Biology Reference
In-Depth Information
reduced biomass, as has already been observed for perennial herbaceous
species (
Casler et al., 2002
). The threshold appears to be around a 20%
reduction: interestingly, this threshold fits with the estimated natural varia-
tion of lignin content measured both within and between tree species
Campbell and Sederoff, 1996
;
Sannigrahi et al., 2009
. Moreover, it appears
that, often, only the most affected trees that could grow normally in near
optimal greenhouse conditions were kept for further small-scale field trial
evaluation, leading sometimes to deceiving results in terms of growth in the
field. This suggests that in field trial evaluations, it would be wiser to keep a
larger number of different transgenic lines representing a large array of lignin
alterations, in order to be able to identify the degree of lignin alteration
interesting for a given use, but still compatible for normal or near-normal
tree growth and development: for example, this approach was successful to
identify a CAD-downregulated transgenic line, exhibiting both normal growth
and wood easier to delignify for paper production (
Pilate et al., 2002
). This is
also the case for a 4CL-downregulated line with reduced lignin content that
grows at least as well as wild-type trees in three different sites (
Stout, 2011
).
It is also worth pointing out that lignification is a highly variable process
strongly dependent on both environmental (external) conditions and the
physiological (internal) state of the trees: effects of environmental conditions
are illustrated by the variations in results obtained from multisite field trials
(
Pilate et al., 2002; Stout, 2011
). Effects of the physiological state of the trees
can be easily estimated by tracking in the different parts of the tree, the pattern
of coloured phenotypes generally observed in lignin-modified trees. From
published results, it seems that the maximum effect is generally observed
at the base of the stem, with eventually a uniform colouration, while this
colouration may be weakest or patchy in other parts of the plants. Likewise,
it seems that at least in some cases, there is an attenuation of the effects of the
transformation over the year or during tree development as observed, for
example, for COMT-downregulated poplars, when comparing OMT activity
in young and older trees (see
Pilate et al., 2002; van Doorsselaere et al., 1995
).
The evolution of the coloured phenotype can be a good way to evaluate the
strength and stability of the modification that may be different according to
the strategy used (antisense, sense cosuppression, RNAi or over-expression).
2. Lignin structure can be modified
The second promising lignin modification concerns the effects on lignin
composition in such a way that lignins will be easier to delignify. Most of
the time, such beneficial results were successfully obtained by increasing the
