Agriculture Reference
In-Depth Information
cells. Thus, it has been proposed that mangrin plays an important role in the
salt-tolerance mechanisms of
B. sexangula
and that the biosynthesis of
mangrin might be an effective means of enhancing salt tolerance in higher
plants. This is the first and most promising salt tolerance gene to be isolated
from mangroves. However, a mangrin homologue isolated from
B. gymnorhiza
was unable to confer enhanced salt tolerance in transgenic
Arabidopsis
(Tada
et al., unpublished data).
Functional screening in
E. coli
has also been used to isolate salt tolerance
genes that encode a cytosolic chaperonin containing TCP-1ʱ (CCTʱ)
homologue [59] and a eukaryotic elongation factor IA (eEF1A) homologue
[60] in
B. sexangula
. In CCTʱ, a functional region of only 218 amino acids,
which contains the apical domain, is necessary for osmotolerance.
Furthermore, this domain exhibits chaperone activity
in vitro
. Therefore, it has
been suggested that CCTʱ facilitates the folding of proteins without ATP or a
cage-like structure, and may play an important role in stress tolerance.
Enhanced tolerance to salt and osmotic stress was not conferred by other plant
eEFIA homologues isolated from the halophytes
Suaeda japonica
and
Salsola
komarovii
or
A. thaliana
.
Transcriptome profiling followed by functional screening in
Agrobacterium
has been applied for the identification of salt tolerance genes
from
B. gymnorhiza
[61].
Agrobacterium tumefaciens
transformed with
expression vectors for 28 cDNAs that were up-regulated in salt-treated roots
were screened for salt tolerance. Among them,
A. tumefaciens
with genes for
lipid transfer, zinc finger and ankyrin repeat proteins, showed enhanced salt
tolerance. Transgenic
Arabidopsis
plants that expressed each of these genes
also exhibited increased tolerance to NaCl compared to wild-type plants.
For a comprehensive analysis of key genes involved in the regulation of
salt tolerance in
B. gymnorhiza
, cDNA expression libraries were constructed
from salt-treated plants using the host organism
A. tumefaciens
[62].
Functional screening of these
Agrobacterium
libraries identified 44 putative
salt tolerance genes. Among them, two cDNA clones, a homologue of the
cyc02
gene from
Catharanthus roseus
and
Bg70
, which belongs to a multigene
family specific to several mangroves, conferred increased salt tolerance to
transgenic
Arabidopsis
. These results suggested that these two genes are
involved in salt tolerance in
B. gymnorhiza
. Other notable clones identified by
Agrobacterium
functional screening included cDNAs encoding lipoic acid
synthase (LAS), the non-specific lipid transfer protein (LTP) precursor, the
H
+
-transporting ATP synthase-like protein, the ribulose 1,5-bisphosphate
carboxylase small subunit, metallothionein-like protein, and the DNAJ heat
