Effects of Sodium Chloride on plant cells; a P-31 and Na-23 NMR system to study salt tolerance

In plant cells, the Na+/H+ antiport at the tonoplast provides a biochemical pathway to transport cytoplasmic Nat into the vacuole. Recently it was sho wn that overexpression of a vacuolar Na+/H+ promotes sustained plant growth at high NaCl levels (Apse et al. Science 285, 1256, 1999). The sequestrati on of Na+ ions into the vacuole can be followed using P-31 and Na-23 NMR sp ectroscopy. Suspension cell cultures are very suitable for this purpose and allow rapid and accurate assessment of the activity of the Na+/H+ antiport and therefore potentially of salt tolerance. Perfusion experiments with ma ize cells that are not particularly salt (NaCl) tolerant showed that during salt stress the cytoplasmic pH remains unchanged while the vacuolar pH sig nificantly increased. During Na+ sequestration into the vacuole, the cytopl asmic pH equilibrates faster than that of the vacuole. Both vacuolar pH and the cellular Na+ uptake rate were dependent on extracellular Na+ for conce ntrations up to approximately 300 mM. For Na+ concentrations greater than o r equal to 300 mM, both vacuolar pH and cellular Na+ uptake became independ ent of the extracellular concentration. This indicates either a saturation of Na+ uptake at the cell surface or a saturation of the Na+/H+ transporter at the tonoplast. Na+ uptake into the cell is accompanied by a rapid incre ase in vacuolar PO43-: broadening of the P-31 resonances and a reduction in glucose monophosphate and UDPG. Crown copyright (C) 2001 Published by Else vier Science Inland Ltd. All rights reserved.