Biology Reference
In-Depth Information
III. EVOLUTIONARY VIEW OF S LIGNIN
BIOSYNTHETIC PATHWAY AND TRANSCRIPTIONAL
REGULATION
A. ENZYMES
In recent years, several studies have deepened investigations into the presence
of S lignin biosynthetic pathway enzymes in different phylogenetic groups.
The obtained results may be crucial in assessing the evolutionary history of
syringyl lignins.
There are two enzymes that divert the flux of coniferyl alcohol to the
synthesis of S lignins: ferulate 5-hydroxylase (F5H) and caffeic acid/
5-hydroxyferulic acid O-methyltransferase (COMT). F5H catalyses the
5-hydroxylation of coniferaldehyde and coniferyl alcohol (
Humphreys et al.,
1999; Meyer et al., 1998
) and had only been identified in angiosperms. How
non-angiosperms were able to form syringyl lignin without the existence of a
F5H was a mystery until recently, when
Weng et al.(2008)
reported the
presence of a F5H-like enzyme in Selaginella moellendorffii. This enzyme
(SmF5H) is a cytochrome P450-dependent monooxygenase, like angiosperm
F5H, and it is able to complement Arabidopsis F5H-deficient mutants. Unlike
angiosperm F5H, SmF5H is able to also catalyse the 3-hydroxylation of
p-coumaraldehyde and p-coumaryl alcohol directly; complementation of the
Arabidopsis C3Hmutant led to an intriguing lignin composed almost solely of S
and H units, practically devoid of G units. Independence from the need for the
enzymes that usually participate in the synthesis of coniferaldehyde and con-
iferyl alcohol (HCT and C3
0
H) points to a non-canonical lignin biosynthesis
pathway in Selaginella, where SmF5H has the capacity to form syringyl lignin
from p-coumaraldehyde and p-coumaryl alcohol (
Weng et al., 2010
). More-
over, SmF5H is not orthologous to angiosperm F5H, and has probably arisen
in a convergent evolution process (
Weng et al., 2008, 2010
). Hence, SmF5H is
functionally similar to angiosperm F5H but phylogenetically independent.
COMT catalyses the O-methylation of 5-hydroxyconiferaldehyde and
5-hydroxyconiferyl alcohol to form sinapaldehyde and sinapyl alcohol.
A novel COMT, distantly related to angiosperm COMTs, has been discov-
ered in S. moellendorffii. Interestingly, the SmCOMT encoding gene clusters
with SmF5H gene and shares an upstream regulatory region that may be
under the control of common cis-regulatory elements. This regulated coordi-
nation does not seem to exist in angiosperms (
Weng et al., 2011
).
The last step of lignin biosynthesis is the oxidation of monolignols, which
is driven by laccases (
Berthet et al., 2011
) and mainly peroxidases (
Fagerstedt
et al., 2010
). Both acidic and cationic peroxidases can oxidize p-coumaryl
