Biology Reference
In-Depth Information
Similar to Voelker's study of hybrid aspen (2010), low-lignin trees exhibited
decreased xylem-specific hydraulic conductivity and increased susceptibility to
embolism, due to collapsed cells caused by reduced strength of xylem cell
walls, these aspects being exacerbated at the drier sites. However, in this case,
4CL-downregulation translated in reduced wood density. In addition, wood of
transgenic trees with reduced lignin content showed a severe reduction in their
modulus of elasticity and their compression strength in the direction parallel to
the grain (
Horvath et al., 2010
). As observed previously, most low-lignin
transgenic trees exhibited loss of apical dominance, stunted growth, stem
dieback, and increased sensitivity to pathogens even though both gas exchange
and photosynthesis rate were not compromised in low-lignin trees. However,
trees from one transgenic line with a strong (37%) lignin reduction produced
more biomass than the control at the Coastal Plain site, and asmuch biomass as
the control at the other two less favourable sites. This may be linked to the high
number of sylleptic branches produced by the trees of this transgenic line as this
number was shown to be indicative of highly productive clones (
Marron et al.,
2006
). In conclusion, this field study with 4CL-downregulated trees suggests
that, even thoughmost low-lignin transgenic trees appeared strongly affected in
their fitness even in favourable site, it appears possible to find some transgenic
events where the modification may still be interesting for technological applica-
tions without apparently affecting the growth and development of the tree.
Such identification requires the evaluation of a large array of different trans-
genic events exhibiting large variations in the effects of the genetic modification
(as previously suggested in
Busov et al., 2005
).
To summarize the results obtained on 4CL-downregulated transgenic
poplars, it appears that the use of the same construct in different poplar
species led to strikingly different results, for example, pPt4CL1-ASPt4CL1
led to strongly affected growth and development in P. tremula
P. alba
whereas it has no such effects in a related species such as P. trichocarpa.
In this case, it seems preferable to use a xylem-specific promoter rather than a
constitutive pCaMV 35S even though the pPt4CL1 promoter does not seem
to be strictly xylem-specific (A. D
´
jardin, M. Martin, F. Laurans, N. Millet,
A.
ˇ
e
´
erovi
´
, J.-C. Lepl
´
, G. Pilate, unpublished results).
As for C4H, there is currently a project in Taiwan aiming to downregulate
4CL in E. camaldulensis by genetic transformation of an antisense eucalyptus
4CL gene driven by a xylem-specific promoter. Five different transgenic lines
have been produced with a 5-18% lignin reduction as compared to the wild-
type plants. In addition, transgenic plants have been planted in the field in
order to monitor tree growth performance, to evaluate variations in wood
composition, and to analyse pulping property of wood chips (
http://
grbsearch.stpi.narl.org.tw/GRB/result.jsp?id
1358296&plan_no
97%E8%
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