Lignins and Abiotic Stresses - Botanical Research - page 246

function(s). ROS produced by many abiotic stresses may be in excess and
therefore likely to cause irreversible damages to the cell. Due to its scaveng-
ing properties, lignin may act as an antioxidant ( Blokhina et al., 2003;
Dizhbite et al., 2004 ). In ozone-exposed leaves of poplar, mesophyll cells
with newly synthesized lignins were located near the necrotic lesion ( Caban´
et al., 2004 ). ROS (H 2 O 2 ) were also shown to be accumulated near the
necrotic spots in birch, tobacco and tomato ( Pellinen et al., 2002;
Wohlgemuth et al., 2002 ). Although the authors suggested that H 2 O 2 might
act as a signal molecule for triggering programmed cell death, it is also
possible that lignin could contribute to maintaining the delicate ROS equi-
librium between signalling and toxicity. Consistent with this hypothesis,
ozone-induced H 2 O 2 production was restricted to the apoplast in ozone-
tolerant poplar, whereas H 2 O 2 accumulation occurred in the cell wall, the
plasma membrane, cytosol and chloroplasts in sensitive plants ( Oksanen
et al., 2004 ). Lignins may contribute, together with other antioxidants, to
limit ROS production to the apoplast.
Lignins are also likely to play a role in acclimation phase defence mechan-
isms by strengthening the cell wall. Both drought and cold stress can lead to
wilting and increased lignification would contribute to improved mechanical
support of the plant aerial structure as well as water transport. Lignification
could also help to reduce cell expansion and plant growth during this phase,
thereby favouring reallocation of carbon resources to other defence mechan-
isms. Lignin may also waterproof plant cell walls and limit the apoplastic
transport, thereby allowing a higher degree of ion selectivity as observed in
the case of salt stress. In this case, lignification (and/or suberization) occurs in
the root endodermis.
Finally, as a major component of the cell wall, lignin may play a role in
metabolic readjustment during the maintenance phase of the stress response
( Fig. 2 ). For example, the late lignification response of poplar stems to ozone
was typically an adjustment response ( Richet et al., 2011 ). In this case, most
of the leaf metabolism was devoted to the acclimation process, thereby
reducing carbon availability to stem and provoking a readjustment of stem
metabolism.
IV. REGULATION OF THE RESPONSE
Because lignin biosynthesis is a metabolically costly process ( Amthor, 2003 )
and its carbon investment is not reversible, the activation of this pathway in
response to abiotic stresses must be tightly regulated. However, few data are
available in the literature.