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 μmol 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 ∼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.

Sodium as nutrient and toxicant

“…Thus, a suppression of [K + ] cyt is one of the clear consequences of sodium's actions (perhaps not necessitating the invoking of the miraculous powers of a "cytosolic K + /Na + ratio"), as is a depolarization of the plasma-membrane potential, both instantaneously, upon first exposure to Na + (Shabala et al 2003(Shabala et al , 2006Mian et al 2011;cf. Bowling andAnsari 1971, 1972;Cheeseman 1982;Nocito et al 2002), and in the longer term (Malagoli et al 2008) (Fig. 1a).…”

“…However, as we have previously pointed out, the relationship of currents obtained from such electrophysiological studies, mostly conducted in patch-clamp configurations in membrane patches and naked protoplasts, to Na + fluxes and accumulation at the whole-plant level has, by no means, been established, and many questions remain . Indeed, in planta fluxes in excess of 100 micromoles per gram (fresh weight) per hour have been repeatedly reported in root systems (Essah et al 2003;Malagoli et al 2008;Møller et al 2009;Wang et al 2009;Wetson and Flowers 2010), and one can show, using established models of cation transport and energization Britto and Kronzucker 2006), that ion fluxes of this magnitude, were they to indeed proceed across plasma membranes, would be associated with a respiratory energy cost vastly in excess of the entire respiratory budget of the plant (Malagoli et al 2008;Britto and Kronzucker 2009;Kronzucker and Britto 2011).…”

Rapid Ammonia Gas Transport Accounts for Futile Transmembrane Cycling under NH3/NH4 + Toxicity in Plant Roots

“…] higher than typical fluxes of mineral (ionic) nutrients (Britto and Kronzucker, 2006). Although fluxes of sodium (Na + ) under toxic (saline) conditions have been reported to reach or exceed such values (Lazof and Cheeseman, 1986;Essah et al, 2003;Malagoli et al, 2008), the validity of these fluxes have recently come into question, particularly with respect to their unrealistic energetic requirements (Britto and Kronzucker, 2009;Kronzucker and Britto, 2011); moreover, such fluxes are generally reported at much higher external substrate concentrations (typically, 100 mM or higher). On the other hand, such energetic limitations do not apply to the passive electroneutral fluxes of NH 3 .…”

The integration of activity in saline environments: problems and perspectives

“…The apparent biophysical aspects of the balance cannot be lightly discounted (e.g. Carden et al 2003); the inward electrochemical gradient on Na + at the plasmalemma, the tendency of Na + to move down the gradient (Cheeseman et al 1985;Xue et al 2011) and the tendency of Na + to cycle rapidly, make it difficult to reconcile measurable fluxes with the energy available from respiration (Malagoli et al 2008;Britto and Kronzucker 2009;Kronzucker and Britto 2011). In addition, even at moderate salinity in both glycophytes and halophytes, there can also be a substantial electrochemical gradient across the tonoplast driving Na + back to the cytosol (Carden et al 2003).…”