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.

Research highlights
► We analyze cytoplasmic Na+-homeostasis in Z. marina cells using microelectrodes.
► We detect Na+/H+ antiport activity at the plasmalemma.
► The Na+/H+ activity is highly selective and shows a high affinity for Na+.
► The antiporter is fundamental for Na+ homeostasis and nutrient uptake in this species.