Sodium Influx and Accumulation in Arabidopsis

Abstract

Arabidopsis is frequently used as a genetic model in plant salt tolerance studies, however, its physiological responses to salinity remain poorly characterized. This study presents a characterization of initial Na ϩ entry and the effects of Ca 2ϩ on plant growth and net Na ϩ accumulation in saline conditions. Unidirectional Na ϩ influx was measured carefully using very short influx times in roots of 12-d-old seedlings. Influx showed three components with distinct sensitivities to Ca 2ϩ , diethylpyrocarbonate, and osmotic pretreatment. Pharmacological agents and known mutants were used to test the contribution of different transport pathways to Na ϩ uptake. Influx was stimulated by 4-aminobutyric acid and glutamic acid; was inhibited by flufenamate, quinine, and cGMP; and was insensitive to modulators of K ϩ and Ca 2ϩ channels. Influx did not differ from wild type in akt1 and hkt1 insertional mutants. These data suggested that influx was mediated by several different types of nonselective cation channels. Na ϩ accumulation in plants grown in 50 mm NaCl was strongly reduced by increasing Ca 2ϩ activity (from 0.05-3.0 mm), and plant survival was improved. However, plant biomass was not affected by shoot Na ϩ concentration, suggesting that in Arabidopsis Na ϩ toxicity is not dependent on shoot Na ϩ accumulation. These data suggest that Arabidopsis is a good model for investigation of Na ϩ transport, but may be of limited utility as a model for the study of Na ϩ toxicity.

Sodium as nutrient and toxicant

“…Indeed, reported unidirectional Na + fluxes across the plasma membrane, as measured with CATE or with short-term tracer uptake experiments, appear at times to be too high to be energetically feasible (Britto and Kronzucker 2009). When coupled with even the small cytosolic exchange half-times for Na + found in the literature (Essah et al 2003), these values can, with CATE, result in cytosolic [Na + ] values an order of magnitude higher than those reported using electrodes see below). Perhaps because of uncertainties with these and other methods, ion-specific microelectrode measurements of cytosolic Na + accumulation have become the new standard (Munns and Tester 2008).…”

“…Interestingly, these are levels at which little enzyme inhibition occurs (Greenway and Osmond 1972;Munns and Tester 2008), so mechanisms of Na + -specific toxicity may need to be sought elsewhere. Of additional importance is the conundrum that these low values, relative to CATE and other methods, suggest that either unidirectional Na + fluxes reported across the membrane using tracers (Britto and Kronzucker 2009), or exchange half-times for cytosolic Na + pools (Essah et al 2003), have been substantially overestimated. Another way to look at this problem is to predict cytosolic pool sizes on the basis of unidirectional influx across the plasma membrane and exchange halftimes for the cytosol (Cram 1969;MacRobbie 1971;Kronzucker 2001, 2003).…”

“…In a pioneering study on Na + transport and accumulation in A. thaliana, Essah et al (2003) showed that, although A. thaliana plants grown under 60-fold differences in external [Ca 2+ ] showed 6-fold differences in per-plant accumulation of Na + (and 4-fold differences in Na + accumulation per gram root), there were no differences in biomass between them. More recently, screening of 12 A. thaliana accessions yielded two coastal populations with reduced AtHKT1 expression that apparently resulted in increased shoot Na + and, surprisingly, greater salt tolerance (Rus et al 2006).…”

“…Schubert and Läuchli 1990;Cramer 1992;Schachtman and Munns 1992;Davenport and Tester 2000;Munns and Tester 2008;Møller and Tester 2007;Møller et al 2009) have suggested that the excessive accumulation of Na + , particularly in shoot tissue, lies at the heart of its toxicity. Indeed, this notion has driven much investigation into the identification of transport systems that either catalyze primary Na + influx into the root (Davenport and Tester 2000;Essah et al 2003), or its distribution within the plant (Møller et al 2009), with the tantalizing prospect of conferring salt tolerance by genetically modifying the influx or localization of Na + . However, as we shall now discuss, evidence is mounting that the assumption that sodium accumulation must be causally linked with its toxicity is not always a safe one.…”

Organic acids enhance the uptake of lead by wheat roots

“…An unamended nutrient solution was used as the control. La(NO 3 ) 3 and CsCl have previously been identified as cation channel blockers by White (1997) and by Essah et al (2003), who also reported appropriate application concentrations. The Pb concentrations in our experiments were maintained at 20 lM.…”