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case, the substrate is an amide rather than an ester highlighting that CYP98As
can also be involved in hydroxycinnamoyl-N-conjugate metabolism. Also the
wheat enzymes, CYP98A11 and CYP98A12, were capable of meta-hydroxylat-
ing an N-conjugate, namely, 4-coumaroyltyramine, and may thus be involved
in the biosynthesis of feruloyltyramine ( Morant et al.,2007 ), which is a com-
mon constituent of the cell wall and accumulates in response to wounding or
pathogen penetration. However, in this case, the activity with the N-conjugate
is much lower compared with the hydroxylation of the shikimate ester ( Morant
et al.,2007 ) shedding a doubt on an in vivo role of CYP98As in feruloyltyramine
biosynthesis. Taken together, it is likely that species- and isoform-specific
differences in substrate specificities of CYP98A enzymes are contributing to
the wide array of hydroxycinnamoyl conjugates found in plants. Combined
with substrate differences of HCT isoforms, which can act on either the sub-
strates or the products of the 3-hydroxylase, it appears plausible that even a
limited number of isoforms can create a large array of conjugates.
3. Are there alternative pathways acting in parallel?
While there is now convincing evidence that the aromatic 3-hydroxylation
in the lignin pathway occurs primarily on the shikimate-ester level of
4-coumarate, it is still a matter of debate if this is the sole pathway leading
to coniferyl and sinapyl monolignols. For once, this pathway leaves caffeic
acid orphaned ( Fig. 1 ); this free acid accumulates in many plants, but one
would have to postulate the existence of an esterase, thioesterase, or aldehyde
dehydrogenase to synthesize the free acid from caffeoyl-shikimate, caffeoyl-
CoA, or caffeyl-aldehyde, respectively. An aldehyde dehydrogenase acting
on sinapaldehyde and coniferaldehyde (yielding free sinapate and ferulate,
respectively) has been identified in Arabidopsis ( Nair et al., 2004 ), but an
equivalent activity yielding free caffeic acid has not been described to date.
Furthermore, 3-hydroxylation activity with free 4-coumarate has been de-
scribed in crude extracts from several plants, and in the case of poplar, the
recombinant C3 0 H was also capable to convert the free acid albeit only when
coexpressed with C4H and only to minute levels ( Chen et al., 2011 ). Further-
more, 4CL enzymes, which activate the free acids to the corresponding
CoA-esters, have a fairly broad substrate range and generally can activate
not only 4-coumarate but also caffeate and ferulate with high efficiency (e.g.
Allina et al. 1998; Ehlting et al. 1999; Hu et al. 1998 ) and 4CL isoforms have
been characterized that specifically activate sinapate to the CoA ester
( Hamada et al., 2004; Hamberger and Hahlbrock, 2004; Lindermayr et al.,
2002 ). Taken together, these observations suggest a role of the free acid in
phenylpropanoid metabolism. More direct evidence for an alternative path-
way comes from the analysis of the Arabidopsis cyp98A3 T-DNA knockout
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