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

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activate the MAPK cascade, as well (Kovtun et al., 2000). Some mechanisms of signaling
pathways involved in salt tolerance may be associated with stretch-activated channels
and ion-specific receptors, with significant role of calcium transporters and components
of Ca 2+ -related signal transductional pathways (Tester and Davenport, 2003).
The Ca 2+ has at least two roles in salt tolerance: 1) a pivotal participation in salt
stress signaling that controls ion homeostasis pathways (Yokoi et al., 2002a), and 2)
direct inhibitory effect on a Na + entry (e.g. Lazof and Bernstein, 1999). The first role was
confirmed by Ca 2+ -dependent activation of phosphatase leading to transcription of the
ENA1 gene, which encodes the P-type ATPase (Mendoza et al., 1994). Components of
signal recognition and transduction pathways initiate the action of a calcium sensor on
a protein kinase that affects the activity of Na + /H + antiporter (Shi et al., 2000).
In Arabidopsis, the SOS1 (Salt Overly Sensitive) locus is essential for Na + and
K + homeostasis, as well as for the control of the long-distance Na + transport and loading
Na + into the xylem under severe and mild salt stress, respectively (Shi et al., 2002a).
SOS1 ion transporter, the SOS2 protein kinase, and its associated Ca 2+ sensor - the
SOS3 constitute a functional module (Quintero et al., 2002). The Arabidopsis SOS2
gene, which is presumed to encode a serine/threonine protein kinase, is required for
intracellular ion homeostasis (Liu et al., 2000). Although vesicles of sos2 and sos3
plants had reduced plasma membrane Na + /H + -exchange activity, transport ability in the
mutants increased with the addition of activated SOS2 protein, indicating that SOS2
and SOS3 are involved in regulation of SOS1 transport activity (Qiu et al., 2002). Moreover,
the recent discovery of SOS4, a novel salt tolerance determinant encoding pyridoxal
kinase, pointed out the role of this gene in Na + and K + homeostasis by modulating
the activities of ion transporters (Shi et al., 2002b). Components of SOS signal pathway,
which was identified to be a pivotal regulator of plant ion homeostasis, operate in the
hierarchical sequence (Hasegawa et al., 2000). This signaling pathway resembles the
yeast calcineurin cascade, controlling Na + fluxes across the plasma membrane (Bressan
et al., 1998).
According to Zhu (2000), the SOS pathway for plant Na + tolerance includes: a)
high salt stress leading to an increase in cytosolic free Ca 2+ concentrations, b) binding
of SOS3 to Ca 2+ which activates the protein kinase SOS2, c) activated SOS3-SOS2
kinase complex increasing the expression of SOS1 and probably other transporter genes
under salt stress, and d) this gene expression (remarkable also at the post-translation
level) and regulation of transporter activity leading to ion homeostasis and, thus enhancing
salt tolerance in plants.
Majority of Ca 2+ -stimulated protein phosphorylation is performed by members
of the Ca 2+ -dependent protein kinase (CDPK) family (Sanders et al., 1999). Many of
transport proteins, such as membrane transporters, H + -ATPases and aquaporins involved
in osmoregulation of the cell, are regulated by CDPKs (Li et al., 1998). Gene
found in rice that encodes a Ca 2+ -dependent protein kinase (designated as OsCDPK7)
was induced by salt stress, maintaining the signaling pathway by unknown post-translation
mechanism (Saijo et al., 2000). The GSK3/shaggy-like protein kinases play an