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proceeds from 4-coumarate via CoA-activation, shikimate-transesterification,
3-hydroxylation, reduction to the aldehyde, methoxylation, 5-hydroxylation,
and dehydrogenation back to the free acid (
Fig. 1
). This is certainly a tremen-
dous and energy-consuming detour compared to direct hydroxylations and
methoxylations of the free acids.
2. F5H is instrumental in changing lignin composition
F5H has a defining role in S-lignin biosynthesis. The fah1 mutant is basically
devoid of S-lignin units, while wild-type plants accumulate about 20%
S-lignin, which is largely limited to the interfascicular fibres and xylem fibres
in the vascular bundles of the inflorescence stems (
Chapple et al., 1992,
Meyer et al., 1998; Patten et al., 2010
). Likewise, downregulation of F5H in
alfalfa (M. sativa) by antisense expression of the M. truncatula F5H gene under
control of a vascular specific PAL promoter from bean led to an overall
twofold decreased S-lignin content, while G-monomer amounts remained
largely unchanged. Together, this resulted only in modest reduction in total
lignin content (
Chen et al., 2006; Reddy et al.,2005
). Antisense suppression of
F5H in alfalfa reduced accumulation of S-lignin in particular in fibres and
parenchymatic cells, while no such reduction was apparent in vascular ele-
ments (
Nakashima et al.,2008
).
Using the reverse approach, overexpression of the Arabidopsis F5H gene
in the fah1 mutant driven by the commonly used 35S promoter complemen-
ted fah1 S-lignin phenotype and resulted in S-lignin being produced. But only
a moderate increase in S-lignin compared to wild type was observed. In
contrast, use of the C4H promoter to drive F5H expression in wild type
plants resulted in a lignin that consisted primarily of S-units (
Marita et al.,
1999, Meyer et al., 1998
), which overaccumulates primarily in interfascicular
fibres, but not in vascular bundles (
Patten et al., 2010
). Thus, F5H availabili-
ty is the limiting factor determining flux into S-lignin in Arabidopsis. Shortly
after, it was shown that in addition to the increased amount of S-lignin, also
5-hydroxyconiferyl alcohol, the immediate product of F5H is incorporated
into lignin (
Ralph et al., 2001
). The resulting 5-hydroxyguaicyl lignin units
only occur as trace amounts in angiosperm lignins. This suggests that
in Arabidopsis COMT, the O-methyltransferase that methoxylates the
5-hydroxy group incorporated by F5H, cannot keep pace with the increased
substrate amounts generated by F5H overexpression, resulting in the accu-
mulation of 5-hydroxyconiferaldyde and 5-hydroxyconiferyl alcohol, which
is then incorporated into the growing lignin polymer. To further corroborate
this, C4H-promoter-driven overexpression of F5H was combined with a
knockout of the COMT gene (the mutant is called omt1). By itself, omt1
mutants show a total loss of S-lignin and incorporate some 5-hydroxyguaicyl
lignin, just as the fah1 mutant does. Additional overexpression of F5H in omt1
