Agriculture Reference
In-Depth Information
to prevent certain skin lesions, like pellagra in
animals and humans (Birch
et al
., 1935). How-
ever, it soon became clear with progressing re-
search that vitamin B actually represents a group
of compounds that are now collectively called
the “vitamin B complex”. B vitamins are classi-
fied nowadays as a group of eight water-soluble
compounds that are chemically quite distinct,
and to which belong vitamin B1 (thiamine), B2
(riboflavin), B3 (niacin), B5 (pantothenic acid),
B6 (pyridoxine), B8 (biotin), B9 (folic acid), and
B12 (various cobalamins) (Roje, 2007). While
none of the B vitamins can be synthesized
de novo
by humans, plants have the necessary enzymatic
machinery to synthesize these vitamins, except-
ing vitamin B
12
(Fitzpatrick
et al
., 2012). The fol-
lowing section will summarize the current know-
ledge about their biosynthesis and functions in
plants and animals. It will also address to what
extent potatoes represent a major resource for
these vitamins, and what the potential is to in-
crease B-vitamin contents in potato tubers as
beneficial phytonutrients in the human diet.
(Roje, 2007). Here, precursors include
2-
methyl-
4-
amino-
5-
hydroxymethylpyrimidine diphosphate
and
4-
methyl-
5-
b-hydroxyethylthiazole phos-
phate, which are condensed by the activity of
thiamine phosphate pyrophosphorylase to thia-
mine monophosphate (TMP). TMP is then further
metabolized to the active cofactor by the con-
certed activities of a TMP phosphatase and a thia-
mine pyrophosphokinase (Rapala-Kozik
et al
.,
2008, 2012; Goyer, 2010).
The RDA values for thiamine consumption
in the USA for adult women and men (ages
18
and older) are 1.1 and 1.2 mg day
-
1
,
respectively
(Table 18.1
) (Otten
et al
., 2006). The thiamine
content in
100
g baked or boiled potato tubers
provides ~10% RDA. Interestingly, potato chips
have a significantly higher content, about 15%
RDA in
100
g. This higher content in potato
chips is consistent for all the other B vitamins
discussed in this chapter, and is likely due to
their dryness in comparison to potato products
with high water content.
The first substantial screening of potato
germplasm found a range of B
1
from 0.062 to
0.090 mg
100
g
-
1
FW (Augustin
et al
., 1978;
Holland
et al
., 1996). More recent screening of
cultivars or primitive germplasm identified geno-
types containing up to 0.132 and 0.232 mg
100
g
-
1
FW of thiamine, respectively (Goyer and
Sweek, 2011). Thiamine concentrations vary
during tuber development, and are consistently
higher in small tubers harvested early in the sea-
son than in large tubers harvested later in the
season in most of the
54
varieties analyzed. Over
a 2.4-fold increase between small and large
tubers was observed for some varieties (Goyer
and Haynes, 2011).
Vitamin B
1
(thiamine)
Vitamin B
1
, or thiamine, has critical roles in its
phosphorylated form, thiamine-diphosphate
(TDP;
Fig. 18.3a
), as a cofactor in carbohydrate
and amino acid metabolism (Jordan, 2003;
Settembre
et al
., 2003; Nosaka, 2006), and in
response to abiotic and biotic stresses (Gossert
et al
., 2008). It is also called “aneurin” in reflec-
tion of neurological disorders that can occur
when deficient (Wang and Yudkin, 1940). In
general, severe vitamin B
1
deficiencies cause
pleiotropic symptoms in humans, including in
the gastrointestinal, muscular, and cardiovascu-
lar systems. The disease is commonly described
as “beriberi”, and affected people become lethar-
gic and fatigued (Lonsdale, 2012). In plants,
thiamine appears also to have critical roles in
both biotic and abiotic stress responses, since
biosynthesis is upregulated on such stresses and
appears to be critical for stress adaptation (Wang
et al
., 2006; Tunc-Ozdemir
et al
., 2009; Rapala-
Kozik
et al
., 2012).
Thiamine biosynthesis in plants is not fully
understood, and current knowledge indicates the
pathway is related to that of bacterial organisms
Stability and broad-sense heritability
Thiamine is one of the few metabolites for which
stability and broad-sense heritability have been
estimated in potato. One study showed that gen-
etic variation accounted for about 50% of the
thiamine variation observed in a group of
39
po-
tato clones grown in four environments, as de-
termined by calculating broad-sense heritability
for this group of clones as 0.49 with a 95% con-
fidence interval of 0.21-0.72 (Goyer and
Haynes, 2011). There were significant genotype
and genotype × environment effects. After ac-
counting for environmental variation,
25
clones
