Biology Reference
In-Depth Information
observed to be particularly abundant in xylem cells in all three organs, as
well as in stem collenchyma cells and root epidermal cells. Histochemical
staining revealed that salt stress induced a significant increase in the quan-
tity of lignified xylem tissue in stems (11
), but did not
appear to stimulate lignin production in individual cells. The authors
suggest that the increased lignification of vascular tissue is necessary to
impart greater selectivity and to compensate for diminished bulk flow along
the apoplastic pathway.
) and roots (2
C. HEAVY METALS
Metal pollutants (Zn, Pb, Cd, Cu, As, etc.) enter the biosphere mainly as a
result of anthropomorphic activities (industry, mining, agriculture, etc.) where
they represent a danger for plant, animal and human well-being (
Sharma and
Dietz, 2009
). Metal toxicity in plants can be assigned to three main actions:
(i) direct interaction with proteins by targeting of structural and catalytic sites,
(ii) stimulation of ROS production and associated oxidative stress and
(iii) displacement of cations from specific binding sites resulting in the disrup-
tion of physiological processes (
Sharma and Dietz, 2009
). Metals can be
classified as heavy metals, toxic metals and metalloids; however, since no
clear agreement exists on exactly what metals belong to which group it is
better to consider each element separately.
1. Copper
Copper (Cu) is known to be essential for lignin biosynthesis and sub-/supra-
optimal levels are often associated with modifications in lignin production
(
Moura et al., 2010
). In one study (
Lin et al., 2005
), Cu treatment of soybean
seedlings was associated with a significant reduction (18-56% depending
upon the concentration) in root growth. Copper also provoked a rapid
increase, then decrease in whole root H
2
O
2
levels, together with an increase
in both laccase and peroxidase activity and a global increase in lignin (thio-
glycolic acid assay). Activities of both anionic and cationic peroxidases
increased and semi-quantitative RT-PCR showed that the expression of
two (out of three tested) peroxidase genes increased following Cu treatment.
Copper ions form part of the catalytic site of laccase enzymes and it is
possible that Cu-induced lignification is associated with increased laccase
activity and gene expression (see
Chapter 5
). Interestingly, copper also plays
a central role in regulating the expression of different laccase genes in
Arabidopsis through a post-transcriptional mechanism implicating different
microRNAs (miRNAs;
Abdel-Ghany and Pilon, 2008
).
