Increased vacuolar Na⁺/H⁺ exchange activity in Salicornia bigelovii Torr. in response to NaCl

Shoots of the halophyte Salicornia bigelovii are larger and more succulent when grown in highly saline environments. This increased growth and water uptake has been correlated with a large and specific cellular accumulation of sodium. In glycophytes, sensitivity to salt has been associated with an inability to remove sodium ions effectively from the cytoplasm in order to protect salt-sensitive metabolic processes. Therefore, in Salicornia bigelovii efficient vacuolar sequestration of sodium may be part of the mechanism underlying salt tolerance. The ability to compartmentalize sodium may result from a stimulation of the proton pumps that provide the driving force for increased sodium transport into the vacuole via a Na⁺/H⁺ exchanger. In current studies, increased vacuolar pyrophosphatase activity (hydrolysis of inorganic pyrophosphate and proton translocation) and protein accumulation were observed in Salicornia bigelovii grown in high concentrations of NaCl. Based on sodium-induced dissipation of a pyrophosphate-dependent pH gradient in vacuolar membrane vesicles, a Na⁺/H⁺ exchange activity was identified and characterized. This activity is sodium concentration-dependent, specific for sodium and lithium, sensitive to methyl-isobutyl amiloride, and independent of an electrical potential. Vacuolar Na⁺/H⁺ exchange activity varied as a function of plant growth in salt. The affinity of the transporter for Na⁺ is almost three times higher in plants grown in high levels of salt (K_m=3.8 and 11.5 mM for plants grown in high and low salt, respectively) suggesting a role for exchange activity in the salt adaptation of Salicornia bigelovii.