Futile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance

Abstract

Globally, over one-third of irrigated land is affected by salinity, including much of the land under lowland rice cultivation in the tropics, seriously compromising yields of this most important of crop species. However, there remains an insufficient understanding of the cellular basis of salt tolerance in rice. Here, three methods of 24Na+ tracer analysis were used to investigate primary Na+ transport at the root plasma membrane in a salt-tolerant rice cultivar (Pokkali) and a salt-sensitive cultivar (IR29). Futile cycling of Na+ at the plasma membrane of intact roots occurred at both low and elevated levels of steady-state Na+ supply ([Na+]ext=1 mM and 25 mM) in both cultivars. At 25 mM [Na+]ext, a toxic condition for IR29, unidirectional influx and efflux of Na+ in this cultivar, but not in Pokkali, became very high [>100 micromol g (root FW)(-1) h(-1)], demonstrating an inability to restrict sodium fluxes. Current models of sodium transport energetics across the plasma membrane in root cells predict that, if the sodium efflux were mediated by Na+/H+ antiport, this toxic scenario would impose a substantial respiratory cost in IR29. This cost is calculated here, and compared with root respiration, which, however, comprised only approximately 50% of what would be required to sustain efflux by the antiporter. This suggests that either the conventional 'leak-pump' model of Na+ transport or the energetic model of proton-linked Na+ transport may require some revision. In addition, the lack of suppression of Na+ influx by both K+ and Ca2+, and by the application of the channel inhibitors Cs+, TEA+, and Ba2+, questions the participation of potassium channels and non-selective cation channels in the observed Na+ fluxes.

Fig. 1.

Accepted ‘pump-leak’ mechanism of sodium transport across the plasma membrane in plant cells. (This figure is available in colour at JXB online.)

Fig. 2.

Efflux of ²⁴Na⁺ from roots of intact seedlings of two cultivars of rice. (A and B) Representative plots of efflux measured using (A) a periodic elution protocol (Method 1) or (B) a subsampling procedure (Method 2). (This figure is available in colour at JXB online.)

Fig. 3.

Time course of apparent Na⁺ influx into roots of intact seedlings of two cultivars of rice, at 25 mM [Na⁺]_ext. Declining influx over time is an indication of the extent of simultaneously occurring efflux.

Fig. 4.

Five minute ‘direct’ influx into roots of intact seedlings of two cultivars of rice (Pokkali, left side; IR29, right side) at 25 mM [Na⁺]_ext, under variable ionic conditions and in the presence of established channel-modifying agents (see Materials and methods). Different letters (a, b) indicate significant differences between means within a given cultivar, and a given individual graph (P <0.05, n >3).

Fig. 5.

²⁴Na⁺ efflux from roots of intact seedlings of IR29 rice, grown at 25 mM [Na⁺]_ext, as sodium sulphate (triangles), or as sodium chloride with (squares) or without (circles) added silicate. (This figure is available in colour at JXB online.)

Fig. 6.

Oxygen consumption by roots of two rice cultivars, under low and high sodium provision. Percentage figures above the bars indicate the proportion of the O₂ flux that would be required to power the unidirectional efflux of Na⁺ from roots (as measured by Method 1 in Table 1), assuming that this flux is energetically active, and powered by an electroneutral H⁺/Na⁺ exchange (see Fig. 1). The hatched area above the 25 mM condition in IR29 indicates the amount by which the O₂ flux falls short of what would be required for active Na⁺ efflux. (This figure is available in colour at JXB online.)