Agriculture Reference
In-Depth Information
More importantly, for some of these fructan accumulating transgenic plants,
it was reported that they were more tolerant to stress. Konstantinova et al. (
2002
)
showed that transgenic tobacco carrying the
Bacillus subtilis
levansucrase gene Sac
B survived freezing stress both in controlled and in field conditions. Transgenic
Lo-
lium
plants carrying wheat FT genes (1-SST or 6-SFT) and accumulating increased
amounts of fructan also demonstrated enhanced freezing resistance at the cellular
level (Hisano et al.
2004
). Rice (
Oryza sativa
L.) is a non-fructan accumulating
plant which is highly sensitive to chilling (Kawakami et al.
2008
). Transgenic seed-
lings carrying wheat 1-SST and accumulating fructan oligo- and polysaccharides
showed enhanced chilling tolerance. Introduction of the 1-SST of lettuce (
Lactuca
sativa
L; Li et al.
2007
) also led to increased freezing tolerance and reduced oxida-
tion of membranes, similarly as observed in the
Arabidopsis
GolS overexpression
plants (Nishizawa et al.
2008
). Taken together, these data strongly suggest that sug-
ars can contribute to stress tolerance by protecting membranes.
4.4 Fructan-Membrane Interactions
Sucrose and trehalose are well-known membrane stabilizers, and also fructans and
RFOs have such properties, while hydroxyethyl starch, glucan and dextran have not.
It is believed that fructans can replace water molecules at the membrane. Sucrose
and trehalose can replace around 18 water molecules while 1-kestose has a volume
equal to about 21 water molecules. For raffinose, this number is about 30 (Valluru
and Van den Ende
2008
). The Fru-Fru linkage (CH
2
-O) in fructans is longer than the
O-linkage in hydroxyethyl starch, glucan and dextran, creating an extra flexibility
to interact with and stabilize membranes (Valluru and Van den Ende
2008
). Inulin
chains are even more flexible than levans (Vereyken et al.
2003
). Using liposomes
as a model system, five fructan classes (DP3, DP4, DP5, DP6 and DP7) and two
DP> 7 fractions were isolated from oat and rye and tested as membrane stabiliza-
tors
in vitro
(Hincha et al.
2007
). The two DP>7 fractions from both species were
unable to protect liposomes, while the fractions containing smaller fructans were
protective to different degrees. Protection showed an optimum at DP4. Intriguingly,
synergistic effects were found when low DP fructans were combined with DP> 7
fructans, suggesting that mixtures of fructans, as they occur in living cells, may
have protective properties that differ significantly from those of the purified frac-
tions. However, no mechanistic insights are yet available to explain these observa-
tions. Accordingly, the capacity to accumulate higher DP fructans has been found in
many stress-tolerant species, such as
Echinops
,
Viguiera
,
Dactylis
,
Lolium
,
Poa
and
Pachysandra
(Van den Ende et al.
2011
and references therein).
Next to protecting the tonoplast, fructans might also protect the plasma mem-
brane. Tonoplast vesicle-derived exocytosis (TVE, see also below in Fig.
13.1
) was
proposed as a mechanism to transport fructans from the vacuole to the apoplast
under stress (Valluru et al.
2008
). No fructan transporters have yet been reported in
