Plasma-membrane electrical responses to salt and osmotic gradients contradict radiotracer kinetics, and reveal Na⁺-transport dynamics in rice (Oryza sativaL.)

• Published in: Planta Vol. 249; no. 4; pp. 1037 - 1051
• Main Authors: Hamam, Ahmed M., Coskun, Devrim, Britto, Dev T., Plett, Darren, Kronzucker, Herbert J.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Science + Business Media 01.04.2019; Springer Berlin Heidelberg; Springer Nature B.V
• Subjects: Agriculture; Biomedical and Life Sciences; Cell Membrane - metabolism; Chlorides - metabolism; Depolarization; Ecology; Efflux; Forestry; H+-transporting ATPase; Life Sciences; Membrane potential; Membrane Potentials; Membranes; ORIGINAL ARTICLE; Oryza - metabolism; Oryza sativa; Osmotic Pressure; Plant Sciences; Potassium Radioisotopes - metabolism; Proton-Translocating ATPases - metabolism; Radioactive tracers; Roots; Seedlings; Sodium - metabolism; Sodium 24; Sodium chloride; Sodium Radioisotopes - metabolism; Transport; Depolarization; Efflux; Influx; Sodium transport; Osmotic; Roots; Salinity stress; Ionic; Membrane potential; Radiotracer; Rapid transmembrane sodium cycling (RTSC); Rice
• ISSN: 0032-0935; 1432-2048
• DOI: 10.1007/s00425-018-3059-7

To investigate the character and mechanisms of Na⁺ influx into roots, Na⁺-dependent changes in plasma-membrane electrical potentials (ΔΔΨ) were measured in root cells of intact rice (Oryza sativa L., cv. Pokkali) seedlings. As external sodium concentrations ([Na⁺]ext) were increased in a step gradient from 0 to 100 mM, membrane potentials depolarized in a saturable manner, fitting a Michaelis–Menten model and contradicting the linear (non-saturating) models developed from radiotracer studies. Clear differences in saturation patterns were found between plants grown under low- and high-nutrient (LN and HN) conditions, with LN plants showing greater depolarization and higher affinity for Na⁺ (i.e., higher Vmax and lower Km) than HN plants. In addition, counterion effects on ΔΔΨ were pronounced in LN plants (with ΔΔΨ decreasing in the order: C I − > S O 4 2 − > H P O 4 2 − ), but not seen in HN plants. When effects of osmotic strength, Cl⁻ influx, K⁻ efflux, and H⁺-ATPase activity on ΔΔΨ were accounted for, resultant Km and Vmax values suggested that a single, dominant Na⁺-transport mechanism was operating under each nutritional condition, with Km values of 1.2 and 16 mM for LN and HN plants, respectively. Comparing saturating patterns of depolarization to linear patterns of ²⁴Na⁺ radiotracer influx leads to the conclusion that electrophysiological and tracer methods do not report the same phenomena and that the current model of rapid transmembrane sodium cycling may require revision.