Vitamins in Fleshy Fruits - Fruit Ripening: Physiology, Signalling and Genomics - page 139

D -Glucose

HK

D -Glucose-6-P

PGI

D -Fructose-6-P

PMI

D -Mannose-6-P

Cell wall polymers

PMM

D -Mannose-1-P

GMP

Methyl- D -galacturonic acid

GDP- D -Mannose

ME

→

5 epimerization GME 3 5 epimerization

myo -Inositol

GDP- L -Gulose GDP- L -Galactose

MIOX

D -Galacturonic acid

GGulP

D -Glucuronic acid GGP

GalUR

L -Gulose-1-P L -Galactose-1-P

GluUR

GulPP GPP

L -Gulonic acid

L -Galactonic acid

L -Gulose L -Galactose

AL

GulDH GDH

AL

L -Gulono-1,4-lactone L -Galactono-1,4-lactone

GalLDH

GR

GulLDH OH

GSH

GSSG

O

HO DHAR

MDHAR

O

Monodehydroascorbate Dehydroascorbate

APX, AO

HO OH

Degradation pathway

L -Ascorbic acid

Fig. 8.3. Biosynthetic routes to L -ascorbic acid and the recycling reactions in plants. The corresponding
enzymes and metabolites are: HK, hexokinase; PGI, D -glucose-6-phosphate isomerase; PMI, D -mannose-6-
phosphate isomerase; PMM, phosphomannomutase; GMP, GDP- D -mannose pyrophosphorylase; GME,
GDP- D -mannose-3 c ,5 c -epimerase; GGP, GDP- L -galactose phosphorylase; GPP, L -galactose-1-phosphate
phosphatase; GDH, L -galactose dehydrogenase; GalLDH, L -galactono-1,4-lactone dehydrogenase; ME,
methylesterase; GalUR, D -galacturonate reductase; AL, aldono-lactonase; GGulP, GDP- L -gulose
phosphorylase; GulPP, L -gulose-1-P phosphatase; GulDH, L -gulose dehydrogenase; GulLDH, L -gulono-1,4-
lactone dehydrogenase; MIOX, myo -inositol oxygenase; GluUR, D -glucuronate reductase; AL, aldono-
lactonase; APX, ascorbate peroxidase; AO, ascorbate oxidase; MDHAR, monodehydroascorbate reductase;
DHAR, dehydroascorbate reductase; GR, glutathione reductase; GSSG, oxidized glutathione; GSH, reduced
glutathione. P, phosphate.

ascorbate (Agius et al. , 2003), is remarkable
as D -galacturonic acid is produced by the
degradation of pectins from the cell wall
during fruit ripening (see Tucker, Chapter
4, this volume). In addition, two
Arabidopsis transcription factors, the F-box
E3 protein AMR1 (ascorbic acid mannose
pathway regulator 1) and the ERF98
(ethylene response factor 98) protein, were
suggested to regulate the expression of the
D -mannose/ L -galactose pathway genes at
the transcriptional level and to modulate
ascorbate levels in the leaf in response to
abiotic stress (Zhang et al. , 2009, 2012).

Such ascorbate biosynthetic and regulatory
genes are putative targets for under-
standing the regulation and manipulation
of the levels of vitamin C in fl eshy fruit
species.

8.5 Enhancing Vitamin Content in Fleshy
Fruits

Carotenoids, tocochromanols (tocopherols
and tocotrienols) and ascorbate are among
the main non-enzymatic antioxidants
found in plants, in which they play crucial