Biology Reference
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mutants resulted in a lignin that still lacks S-lignin but contains up to a 70%
5-hydroxyguaicyl units, a novel type of lignin not normally found in plants
(
Vanholme et al., 2010; Weng et al.,2010b
). Together, these results highlighted
the tremendous plasticity of plants in creating the lignin polymer.
The original cloning of F5H quickly prompted work in other plant species
on the impact of F5H misexpression on lignin composition. Overexpression
of F5H in either tobacco (Nicotiana tabacum) or poplar (P. tremula
alba)
confirmed the Arabidopsis results and led to an increased S-proportion in
woody tissues (
Franke et al., 2000
). In poplar, COMT is apparently not
limiting, but efficiently methoxylated the additional 5-coniferaldehyde
provided by F5H overexpression. In consequence, transgenic poplar trees
produced a lignin that contained up to 97% S-units compared to about 65%
in wild-type wood. The resulting lignin is more simple and likely entirely
unbranched and has a lower degree of polymerization indicated by a reduced
molecular mass (
Franke et al., 2000; Steward et al., 2009
). C4H-promoter-
driven F5H expression in poplar did neither affect total lignin content nor
cell wall polysaccharide deposition and had no apparent effect on overall
plant or wood morphology (
Huntley et al., 2003
). Testing this transgenic
wood under different kraft pulping conditions suggests that pulp throughputs
at mills could be increased by more than 60% resulting also in a significant
decrease in environmentally deleterious by-products (
Huntley et al.,2003
).
Similar results were obtained when the sweetgum F5H was expressed under
control of a xylem specific 4CL promoter in aspen (Populus tremuloides). This
increased the S-proportion in lignin but had little effect on total lignin
amounts. But when 4CL activity was downregulated in addition, a reduced
total lignin with an increased S/G ratio was generated (
Li et al.,2003
).
3. Convergent evolution of the pathway
CYP84A family members are present in all angiosperm genomes published
to date usually in single- or low-copy numbers not exceeding four members.
But they are absent from the genomes of the moss P. patens and the lycopod
S. moellendorffii (
Mizutani and Ohta, 2010
). Also, to date, no CYP84A genes
have been identified in large-scale transcriptome sequencing projects of con-
ifers. This is consistent with the notion that S-lignin is generally thought to be
unique to angiosperms, while gymnosperm and fern lignins generally are
composed of H- and G-lignin only. However, S-lignin has been identified
in a few gymnosperm and fern species and also in the lycopod S. moellen-
dorffii (
Sarkanen and Ludwig, 1971; Weng et al., 2008a
). Lycopods and
angiosperms diverged
400 million years ago, and there is now increasing
evidence that S-lignin biosynthesis evolved independently in either lineage.
A Selaginella P450 belonging to a distinct family from either CYP84A or
CYP98A, named CYP788A1 and defining the CYP788 family, can catalyse
