Biology Reference
In-Depth Information
III. LIGNIN ROLE IN RESPONSE TO
ABIOTIC STRESSES
Phenylpropanoid metabolism is clearly stimulated by abiotic stresses such as
drought, salinity, ozone and heavy metals. However, experiments clearly
demonstrating a subsequent increase in lignin synthesis are not so abundant.
In fact, this latter parameter was not assessed in many studies. In other
investigations, stress-induced phenylpropanoid metabolism was clearly asso-
ciated with the synthesis of secondary phenolic compounds and not lignin.
Nevertheless, certain studies have clearly demonstrated that lignin content
increases in response to abiotic stresses. Such observations raise questions
about the biological role(s) of lignin synthesized in response to abiotic stresses.
Lignin could be involved in the first alarm phase during stress response
(
Fig. 2
). Most abiotic stresses result in the production of ROS that can
damage the cell when produced in excess but can also act as stress sensor
and signal transduction molecules (
Foyer and Noctor, 2005
). Moreover,
ROS generated in the apoplast may react with cell wall aromatic compounds
such as lignin to generate signalling molecules. Interestingly, ozone has been
shown to cause oxidative modification of the cell wall and the release of wall-
bound phenolics in tomato plants and cell wall extracts from tomato leaves.
These molecules were proposed to act as signalling molecules to trigger
defence reactions (
Wiese and Pell, 2003
). The hypothesis of ozone perception
by the cell wall is consistent with the rapid increase of cytosolic-free calcium
observed in A. thaliana exposed to ozone (
Evans et al., 2005
). However, the
direct involvement of lignins in this signalling process still needs to be
demonstrated.
The most plausible role for lignins is probably in the defence mechanisms
utilized during the acclimation phase of the stress response (
Fig. 2
). The rapid
stimulation of the phenylpropanoid pathway in response to various stresses
(
Koch et al., 1998
) is in favour of such a role, even though associated lignin
synthesis still needs to be demonstrated in many cases. Further proof for a
role in defence is that the lignin increase in response to abiotic stresses is
related to stress intensity and damages (
Betz et al., 2009a; Caban
´
et al., 2004;
Frei et al., 2011
). Finally, it is interesting to note that—when analysed—the
structure of newly synthesized, stress-induced lignins are different from those
of constitutive lignins (
Betz et al., 2009a; Caban´ et al., 2004; Finger-Teixeira
et al., 2010; Frankenstein et al., 2006; Pitre et al., 2007
). These findings could
suggest that the newly synthesized lignins possess different properties from
constitutive lignins that allow them to play a more efficient role in defence.
If stress-induced lignins do indeed play a role in the acclimation phase
defence mechanisms, then questions still remain as to their biological
