Biology Reference
In-Depth Information
than in control xylem, suggesting that the stressed cell modifies its pattern of
lignin deposition when confronted to drought stress.
B. SALINITY
Salt stress is caused by the presence of high concentrations of salt ions—
mainly sodium and chloride. Analysing the effects of this stress on plants is
difficult since the high salt ion concentrations provoke not only a chemical
(ionic) imbalance in plant cells, but also induce a drought stress by increasing
the water potential (
) of the soil. Major resistance mechanisms to salt stress
are associated with transporter-mediated export/sequestration of Na
þ
ions
across the plasma membrane and tonoplast, respectively (
Zhu, 2003
).
Although a number of studies have generated experimental results suggesting
that lignification might play a role in salt tolerance, direct evidence linking
salt stress to modifications in the lignification process is difficult to find in the
literature. For example, metabolic profiling of Arabidopsis cell cultures sug-
gested that the methylation cycle (supplying methyl groups), the phenylpro-
panoid pathway and glycinebetain biosynthesis were all synergistically
induced as a short-term response to salt stress (
Kim et al., 2007
). However,
phenylpropanoids are not just destined to lignin biosynthesis and the obser-
vation of increased carbon flow in this metabolic pathway is not definite
proof of an effect on lignification.
In another study (
Quiroga et al., 2000
), the effect of salt stress (100 mM
NaCl) on the expression of a tomato peroxidase gene (TPX1) was investi-
gated. Under non-stress conditions, this gene is specifically expressed in root
tissues and Northern blot analyses indicated that the gene was upregulated
by salt stress. The same gene is also activated in aerial tissues following
wounding and its overexpression in tomato leaves is associated with
increased lignification. In situ hybridization of roots from non-stressed to-
mato plants showed that TPX1 transcripts accumulated in the endodermis
and protoxylem in the apical zone (first 0.5 cm), in the endodermis and
hypodermis of the medium zone (8-10 cm from root tip) and in the endoder-
mis and exodermis of the basal zone (13-15 cm from root tip). Since these
tissues are lignified and/or suberized, the observed expression pattern of the
TPX1 gene is coherent with a role in root lignification/suberization. Salt
stress modified TPX1 expression in tomato roots; the TPX1 gene was re-
pressed in the apical root zone, stimulated in the medium zone and decreased
in the exodermis, but not the endodermis of basal root samples. Unfortu-
nately, lignin/suberin levels were not evaluated in the roots of salt-stressed
plants, and it is therefore not possible to know whether alterations in TPX1
expression were associated with modified lignification.
C
