Environmental Engineering Reference
In-Depth Information
accumulate carbohydrates such as sugars and starch to cope with salt stress (Parida et al.
2002). In conclusion, to pull through salt stress successfully, plants have to accumulate high
concentrations of compatible solutes to maintain low water potential in cells.
I
ON
T
OXICITY AND
I
ON
H
OMEOSTASIS
Due to those similarities in physicochemical properties, Na
+
competes with K
+
to bind to
the active site of the enzymes causing the inactivation or degeneration of the enzymes and the
disorder of protein synthesis and ribosome functions (Munns and Tester 2008). The toxic
position of Na
+
is mainly in leaf blade, since Na
+
in the transpiration stream is deposited here
nor in the roots (Munns 2002). Slabu et al. indicated that the functions of chloroplast were
disturbed when K
+
was replaced by Na
+
, resulting in uncontrolled water losses (Slabu et al.
2009). Although Cl
-
is a necessary micronutrient for cells in regulating enzyme activity,
maintaining membrane protein and pH gradients, it is toxic to cells at high level (White and
Broadley 2001; Xu et al. 1999). Accumulated Na
+
in shoot is associated with a declining in
stomatal conductance, while Cl
-
in high concentration can damage chloropyll and inhibit PS II
(Tavakkoli et al. 2011). More seriously, the toxic effects of Na
+
and Cl
-
have a cumulative
effect. Hence, balancing intracellular ion concentrations by maintaining high K
+
and low Na
+
concentration in the cytosol are the foremost for plants (Yan et al. 2013).
Fortunately, during the lengthy evolution, most plants have acquired capabilities to
regulate ions accumulation and to uptake ions selectively. Plants can exclude the toxic excess
Na
+
ions from cytosol (Janicka-Russak et al. 2013) and almost all the plants are able to take
up water and exclude Na
+
and Cl
-
from soil effectively (Munns 2005). Some plants especially
halophytes own succulent leaves and stems to dilute the concentration of the toxic ions and
others evolve salt glands or bladders to secret excess ions (Yan et al. 2013). Studies in wheat
and wild soybean with better salt-tolerance demonstrated that the Na
+
concentration in shoot
is kept lower through withholding it in root (Islam et al. 2007; Luo et al. 2005). That is to say,
to exclude Na
+
from the cytoplasm and to accumulate Na
+
in vacuolar efficiently are the most
important steps to maintain ion homeostasis in cells (Silva and Gerós 2009). Either exclusion
by roots or compartmentalization into vacuoles, the transport of ions is mediated by H
+
-
ATPase, ion channels and co-transporters in plants. The plasma membrane Na
+
/H
+
antiporter
SOS1 and the tonoplast membrane antiporters NHXs are the most important transporters in
salt stress and are investigated extensively and thoroughly (Zhu 2003).
Table 1. Improving plant salt tolerance by bioengineering ion homeostasis
Gene symbol
Donor
Acceptor
Gene function
Elevated character
Reference
Na+/H+ antiporters in
tonoplast, exchange Na+
or Li+ for H+
biomass production, grain
output and leaf
K+ accumulation
Vacuolar Na+/H+
Antiporter AtNHX1
A. thaliana
Wheat
Xue et al. (2004)
Na+/H+ antiporters in
tonoplast, exchange Na+
or Li+ for H+
Vacuolar Na+/H+
antiporter AtNHX1
Activity of vacuolar
Na+/H+ antiporter
A. thaliana
Tall fescue
Zhao et al. (2007)
