Biology Reference
In-Depth Information
III. LIGNIN MANIPULATIONS IN CONIFERS
Genetic and biochemical studies in the last 10-15 years have resulted in
substantial revisions and refinements of our understanding of the monolignol
pathway in plants. The molecular function of many genes associated with
lignin biosynthesis has been established and their impact on lignin content
and composition tested using recombinant approaches. The vast majority of
these lignin-related studies were undertaken in angiosperm species and are
reviewed elsewhere (
Boerjan et al., 2003; Bonawitz and Chapple, 2010; Boudet,
2007; Chiang, 2006; Halpin, 2004; Higuchi, 2006; Rastogi and Dwivedi, 2008;
Umezawa, 2010; Vanholme et al.,2008
).
Conifer lignins derive almost exclusively from p-coumaryl and coniferyl
alcohols. Both monolignols enable the formation of a 'condensed' lignin
polymer, which hampers processing of lignocellulosic material derived from
conifer species. In addition, lignin content in conifer wood is relatively high
and can reach levels of more than 30% (w/w) (
Nanayakkara et al., 2009;
Timell, 1986
). Both those features could make it from a biotechnological
perspective desirable to modify lignin in conifers.
To date, only a small number of studies has been published that investi-
gated lignin biosynthesis and the potential for lignin manipulations in
conifers (
M¨ ller et al., 2005; Wadenb¨ck et al., 2008; Wagner et al., 2007,
2009, 2011
). A significant number of lignin-related genes have not been
investigated or results have not been published, somewhat limiting our
understanding of monolignol biosynthesis and lignification in conifers.
Two quite different experimental platforms have been used to investigate
lignin biosynthesis in conifers: transgenic plants (
Wadenb¨ck et al., 2008;
Wagner et al., 2009
), and transformable callus cultures that are capable of
producing tracheary elements (TEs) (
M¨ ller et al., 2005; Wagner et al., 2007,
2009, 2011
). Each system has advantages and disadvantages for lignin-
related studies. Plants provide more phenotypic information compared to
TEs due to tissue formation and their need to respond to environmental
stimuli. However, generation of transgenic conifers is difficult and slow, and
transgenic plants seem to not tolerate substantial reductions in lignin content
(
Wagner et al., 2009
). Transformed TE cultures are faster and easier to
generate, but obtainable phenotypic information is more limited. The TE
system is, despite this limitation, well suited for lignin-related studies as
differentiated TEs have a lignin content and composition that is similar to
that of tracheids and even severe lignin modifications and reductions in lignin
content do not compromise cell viability or TE formation (
Wagner et al.,
2007, 2009, 2011
). This helps to assess what impact lignin-related genes have
on lignin content, composition and structure in conifers such as pine. Finally,
