Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Salt Stress
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1997), is speculated to be important for Na + uptake in conditions of high salinity
(Maathuis and Amtmann, 1999). The LCT1 expression in wheat resulted in suppressed
uptake of Na + by K + and Ca 2+ , indicating that LCT1 may represent a molecular link
between Ca 2+ and Na + uptake into plant cells (Amtmann et al., 2001). Low affinity K +
carriers, such as AKT1 are inward rectifying channels that activate K + influx and display
a high K + /Na + selectivity ratio, but also can mediate a significant uptake of sodium
ions under high salinity (Blumwald et al., 2000).
Ion channels are distinguished form carriers by their capacity to catalyze transmembrane
ion movement without conformational change. Ion channels are integral
components of all membranes operating as dynamic ion transport systems coupled via
membrane electrical activities (White, 1999). There are four major groups of ion channels,
classified according to the gating mechanism (Krol and Trebacz, 2000): 1) ligandgated
(able to bind intracellular second messengers), 2) voltage-gated (responsible for
signal transmission and transduction in response to changes in the membrane potential),
3) stretch-activated (additional transmembrane receptors) and, 4) light -activated
(involved in light signal transduction). Since sodium-specific channels in plants have
not been reported yet, it seems that sodium moves through the general cation channels
with different permeability for particular ions (Schachtman et al., 1991, Amtmann and
Sanders, 1999).
Competition between Na + and K + for intracellular influx by utilization of the
common ion channels has physiological basis, since K + is essential co-factor of many
enzymes. Sodium is an inhibitor of the enzymatic activity even in the halophytes, but at
the same time it is the most available and the cheapest vacuolar osmolyte for plants in
saline environments. Regarding control of sodium uptake and transport, especially
from the aspect of K/Na selectivity, the voltage-gated plasmalemma K + channels have
very important role and are classified into the inward (K + in ) and outward (K+ out ) rectifiers.
The K + in (KIRCc) and K+ out
(KORCs) channels are activated by hyperpolarizing
potentials and by membrane depolarization, respectively, and controlled by cytosolic
Ca 2+ , ATP and pH in different ways (Grabov and Blatt, 1997).
Outward rectifiers of the root parenchyma cells are responsible for xylem loading
(Maathuis et al., 1997). Salt-induced depolarization of the root plasma membrane
may activate outward rectifying potassium channel, enabling the diffusion of Na + into
the cells down its electrochemical gradient (Schachtman et al., 1991). Two classes of the
outward rectifying channels have been cloned from plants: first is energized by Na + ,
whereas the second class of K + transporters is comprised of a large gene family, expressing
dual affinity for K + (Schachtman, 2000). Inward rectifiers are very sensitive for
K + over Na + (Amtmann and Sanders, 1999). Inward rectified cation channels (which
close on depolarization) are involved in salt adaptation by reducing the permeability for
sodium and potassium ions, leading to decrease in the entry of Na + ions and the leakage
of K + ions out of the cells under high salinity conditions (Jacoby, 1994). Sodium ion is
a competitor for uptake through the K + inward rectifying channels at the plasma membrane,
such as AKT1, which is the member of the Shaker-type family (Schachtman,