Agriculture Reference
In-Depth Information
good candidate to act as a ROS scavenger in their vacuoles. Of course, such vacu-
olar antioxidant mechanisms occur in concert with the classic cytosolic antioxidant
mechanisms (Van den Ende and Valluru
2009
). So, at lower sucrose concentrations,
sucrose might preferably act as a signal molecule, while it might become a ROS
scavenger at high concentrations (Bolouri-Moghaddam et al.
2010
).
Accordingly, transgenic potato plants carrying a yeast invertase gene (В33-
inv plants), with decreased sucrose efflux and 20-30 % higher total sugar con-
tents, showed enhanced cold tolerance and lower level of malondialdehyde (MDA;
Sinkevich et al.
2010
). MDA is an indicator of lipid peroxidation (LPO). The re-
duced MDA levels suggest that there is less ROS-mediated membrane damage in
the transgenic potatoes, despite the higher levels of superoxide found in stressed
B33-inv plants. These authors speculated that sugars act as primary antioxidants,
and that the generated sugar radicals are reduced by AsA.
A group of sugar alcohols (such as mannitol, inositol, sorbitol) also possesses
ROS scavenging capacities (Shen et al.
1997
; Stoyanova et al.
2011
). Mannitol pro-
tects thioredoxin, ferredoxin, GSH and the thiol-regulated enzyme phosphoribulo-
kinase in
Nicotiana tabacum
. Genetically engineered tobacco plants with increased
chloroplastic mannitol showed increased tolerance under paraquat treatments. In
the same experimental setup, mannitol did not reduce
•
OH radical production in the
chloroplast, but it increased the capacity to scavenge these radicals protecting the
cells against oxidative damage (Shen et al.
1997
). Furthermore, mannitol accumu-
lation had no harmful effects on these plants. This means that no sugar-mediated
negative feedback on photosynthesis was observed, as is the case for metabolizable
sugars such as glucose, fructose and sucrose (Bolouri-Moghaddam et al.
2010
).
Trehalose also can act as a ROS scavenger
in vitro
(Stoyanova et al.
2011
) and
it was demonstrated that this sugar acts as a ROS scavenger
in vivo
in yeast (Nery
et al.
2008
). Exposing yeast to exogenous H
2
O
2
leads to trehalose accumulation,
which reduces the oxidant-induced modifications of proteins and the levels of lipid
peroxidation. Transgenic rice plants accumulating increasing levels of trehalose
showed increased tolerance to salt, drought and low-temperature stresses. More-
over, several transgenic lines exhibited sustained plant growth, less photo-oxidative
damage and a more favorable mineral balance under stress (Garg et al.
2002
).
6.3 Fructans: A Role in Vacuolar Antioxidant Mechanisms?
Fructans as water-soluble vacuolar oligo- and polysaccharides are probably good
candidates to act as vacuolar ROS scavengers. Fructans can accumulate to a great
extent in plant tissues (up to 20 % on a fresh weight basis; Van Laere and Van den
Ende
2002
). This kind of fructan levels cannot be solubilized
in vitro
, but appar-
ently they can be kept in a solubilized (gelly-like) state in the vacuole, where they
might interact profoundly with the inner side of the tonoplast. Under stress, when
the redox equilibrium is disturbed, a spike in ROS occurs. The excess cytoplasmic
H
2
O
2
, derived from ROS produced in chloroplasts or other cell compartments, can
