Biology Reference
In-Depth Information
level, and structural models for conifer lignin have been established. Recent years
have seen significant advances in our molecular-level understanding of lignification,
and in conifer monolignol biosynthesis. The majority of the genes involved have been
identified and the molecular functions of several have been experimentally verified.
Suppression of lignin-related genes confirmed that lignin is vital for plant fitness and
vascular integrity in conifers and established that conifers do not tolerate substantial
reductions in lignin content. Significant gaps in our understanding of conifer lignifi-
cation nevertheless remain. Aspects of lignification about which we still know relatively
little include: the regulatory cascades that trigger lignification, metabolic connections
between monolignol biosynthesis and other metabolic processes, the cellular biology
of monolignol biosynthesis, the transport of monolignols to the apoplast, the role of
monolignol glucosides in lignification, the process of lignin initiation, and the interac-
tion of lignin with other cell wall polymers such as non-cellulosic polysaccharides.
These significant gaps in our understanding provide ample opportunity for new and
exciting discoveries on lignification in conifers.
I. INTRODUCTION
Coniferous gymnosperm is a term that describes a plant division containing
many well-known tree species including pine, spruce, fir, cypress, cedar,
redwood, hemlock and larch. Some of these evolutionarily ancient tree
species still dominate vast areas of land, in particular the boreal forests of
the northern hemisphere. Conifers not only have great ecological but also
significant economic value, primarily for the production of timber and
paper. The wood of gymnosperms (including conifers) is known as soft-
wood, which differentiates it from hardwood, the wood from arborescent
angiosperms.
The cellular composition of softwood differs in many respects from that
of hardwood. The physiological and structural roles of vessel elements and
wood fibres in hardwoods are assumed by tracheids in softwoods (
Core
et al., 1979
). Tracheids are long fibrous cells with lignified cell walls that
generally make up more than 90% of conifer wood (
Fig. 1
). Conifer trac-
heids have a typical three-layered secondary cell wall surrounded by a
highly lignified primary wall/middle lamella. The three layers of the second-
ary wall are known as S1, S2 and S3, the S2 layer being the thickest. The
three layers are characterised not only by differences in thickness but also
by differences in lignification (
Donaldson, 2001
). In normal wood, the S1
layer is often the least lignified part of the cell wall, whereas the S3 layer is
typically more highly lignified than the S2 layer but not as highly lignified as
the middle lamella. In compression wood, the pattern of lignification is
altered with reduced lignification in the middle lamella/primary wall, and
increased lignification in the outer part of the S2 layer, a region known
as the S2L layer (
Cˆ t´ et al., 1968; Donaldson et al., 1999
). The inner part of
