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Maintaining low levels of sodium ions in the cell cytosol is critical for plant growth and development. Biochemical studies suggest that [Na.sup.+]/[H.sup.+] 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.sup.+]/[H.sup.+]-exchange activity in wild-type and sos plants using highly purified plasma membrane vesicles. The results showed that plasma membrane [Na.sup.+]/[H.sup.+]-exchange activity was present in wild-type plants treated with 250 mM NaCI, 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.sup.+]/[H.sup.+]-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.sup.+]/[H.sup.+]-exchange activity, transport activity in both increased with the addition of activated SOS2 protein. These results demonstrate that SOS1 contributes to plasma membrane [Na.sup.+]/[H.sup.+] exchange and that SOS2 and SOS3 regulate SOS1 transport activity.

Ca2+ influx into moss protoplasts was estimated to confirm the existence of a dihydropyridine (DHP)-sensitive Ca2+ transport system. Voltage-dependent DHP-sensitive Ca2+ channels on the plasma membrane transport Ca2+ to moss protoplasts. The mediation of this plasma membrane Ca2+ channel is an effect of cytokinin in the induction of bud formation.

The phytohormone abscisic acid (ABA) plays an essential role in plant development and during the response of the plant to abiotic stress. In this study, we report that the R2R3-type transcription factor MYB30 is involved in the regulation of ABA signaling. Arabidopsis mutants lacking MYB30 are hypersensitive to ABA during germination and seedling growth. A K283R substitution in MYB30 blocks its SUMO E3 ligase SIZ1-mediated sumoylation in Arabidopsis protoplasts, indicating that MYB30 is sumoylated by SIZ1 and that K283 is the principal site for small ubiquitin-like modifier conjugation. Expression of MYB30K283R in myb30 partially rescues the mutant ABA-hypersensitive phenotype, but expression of wild-type MYB30 complements the mutant phenotype. Overexpression of MYB30 in wild-type results in an ABA-insensitive phenotype, whereas overexpression of MYB30 in the sizl mutant does not alter sizl hypersensitivity to ABA. The sizt-2 myb30-2 double-mutant exhibits greater ABA sensitivity than either single mutant, but a mutation in the SIZ1-sumoylated ABI5 transcription factor suppresses the ABA hypersensitivity of myb30-2 to wild-type levels. Our results suggest that coordination of ABI5 and MYB30 sumoylation by SIZ1 may balance gene expression, which is required for regulation of ABA signaling during seed germination. [ABSTRACT FROM AUTHOR]

Examines the regulation of salt overly sensitive pathway (SOS1), a plasma membrane in Arabidopsis thaliana. Transport of sodium ions across the plasma membrane; Influence of activated SOS2 in the increase exchange activity of sodium ions; Effects of sodium ions on cellular metabolism and ion homeostasis.