Measurement of Differential Na+ Efflux from Apical and Bulk Root Zones of Intact Barley and Arabidopsis Plants

Rapid sodium cycling across the plasma membrane of root cells is widely thought to be associated with Na(+) toxicity in plants. However, the efflux component of this cycling is not well understood. Efflux of Na(+) from root cells is believed to be mediated by Salt Overly-Sensitive-1, although expression of this Na(+)/H(+) antiporter has been localized to the vascular tissue and root meristem. Here, we used a chambered cuvette system in which the distal root of intact salinized barley and Arabidopsis thaliana plants (wild-type and sos1) were isolated from the bulk of the root by a silicone-acrylic barrier, so that we could compare patterns of (24)Na(+) efflux in these two regions of root. In barley, steady-state release of (24)Na(+) was about four times higher from the distal root than from the bulk roots. In the distal root, (24)Na(+) release was pronouncedly decreased by elevated pH (9.2), while the bulk-root release was not significantly affected. In A. thaliana, tracer efflux was about three times higher from the wild-type distal root than from the wild-type bulk root and also three to four times higher than both distal- and bulk-root fluxes of Atsos1 mutants. Elevated pH also greatly reduced the efflux from wild-type roots. These findings support a significant role of SOS1-mediated Na(+) efflux in the distal root, but not in the bulk root.