Biology Reference
In-Depth Information
IV. SYNTHESIS, CONCLUSION AND PROSPECTS
The results obtained from lignin-modified transgenic trees emphasize the
need for running field trials to evaluate in real conditions the effects (some-
times unforeseen) of genetic modifications.
As stated in the corpus reviewed in the previous section, nearly all the
genes coding for enzymes from the lignin biosynthetic pathway have been
identified, excepting genes coding for proteins involved in monolignol trans-
location to the extracellular matrix before lignin polymerization. Transgenic
trees modified for these genes have been generated and evaluated for the
effects of their downregulation or over-expression on lignin metabolism, on
wood properties with regards to industrial uses (mostly pulp and paper
making and now biofuel production) and finally on tree growth and devel-
opment. A few recent studies have been targeted to transcription factors
known to regulate lignin biosynthesis while a few others not directly targeted
to lignin modification resulted in transgenic trees altered in lignin metabolism
and some new approaches still at their infancy aim to produce synthetic lignins
specially designed for a given industrial use. Finally, it is worth underlining that
there is virtually no report on the production of trees with increased lignin
amounts. Likewise, nobody has apparently been successful in producing a
conifer tree able to synthesize S units by the integration of adequate angio-
sperm enzymes to process the different substrates leading to the production of
sinapyl alcohol. This strongly suggests that this inability of conifers to produce
S units may not be limited just to the absence of the suitable enzymes.
A. LESSONS FROM THE PAST STUDIES
Among all the research efforts invested over these past 20 years, only a rather
limited number of target genes gave promising results in the field, that is, a
reduction of lignin content or a modification of the lignin polymer making it
more amenable to efficient delignification.
1. It is easy to reduce lignin content but often with negative effect on growth
A number of studies were very successful in producing transgenic trees with
different degrees of reduction in lignin content through modulation of lignin
biosynthetic enzymes (e.g. C4H, C3H, 4CL, CCoAOMT, CCR, CAD and
peroxidase). Such lignin reduction appears most of the time beneficial for
pulp and paper production. Nevertheless, field trial evaluation of transgenic
trees with reduced lignin contents indicated that beyond a certain level,
lignin reduction resulted in major detrimental effects on the growth and
development of the trees, inducing an increase in winter mortality and a
