Agriculture Reference
In-Depth Information
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Fig. 13.2
Possible vacuolar scavenging mechanisms of fructans and sucrose in oxidative stress
defense.
A
apoplast,
AsA
ascorbate,
C
cytoplasm,
DHA
dehydroascorbate,
GSH
reduced glutathione,
GSSG
oxidized glutathione,
H
2
O
2
hydrogen peroxide,
O
2
•-
superoxide ion,
•
OH
hydroxyl radi-
cal,
OFFRs
oxidized fructan-free radicals,
OSFRs
oxidized sucrose-free radicals,
OPC
oxidized
phenolic compounds,
PC
phenolic compounds,
PRX
peroxidase,
SOD
superoxide dismutase,
TVE
tonoplast vesicle-derived exocytosis,
V
vacuole
be directed to the vacuole (plant cell “detoxification factory” and “dump site”).
H
2
O
2
can diffuse through the tonoplast directly and/or through aquaporins. Another
possible way to transmit the oxidative stress from the cytosol into the vacuole is
through a putative vacuolar NADPH oxidase. Carter et al. (
2004
) reported this en-
zyme at the tonoplast but its localization was never confirmed by other studies
(Fig.
13.2
). This NADPH oxidase may be positioned in the neighbourhood of SOD
and tonoplast-bound class III peroxidases (Fig.
13.1
) catalyzing the reduction of
H
2
O
2
. They use various substrates as electron donors, such as phenolic compounds,
lignin precursors, auxin or secondary metabolites. However, as by-products of the
so-called hydroxylic cycle of these peroxidase enzymes, the dangerous
•
OH and
•
OOH can be produced. The localization of fructans along the tonoplast make them
ideally positioned to stabilize the tonoplast, but also to temporarily scavenge the
aggressive
•
OH and
•
OOH radicals that are produced in the vicinity of these mem-
branes (Fig.
13.1
). The neutralization of these highly toxic radicals by the fructans
or other vacuolar sugars⁄sugar-like compounds results into (less harmful) radicals
(Fig.
13.1
). It has been proposed that such sugar radicals could be recycled back into
sugars with the help of phenolic compounds or anthocyanins with the use of AsA,