Agriculture Reference
In-Depth Information
duction and the subsequent oxidative stress. ABA and CaM likely restricted root-to-
shoot salt transport by reducing water flow (Li et al.
2009
).
There is experimental evidence that salt stress affects the integrity of cellular
membranes, activities of enzymes and the functioning of the plant photosynthetic
apparatus (Serrano et al.
1999
). An important cause of this damage is the production
of reactive oxygen species (ROS; Smirnoff
1993
). Oxidative stress generates ROS
such as superoxide, hydroxyl and peroxyl radicals and the balance between antioxi-
dation and oxidation is believed to be a critical concept for maintaining a healthy
biological system (Jithesh et al.
2006b
).
A number of reviews have concentrated on the link between salt stress and anti-
oxidative pathways in plants (Bohnert and Jensen
1996
; Dat et al.
2000
; Van Breuse-
gem et al.
2001
; Arora et al.
2002
; Borsani et al.
2003
). The plant antioxidative
stress pathway comprises two components, the non-enzymatic and the enzymatic
components. The non-enzymatic component consists of antioxidants such as to-
copherol, carotenoids, ascorbate and glutathione that are free-radical-scavenging
molecules (Salin
1987
). The enzymatic component consists of enzymes such as
superoxide dismutase, catalase, ascorbate peroxidase, monohydroascorbate reduc-
tase, dehydroascorbate reductase and glutathione reductase (Salin
1987
). Apart
from these, an iron-storage protein, ferritin, is also involved in the reactive oxygen-
scavenging network (Morel and Barouki
1999
; Mittler et al.
2004
).
Most of the early studies in mangroves have dealt with the effects of salinity on
photosynthesis (Ball and Farquhar
1984
) and respiration (Burchett et al.
1989
; Fu-
kushima et al.
1997
). However, recently, there has been a growing interest in the ef-
fect of salinity and its relation to antioxidant enzyme status in mangroves and their
associates (Cherian et al.
1999
; Takemura et al.
2000
; Cherian and Reddy
2003
;
Parida et al.
2004
; Jithesh et al.
2006a
). Parida et al. (
2004
) assessed the activities
of some antioxidative enzymes and levels of antioxidants in
Bruguiera parviflora
and suggested that under salinity stress plants are protected against activated oxy-
gen species by the elevated levels of certain antioxidative enzymes, thus avoiding
lipid peroxidation during salt exposure and differential changes in the levels of the
isoforms due to NaCl treatment may be useful as markers for recognizing salt toler-
ance in mangroves.
The morphological, physiological and biochemical studies done in the past have
not clearly explained the salt-adaptation mechanism and its evolution. Recently,
some progresses have been achieved in understanding the mechanism of salt ad-
aptation in mangroves on a molecular level.
Avicennia marina
is one of the well-
studied mangroves because of its characters of salt secretion and high salt tolerance.
A. marina
deals with salt stress through accumulating betaine serving as an osmo-
lyte. Hibino et al. (
2001
) first identified and cloned the
BADH
gene that is involved
in betaine synthesis in
A. marina
.
BADH
was up-regulated under salt stress, and this
tendency was consistent with the accumulation of betaine in
A. marina
. Two other
genes,
AmT1
and
AmT2
(coding for Betaine/Proline transporter) were also isolated
from
A. marina
later (Waditee et al.
2002
). Jithesh et al. (
2006a
) reported that in
A. marina
high salinity did not lead to transcriptional change of gene
Sod1
, encoding
enzyme Cu/Zn-SOD, but osmotic stress decreased transcript level of this gene and
