Differential Sodium and Potassium Transport Selectivities of the Rice OsHKT2;1 and OsHKT2;2 Transporters in Plant Cells    

Abstract

Na(+) and K(+) homeostasis are crucial for plant growth and development. Two HKT transporter/channel classes have been characterized that mediate either Na(+) transport or Na(+) and K(+) transport when expressed in Xenopus laevis oocytes and yeast. However, the Na(+)/K(+) selectivities of the K(+)-permeable HKT transporters have not yet been studied in plant cells. One study expressing 5' untranslated region-modified HKT constructs in yeast has questioned the relevance of cation selectivities found in heterologous systems for selectivity predictions in plant cells. Therefore, here we analyze two highly homologous rice (Oryza sativa) HKT transporters in plant cells, OsHKT2;1 and OsHKT2;2, that show differential K(+) permeabilities in heterologous systems. Upon stable expression in cultured tobacco (Nicotiana tabacum) Bright-Yellow 2 cells, OsHKT2;1 mediated Na(+) uptake, but little Rb(+) uptake, consistent with earlier studies and new findings presented here in oocytes. In contrast, OsHKT2;2 mediated Na(+)-K(+) cotransport in plant cells such that extracellular K(+) stimulated OsHKT2;2-mediated Na(+) influx and vice versa. Furthermore, at millimolar Na(+) concentrations, OsHKT2;2 mediated Na(+) influx into plant cells without adding extracellular K(+). This study shows that the Na(+)/K(+) selectivities of these HKT transporters in plant cells coincide closely with the selectivities in oocytes and yeast. In addition, the presence of external K(+) and Ca(2+) down-regulated OsHKT2;1-mediated Na(+) influx in two plant systems, Bright-Yellow 2 cells and intact rice roots, and also in Xenopus oocytes. Moreover, OsHKT transporter selectivities in plant cells are shown to depend on the imposed cationic conditions, supporting the model that HKT transporters are multi-ion pores.

Calcium regulates K⁺/Na⁺ homeostasis in rice (Oryza sativa L.) under saline conditions

“…Ca 2+ was demonstrated to inhibit Na + transport through NSCCs and LCT1 Tester 2000, Amtmann et al 2001). Recent research showed that Ca 2+ inhibited OsHKT2; 1-mediated Na + influx into plant cells (Yao et al 2010). Therefore, Ca 2+ plays important role in regulating apoplast and symplast pathways involved in Na + transport.…”

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

“…These results have led to hypothesize that HKT2;1 subgroup transporters only work as Na + uniporters in planta (Haro et al, 2005;Horie et al, 2007), and that the Na + -K + symport mode evidenced in yeast or Xenopus oocytes is artifactual, resulting from the heterologous context, or anecdotal in planta. It should however be noted that the rice OsHKT2;2 transporter has recently been shown to have the capacity to mediate K + transport in a plant context, when overexpressed in cultured tobacco (Nicotiana tabacum) cells (Yao et al, 2010). Since OsHKT2;4 appears highly K + permeable in all its conducting modes and can even mediate pure K + transport at low external Na + concentrations when expressed in Xenopus oocytes (Figs.…”

Sodium as nutrient and toxicant

“…There is no doubt that Na + , when appearing suddenly at high concentrations, in "shock" scenarios (see discussion in Cheeseman 2013), carries osmotic consequences that disrupt, typically temporarily, the membrane integrity of roots (Britto et al 2010;Coskun et al 2013), or also those of shoots, such as shown in rice (Flowers et al 1991), preceding, or perhaps coinciding with, more "ion-specific" effects (see later discussions). In addition to this well-recognized osmotic effect, a second parsimonious explanation may lie in the disruption of potassium homeostasis-one that may reasonably and fruitfully supplant alternative, to date ill-substantiated , hypotheses, such as those of "toxic" Na + fluxes or cytosolic K + /Na + ratios (Maathuis and Amtmann 1999;Davenport and Tester 2000;Yao et al 2010; see below). The decline in cytosolic [K + ] ([K + ] cyt ) under saline conditions is well documented (Hajibagheri et al 1987(Hajibagheri et al , 1988Binzel et al 1988;Schroeppel-Meier and Kaiser 1988;Speer and Kaiser 1991;Hajibagheri and Flowers 2001;Carden et al 2003;Kronzucker et al 2006; see also Fig.…”