Na+ transport in plants

The ability of plants to grow in high NaCl concentrations is associated with the ability of the plants to transport, compartmentalize, extrude, and mobilize Na(+) ions. While the influx and efflux at the roots establish the steady state rate of entry of Na(+) into the plant, the compartmentation of Na(+) into the cell vacuoles and the radial transport of Na(+) to the stele and its loading into the xylem establish the homeostatic control of Na(+) in the cytosol of the root cells. Removal of Na(+) from the transpirational stream, its distribution within the plant and its progressive accumulation in the leaf vacuoles, will determine the ability to deal with the toxic effects of Na(+). The aim of this review is to highlight and discuss the recent progress in understanding of Na(+) transport in plants.     

Vacuolar cation/H+ exchange, ion homeostasis, and leaf development are altered in a T-DNA insertional mutant of AtNHX1, the Arabidopsis vacuolar Na+/H+ antiporter

The function of vacuolar Na+/H+ antiporter(s) in plants has been studied primarily in the context of salinity tolerance. By facilitating the accumulation of Na+ away from the cytosol, plant cells can avert ion toxicity and also utilize vacuolar Na+ as osmoticum to maintain turgor. As many genes encoding these antiporters have been cloned from salt-sensitive plants, it is likely that they function in some capacity other than salinity tolerance. The wide expression pattern of Arabidopsis thaliana sodium proton exchanger 1 (AtNHX1) in this study supports this hypothesis. Here, we report the isolation of a T-DNA insertional mutant of AtNHX1, a vacuolar Na+/H+ antiporter in Arabidopsis. Vacuoles isolated from leaves of the nhx1 plants had a much lower Na+/H+ and K+/H+ exchange activity. nhx1 plants also showed an altered leaf development, with reduction in the frequency of large epidermal cells and a reduction in overall leaf area compared to wild-type plants. The overexpression of AtNHX1 in the nhx1 background complemented these phenotypes. In the presence of NaCl, nhx1 seedling establishment was impaired. These results place AtNHX1 as the dominant K+ and Na+/H+ antiporter in leaf vacuoles in Arabidopsis and also suggest that its contribution to ion homeostasis is important for not only salinity tolerance but development as well.     

Importance of glucose-6-phosphate dehydrogenase activity in cell death

The intracellular redox potential plays an important role incell survival. The principal intracellular reductant NADPH is mainlyproduced by the pentose phosphate pathway by glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme, and by6-phosphogluconate dehydrogenase. Considering the importance of NADPH,we hypothesized that G6PDH plays a critical role in cell death. Ourresults show that 1) G6PDHinhibitors potentiatedH₂O₂-inducedcell death; 2) overexpression ofG6PDH increased resistance toH₂O₂-induced cell death; 3) serum deprivation, astimulator of cell death, was associated with decreased G6PDH activityand resulted in elevated reactive oxygen species (ROS);4) additions of substrates for G6PDHto serum-deprived cells almost completely abrogated the serumdeprivation-induced rise in ROS; 5)consequences of G6PDH inhibition included a significant increase inapoptosis, loss of protein thiols, and degradation of G6PDH; and6) G6PDH inhibition caused changesin mitogen-activated protein kinase phosphorylation that were similarto the changes seen withH₂O₂.We conclude that G6PDH plays a critical role in cell death by affectingthe redox potential.     

High glucose inhibits glucose-6-phosphate dehydrogenase via cAMP in aortic endothelial cells

Recent studies have shown that hyperglycemia is a principal cause of cellular damage in patients with diabetes mellitus. A major consequence of hyperglycemia is increased oxidative stress. Glucose-6-phosphate dehydrogenase (G6PD) plays an essential role in the regulation of oxidative stress by primarily regulating NADPH, the main intracellular reductant. In this paper we show that increased glucose (10-25 mm) caused inhibition of G6PD resulting in decreased NADPH levels in bovine aortic endothelial cells (BAEC). Inhibition was seen within 15 min. High glucose-induced inhibition of G6PD predisposed cells to cell death. High glucose via increased activity of adenylate cyclase also stimulated an increase in cAMP levels in BAEC. Agents that increased cAMP caused a decrease in G6PD activity. Inhibition of cAMP-dependent protein kinase A ameliorated the high glucose-induced inhibition of G6PD. Finally, high glucose stimulated phosphorylation of G6PD. These results suggest that, in BAEC, high glucose stimulated increased cAMP, which led to increased protein kinase A activity, phosphorylation of G6PD, and inhibition of G6PD activity. We conclude that these changes in G6PD activity play an important role in high glucose-induced cell damage/death.     

Engineering salt tolerance in plants

Recent progress has been made in the identification and characterization of the mechanisms that allow plants to tolerate high salt concentrations. The understanding of metabolic fluxes and the main constraints for the production of compatible solutes (i.e. feedback inhibition and the limitation of substrate supply) open up the possibility of genetically engineering entire pathways that could lead to the production of osmoprotectants. This, together with the identification of the different sodium transporters (in particular vacuolar and plasma membrane Na(+)/H(+) antiporters) that could provide the needed ion homeostasis during salt stress, opens the possibility of engineering crop plants with improved salt tolerance.     

Sodium transport in plant cells

Salinity limits plant growth and impairs agricultural productivity. There is a wide spectrum of plant responses to salinity that are defined by a range of adaptations at the cellular and the whole-plant levels, however, the mechanisms of sodium transport appear to be fundamentally similar. At the cellular level, sodium ions gain entry via several plasma membrane channels. As cytoplasmic sodium is toxic above threshold levels, it is extruded by plasma membrane Na(+)/H(+) antiports that are energized by the proton gradient generated by the plasma membrane ATPase. Cytoplasmic Na(+) may also be compartmentalized by vacuolar Na(+)/H(+) antiports. These transporters are energized by the proton gradient generated by the vacuolar H(+)-ATPase and H(+)-PPiase. Here, the mechanisms of sodium entry, extrusion, and compartmentation are reviewed, with a discussion of recent progress on the cloning and characterization, directly in planta and in yeast, of some of the proteins involved in sodium transport.