Agriculture Reference
In-Depth Information
contents (Chakauya
et al
., 2008); therefore, it
may not be desired to increase pantothenate
levels in plants.
ribulose-
5-
phosphate or ribose-
5-
phosphate), a
triose-phosphate (either glycerinaldehyde-
3-
phosphate or dihydroxyacetone-phosphate), and
glutamate to synthesize PLP. The pathway depends
on two enzymes, PDX1 and PDX2 (for pyridoxine
biosynthesis
1
and 2), which assemble to a mul-
timeric PLP synthase complex of
12
PDX1 and
12
PDX2 proteins (Strohmeier
et al
., 2006).
A recent study indicated that B
6
deficiency
was relatively common in the USA (Morris
et al
., 2008). Potato is a very good source of
vitamin B
6
, and analysis of B
6
content in pro-
cessed food demonstrates that the vitamin can
also withstand high temperatures like baking
or cooking. USDA nutritional values indicate
that
100
g of baked or boiled potatoes provide
~20% of the RDA values for adult women and
men (1.3-1.5 and 1.3-1.7 mg day
-
1
,
respect-
ively;
Table 18.1
) (Otten
et al
., 2006). A recent
analysis of vitamin B
6
variation among differ-
ent genotypes of immature (baby) potatoes
showed a 30% variation from 16.9 μg vitamin
B
6
g
-
1
DW (PORO07PG63-1) to
22
μg (Ruby
Crescent). In addition, analysis of mature tuber
genotypes revealed a similar degree of vari-
ation from 18.6 μg (PORO07PG63-1) to
27
μg
(Clearwater Russet) (Mooney
et al
., 2013). This
study underscores not only the existing vari-
ation of vitamin B
6
in different cultivars, and
that potato is a very good nutritional resource
for vitamin B
6
, but also the potential to use
these genotypes for breeding efforts to improve
phytonutrient content in potato. Increasing
vitamin B
6
content in plants by transgenic ap-
proaches has been demonstrated recently by
several groups (Chen and Xiong, 2009; Leuen-
dorf
et al
., 2010; Raschke
et al
., 2011). Inter-
estingly, increased B
6
content also correlated
positively with seed size (Raschke
et al
., 2011),
and it would be interesting to see whether simi-
lar results could be accomplished with potatoes.
Based on the comparably easy transformation
of potato, it would be practical to generate
transgenic plants with
dominant
traits that lead
to elevated or reduced B
6
contents; this would
also bypass the comparably difficult genetic
situation of this crop. An interesting aspect
about B
6
is its involvement in a-glucan phos-
phorylase activity, a PLP-dependent key en-
zyme in starch degradation (Mori
et al
., 1991).
It would be informative to investigate whether
reduced B
6
content in the tuber can affect
Vitamin B
6
(pyridoxine)
Vitamin B
6
is the generic term for a group of
three chemically related compounds, pyridoxine
(PN), pyridoxal (PL), and pyridoxamine (PM),
which only differ in a variable group at the
4¢
position. PN carries a hydroxyl group, while PL
and PM have an aldehyde and an amino group,
can be phosphorylated at their
5¢
position,
which is a required step to convert them to their
biologically active forms as cofactors in enzym-
atic reactions. Pyridoxal-
5-
phosphate (PLP) is
thought to be the primary isoform (Amadasi
et al
., 2007).
Vitamin B
6
is required for more than
140
biochemical reactions in the cell that are related
mainly to amino acid and carbohydrate metab-
olism (Percudani and Peracchi, 2009), but it
also plays a role in the biosynthesis of other vita-
mins like B
8
, B
9
, and B
12
(Battersby and Leeper,
1998; Basset
et al
., 2004; Pinon
et al
., 2005).
Because of its versatile functions as a cofactor, it
is not surprising that the vitamin is critical for
human health. Insufficient supply of the vita-
min can lead to neurological disorders, diabetes,
and skin diseases like pellagra (for a comprehen-
sive overview on vitamin B
6
and human health
issues see Hellmann and Mooney, 2010). Note-
worthy is that the vitamin is also an antioxidant
that rivals other vitamins like C and E in its anti-
oxidative capacity (Ehrenshaft
et al
., 1998; Bilski
et al
., 2000).
Two different biosynthetic pathways are
known in plants. One is the “salvage pathway”
that allows “recycling” or conversion of B
6
vitam-
ers to their active phosphorylated forms, and
which depends on the concerted activities of vita-
min B
6
dehydrogenases and kinases (Denslow
et al
., 2005; di Salvo
et al
., 2011). The second
de
novo
biosynthesis pathway has only recently been
characterized in plants (Tambasco-Studart
et al
.,
2005), but since then has been studied inten-
sively by several groups (Chen and Xiong, 2005,
2009; Wagner
et al
., 2006; Leuendorf
et al
.,
2010). It employs a pentose-phosphate (either
