Agriculture Reference
In-Depth Information
stress-induced up-regulation of the Na
+
/H
+
antiporter has been reported in
A.
corniculatum
[19]. Components of SOS pathway have been shown to be
functional not only in glycophytic plants, such as
Arabidopsis
, but also in
halophytes, such as
Thellungiella halophila
[37]. Thus, this pathway may also
be important for maintaining Na
+
homeostasis in mangroves. Tanaka
et al.
[38] identified the gene encoding the vacuolar Na
+
/H
+
antiporter in
B.
sexangula
. This species has been shown to maintain osmotic homeostasis by
compartmentalization of Na
+
into vacuoles, a process that is regulated by the
tonoplast H
+
-ATPase and vacuolar acid phosphatase [6]. These results suggest
that the salt tolerance mechanisms employed by glycophytes may also be
active in halophytes and at least partially responsible for the salt tolerance
observed in mangroves. However, the Na
+
/H
+
antiporter homologs that have
been identified in
Bruguiera
gymnorhiza
are not up-regulated under salt stress
conditions [39]. In addition,
B.
gymnorhiza
exhibits rapid Na
+
accumulation
upon the initiation of salt stress, and its leaves contain 90% more Na
+
and 40%
more Cl
−
than those of
Kandelia candel
[40]. X-ray microanalysis of leaf
mesophyll cells shows evidence of distinct vacuolar compartmentalization of
Na
+
in
K. candel
; conversely,
B. gymnorhiza
seedlings subjected to 100 mM
NaCl compartmentalize Cl
−
, suggesting a distinct role for the Na
+
/H
+
antiporter among mangroves. In addition, the apoplast is another candidate for
sodium compartmentalization by the Na
+
/H
+
antiporter.
2.3. Induction of Antioxidative Enzymes
Abiotic stress factors including salinity, high light intensity, high
temperature and heavy metals lead to oxidative stress and the formation of
reactive oxygen species (ROS), such as superoxide (O
2
-
), hydrogen peroxide
(H
2
O
2
), hydroxyl radicals and singlet oxygen (O
2
1
), which cause extensive
cellular damage and inhibit photosynthesis [41]. These cytotoxic ROS disrupt
normal metabolism through oxidative damage to lipids, proteins and nucleic
acids [42]. Mangroves with high levels of antioxidants, either constitutive or
induced, have been reported to exhibit greater tolerance to oxidative damage
[13, 42, 43]. The activities of antioxidative enzymes such as catalase (CAT),
ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione
reductase (GR), and superoxide dismutase (SOD) increase under high salinity,
and there is a correlation between the levels of these enzymes and salt
tolerance in mangroves [13,42]. Transgenic tobacco plants that over-expressed
APX in their chloroplasts showed enhanced tolerance to salt, PEG and water
