Abstract
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 24Na+ efflux in these two regions of root. In barley, steady-state release of 24Na+ was about four times higher from the distal root than from the bulk roots. In the distal root, 24Na+ 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.
Highlights
The entry of Na+ into plant cells is considered to be central to Na+ stress and toxicity, in large part due to its interference with cytosolic K+ homeostasis (Maathuis and Amtmann, 1999; Munns and Tester, 2008)
Significantly (60–80%) more tracer was released by the distal root of wild-type plants than from their bulk roots, or from the sos1 distal root and bulk roots, or the wildtype distal root at pH 9.2
Tracer released by the distal root of sos1 mutants was not significantly different from that released by sos1 bulk roots, nor did pH treatment significantly affect tracer release from bulk roots of wild-type plants
Summary
The entry of Na+ into plant cells is considered to be central to Na+ stress and toxicity, in large part due to its interference with cytosolic K+ homeostasis (Maathuis and Amtmann, 1999; Munns and Tester, 2008). Under saline (sodic) conditions, roots of many plant species appear to have little control over the thermodynamically passive, unidirectional influx of Na+ across the plasma membrane; this is suggested by the exceedingly high flux values that are frequently reported for this ion when it is present at high external concentrations (e.g., Lazof and Cheeseman, 1986; Davenport and Tester, 2000; Essah et al, 2003; Malagoli et al, 2008; Wang et al, 2009; Kronzucker and Britto, 2011) To counteract such large influxes of Na+, and keep cytosolic Na+ concentrations low (Carden et al, 2001), a powerful Na+ efflux system is thought to operate in the plasma membrane of root cells, mediating the extrusion of Na+ (Malagoli et al, 2008). In a recent comprehensive review of Na+ efflux from roots of higher plants, we have referred to this cycling phenomenon as “Rapid Transmembrane Sodium Cycling,” or RTSC (Britto and Kronzucker, 2015).
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