Agriculture Reference
In-Depth Information
by these studies. Despite these findings, the molecular mechanisms of salt
tolerance in mangrove plants remain incompletely understood. A
potential pitfall of the strategies used in these studies is that they are
based on the over-expression of specific mangrove genes. On the other
hand, there have been several reports on the involvement of repression or
silencing of genes by miRNAs and repressor proteins in salt tolerance of
glycophytes. These findings raise the possibility that the regulation of
gene/protein silencing is the key to understanding salt tolerance in
halophytes, including mangroves. New approaches, such as
metabolomics, might provide novel insight into the mechanisms of
mangrove salt tolerance.
1.
I
NTRODUCTION
Salinity stress is one of the most significant limiting factors in agricultural
crop productivity [1]. Hence, improving the salt tolerance of crops is essential
for sustainable food production. Mangroves are tolerant to high salt stress and
have likely acquired specific genes for the regulation of salt tolerance.
Understanding the molecular mechanisms that mediate salt tolerance in
mangroves will contribute to the knowledge necessary to breed or genetically
engineer salt-tolerant crops.
Mangroves are a heterogeneous group of 80 taxonomically diverse
species, spanning 30 genera and 20 families [2]. It is interesting to note that
these phylogenically distant plants have developed similarly high levels of salt
tolerance. It has not been determined whether there is a common mechanism
that is primarily responsible for salt tolerance. Mangroves are divided into two
distinct groups on the basis of their salt management strategies: ―secretors‖,
which have salt glands or salt hairs, and ―non-secretors‖ [3]. Some secretor
species
form salt gland structures to secrete excess salt, whereas other species
produce structures that are analogous to salt glands. Salt secretion from leaves
is related to salt tolerance [4]; however, the degree to which salt secretion
contributes to the salt tolerance of secreting mangroves is still unclear. Non-
secretors are further classified into two groups: salt excluders and salt
accumulators. Even with exclusion of most of the salt in salt excluders, the
concentrations of Na
+
and Cl
−
within the plant tissues are higher than in non-
mangrove species [5]. To cope with the detrimental effects of salt stress and
maintain ion homeostasis in the cytoplasm, mangrove plants sequester Na
+
and
Cl
−
into vacuoles in hypodermal storage tissue in the leaves [6-9], and some
species deposit salt in the bark of stems and roots [10]. Overall, however, there
