Biology Reference
In-Depth Information
defence compounds, or signalling molecules (
Tzin and Galili, 2010
). Thus,
flow through the pathway must be regulated tightly to ensure production of
large amounts of precursors for lignin biosynthesis when and where needed,
but also to ensure sufficient availability of precursors for less abundant
products. Regulation of the pathway clearly occurs at the transcriptional
level, evident by the temporal and spatial variation of gene expression during
development and in response to environmental clues. Most genes encoding
phenylpropanoid enzymes are highly expressed in tissues and organs under-
going lignification and many are induced by biotic and abiotic stresses. Most
lignin biosynthetic genes share a common expression pattern when compared
across hundreds of developmental samples (
Ehlting et al., 2008
) suggesting
transcriptional coregulation by the same set of gene regulatory proteins.
In recent years, the complex transcription factor network controlling lignin
biosynthetic gene expression became more and more unravelled and this is
the topic of Chapter
6
.
II. CYTOCHROME P450 HYDROXYLASES
IN MONOLIGNOL BIOSYNTHESIS
All three hydroxylations of the phenylpropanoid pathway are catalysed by
distinct cytochrome P450 monooxygenases (CYP or P450). Systematic no-
menclature of P450 is based on protein sequence identity and phylogeny
(
Nelson et al., 1996
). Members of the same family (e.g., CYP73) usually share
at least 40% identity, and subfamilies (e.g., CYP73A) share at least 55%
identity. Within a family, individual genes are numbered in the order of
identification regardless of the species they originate from. P450s are a
ubiquitous class of enzymes found both in eukaryotes and prokaryotes. In
plants, they catalyse a wide variety of redox reactions both in primary and
secondary metabolism and are encoded by large gene families encompassing
several hundred members (
Bak et al., 2011; Mizutani and Ohta, 2010
). P450-
mediated reactions are essentially irreversible and are strategically located at
important branch points in many metabolic networks. Thus, they are the
major 'gatekeepers' that irreversibly channel carbon into distinct sub-
branches of metabolic networks. Indeed, it has been shown that the three
hydroxylases involved in the phenylpropanoid pathway are the rate-limiting
steps defining flow into the pathway itself, into the G-monolignol pathway,
and into the S-lignin specific branch (
Anterola and Lewis, 2002
). Within the
phenylpropanoid pathway, cinnamate 4-hydroxylase (C4H) constitutes the
CYP73A family and catalyses the para- or 4-hydroxylation of cinnamic acid;
4-coumaroylshikimate 3
0
-hydroxylase (C3
0
H) belongs to the CYP98A family
