Biology Reference
In-Depth Information
H. CINNAMOYL COA REDUCTASE
Cinnamoyl CoA reductase (CCR) converts hydroxycinnamoyl CoA esters
to their corresponding aldehydes, the penultimate step in monolignol
biosynthesis.
Downregulation of CCR was obtained in transgenic poplars (INRA 717-
1B4, P. tremula
P. alba) by the integration of a sense or antisense copy of a
P. trichocarpa CCR sequence, shown to be the only one strongly expressed in
differentiating xylem (
Lepl´ et al., 2007
). About 5% of the transformed plants
were dwarf and unable to be acclimatized, suggesting deleterious effects
when CCR downregulation was too strong. Less affected transgenic lines,
still with a 3-4% CCR transcript residual level in some wood areas, were able
to grow normally in the greenhouse. These plants were shown to be strongly
affected in lignin metabolism, with up to a 50% reduction in lignin content.
Lignin structure was more condensed apparently due to a decrease in S unit
incorporation. In addition, the decreased flux of feruloyl-CoA to lignin
resulting from CCR downregulation appeared to be compensated by an
increased flux toward ferulic acid, further detoxified by its glucosylation.
Lignin alterations appeared confined in patchy areas where active xylem
exhibited an orange-brown colouration potentially due to the incorpora-
tion of ferulic acid in the lignin polymer (
Lepl´ et al., 2007
). Accordingly,
the cohesion of the cell wall was strongly altered in these areas, with the
occurrence of additional concentric sublayers within the S2 layer of fibres.
In this respect, it was hypothesized that ferulic acid deposition in the wall
was actively induced to strengthen the cell wall. Transcriptomic and meta-
bolomic evaluation found an oxidative stress response likely to be induced
by cell wall alterations. These analyses also revealed that, in addition to
lignins, hemicellulose and pectin were submitted to increased breakdown or
remodeling and decreased synthesis, with a concomitant increase in cellu-
lose content.
CCR-downregulated trees from the most affected lines that were able to
grow normally in the greenhouse were evaluated in a field trial. Upon sample
collection after 5 years, the coloured phenotype was mostly concentrated in
the basal part of branches and stems. Wood collected from 4-year-old field-
grown trees altered by CCR gene silencing (with a sense construct) appeared
easier to delignify than control trees using lower alkali concentrations,
although pulp yield was decreased. In addition, the cell wall alterations
resulting from the genetic modification were associated with a consistent
reduction in tree growth, both in height and diameter. CCR-silenced poplars
are still grown in the field, both in France and Belgium (
Custers, 2009
), for
further evaluation of the potential increased ability of their wood to be
