Biology Reference
In-Depth Information
1997
) and the poplar class I genes are xylem enriched with some expression
in roots (
Lu et al., 2006; Shi et al., 2010
). C4H is generally coexpressed
with other genes encoding enzymes of the phenylpropanoid pathway, both
spatially across different organs and temporally, for example, following a
diurnal rhythm or in response to multiple stress treatments (
Abdulrazzak
et al., 2006; Ehlting et al., 2005, 2008; Rogers et al.,2005
). But also the class
II C4H from French bean has been related to lignification based on coex-
pression with PAL (
Nedelkina et al., 1999
).C4Hisalsoinvolvedinthe
production of a plethora of phenylpropanoids other than lignin. These
include, among many others, defence-related compounds such as furano-
coumarins, flavonoids, and other phytoalexins and diverse UV-protective
compounds including flavonoids and sinapoyl esters (
Vogt, 2010
). There-
fore, it is not surprising that C4H expression has been found induced by
pathogens and elicitors of defence responses, as well as in response to
wounding, UV-containing light, nutrient deprivation, drought, and other
environmental stresses (for review, see
Dixon et al., 2002; Ehlting et al.,
2006; Hahlbrock and Scheel, 1989; Vogt, 2010
). The central and rate-
limiting position of C4H in the general phenylpropanoid pathway is also
the driving force for its characterization in a number of non-model plants
aimed to understand the pathways to specialized phenolic compounds,
especially those with potentials as neutra- and pharmaceuticals; some re-
cent examples include radish (Raphanus sativa), Astragalus membranaceus,
and Angelica gigas (
Park et al., 2010, 2011; Xu et al.,2011
). Towards
manipulating the accumulation of compounds of interest, transgenic
approaches have been employed recently as well. For example, overexpres-
sion of C4H in hairy roots system in A. gigas, a medicinal plant producing
pyranocoumarins, leads to increased production of these bioactive com-
pounds (
Park et al., 2012
).
4. Blocking the way into the lignin pathway: Mutant analyses
Several approaches aimed at altering C4H activity have been employed to
study its impact on lignin accumulation and composition and the consequence
on chemical and physical cell wall properties. Using C4H coding regions for
misregulation through sense-, antisense-, or RNAi-based approaches yielded
surprisingly different effects on lignin amounts and composition depending on
the species and experimental approach used: Overexpression of an alfalfa
(M.
sativa)classIC4H in tobacco resulted in a slight C4H activity increase,
but had little effect on lignin composition. In contrast, downregulation resulted
in reduced total lignin (
Sewalt et al.,1997
). Likewise, C4H downregulation
using the Medicago truncatula class I C4H controlled by a vascular tissue-
specific PAL promoter in transgenic alfalfa resulted in a strong total lignin
