Biology Reference
In-Depth Information
ABSTRACT
Lignins, major components of the vascular plant cell wall, provided the mechanical
support that allowed the development of upright plants adapted to a terrestrial
habitat. Their biosynthesis through the phenylpropanoid and monolignol pathways
has been extensively studied and significant advances have recently been made in
understanding the regulation of this process. Lignin deposition is also modified
in response to both abiotic and biotic stresses. Here, we present an overview of lignin
biosynthesis in response to various abiotic stresses: drought, salinity, heavy metals,
wounding, low temperature, ozone, UV-B radiation, light, elevated CO
2
and nitrogen
stress. Although the stimulation of the phenylpropanoid pathway is a common
feature of stress response, the subsequent synthesis of lignin is only demonstrated in
some cases. The roles of lignins in different phases of abiotic stress response are
discussed as well as the regulation of their synthesis under stress.
I. INTRODUCTION
A. LIGNINS
Lignins are complex phenolic polymers that provide strength, rigidity and
hydrophobicity to certain plant cell walls (
Boerjan et al., 2003; Ralph et al.,
2004; Rogers and Campbell, 2004
). The presence of lignin in vascular tissues
allows plants to stand upright and to withstand the pressure of water trans-
port. The emergence of lignin biosynthesis during evolution is considered to
be crucial for the development of land plants (
Weng and Chapple, 2010
). Due
to the high amounts of this compound in tree wood, lignin is the second most
abundant biopolymer on Earth after cellulose and represents a significant
carbon sink (
Boudet et al., 2003
).
The lignin biosynthesis pathway has been extensively studied over the past
two decades and it is relatively well understood even if some gaps remain
(
Bonawitz and Chapple, 2010
). Lignin is synthesized through the oxidative
polymerization of the three most abundant lignin monomers or monolignols,
namely p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol (
Fig. 1
).
Upon incorporation in the lignin polymer, these monolignols give rise to
p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units. The lignin struc-
ture varies substantially between cell types and species according to the
relative proportion of each unit and the different kinds of interunit bounds
(
Neutelings, 2011; Ralph et al., 2004
). The term lignin therefore refers to a
class of biopolymers with considerable diversity and it is probably more
accurate to use the plural form lignins.
Monolignols are synthesized through a metabolic grid involving 10
different steps and begins with the general phenylpropanoid pathway
(
Fig. 1
). Phenylalanine ammonia-lyase (PAL) catalyses the first step of
