Agriculture Reference
In-Depth Information
Proline is synthesized from L-glutamate by two successive reducing reactions,
which are catalyzed by Δ
1
-pyrroline-5-carboxylate synthase (P5CS) and P5C
reductase (P5CR), respectively [24]. In the glutamate pathway, the γ-glutamyl
kinase activity of P5CS is the rate-limiting step [25]. The molecular
mechanisms that mediate proline accumulation may result from a significant
up-regulation of
AcP5CS
expression in leaves of
A. corniculatum
during
salinity stress [19] (Table 1). Salt stress-enhanced expression of
P5CS
has also
been observed in
Arabidopsis
[26]. The other precursor of proline biosynthesis
is ornithine, which is transaminated to P5C by the mitochondrial enzyme
ornithine-Δ-aminotransferase (OAT) [27].
In
B. gymnorhiza
, salt treatment increases the transcription and activity of
the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphate
2-phosphatase, which is believed to be involved in osmotic regulation via
sucrose synthesis [28, 29] (Table 1).
2.2. Maintenance of Ion Homeostasis
In addition to compatible solutes, mangroves and other halophytes also
use Na
+
as an energetically cheap osmoticum [30]. In
A. germinans
,
Laguncularia racemosa
and
Rhizophora
spp., leaf sap osmolarity is mostly
explained by the concentration of Na
+
and Cl
−
[31]. In mangroves, a high
osmotic potential is essential for the uptake of seawater against the high
negative water potential. After accumulating excess amounts of Na
+
, plants
must maintain low cytosolic Na
+
levels and a high cytosolic K
+
/Na
+
ratio
because, as with glycophytes, cytosolic enzymes in halophytes are sensitive to
high Na
+
concentrations [21]. In mangroves, salt accumulation occurs via the
sequestration of Na
+
and Cl
−
into vacuoles in hypodermal storage tissue in the
leaves [6-9].
In non-halophytes, Na
+
compartmentalization is well characterized. This
compartmentalization is mediated by the salt overly sensitive (SOS) pathway
in
Arabidopsis
, through which toxic amounts of Na
+
are sequestered into
vacuoles via Na
+
/H
+
antiporters [32]. In the SOS pathway, SOS1, an NHX-
type Na
+
/H
+
antiporter that regulates cytosolic Na
+
concentrations, is
stimulated by the SOS3-SOS2 complex [33], which is comprised of the protein
kinase SOS2 and a calcineurin-like protein SOS3 and activated by Ca
2+
binding [34]. It was shown that overexpression of vacuolar Na
+
/H
+
antiporters
could enable transgenic
Arabidopsis
to grow in 200 mM NaCl [35, 36].
Although the SOS pathway has not been well studied in mangroves, salt
