Regulation of SOS1, a plasma membrane Na+/H+ exchanger in
Arabidopsis thaliana, by SOS2 and SOS3.
Qiu QS, Guo Y, Dietrich MA, Schumaker KS, Zhu JK.
Department of Plant Sciences, University of Arizona, Tucson, AZ 85721, USA.
Maintaining low levels of sodium ions in the cell cytosol is critical for plant
growth and development. Biochemical studies suggest that Na(+)/H(+) exchangers
in the plasma membrane of plant cells contribute to cellular sodium homeostasis
by transporting sodium ions out of the cell; however, these exchangers have not
been identified at the molecular level. Genetic analysis has linked components
of the salt overly sensitive pathway (SOS1-3) to salt tolerance in Arabidopsis
thaliana. The predicted SOS1 protein sequence and comparisons of sodium ion
accumulation in wild-type and sos1 plants suggest that SOS1 is involved directly
in the transport of sodium ions across the plasma membrane. To demonstrate the
transport capability of SOS1, we studied Na(+)/H(+)-exchange activity in
wild-type and sos plants using highly purified plasma membrane vesicles. The
results showed that plasma membrane Na(+)/H(+)-exchange activity was present in
wild-type plants treated with 250 mM NaCl, but this transport activity was
reduced by 80% in similarly treated sos1 plants. In vitro addition of activated
SOS2 protein (a protein kinase) increased Na(+)/H(+)-exchange activity in
salt-treated wild-type plants 2-fold relative to transport without added
protein. However, the addition of activated SOS2 did not have any stimulatory
effect on the exchange activity in sos1 plants. Although vesicles of sos2 and
sos3 plants had reduced plasma membrane Na(+)/H(+)-exchange activity, transport
activity in both increased with the addition of activated SOS2 protein. These
results demonstrate that SOS1 contributes to plasma membrane Na(+)/H(+) exchange
and that SOS2 and SOS3 regulate SOS1 transport activity.