Biology Reference
In-Depth Information
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D. HYDROXYCINNAMOYL-COA:SHIKIMATE/QUINATE
HYDROXYCINNAMOYLTRANSFERASE AND P-COUMARATE 3-HYDROXYLASE
The first hydroxycinnamoyl-CoA:shikimate/quinate hydroxycinnamoyl-
transferase (HCT) was recently purified and the corresponding gene cloned
from tobacco ( Hoffmann et al., 2003 ) and later on in P. radiata ( Wagner
et al., 2007 ). This enzyme converts p-coumaroyl-CoA and caffeoyl-CoA to
their corresponding shikimate or quinate esters and catalyses the reverse
reaction as well. These shikimate and quinate esters of p-coumaroyl-CoA
have been shown to be preferred substrates for p-coumarate 3-hydroxylase
(C3H), which belongs to the CYP98A group of the cytochrome P450-
dependent monooxygenase family. This enzyme converts these substrates
into their corresponding caffeoyl esters ( Schoch et al., 2001 ) leading to the
synthesis of G and S monolignols. Due to the relatively recent isolation of
these two enzymes, only a few publications report on the evaluation of
transgenic trees affected in the expression of either of these genes and none
of these studies deals with field trials.
RNAi-mediated silencing of a P. radiata HCT gene resulted in a strong
decrease (up to 42%) in lignin content, a strong increase in H units (from
traces up to 31%) and modifications in the lignin polymer structure ( Wagner
et al., 2007 ): these observations were carried out on pine tracheary elements
as transgenic plants were not regenerated.
In the most affected transgenic lines, RNAi suppression of C3H in hybrid
poplar (P. alba
Populus grandidentata) led to reduced (up to 55%) lignin
content ( Coleman et al., 2008a ) and altered S/G/H lignin monomer ratios,
resulting from a decrease in G monomers compensated by an increase in
H units, while, surprisingly, S lignin remained relatively constant. Likewise,
lignin modifications were associated with concomitant increases in cellulose
and arabinoxylan (a common hemicellulose from angiosperm cell walls), as
well as an increase in the pool of different phenyl glycosides. While irregular
patterning of lignin deposition occurs all along the stem, lignification was
most notably reduced around vessel elements. Overall, these modifications
resulted in plants with impaired plant growth both in height and diameter,
with smaller stems, reduced root biomass and altered leaf morphology and
architecture ( Coleman et al., 2008b ). These altered phenotypes at least partly
resulted from a strongly reduced cell wall lignification and its consequences,
as already observed in 4CL-downregulated trees: collapsed xylem vessels,
reduced hydraulic conductivity and higher susceptibility to embolism.
 
 
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