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In-Depth Information
negative feedback control (Diretto
et al
., 2010).
Mixed results have been reported about the ex-
tent that tuber carotenoid expression is con-
trolled at the transcriptional level (Goo
et al
.,
2009; Zhou
et al
., 2011; Payyavula
et al
., 2012).
Substantial differences were observed in gene-
metabolite relationships in leaves versus tubers
(Diretto
et al
., 2010). Recent evidence suggests
the expression of some carotenoid genes may be
influenced by tuber sugar concentrations (Zhou
et al
., 2011).
Carotenoids have provitamin A activity and
decrease the risk of several diseases (Fraser and
Bramley, 2004). The most abundant potato ca-
rotenoids, lutein and zeaxanthin, are important
for eye health, and reduce the risk of age-related
macular degeneration (Chucair
et al
., 2007; Tan
et al
., 2008). The type and quantity of potato ca-
rotenoids varies by cultivar, and the yellow to al-
most orange flesh color found in some potatoes is
due to carotenoids. Lutein amounts influence
the intensity of yellow coloration, and zeaxan-
thin influences the orange coloration (Brown
et al
., 1993). Analysis of eleven
Chy2
(b-carotene
hydroxylase 2) alleles in potato showed that just
one dominant allele was responsible for chan-
ging white flesh to yellow flesh (Wolters
et al
.,
2010). Five
Zep
(zeaxanthin epoxidase) alleles
were detected, one of which was recessive and
responsible for the orange flesh color, but only in
the presence of the dominant
Chy2
. The authors
suggested the reduced Zep activity in or-
ange-flesh genotypes was not due to reduced en-
zymatic activity but to reduced amounts due to
inefficient splicing (Wolters
et al
., 2010). An
additional level of control over tuber carotenoid
accumulation may be exerted by the suborga-
nellar compartmentation, as Chy2 and PSY2
were localized to chloroplasts in leaves, but ves-
icular structures inside amyloplasts in tubers
(Pasare
et al
., 2013). Tuber carotenoid concen-
trations are also controlled by catabolism; reducing
the expression of a carotenoid cleavage dioxy-
genase resulted in a two- to fivefold increase in
carotenoids (Campbell
et al
., 2010).
Some cultivated diploid potatoes derived
from
Solanum stenotomum
and
Solanum phureja
contained 2000 μg
100
g
-
1
FW of zeaxanthin
(Brown
et al
., 1993). Total carotenoid concen-
trations in
74
Andean landraces ranged from
3
to
36
μg g
-
1
DW (Andre
et al.
, 2007a). A screen
of
24
Andean cultivars identified genotypes
with almost
18
μg g
-
1
DW each of lutein and ze-
axanthin and over
2
μg g
-
1
DW of b-carotene
(Andre
et al
., 2007b). Sixty mostly Spanish culti-
vars were found to contain 0.50-15.5 μg g
-
1
DW, but curiously no zeaxanthin was found,
possibly due to prolonged storage in the dark
(Fernandez-Orozco
et al
., 2013). White-flesh po-
tatoes contain less carotenoids than the yellow
or orange cultivars. One study found white culti-
vars had
27-74
μg
100
g
-
1
FW of carotenoids
(Iwanzik
et al
., 1983).
Burgos
et al
., reported that violaxanthin
and antheraxanthin in tubers decreased after
boiling, but lutein and zeaxanthin did not
(Burgos
et al
., 2012). In addition to carotenoid
degradation, substantial isomerization was ob-
served in boiled tubers (Burmeister
et al
., 2011).
Declines in carotenoids during cold storage can
occur (Morris
et al
., 2004; Griffiths
et al
., 2007).
The broad-sense heritability in potato was
0.96 for total carotenoids, suggesting that breed-
ing should be able to increase the amounts
significantly (Haynes
et al
., 2010). Besides
breeding, transgenic approaches can increase
potato carotenoids substantially. A bacterial
PSY
overexpressed in tubers increased carotenoids
from 5.6 to
35
μg g
-
1
DW and b-carotene con-
centrations from trace amounts to
11
μg g
-
1
DW
(Ducreux
et al
., 2005). Overexpression of the
cauliflower
Or
gene in Desiree resulted in a six-
fold increase in tuber carotenoids to about
20-
25
μg g
-
1
DW (Lu
et al
., 2006). Overexpression of
three bacterial genes in the cultivar Desiree re-
sulted in a
20-
fold increase in total carotenoids
to
114
μg g
-
1
DW and an impressive 3600-fold
increase in b-carotene to
47
μg g
-
1
DW (Diretto
et al
., 2007).
18.9 Conclusion
Since their introduction from the Andes, the cul-
tivation of potatoes in the West has helped cur-
tail hunger for over
200
years, and they have
been the staple food of generations of people. Po-
tatoes have even been linked to marked popula-
tion growth in Europe in the
17-
1800s. Today,
the potato finds itself in unfamiliar territory, as
some nutritionists question the healthfulness of
carbohydrate-rich staples like potatoes and
speculate based only on epidemiological data
that potatoes are linked to increasing diabetes
