Sodium Uptake in Arabidopsis Roots Is Regulated by Cyclic Nucleotides

Sodium uptake from the soil is a major cause of salinity toxicity in plants, yet little is known about the mechanisms that underlie Na+ influx. We have characterized voltage independent channels (VICs) in Arabidopsis roots that are thought to contribute to Na+ entry. VICs showed no selectivity among monovalent cations, and their gating was found to be voltage independent. However, VIC open probability showed sensitivity to cyclic nucleotides. The presence of micromolar concentrations of cAMP or cGMP at the cytoplasmic side of the plasma membrane evoked a rapid decrease in channel open probability. In accord with predictions from electrophysiological data, our results show that short-term unidirectional Na+ influx is also reduced in the presence of cyclic nucleotides. Moreover, addition of membrane permeable cyclic nucleotides during growth assays improved plant salinity tolerance, which corresponded with lower levels of Na+ accumulation in plants. In summary, these data imply that Arabidopsis plants may contain a cyclic nucleotide-based signaling pathway that directly affects Na+ transport via VICs.