Biology Reference
In-Depth Information
The recent characterization of transcription factors involved in controlling the
expression of genes encoding lignin biosynthesis enzymes is enabling a better
understanding of the regulation of this complex process (
Zhao and Dixon,
2011
). Lignins also play an important role in plant defence, by acting as a
physical barrier against pathogens. More recent studies also suggest a role for
phenylpropanoids in chemical defence mechanisms (
Naoumkina et al., 2010
).
Abiotic stresses are also known to modulate lignification and lignins are
supposed to play a role in abiotic stress tolerance. The objective of this review
is to provide an overview of our current knowledge about this topic.
B. ABIOTIC STRESSES
Abiotic stresses are defined as environmental factors that reduce cell activity
and plant growth and cause a profound reorganization of plant metabolism
(
Lichtenthaler et al., 1998
). Abiotic stresses include various natural or an-
thropogenic factors including light, UV, heat, low temperatures, drought,
flooding, mineral deficiencies or excess, pollutants, ozone, heavy metals and
elevated CO
2
. Plants respond to stresses through a dynamic process that can
be divided into four phases according to the unifying stress concept (
Fig. 2
;
Kosova et al., 2011; Larcher, 2003; Lichtenthaler et al., 1998
). Before the
stress application, plants are in a physiological standard situation that is
optimum in its location. At the beginning of the stress, the plants are in an
alarm phase: the plant vitality declines, the tolerance level is minimum and
stress-signalling pathways are induced. Acute damage and senescence may
occur if the stress is too high and the plant possesses low tolerance mechan-
isms. If the plant survives, this phase is followed by an acclimation phase that
corresponds to a new physiological standard with a maximum resistance to
stress. This phase is associated with the activation of protection, detoxifica-
tion and repair mechanisms and may result in readjustments leading to a
maintenance phase if the new physiological standard is stable under stress.
Plant metabolism undergoes profound reorganizations due to the reorienta-
tion of metabolism to stress acclimation. This phase may be stabilized and
maintained under prolonged exposure to stress. If the plant is unable to
maintain this phase or the stress intensity is too high, the vitality declines
and chronic damage occurs leading to cell death. This is the exhaustion
phase. When the stressor is removed before the beginning of the senescence,
a regeneration phase may be observed leading to new physiological stan-
dards. The sequence of these phases depends on stress duration and intensity
and plant tolerance capacity. Specific modifications in cell metabolism may
be observed at each stage and different roles can be assigned to alterations in
