Physiological evidence for a sodium-dependent high-affinity phosphate and nitrate transport at the plasma membrane of leaf and root cells of Zostera marina L.
Zostera marina L. is an angiosperm that grows in a medium in which inorganic phosphate (P(i)) and nitrate (NO(3)(-)) are present in micromolar concentrations and must be absorbed against a steep electrochemical potential gradient. The operation of a Na(+)-dependent NO(3)(-) transport was previously demonstrated in leaf cells of this plant, suggesting that other Na(+)-coupled systems could mediate the uptake of anions. To address this question, P(i) transport was studied in leaves and roots of Z. marina, as well as NO(3)(-) uptake in roots. Electrophysiological studies demonstrated that micromolar concentrations of P(i) induced depolarizations of the plasma membrane of root cells. However, this effect was not observed in leaf cells. P(i)-induced depolarizations showed Michaelis-Menten kinetics (K(m)=1.5+/-0.6 microM P(i); D(max)=7.8+/-0.8 mV), and were not observed in the absence of Na(+). However, depolarizations were restored when Na(+) was resupplied. NO(3)(-) additions also evoked depolarizations of the plasma membrane of root cells only in the presence of Na(+). Both NO(3)(-)- and P(i)-induced depolarizations were accompanied by an increase in cytoplasmic Na(+) activity, detected by Na(+)-sensitive microelectrodes. P(i) net uptake (measured in depletion experiments) was stimulated by Na(+). These results strongly suggest that P(i) uptake in roots of Z. marina is mediated by a high-affinity Na(+)-dependent transport system. Both NO(3)(-) and P(i) transport systems exploit the steep inwardly directed electrochemical potential gradient for Na(+), considering the low cytoplasmic Na(+) activity (10.7+/-3.3 mM Na(+)) and the high external Na(+) concentration (500 mM Na(+)).
Rubio
L
L
Departamento Biología Vegetal, Facultad de Ciencias, Universidad de Málaga, Campus de Teatinos s/n, S-29071, Málaga, Spain. Lrubio@uma.es
Linares-Rueda
A
A
García-Sánchez
M J
MJ
Fernández
J A
JA
D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000502:physiology*; D008564:Membrane Potentials; D009566:Nitrates / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism*; D019441:Plant Epidermis / Q000502:physiology; D018515:Plant Leaves / Q000378:metabolism*; D010940:Plant Proteins / Q000502:physiology; D018517:Plant Roots / Q000378:metabolism*; D012964:Sodium / Q000502:physiology; D035962:Zosteraceae / Q000378:metabolism*
D002352:Carrier Proteins; D009566:Nitrates; D010710:Phosphates; D010940:Plant Proteins; D012964:Sodium