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In-Depth Information
For example, laccases might play a more prominent role in lignin polymer-
isation in conifers than they do in angiosperms, as laccases are among
the most abundant proteins in lignin-rich compression wood in conifers
(
Mast et al., 2010
; Wagner et al., unpublished results). In addition, conifers
and angiosperms differ in their responses to lignin manipulations. For
example, enhanced bark formation, production of axial parenchyma and
incorporation of caffeyl alcohol are all phenotypes that were only observed
in conifers. These differences point towards physiological and metabolic
responses in conifers that differ from those in angiosperms, and those
differences are exposed when the monolignol pathway in conifers is
manipulated. Furthermore, unlike angiosperms, conifers seem unable to
tolerate reductions in lignin content and are therefore less amenable to
lignin down-regulation than hardwood species. This observation is consis-
tent with the narrow range in lignin content in pine wild-type populations
(
Campbell and Sederoff, 1996
). The reason for the inability of conifers to
tolerate low lignin levels could be associated with their wood anatomy.
Reductions in lignin content impact tracheids and therefore the structural
integrity and water conduction at the same time, as the cellular diversifica-
tion into structural and conducting elements found in angiosperm species
does not exist in conifers.
The number of recombinant studies that have focused on lignin biosynthesis
in conifers is still quite small, but these studies have already highlighted limita-
tions in our understanding of the biosynthesis of monolignols in conifers. For
example, pine CCoAOMT-RNAi lines still contain more than 80% of the wild-
type lignin content despite substantial CCoAOMT suppression (
Wagner et al.,
2011
), suggesting the presence of other enzymes that can support methylation of
pathway intermediates. In addition, the P. taeda cad-n1 mutant produces sub-
stantial amounts of coniferyl and dihydroconiferyl alcohols despite an almost
complete lack of CAD activity (
Sederoff et al.,1999
), pointing towards an
unknown dehydrogenase that can support hydroxycinnamaldehyde reduction.
In conclusion, our molecular understanding of lignification in conifers has
benefited significantly from lignin-related studies in angiosperm species, but
nevertheless remains quite limited at present. Many molecular aspects of
lignification in conifers await exploration.
ACKNOWLEDGMENTS
The authors like to thank the New Zealand Ministry of Science and Innova-
tion, Scion for financial support, and Elspeth MacRae, Scion for critical
reading of this manuscript. J. R. was funded in part by the DOE Great Lakes
Bioenergy Research Center (DOE Office of Science BER DE-FC02-
07ER64494).
