Agriculture Reference
In-Depth Information
have been few reports on the molecular dissection of mechanisms of salt
exclusion. Other means by which mangroves cope with salt stress include the
accumulation of compatible solutes and the induction of antioxidative
enzymes. Recently, genome-wide investigations into the genes involved in the
salt tolerance mechanisms of mangroves have been conducted. These studies
include transcriptome and proteome analysis, subtractive screening of cDNAs,
and functional screening of cDNAs in microorganism hosts. This review
focuses on the insights revealed by molecular studies aimed at understanding
salinity tolerance in mangroves and their future directions.
2.
M
OLECULAR
R
ESEARCH ON
M
ANGROVE
S
ALT
T
OLERANCE
2.1. Accumulation of Compatible Solutes
An important biochemical mechanism by which mangroves cope with the
high osmolarity of salt is through the accumulation of low molecular mass
compounds termed compatible solutes, or osmolites [11-20]. Compatible
solutes that accumulate in the cytoplasm balance the osmotic pressure caused
by high concentrations of sodium and chloride ions in the vacuoles, without
interfering with normal biochemical reactions. Compatible solutes fall into
several major categories, including carbohydrates or their derivatives, and
amino acids or their derivatives. In many halophytes, proline or glycine
betaine accumulate in concentrations high enough to create osmotic pressures
over 0.1 MPa [21]. In mangroves, pinitol, chiro-inositol, asparagine,
quebrachitol and mannitol are also considered to be compatible solutes [12,
14, 17, 18, 20]. Pinitol and mannitol are the most common compatible solutes
in a number of mangrove species [12, 17, 18]. Molecular researches on the
production of compatible solutes in mangroves are listed in Table 1.
Glycine betaine, the most common compatible solute in plants, is found in
mangrove species [11, 16]. In higher plants, glycine betaine is synthesized
from choline in a two-step oxidation reaction catalyzed by choline
monooxygenase (CMO) and betaine aldehyde dehydrogenase (BADH),
respectively [22].
BADH
genes involved in glycine betaine synthesis in
Avicennia marina
have been cloned [16] (Table 1). Of the two
BADH
genes
identified, one was up-regulated under salt stress, and this up-regulation
