School of BioSciences - Research Publications
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ItemFutile Na+ cycling at the root plasma membrane in rice (Oryza sativa L.): kinetics, energetics, and relationship to salinity tolerance(OXFORD UNIV PRESS, 2008-11)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.
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ItemNon-reciprocal interactions between K+ and Na+ ions in barley (Hordeum vulgare L.)(OXFORD UNIV PRESS, 2008-07)The interaction of sodium and potassium ions in the context of the primary entry of Na(+) into plant cells, and the subsequent development of sodium toxicity, has been the subject of much recent attention. In the present study, the technique of compartmental analysis with the radiotracers (42)K(+) and (24)Na(+) was applied in intact seedlings of barley (Hordeum vulgare L.) to test the hypothesis that elevated levels of K(+) in the growth medium will reduce both rapid, futile Na(+) cycling at the plasma membrane, and Na(+) build-up in the cytosol of root cells, under saline conditions (100 mM NaCl). We reject this hypothesis, showing that, over a wide (400-fold) range of K(+) supply, K(+) neither reduces the primary fluxes of Na(+) at the root plasma membrane nor suppresses Na(+) accumulation in the cytosol. By contrast, 100 mM NaCl suppressed the cytosolic K(+) pool by 47-73%, and also substantially decreased low-affinity K(+) transport across the plasma membrane. We confirm that the cytosolic [K(+)]:[Na(+)] ratio is a poor predictor of growth performance under saline conditions, while a good correlation is seen between growth and the tissue ratios of the two ions. The data provide insight into the mechanisms that mediate the toxic influx of sodium across the root plasma membrane under salinity stress, demonstrating that, in the glycophyte barley, K(+) and Na(+) are unlikely to share a common low-affinity pathway for entry into the plant cell.
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ItemNH4+-stimulated and -inhibited components of K+ transport in rice (Oryza sativa L.)(OXFORD UNIV PRESS, 2008-09)The disruption of K(+) transport and accumulation is symptomatic of NH(4)(+) toxicity in plants. In this study, the influence of K(+) supply (0.02-40 mM) and nitrogen source (10 mM NH(4)(+) or NO(3)(-)) on root plasma membrane K(+) fluxes and cytosolic K(+) pools, plant growth, and whole-plant K(+) distribution in the NH(4)(+)-tolerant plant species rice (Oryza sativa L.) was examined. Using the radiotracer (42)K(+), tissue mineral analysis, and growth data, it is shown that rice is affected by NH(4)(+) toxicity under high-affinity K(+) transport conditions. Substantial recovery of growth was seen as [K(+)](ext) was increased from 0.02 mM to 0.1 mM, and, at 1.5 mM, growth was superior on NH(4)(+). Growth recovery at these concentrations was accompanied by greater influx of K(+) into root cells, translocation of K(+) to the shoot, and tissue K(+). Elevating the K(+) supply also resulted in a significant reduction of NH(4)(+) influx, as measured by (13)N radiotracing. In the low-affinity K(+) transport range, NH(4)(+) stimulated K(+) influx relative to NO(3)(-) controls. It is concluded that rice, despite its well-known tolerance to NH(4)(+), nevertheless displays considerable growth suppression and disruption of K(+) homeostasis under this N regime at low [K(+)](ext), but displays efficient recovery from NH(4)(+) inhibition, and indeed a stimulation of K(+) acquisition, when [K(+)](ext) is increased in the presence of NH(4)(+).