Evidence for a sodium efflux mechanism in the leaf cells of the seagrass Zostera marina L.

The efficient exclusion of excess Na(+) from the cytoplasm is of vital importance in cells from the seagrass Zostera marina L. a vascular plant that lives in seawater and exhibits Na(+)-dependent high affinity nitrate and phosphate uptake systems. Intracellular measurements with Na(+)-selective microelectrodes show a low cytosolic Na+ concentration in leaf cells (13.2 +/- 2.8 mM Na(+)) of this species. This value, similar to the concentrations detected in terrestrial glycophyte plants, increases about 10 mM in the presence of respiration inhibitors. A rise in sodium concentration is also detected when external pH is increased, whereas a decrease is recorded at lower external pH levels. These results suggest that H(+)-dependent systems are involved in the maintenance of a low cytosolic Na(+) concentration in Z. marina cells. To investigate this further, plasma membrane vesicles were isolated from leaves and cation/H(+) antiport activities were analyzed using a fluorescent probe. Na(+)/H(+) exchange activity was detected and showed saturation around 35 mM Na(+) This activity was highly selective for Na(+) as no exchange activity was detected with Rb(+) or TMA(+), whereas Li(+) showed only 25% of the Na(+) activity. Furthermore, K(+) did not affect Na(+)/H(+) exchange activity, which showed similar Km values in the presence of 5 mM K(+) (4.1 +/- 2.1 mM Na(+)) or 50 mM K(+) (3.9 +/- 2.6 mM Na(+)). These Km values are lower than the mean sodium concentration measured in the cytoplasm of Z. marina cells, indicating that the presence of Na(+)/H(+) exchangers in the plasma membrane, whose activity has been detected for the first time in a seagrass, can be a very effective Na(+) efflux mechanism in this plant. This activity can be crucial both for salinity tolerance and for the energetization of nutrient uptake in this species. (C) 2011 Elsevier B.V. All rights reserved.