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

• Published in: Journal of experimental marine biology and ecology 2011-06, Vol.402 (1), p.56-64
• Main Authors: Rubio, Lourdes, Belver, Andrés, Venema, Kees, Jesús García-Sánchez, María, Fernández, José Antonio
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Cytosolic sodium concentration; Fundamental and applied biological sciences. Psychology; Marine; Microelectrodes; Plasma membrane vesicles; Salinity tolerance; Sea water ecosystems; Sodium–proton antiport; Synecology; Zostera marina
• ISSN: 0022-0981; 1879-1697
• DOI: 10.1016/j.jembe.2011.03.016

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 K m 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 K m 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. ► 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.