a moiety of the coen-
zyme A (CoA) that serves as a prosthetic group
to the acyl carrier protein and participates as an
enzymatic cofactor in various biochemical re-
actions related mostly to lipid and fatty acid me-
tabolism (Rebeille
et al
., 2007). Pantothenate is
synthesized
de novo
in bacteria and plants, and
biochemical studies indicate similar pathways
are used (Coxon
et al
., 2005; Roje, 2007). Imme-
diate precursors of pantothenate in both organ-
isms appear to be b-alanine and l-pantoate,
which are condensed to vitamin B
5
with ATP
consumption by pantothenate synthetase
(Chakauya
et al
., 2008; Jonczyk
et al
., 2008).
Insufficient pantothenate appears to be un-
common among humans and is rarely re-
ported, but symptoms may include burning
foot syndrome, irritability, fatigue, and apathy
(Tahiliani and Beinlich, 1991; Bender, 1999).
Based on its critical function in lipid and fatty
acid metabolism, it is not surprising that loss of
the biosynthetic pathway is lethal, as shown by
Arabidopsis deficient in pantothenate synthase
that become arrested in embryo development
(Jonczyk
et al
., 2008).
Likely because malnutrition with vitamin
B
5
is not a common problem, limited data about
adequate intake are available, which is esti-
mated to be
5
mg day
-
1
for adult women and
men (
Table 18.1
)
. A
100
g serving of baked or
boiled potatoes provides around 10% of the
RDA (
Table 18.2
). Variation among potato ger-
mplasm is unknown. Recent findings showed
that genetic engineering of the pantothenate
content was possible in plants (Chakauya
et al
.,
2008). However, the vitamin B
5
levels in trans-
genic oilseed rape (
Brassica napus
) appear to cor-
relate rather negatively with triacyl-glyceride
