Biology Reference
In-Depth Information
2. Cadmium
Cadmium (Cd) is another metal pollutant impacting directly or indirectly on
different plant physiological processes and plants have evolved a wide range of
different mechanisms to increase their tolerance to this metal (
Xiong et al.,
2009
). Studies (
Wang et al., 2008
) showed that up to 60% of total cadmium
taken up by plants is located in the cell wall of Ramie and affects the structure
of primary cell wall polysaccharides in flax hypocotyls (
Douchiche et al.,
2010
). A recent study (
Finger-Teixeira et al., 2010
) showed that cadmium
treatment of soybean plants was associated with reduced root elongation
and fresh-/dry-weight production, as well as an increase in PAL activity,
H
2
O
2
levels and both soluble- and cell wall-bound peroxidase activity. Deter-
mination of lignin levels (thioglycolic acid assay) revealed a dramatic increase
(16.1-131%) in cell wall residue (CWR) lignin content. In agreement with these
results, subsequent nitrobenzene oxidation indicated an increase in total
(H
S) monomer yield when expressed on a CWR basis. When total
monomer yield was based on lignin content it was decreased, suggesting that
the lignin structure was modified (increased condensation). In agreement with
this observation, the percentage of H units increased.
þ
G
þ
3. Zinc
The metal zinc (Zn) serves as a cofactor for more than 300 enzymes and is
therefore an essential micronutrient for both plants and animals (
Guerinot
and Eide, 1999
). In a global transcriptomic approach (
van de Mortel et al.,
2006
), the differential expression of Arabidopsis thaliana—a non-zinc accu-
mulator and Thlaspi caerulescens a zinc hyperaccumulator—under different
zinc concentrations revealed that more than 2000 genes were significantly
differentially expressed (
5-fold) between the two species. In addition to
genes known to be involved in metal homeostasis (Zn transporters, defen-
sins), 24 genes with a potential function in lignification (
Ehlting et al.,
2005
)—including transcripts corresponding to F5H, CAD, COMT, 4CL
and laccase genes—as well as 13 genes involved in suberin synthesis
(
Costaglioli et al., 2005
) were more highly expressed in T. caerulescens as
compared to A. thaliana. Subsequent microscopic examination by autofluor-
escence revealed higher autofluorescence of the root endodermis (and occa-
sional presence of a second cell layer) in T. caerulescens suggesting a higher
degree of lignin/suberin biosynthesis. Autofluorescent cell wall thickenings
are deposed in a U-shape in radial walls and inner tangential walls. Similar
thickenings have been observed in the salt-adapted crucifer Thellungiella
halophila (
Inan et al., 2004
). The authors observed that the U-shaped depos-
its occurred mainly in the cell walls of the older non-absorbing root region
and hypothesized that increased lignin/suberin deposition was mainly linked
