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

Salt Stress
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together with the AtNHX1, are the salt-tolerance determinants, acting in facilitation of
Na + compartmentation and maintenance of intracellular K + status (Yokoi et al., 2002b).
Migration pattern of antiporter gene isolated from the halophyte Atriplex
gmelini (AgNHX1) correlates with H + -pyrophosphatase, indicating its role in vacuolar
compartmentation (Hamada et al., 2001). The product of the novel gene isolated from
rice, the OsNHX1, functions as the Na + /H + exchanger and plays an important role in salt
tolerance (Fukuda et al., 1999). Expression of Na + /H + antiporter gene found in citrus
(cNHX1) was markedly induced by salt stress, supporting its role in salt tolerance
(Porat et al., 2002).
Transcription of NHX1 gene and a number of related genes in Arabidopsis
was up-regulated by drought and/or salinity stress (Yokoi et al., 2002a), which is partially
dependent on ABA biosynthesis and ABA signaling (Shi and Zhu, 2002). It
seems that up-regulation of tonoplast transporters is associated with pleiotropic upregulation
of other genes, or of the activity of the gene products (Tester and Davenport,
2003). Besides the AtNHX family, another gene family has also been identified in
Arabidopsis to encode the cation exchangers (CAX genes) responsible for modulation
of ion fluxes across the vacuolar membrane (Cheng et al., 2002).
The significance of Na + /H + antiporters in sodium translocation into the vacuoles
has been reported for halophytic plants, such as common ice plant (Barkla et al.,
2002). Nevertheless, some plants do not have Na + /H + antiporters, which means that
such species must rely on some other pathways and mechanisms to reduce sodium
uptake, probably by utilization of K + channels and/or transporters that are more selective
for K + (Blumwald et al., 2000).
The ability for harmonizing the sodium uptake, transport, sequestration and
maintenance of energy pools is fundamental in salt tolerance. For better understanding
of the crucial mechanisms involved in regulation of the activity of diverse channel
types, the facilitating ion fluxes across the cell membranes, both in salt-sensitive and
salt-tolerant species, will be major issue in the future studies on salt tolerance in plants.
8.3. Signaling Pathways of Salt Stress Tolerance
Metabolic pathways triggered by the receptor perceiving salt stress leading to the
alterations in gene transcription and protein activity mediated by various signaling
components, have been reviewed (Hasegawa et al., 2000, Zhu, 2000, Tester and Davenport,
2003). Candidate receptors for ionic stress could be various ion transporters.
Related changes in receptor occupancy or receptor clustering may activate the ionic
stress signaling pathways (Xiong and Zhu, 2002).
Mitogen-activated protein kinase (MAPK) cascade and triggering of conformational
alterations in membrane proteins are known as signaling modules initiated by
osmotic stress (Xiong and Zhu, 2002). Several proteins found in Arabidopsis are functional
components of an osmotic stress MAPK cascade (Hasegawa et al., 2000). Accumulation
of the reactive oxygen species (ROS), caused by salinity stress, seems to