Na+/H+ antiport activity in tonoplast vesicles isolated from sunflower roots induced by NaCl stress

Na+ transport across the tonoplast and its accumulation in the vacuoles is of crucial importance for plant adaptation to salinity. Mild and severe salt stress increased both ATP- and PPi-dependent H+ transport in tonoplast vesicles from sunflower seedling roots, suggesting the possibility that a Na+/H+ antiport system could be operating in such vesicles under salt conditions (E. Ballesteros et al. 1996. Physiol. Plant. 97: 259-268). During a mild salt stress, Na+ was mainly accumulated in the roots. Under a more severe salt treatment, Na+ was equally distributed in shoots and roots. In contrast to what was observed with Na+, all the salt treatments reduced the shoot K+ content. Dissipation by Na+ of the H+ gradient generated by the tonoplast H+-ATPase, monitored its fluorescence quenching of acridine orange, was used to measure Na+/H+ exchange across tonoplast-enriched vesicles isolated by sucrose gradient centrifugation from sunflower (Helianthus annuus L.) roots treated for 3 days with different NaCl regimes. Salt treatments induced a Na+/H+ exchange activity which displayed saturation kinetics for Na+ added to the assay medium. This activity was partially inhibited by 125 mu M amiloride, a competitive inhibitor of Na+/H+ antiports. No Na+/F+ exchange was detected in vesicles from control roots. The activity was specific for Na+, since K+ added to the assay medium slightly dissipated H+ gradients and displayed non-saturating kinetics for all salt treatments. Apparent K-m for Na+/H+ exchange in tonoplast vesicles from 150 mM NaCl-treated roots was lower than that of 75 mM NaCl-treated roots, V-max remaining unchanged. The results suggest that the existence of a specific Na+/H+ exchange activity in tonoplast-enriched vesicle fractions, induced by salt stress, could represent an adaptative response in sunflower plants, moderately tolerant to salinity.