Biology Reference
In-Depth Information
with preventing excess metal efflux from the vascular cylinder rather than
controlling metal influx.
4. Aluminium
Aluminium (Al) limits plant growth and productivity in acidic soils. Al
toxicity is associated with ROS production and lipid peroxidation, and is
also characterized by the inhibition of root elongation that is believed to
contribute greatly to reduced plant productivity (
Kochian, 1995
). Another
study (
Ma et al., 2011
) showed that Al treatment of rice seedlings inhibited
root elongation to a greater extent in the Al-sensitive variety IR64 when
compared to the Al-tolerant variety azucena. Al treatment also significantly
increased lignin content as determined by the acetyl bromide method in the
Al-sensitive, but not the Al-tolerant variety. Increased lignin accumulation
was also associated with a significant increase in H
2
O
2
production, as well as
in the quantity of cell wall-associated peroxidase activity. The authors sug-
gest that Al stimulates lignification in the roots of Al-sensitive varieties by
increasing H
2
O
2
production. A similar analysis (
You et al., 2011
) compared
global expressions in root apices of an Al-tolerant genotype of soybean after
4 h in the presence and absence of 30
mol L
1
AlCl
3
. Aluminium treatment
upregulated a number of cell wall-related genes including lignin genes and
the authors hypothesize that the expression of these genes might be related to
Al-induced root growth inhibition.
m
D. WOUNDING
Often associated with insect or herbivore activities, mechanical wounding
exposes plant tissues to dehydration and opportunist pathogen attack by
fungi, bacteria and viruses. It is therefore vitally important that plants
rapidly protect the exposed tissues. Both lignification and suberization of
plant cell walls play an important role in this process and they are often
associated with the formation of a wound (necrophylactic) periderm (
Biggs,
1986; Vance et al., 1980
). Autofluorescence and histochemical staining of
wounded eucalyptus seedlings (
Hawkins and Boudet, 1996
) showed that
both lignin and suberin could be detected after 24 h in the xylem wound
zone and after 72 h in the cortex. Histochemical staining suggested that the
lignin formed during these early defence responses were relatively poor in
S units as previously observed (
Biggs, 1986
).
Wounding has been shown to induce large-scale changes in gene expres-
sion profiles (
Cheong et al., 2002
) including genes encoding enzymes in the
phenylpropanoid pathway (
Dixon and Paiva, 1995
). In one particularly
detailed study, semi-quantitative RT-PCR and promoter-GUS fusions were
