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but is barely responsive to wounding (
Betz et al., 2001
). This specialized
function, however, is clearly not the biosynthesis of the large group of
flavonoid-derived compounds as these accumulate also in species lacking
class II isoforms. And to date, no functional differences between class I
and class II isoforms have been clearly established. Indeed, downregulation
of either the class I or a class II gene in transgenic tobacco led to reduced
lignin deposition (
Blee et al., 2001
), arguing against an exclusive role of class
I proteins in developmental lignification in this species. Also in terms of
enzymatic properties, no clear differences between class I and class II
enzymes have been reported. The class II C4H (CYP73A15) from French
bean (Phaseolus vulgaris) encodes a true C4H with enzymatic properties
comparable to that of class I sequences, although expression in yeast required
replacement of the N-terminal region with a proper ER-targeting sequence
(
Nedelkina et al., 1999
).
While an ancient gene duplication gave rise to the angiosperm class I and
class II sequences, independent duplications occurred in Bryophytes and
Gymnosperms (
Fig. 2
). CYP73 sequences from gymnosperms are more
similar to class I sequences (
80% identical on the amino acid level) than
to class II proteins (
65% similarity). A putative C4H sequence is also
present in the lycopod Selaginella moellendorffii and this sequence is located
at the base of the vascular plant clade consistent with its phylogenetic
position (
Fig. 2
). Four CYP73 homologues are present in the moss P. patens,
which form a distinct clade of paralogs in phylogenetic reconstructions
(
Fig. 2
;
Ehlting et al., 2006
). In contrast, no CYP73A members are present
in available green algal genomes, suggesting that the core phenylpropanoid
pathway evolved early in land plant evolution predating the occurrence of
vascular plants. Supporting this hypothesis, the same pattern, that is, pres-
ence in the moss P. patens and absence in green algal genomes, was also
found in genome-wide and functional analyses of the 4CL gene families (
De
Azevedo Souza et al., 2008; Silber et al., 2008
). Likewise, genes likely encod-
ing PAL are present in moss but are absent in green algae based on BLAST
analyses against the Phytozome database (
Goodstein et al., 2012
).
3. Expression patterns
Many studies have characterized C4H expression patterns in diverse spe-
cies, although most genes analysed belong to class I. In general, C4H
appears expressed throughout the plant, but it is expressed to higher levels
in organs with ongoing lignification or correlated with other phenylpropa-
noid biosynthetic activity, especially flavonoids. For example, the Zinnia
elegans CYP73A12 is induced during differentiation of tracheary elements
(
Ye, 1996
); the Arabidopsis C4H gene is predominantly expressed in stems
and roots, as well as in leave veins (
Bell-Lelong et al., 1997; Mizutani et al.,
