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of oxidative stress. In the last few years, also plant oligosaccharides have been
proposed as emerging antioxidants in plants (Nishizawa et al. 2008 ; Foyer and Shi-
geoka 2011 ; Sinkevich et al. 2010 ; Stoyanova et al. 2011 ). They might play a role in
scavenging hydroxyl and superoxide radicals.
6.1   RFOs and Galactinol as Antioxidants in Arabidopsis
In in vitro tests, the hydroxyl radical scavenging capacity of galactinol and raf-
finose was superior compared to those of typical antioxidants, such as AsA, GSH
and citrulline (Nishizawa et al. 2008 ). The concentrations that reduce hydroxylation
of salicylate by 50 % (ID 50 ; the concentration of a compound required to inhibit
OH catalysed hydroxylation of salicylate (or terephthalate) by 50 % of the maxi-
mum yield observed in the absence of the compound) of galactinol, raffinose and
stachyose were 3.1 ± 0.3, 2.9 ± 0.2 and 2.2 ± 0.1 mM, respectively (Nishizawa et al.
2008 ). For comparison, the concentrations of galactinol, raffinose and stachyose are
7.4, 0.74 and 3.6 mM in Arabidopsis seeds. This suggests that the initial intracel-
lular levels of galactinol and stachyose might be in a good range to protect cellular
components from oxidative damage (Nishizawa et al. 2008 ). Furthermore, raffi-
nose concentrations in chloroplasts of stressed plants are estimated between 0.27
and 1.35 mM, comparable to the levels of AsA and GSH (Nishizawa et al. 2008 ).
Stoyanova et al. ( 2011 ) reported an ID 50 of 0.5152 mM for raffinose (hydroxyl radi-
cal) scavenging in vitro , suggesting that raffinose could act as a direct scavenger of
hydroxyl radicals in chloroplasts too.
As already explained above, additional evidence for a role of galactinol and raffi-
nose as antioxidants in plants was generated by paraquat treatments on plants over-
expressing GolS (GolS1, GolS2, GolS4) and RafS. These transgenic plants showed
more effective ROS scavenging capacity and oxidative stress tolerance compared
to wild-type plants (Nishizawa et al. 2008 ). It can be speculated that RFOs might be
operating as ROS scavengers in chloroplasts which is a major point of ROS produc-
tion under different stress conditions (Foyer and Shigeoka 2011 ). The oxidized RFO
radicals might be regenerated to RFOs by ascorbic acid (AsA) or other reducing
antioxidants.
6.2   Disaccharides and Sugar Alcohols as Antioxidants
In vitro studies further demonstrated that the ID 50 values of other sugars and sug-
ar alcohols were even better than the ones obtained for raffinose and galactinol.
Compared to mannitol, a widely recognized osmoprotectant, sucrose and especially
inulin-type fructans show excellent antioxidant properties in vitro (Stoyanova et al.
2011 ). Some plant species such as sugarcane and sugar beet accumulate sucrose
to extremely high levels (2 M) in their vacuoles. Therefore, sucrose might be a
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