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Rice Genomics and Genetics 2012, Vol.3, No.7, 39
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49
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40
1.1 Maintaining the integrity of membrane system
Under salt stress, the high salt solution first impairs
the cell membrane by influencing the membrane’s
permeability, composition and the role playing during
transporting. These traits are obviously different
when rice is cultivated in favorable conditions such
as the increased permeability and the intensified
peroxidation of membrane lipids, and these variations
all obstruct normal physiological and biochemical
functions of the plasma membrane, further affect the
metabolism of the whole cell (Bing et al., 2008).
Reactive oxygen including O
2-
and H
2
O
2
, can be
produced in various ways such as the enzymatic
reaction of plants, the electron transfer process of
chloroplasts and mitochondria and the auto-oxidation
of some low-molecular-weight organic compounds.
However, they would be wiped out quickly by
protective enzymes, saying SOD, POD, CAT and APX,
leading to a physiological balance. POD, CAT and
SOD constitute system of protective enzymes in plant.
They worked in coordination to eliminate reactive
oxygen produced by lipid peroxidation to protect the
membrane structure, finally (Sun et al., 2008; Shu and
Chen, 2000).
1.2 Ionic compartmentation
Salt-tolerate plants can resist or reduce the damage
posed by salt-stress via regulating the ions absorption
and compartmentation. Under this situation, the
excessive ions accumulated in plants make many
functional proteins and enzymes inactive leading up to
a failure in cellular metabolism. However, plants do
have a natural ability to protect themselves from
injury produced by vast amount of inorganic ions Na
+
.
Because most inorganic ions accumulated in cells are
transported and stored in the vacuoles, thus plants
can maintain normal growth in certain concentrations
of salt solution, which is called the ionic
compartmentation. Studies also show that this kind of
compartmentation phenomenon exists in both strong
and poor salt-tolerate plants, or even common plants,
which illuminates that it might be a generally obtained
ability in all plants, and it depends on the proton
pumps located in membrane of vacuoles, they
transport H
+
to the outside of vacuoles from inside
while Na
+
was transported conversely. In this way,
Na
+
was accumulated in vacuoles. The power of
transfer comes from gradient of proton concentration
and electrochemical potential (Bing et al., 2008; Ren
et al., 2005).
1.3 Plant osmotic regulation
Under the salt-stress, plants are easy to lose water and
then are damaged badly. Due to the osmotic pressure
of the external environment of high salt is low, it was
forming a strong osmotic pressure difference between
the plant internal environment and the external
environment, which makes plants are difficult to
absorb water, thus resulting in a phenomenon of water
deficit. To avoid cell dehydration and maintain a
stable water balance inside and outside cells, plant
mainly increases the concentration of intracellular
solute (small molecular soluble organics or inorganic
ions) and decreases intracellular osmotic potential to
regulate osmotic potential difference between cells.
The osmotic adjustment of plant includes organic
methods and inorganic methods. The first one
involves proline, betaine, soluble carbohydrates, sugar
alcohols and polyamines, etc. while the inorganic
methods mainly involve inorganic ions such as Na
+
,
K
+
, Ca
2+
, Cl
-
and so on (Bing et al., 2008).
1.3.1 Ionic regulation
In order to maintain a stable micro-environment
within cytoplasm, plant regulates the relative
concentration of inorganic ions (mainly K
+
and Na
+
)
to adjust cell turgor, cell volume, intracellular pH
value, ionic strength and many other crucial
physiological parameters. Normally, in its entire
physiological process, most plant cells accumulate K
+
while transport Na
+
outside to the cells leading to a
high K
+
/Na
+
rate across membrane, which is helpful
for K
+
to perform its unique functions. But, how do
plant cells to maintain high K
+
/Na
+
within cells?
Studies have confirmed that two K
+
absorption
systems have existed on cell membrane—low-affinity
K
+
absorption system and high-affinity K
+
absorption
system. The low-affinity K
+
absorption system is a K
+
channel and K
+
is transported through following a
smooth concentration gradient from high to low. The
high-affinity K
+
absorption system is equipped with a
symport-device and K
+
can be transported against the
concentration gradient depending on the energy