Physiological evidence for a sodium-dependent high-affinity phosphate and nitrate transport at the plasma membrane of leaf and root cells of Zostera marina L.

Abstract

Zostera marina L. is an angiosperm that grows in a medium in which inorganic phosphate (P(i)) and nitrate (NO(3)(-)) are present in micromolar concentrations and must be absorbed against a steep electrochemical potential gradient. The operation of a Na(+)-dependent NO(3)(-) transport was previously demonstrated in leaf cells of this plant, suggesting that other Na(+)-coupled systems could mediate the uptake of anions. To address this question, P(i) transport was studied in leaves and roots of Z. marina, as well as NO(3)(-) uptake in roots. Electrophysiological studies demonstrated that micromolar concentrations of P(i) induced depolarizations of the plasma membrane of root cells. However, this effect was not observed in leaf cells. P(i)-induced depolarizations showed Michaelis-Menten kinetics (K(m)=1.5+/-0.6 microM P(i); D(max)=7.8+/-0.8 mV), and were not observed in the absence of Na(+). However, depolarizations were restored when Na(+) was resupplied. NO(3)(-) additions also evoked depolarizations of the plasma membrane of root cells only in the presence of Na(+). Both NO(3)(-)- and P(i)-induced depolarizations were accompanied by an increase in cytoplasmic Na(+) activity, detected by Na(+)-sensitive microelectrodes. P(i) net uptake (measured in depletion experiments) was stimulated by Na(+). These results strongly suggest that P(i) uptake in roots of Z. marina is mediated by a high-affinity Na(+)-dependent transport system. Both NO(3)(-) and P(i) transport systems exploit the steep inwardly directed electrochemical potential gradient for Na(+), considering the low cytoplasmic Na(+) activity (10.7+/-3.3 mM Na(+)) and the high external Na(+) concentration (500 mM Na(+)).

Sodium as nutrient and toxicant

“…If we use results from Essah et al (2003) in an example, in which an exchange half-time of 5 min is endorsed (Cheeseman 1982), and a unidirectional influx of as high as 300μmol g −1 (fresh wt) h −1 is reported (at 200 mM external [Na + ]; see their Fig. 2), a cytosolic concentration of Na + is predicted to be about 700 mM, more than two orders of magnitude higher than some reported microelectrode measurements at the same, or higher, external Na + concentrations (Carden et al 2003;Rubio et al 2005).…”

“…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). Surprisingly, however, given that cytosolic [Na + ] is considered to be a critical factor in Na + toxicity, very few studies have measured it using microelectrode technology (Carden et al 2003;Rubio et al 2005); clearly, much more progress needs to be made in this area.…”

“…The study by Rubio et al (2005) is particularly striking as it was conducted in the marine angiosperm Zostera marina, and found only 10.7 mM cytosolic Na + in an artificial seawater medium (500 mM [Na + ]). Less of an extreme gradient was found by Carden et al (2003) in barley seedlings exposed to 200 mM NaCl, but here too the cytosolic Na + activity value was low, maximally, 29 mM.…”

The response of primary producer assemblages to mitigation measures to reduce eutrophication in a temperate estuary

“…This is possible due to an efficient uptake of nutrients from the water column and sediment pore water and to a mechanism of conservation, where older leaves act as nutrient sinks which afterwards are translocated to more actively growing and nutrient demanding tissues (Pedersen and Borum, 1992). Nutrients are taken up directly from the water column through leaves and from the sediment pore water through roots (Zimmerman et al, 1987;Hemminga et al, 1994;Bocci et al, 1997;Hemminga, 1998;Sfriso and Marcomini, 1999) but with differences in uptake (Rubio et al, 2005). Sediments are considered the primary nutrient source for seagrass roots, yet they may not have the capacity to support total nutrient requirements seeing as the uptake can be limited by diffusion (Touchette and Burkholder, 2000).…”

The AtNHX1 exchanger mediates potassium compartmentation in vacuoles of transgenic tomato

“…Second, cytosolic K + activities were directly measured by double-barreled K + -selective microelectrodes. The microelectrode pre-treatment and backfilling was similar to the H + -or Na + -selective microelectrodes described previously (Ferná ndez et al, 1999;Rubio et al, 2005). The microelectrodes were filled with a K + -sensor cocktail containing potassium ionophore I (cocktail B, cat.…”