Na+-Dependent High-Affinity Nitrate, Phosphate and Amino Acids Transport in Leaf Cells of the Seagrass Posidonia oceanica (L.) Delile

Abstract

Posidonia oceanica (L.) Delile is a seagrass, the only group of vascular plants to colonize the marine environment. Seawater is an extreme yet stable environment characterized by high salinity, alkaline pH and low availability of essential nutrients, such as nitrate and phosphate. Classical depletion experiments, membrane potential and cytosolic sodium measurements were used to characterize the high-affinity NO3−, Pi and amino acids uptake mechanisms in this species. Net uptake rates of both NO3− and Pi were reduced by more than 70% in the absence of Na+. Micromolar concentrations of NO3− depolarized mesophyll leaf cells plasma membrane. Depolarizations showed saturation kinetics (Km = 8.7 ± 1 μM NO3−), which were not observed in the absence of Na+. NO3− induced depolarizations at increasing Na+ also showed saturation kinetics (Km = 7.2 ± 2 mM Na+). Cytosolic Na+ measured in P. oceanica leaf cells (17 ± 2 mM Na+) increased by 0.4 ± 0.2 mM Na+ upon the addition of 100 μM NO3−. Na+-dependence was also observed for high-affinity l-ala and l-cys uptake and high-affinity Pi transport. All together, these results strongly suggest that NO3−, amino acids and Pi uptake in P. oceanica leaf cells are mediated by high-affinity Na+-dependent transport systems. This mechanism seems to be a key step in the process of adaptation of seagrasses to the marine environment.

Short-Term Response of Cytosolic NO3− to Inorganic Carbon Increase in Posidonia oceanica Leaf Cells

“…A higher NO − 3 c in A. thaliana nia1nia2 mesophyll leaf cells and a similar time course of cytosolic increase to the rate of NR activity change in response to illumination transitions (half-life of 2 to 15 min in spinach; Huber et al, 1992;Kaiser et al, 1992;Riens and Heldt, 1992) support the evidence for the role of NR in regulating NO − 3 c (Cookson et al, 2005). In P. oceanica, the NO − 3 c increase observed in the dark could also explain the one-half diminution of the maximum high affinity NO − 3 uptake observed previously in mesophyll leaf cells of this plant (Rubio et al, 2018), due to the apparent substrate inhibition of the transporter.…”

“…Similar high-affinity and Na + -dependent uptake mechanisms also operate in P. oceanica for both nutrients and some amino acids ( Rubio et al., 2018 ). In both cases, the low semi-saturation constants observed (2.3 and 8.7 µM for Z. marina and P. oceanica , respectively: García-Sánchez et al., 2000 ; Rubio et al., 2018 ) indicate that those systems are very efficient for uptake at the very low concentrations of in seagrass meadows ( Touchette and Burkholder, 2000 ; Romero et al., 2006 ). Nevertheless, those systems are energetically expensive because seagrass leaf cells have to keep low homeostatic Na + concentrations in the cytosol to maintain the Na + motive force ( Rubio et al., 2011 ; Rubio et al., 2018 ).…”

“…In both cases, the low semi-saturation constants observed (2.3 and 8.7 µM for Z. marina and P. oceanica , respectively: García-Sánchez et al., 2000 ; Rubio et al., 2018 ) indicate that those systems are very efficient for uptake at the very low concentrations of in seagrass meadows ( Touchette and Burkholder, 2000 ; Romero et al., 2006 ). Nevertheless, those systems are energetically expensive because seagrass leaf cells have to keep low homeostatic Na + concentrations in the cytosol to maintain the Na + motive force ( Rubio et al., 2011 ; Rubio et al., 2018 ). In this scenario, namely low availability of and the energetically expensive mechanism for its high-affinity uptake, the maintenance of inside the cells appears critical.…”

“…In the Mediterranean, Posidonia oceanica is an endemic coastal species of huge ecological importance (Aires et al, 2011). Similar high-affinity and Na + -dependent uptake mechanisms also operate in P. oceanica for both nutrients and some amino acids (Rubio et al, 2018). In both cases, the low semi-saturation constants observed (2.3 and 8.7 µM NO − 3 for Z. marina and P. oceanica, respectively: Garcıá-Sańchez et al, 2000;Rubio et al, 2018) indicate that those systems are very efficient for NO − 3 uptake at the very low concentrations of NO − 3 in seagrass meadows (Touchette and Burkholder, 2000;Romero et al, 2006).…”

Molecular Characterization of ZosmaNRT2, the Putative Sodium Dependent High-Affinity Nitrate Transporter of Zostera marina L.

“…In the case of nitrate, the concentration in seagrass environments is down to 5 µM [1], suggesting that high-affinity transporters developed by terrestrial plants would work in seagrasses, facing both challenges of salinity and alkaline conditions. As vascular plants, these species conserve H + -ATPase as the primary pump to energize the plasma membrane [2,3]; nevertheless, in contrast to terrestrial plants that use H + symporters for high-affinity nutrient uptake, seagrasses were the first angiosperms in which high-affinity NO 3 − , Pi, and amino acids have been described as Na + -dependent mechanisms [4,5,6].…”