Energy Facilitated Na Uptake in Excised Corn Roots via P and Na NMR

23Na NMR microimaging: a tool for non-invasive monitoring of sodium distribution in living plants

Detailed knowledge of the sodium (Na) distribution within the tissues of highly salt-tolerant Australian native species could help in understanding the physiological adaptations of salt-tolerance or salt-sensitive plants. ²³Na nuclear magnetic resonance (NMR) microimaging is presented as a tool to achieve this goal. Maps of the Na distribution in stem tissue were obtained with an in-plane resolution of approximately125 µm and a slice thickness of 4 mm. Simultaneously recorded high resolution ¹H NMR images showing water distribution in the same slice with 31 µm in-plane resolution and 1 mm slice thickness, were used as an anatomical reference together with optical micrographs that were taken immediately after the NMR experiments were completed. To quantify the Na concentration, reference capillaries with known NaCl concentrations were located in the NMR probe together with the plant sample. Average concentration values calculated from signal intensities in the tissue and the capillaries were compared with concentration values obtained from atomic emission photometry and optical microscopy performed on digested stem sections harvested immediately after NMR experiments. Results showed that ²³Na NMR microimaging has great potential for physiological studies of salt stress at the macroscopic level, and may become a unique tool for diagnosing salt tolerance and sensitivity.

23Na and (1)H NMR microimaging of intact plants

(23)Na NMR microimaging is described to map, for the first time, the sodium distribution in living plants. As an example, the response of 6-day-old seedlings of Ricinus communis to exposure to sodium chloride concentrations from 5 to 300 mM was observed in vivo using (23)Na as well as (1)H NMR microimaging. Experiments were performed at 11.75 T with a double resonant (23)Na-(1)H probehead. The probehead was homebuilt and equipped with a climate chamber. T(1) and T(2) of (23)Na were measured in the cross section of the hypocotyl. Within 85 min (23)Na images with an in-plane resolution of 156 x 156 micrometer were acquired. With this spatial information, the different types of tissue in the hypocotyl can be discerned. The measurement time appears to be short compared to the time scale of sodium uptake and accumulation in the plant so that the kinetics of salt stress can be followed. In conclusion, (23)Na NMR microimaging promises great potential for physiological studies of the consequences of salt stress on the macroscopic level and thus may become a unique tool for characterizing plants with respect to salt tolerance and salt sensitivity.

An in Vivo Nuclear Magnetic Resonance Investigation of Ion Transport in Maize (Zea mays) and Spartina anglica Roots during Exposure to High Salt Concentrations

The response of maize (Zea mays L.) and Spartina anglica root tips to exposure to sodium chloride concentrations in the range 0 to 500 mM was investigated using 23Na and 31P nuclear magnetic resonance spectroscopy (NMR). Changes in the chemical shift of the pH-dependent 31P-NMR signals from the cytoplasmic and vacuolar orthophosphate pools were correlated with the uptake of sodium, and after allowing for a number of complicating factors we concluded that these chemical shift changes indicated the occurrence of a small cytoplasmic alkalinization (0.1-0.2 pH units) and a larger vacuolar alkalinization (0.6 pH units) in maize root tips exposed to salt concentrations greater than 200 mM. The data were interpreted in terms of the ion transport processes that may be important during salt stress, and we concluded that the vacuolar alkalinization provided evidence for the operation of a tonoplast Na+/H+-antiport with an activity that exceeded the activity of the tonoplast H+ pumps. The intracellular pH values stabilized during prolonged treatment with high salt concentrations, and this observation was linked to the recent demonstration (Y. Nakamura, K. Kasamo, N. Shimosato, M. Sakata, E. Ohta [1992] Plant Cell Physiol 33: 139-149) of the salt-induced activation of the tonoplast H+- ATPase. Sodium vanadate, an inhibitor of the plasmalemma H+- ATPase, stimulated the net uptake of sodium by maize root tips, and this was interpreted in terms of a reduction in active sodium efflux from the tissue. S. anglica root tips accumulated sodium more slowly than did maize, with no change in cytoplasmic pH and a relatively small change (0.3 pH units) in vacuolar pH, and it appears that salt tolerance in Spartina is based in part on its ability to prevent the net influx of sodium chloride.

Hypoxia does not affect rate of ATP synthesis and energy metabolism in rice shoot tips as measured by 31P NMR in vivo

The cytoplasmic pH, concentrations of phosphate metabolites, and rate of ATP synthesis were measured in vivo in excised rice shoot tips under normoxic and hypoxic conditions using 31P NMR. When supplied with glucose, the shoot tips grew rapidly and were relatively unaffected by hypoxia. The cytoplasmic pH decreased transiently by only 0.2 units during hypoxia, and the concentration of ATP was maintained to at least 90% of the normoxic level. Most importantly, the unidirectional rate constant of ATP synthesis from free phosphate decreased less than 25% during hypoxia. This is in contrast to other actively growing tissues such as the maize root tip. gamma-Aminobutyrate was the major nonvolatile fermentation end product after 22 h of hypoxia. Other hypoxia-induced changes included a modest increase in [Ala] and [succinate] as well as a substantial decrease in [malate].

In vivo 133Cs-NMR a probe for studying subcellular compartmentation and ion uptake in maize root tissue

Three 133Cs-NMR signals were observed in the spectra of CsCl-perfused and CsCl-grown maize seedling root tips. Two relatively broad lower field resonances were assigned to the subcellular, compartmented Cs+ in the cytoplasm and vacuole, respectively. The rate of area increase of the broader cytoplasmic Cs resonance was about 9-times faster than that of the vacuolar signal during the first 300 min of tissue perfusion with CsCl. In addition, the spin lattice relaxation time of the cytoplasmic Cs resonance was approx. 3-times shorter than that of the extracellular resonance, while the Cs+ signal associated with the metabolically less active vacuolar compartment exhibited a relaxation time comparable to that of the extracellular signal. 133Cs spectra of excised, maize root tips and excised top sections of the root adjacent to the kernel, each grown in 10 mM CsCl showed a difference in the relative areas of the Cs resonance corresponding to the distinct cytoplasm/vacuole volume ratio of these well differentiated sections of the root. The high correlation of counterion concentration with 133Cs chemical shifts suggested that the larger downfield shift exhibited by the cytoplasmic confined Cs+ was due principally to the higher ionic strength and protein content in this compartment. Such observations indicate that 133Cs-NMR might be employed for studying ionic strength, and osmotic pressure associated chemical shifts and the transport properties of Cs+ (perhaps as an analogue for K+) in subcellular compartments of plant tissues.

In vivo 23Na and 31P NMR measurement of a tonoplast Na+/H+ exchange process and its characteristics in two barley cultivars

A Na+ uptake-associated vacuolar alkalinization was observed in roots of two barley cultivars (Arivat and the more salt-tolerant California Mariout) by using 23Na and 31P in vivo NMR spectroscopy. A NaCl uptake-associated broadening was also noted for both vacuolar Pi and intracellular Na NMR peaks, consistent with Na+ uptake into the same compartment as the vacuolar Pi. A close coupling of Na+ with H+ transport (presumably the Na+/H+ antiport) in vivo was evidenced by qualitative and quantitative correlations between Na+ accumulation and vacuolar alkalinization for both cultivars. Prolongation of the low NaCl pretreatment (30 mM) increased the activity of the putative antiport in Arivat but reduced it in California Mariout. This putative antiport also showed a dependence on NaCl concentration for California Mariout but not for Arivat. No cytoplasmic acidification accompanied the antiporter activity for either cultivar. The response of adenosine phosphates indicated that ATP utilization exceeded the capacity for ATP synthesis in Arivat, but the two processes seemed balanced in California Mariout. These comparisons provide clues to the role of the tonoplast Na+/H+ antiport and compensatory cytoplasmic adjustments including pH, osmolytes, and energy phosphates in governing the different salt tolerance of the two cultivars.

An in vivo 1H and 31P NMR investigation of the effect of nitrate on hypoxic metabolism in maize roots

The effect of nitrate on the short-term hypoxic response and recovery of flooded mature maize roots has been investigated in vivo by 1H and 31P NMR and in vitro by 1H NMR and gas chromatography-mass spectrometry. Employing 1H NMR in addition to 31P NMR extended the number of identifiable compounds in vivo from 4 to 15, while in vitro two-dimensional NMR and gas chromatography-mass spectrometry aided rigorous in vivo 1H NMR resonance assignments and quantitation of 24 compounds. In the absence of nitrate, the concentrations of key metabolites including alanine, ethanol, gamma-aminobutyrate, lactate, succinate, and sucrose changed during 8 h of hypoxia in a manner consistent with reduced tricarboxylic acid cycle activity and diversion to glycolytic fermentation. The pH drop in the cytoplasm during hypoxia was rapid, about 0.2 unit, and diminished quickly upon recovery. Rapid recovery of ethanol, succinate, and sucrose levels was also observed, which indicates a return to normal aerobic metabolism. Although the hypoxic response itself, including pH, was not greatly affected by the presence of nitrate, nitrate reduced the amount of fermentation end products produced, helped maintain a higher free NTP concentration during hypoxia, and increased the rate of overall recovery from hypoxia. These findings suggest the presence of a nitrate-induced maintenance-level respiration in hypoxic maize roots, which helps explain the protection imparted by nitrate to flooded hypoxic maize plants.

Na-NMR Studies of the Intracellular Sodium Ion Concentration in the Halotolerant Alga Dunaliella salina

The Intracellular Na(+) concentration in the halotolerant alga Dunaliella salina was measured in intact cells by (23)Na-NMR spectroscopy, utilizing the dysprosium tripolyphosphate complex as a sodium shift reagent, and was found to be 88 +/- 28 millimolar. Intracellular sodium ion content and intracellular volume were the same, within the experimental error, in cells adapted to grow in media containing between 0.1 and 4.0 molar NaCl. These values assume extracellular and intracellular NMR visibilities of the (23)Na nuclei of 100 and 40%, respectively. The relaxation rate of intracellular sodium was enhanced with increasing salinity of the growth medium, in parallel to the intracellular osmosity due to the presence of glycerol, indicating that Na(+) ions and glycerol are codistribbuted within the cell volume.