Agriculture Reference
In-Depth Information
frost, suggesting that the increased galactinol levels in these lines could compensate
for the impaired raffinose levels (Korn et al. 2010 ).
Overexpression of drought-inducible GolS1 and GolS2 genes in Arabidopsis
also led to increased galactinol and raffinose levels, and showed reduced tran-
spiration from leaves to improve drought tolerance (Taji et al. 2002 ) . Moreover,
the intracellular levels of galactinol and raffinose in these transgenic plants were
correlated with increased tolerance to paraquat treatment and salinity or chilling
(Nishizawa et al. 2008 ). However, the debate is still on over the exact mechanism
of action of these metabolites. They might act as signals but more likely they act
as true ROS scavengers when they accumulate at high concentrations at particular
locations (e.g., in the vicinity of chloroplast thylakoid membranes; Foyer and Shi-
geoka 2011 ). Intriguingly, introduction of a StaS from adzuki bean in Arabidopsis
did not lead to increased freezing tolerance (Iftime et al. 2011 ). Probably stachyose
is not at the correct place (cytosol) to provide protection. It can be speculated that
Arabidopsis lacks a stachyose transporter in the chloroplastic envelope. However,
recent evidence was generated for the presence of a raffinose transporter in the
chloroplastic envelope (Schneider and Keller 2009 ).
4.3   Transgenic Fructan Plants
The reviews of Cairns ( 2003 ) and Banguela and Hernández ( 2006 ) summarize
the efforts to introduce fructan metabolism in (mostly) non-fructan accumulating
species. In a first phase, focus was on introducing bacterial levansucrases in such
species (Cairns 2003 ). However, much debate arose on the correct delivery of these
levansucrases to the vacuole and to plastids (mistargeting to the ER; putative ER
contamination in plastid preparations, etc.). In many cases, only low levels of fruc-
tan were found (max 10 % on dry weight basis) since higher levels appeared to be-
come toxic for these plants (Cairns 2003 ). Recently, Banguela et al. ( 2011 ) fused the
preproprotein of onion 1-SST to the levansucrase of Gluconacetobacter diazotro-
phicus and introduced this in tobacco, in order to obtain a correct vacuolar delivery.
Some transgenic lines accumulated levans up to 70 % on dry weight basis, and they
showed phenotypic changes (leaf bleaching) during plant development. However,
only slight leaf bleaching was observed in plants accumulating up to 30 % levans
on dry weight basis (Banguela et al. 2011 ).
In the second phase, interest shifted to introducing plant FTs in non-fructan
plants (Sevenier et al. 1998 ; Hellwege et al. 2000 ; Stoop et al. 2007 ; Li et al. 2007 ;
Kawakami et al. 2008 ; Pan et al. 2009 ) or in plants accumulating other types of
fructans (Vijn et al. 1997 ; Hisano et al. 2004 ; Gadegaard et al. 2008 ). In most cases,
no detrimental effects on the plants phenotype were reported in these experiments.
This might be explained by the correct vacuolar targeting of these enzymes and,
perhaps, by the presence of endogenous FEHs being able to degrade the fructans at
specific phases of plant development (e.g., pre-flowering stages).
Search WWH ::




Custom Search