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reduction, but again almost exclusively S-lignin units were affected (
Reddy
et al.,2005
). Given that C4H is involved in the synthesis of all lignin mono-
mers, it is surprising that C4H downregulation in this system mainly affected
S-lignin, while the decrease in G residues was much smaller. But also using the
class II French bean CYP73A15 for sense- or antisense suppression in tobacco
led to reduced C4H activity and reduced total lignin amounts with decreased
S-proportion (
Blee et al.,2001
) comparable to that found using the class I
alfalfa C4H for suppression. Together, these earlier results suggested that both
class I and class II C4Hs primarily impact syringyl derived lignin, although an
equal impact on both G- and S-units was expected. However, more recent
studies complicate the situation even further: downregulation of a class I C4H
in hybrid aspen (Populus tremula
tremuloides) by RNAi resulted in an overall
reduction of total lignin, but affected both S- and G-units equally. This led to a
decrease in wood density and in consequence to a decrease in wood stiffness,
while tensile strength was only weakly affected (
Bjurhager et al.,2010
).
Detailed analyses of a series of point mutations in the single Arabidopsis
C4H genes (the mutant is called ref3 for reduced epidermal fluorescence 3)
showed that in this species a reduction in C4H activity coincides with a
reduction of G-lignin, while S-lignin remains largely unaffected, thus leading
to an increased S/G ratio (
Schilmiller et al.,2009
). These seemingly contradic-
tory results still remain to be explained. Differences in the experimental
setups—different genetic manipulations, none of which results in complete
loss-of-function alleles, different analytical tools used to determine lignin
amounts and composition, and species-specific variations, for example, in
gene family composition, all may contribute. But these factors do not explain
in general why a reduction in C4H activity, needed for all lignin units, does
affect S- and G-lignin biosynthesis differentially. However, manipulations of
the other general phenylpropanoid enzymes, namely, PAL and 4CL, also
resulted in specific alteration of S/G ratios: PAL downregulation in both
tobacco and Arabidopsis led to an overall decrease in total lignin with an
increased S/G ratio (
Rohde et al.,2004;Sewaltet al.,1997
), thus mirroring
the situation in Arabidopsis but opposite to the tobacco results obtained
with C4H. In contrast, downregulation of 4CL in hybrid aspen leads to a
decreased S/G ratio (
Voelker et al.,2010
), while in Arabidopsis and tobacco
increased S/G ratios were observed (
Kajita et al., 1996; Lee et al.,1997
), in each
case again accompanied by a reduction in total lignin. At least in the cases
where transgenic approaches were used, it is possible that the efficiency of
downregulation varies between cell types depending on the promoter used. For
example, fibres and vessels typically differ in their S/G ratios, and differences in
downregulation efficiency between these cell types could explain an overall
change in the ratio determined. However, this explanation does not hold in the
