Agriculture Reference
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ascorbate redox state seems to be a critical
factor regulating various processes, stress
responses and plant-pathogen interactions
(Davey et al. , 2000; Sanmartin et al. , 2003;
Fotopoulos et al. , 2006, 2008; Ioannidi et
al. , 2009; Fotopoulos and Kanellis, 2013).
The main operational ascorbate bio-
synthetic pathway in tomato ( Solanum
lycopersicum ) fruit tissues was recently
shown to be the mannose/ L -galactose one
(Mellidou et al. , 2012). A combination of
metabolite analyses, non-labelled and
radio-labelled substrate feeding experi-
ments, enzyme activity measurements and
gene expression studies showed that, in the
low-AsA cultivar ('Ailsa Craig'), alternative
routes of AsA biosynthesis may sup-
plement biosynthesis via L -galactose, whilst
in the high-AsA cultivar ('Santorini'),
enhanced AsA recycling activities appeared
to be responsible for AsA accumulation in
the later stages of ripening. Furthermore, it
was shown that GDP- L -galactose phos-
phorylase 1 ( SlGGP1 ) and two orthologues
of NAD(P)-dependent monodehydroascor-
bate reductase ( SlMDHAR ) were closely
correlated with AsA+DHA concentrations
during ripening and are potentially good
candidates for breeding purposes and
marker selection (Mellidou et al. , 2012). It
was suggested that the AsA pool is always
regulated by the combination of synthesis,
recycling and breakdown, without ex-
cluding a possible feedback inhibition of
biosynthesis. It should be noted that no
accumulation of a high ratio of AsA:total
AsA early in fruit development and
ripening has been recorded, even though
AsA biosynthetic capacity is enhanced.
However, AsA can accumulate later, with
the onset of ripening (breaker stage), as a
result of increased biosynthesis (Ioannidi
et al. , 2009), recycling and decreased
breakdown (Mellidou et al. , 2012).
Based on previous data (Ioannidi et al. ,
2009; Mellidou et al. , 2012), overexpression
of GDP- L -galactose phosphorylase ( GGP or
VTC2 ) and L -galactose-1-phosphate phos-
phatase ( GPP or VTC4 ) under the control of
three different promoters ( 35S , a con-
stitutive promoter; phosphoenolpyruvate
carboxylase ( PPC2 ), a green fruit-specifi c
promoter; and polygalacturonase ( PG ), a
ripening fruit-specifi c promoter) resulted in
T3 transgenic red tomato fruit with
threefold increased levels of AsA (Kanellis
et al. , unpublished data). Lastly, sup-
pression of ascorbate oxidase in melon
resulted in signifi cant increase in AsA, but
at the same time, it caused a reduction in
fruit size in agreement with suggested
participation of ascorbate oxidase in the
fast growth of Curcubitaceae fruit (Kanellis
et al. , unpublished data). An analytical
description of vitamin C biosynthesis and
regulation in fl eshy fruits is provided by
Baldet et al. , Chapter 8, this volume.
7.2.4 Vitamin E
Vitamin E includes tocopherols and
tocotrienols. These occur in eight different
forms (four tocopherols and four
tocotrienols). All the isomers have
aromatic rings with a hydroxyl group that
can donate hydrogen atoms to reduce ROS.
The different isomers are named D , E , J and
G related to the number and position of
methyl groups in the ring. Each of the
forms has its own vitamin E activity, with
D -tocopherol being the more active. In
general, vitamin E levels are more
abundant in oily seeds, olives, nuts,
peanuts, avocados and almonds. Vitamin E
is highly susceptible to oxidation during
storage and processing. An analytical
description of vitamin E biosynthesis and
regulation in fl eshy fruits is provided by
Baldet et al. , Chapter 8, this volume.
7.2.5 Terpenes
Terpenes are a large group of phyto-
chemicals and are derived from isoprene
units (C5H8) that can be linked to form
carbon skeletons such as C5, C10, C15, C20
up to C40. According to Nobelist Leopold
Ruzicka, the isoprene units may be linked
together 'head to tail' to form linear chains
or they may be arranged to form rings. The
number of isoprene units (C5H8) n is used
 
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