The putative plasma membrane Na(+)/H(+) antiporter SOS1
controls long-distance Na(+) transport in plants.
Shi H, Quintero FJ, Pardo JM, Zhu JK.
Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, USA.
The salt tolerance locus SOS1 from Arabidopsis has been shown to encode a
putative plasma membrane Na(+)/H(+) antiporter. In this study, we examined the
tissue-specific pattern of gene expression as well as the Na(+) transport
activity and subcellular localization of SOS1. When expressed in a yeast mutant
deficient in endogenous Na(+) transporters, SOS1 was able to reduce Na(+)
accumulation and improve salt tolerance of the mutant cells. Confocal imaging of
a SOS1-green fluorescent protein fusion protein in transgenic Arabidopsis plants
indicated that SOS1 is localized in the plasma membrane. Analysis of SOS1
promoter-beta-glucuronidase transgenic Arabidopsis plants revealed preferential
expression of SOS1 in epidermal cells at the root tip and in parenchyma cells at
the xylem/symplast boundary of roots, stems, and leaves. Under mild salt stress
(25 mM NaCl), sos1 mutant shoot accumulated less Na(+) than did the wild-type
shoot. However, under severe salt stress (100 mM NaCl), sos1 mutant plants
accumulated more Na(+) than did the wild type. There also was greater Na(+)
content in the xylem sap of sos1 mutant plants exposed to 100 mM NaCl. These
results suggest that SOS1 is critical for controlling long-distance Na(+)
transport from root to shoot. We present a model in which SOS1 functions in
retrieving Na(+) from the xylem stream under severe salt stress, whereas under
mild salt stress it may function in loading Na(+) into the xylem.