Agriculture Reference
In-Depth Information
several mangrove species [55, 67-69]. Clones for
dehydrin
and
phytosulphokine
were abundant in the ESTs of
A. marina
[67]. Dehydrins are
induced by dehydration, low temperature, osmotic stress, seed drying, and/or
exposure to abscisic acid [70, 71]. Phytosulphokine-ʱ is a tyrosine-sulfated
mitogenic pentapeptide that was first isolated from asparagus mesophyll cell
cultures [72] that plays a role in cell division and development. These results
suggest that dehydrin and phytosulphokine may play roles in the salt tolerance
of
A. marina.
The salt-secreting mangrove
A. ebracteatus
was also used to
generate ESTs for the isolation of genes involved in salt tolerance [68].
The molecular characteristics of mangrove salt tolerance have been most
intensively investigated in
B. gymnorhiza
. A total of 26,400 ESTs have been
generated and sequenced from five different cDNA libraries constructed from
the leaves and roots of this species [69]. These sequences were processed into
14,842 high-quality sequences, and clustered and assembled into 6943 unique
genes. Expression profiles were then generated based on the EST frequency in
each cDNA library. To identify genes of potential importance for salt tolerance
in
B. gymnorhiza
, transcriptional analysis was performed using microarray
analysis [73]. Calcium-dependent protein kinase (CDPK), peroxidase and
vacuolar ATP synthase subunit B were highly up-regulated in both low-leaves
and roots.
CDPK
genes have been reported to be involved in salt stress- and
low temperature-induced transcription [74], and overexpression of the
CDPK
gene in rice has been shown to increase tolerance to low temperature, drought
and high salt conditions [75]. A peroxidase-encoding gene was also up-
regulated in leaves and roots, and may contribute to detoxifying activity in
plants by scavenging ROS. High-level induction of the gene that encodes
vacuolar ATP synthase subunit B may reflect the proton generation required
for the activity of the vacuolar Na
+
/H
+
antiporter [73].
PR-6
expression was
mainly up-regulated in the roots, whereas after salt treatment,
Bg70
, which is
unique to severl mangroves,
was up-regulated in leaves and down-regulated in
roots, suggesting that Bg70
is involved in salt-adaptation in the mangrove.
Differences in the transcriptional responses to salt and osmotic stress in
B.
gymnorhiza
were monitored by an oligonucleotide microarray, in which
sorbitol- and salt-responding genes were clearly distinct and the former was
not merely a subset of the latter [39]. In this study, Na
+
/H
+
antiporter
homologs were not up-regulated under salt stress conditions.
Recently, normalized cDNA libraries of two mangroves,
R. mangle
and
Heritiera littoralis
, were analyzed by pyrosequencing and a total of 537,635
sequences were assembled
de novo
and annotated as >13,000 distinct gene
models for each species [55]. Despite normalization, a number of ESTs were
