Phosphate uptake was studied by determining [(32)P]phosphate influx and by measurements of the electrical membrane potential in duckweed (Lemna gibba L.). Phosphate-induced membrane depolarization (ΔE m ) was controlled by the intracellular phosphate content, thus maximal ΔE m by 1 mM H2PO 4 (-) was up to 133 mV after 15d of phosphate starvation. The ΔE m was strongly dependent on the extracellular pH, with a sharp optimum at pH 5.7. It is suggested that phosphate uptake is energized by the electrochemical proton gradient, proceeding by a 2H(+)/H2PO 4 (-) contransport mechanism. This is supported also by the fusicoccin stimulation of phosphate influx. Kinetics of phosphate influx and of ΔE m , which represent mere plasmalemma transport, are best described by two Michaelis-Menten terms without any linear components.
High rates of phosphate uptake into phosphate-starved Lemna gibba L.Gl were correlated with a high membrane potential (pd = -220 milivolts). In plants maintaining a low pd (-110 Phosphate uptake by unicellular algae is stimulated up to 10-fold by Na+ or Li' ions in the neutral and alkaline pH range (23,24). The same was found in fungi (18,19) and was interpreted as a Na+-phosphate co-transport system (18). In addition, it has been shown in yeast and in bacteria that, in the acidic pH range, phosphate absorption is accompanied by an H+ influx with a stoichiometry of 1.5 to 3 H+/phosphate transported (2,5,8). Phosphate uptake seems to proceed by a cotransport mechanism along an electrochemical transmembrane Na+ or H+ potential gradient similar to the described sugar-H+ cotransport in lower plants (10,20).For higher plants, it was suggested that an H+ cotransport occurs with sugars, amino acids, and nitrate (15,22). This was concluded from transient alkalinization of the external medium and from the transient depolarization of the plasmalemma at the onset of solute transport (7,(14)(15)(16)22). The results were regarded to be consistent with the hypothesis ofMitchell (13), who proposed a solute-H+ cotransport along
The membrane potential (pd) of duck weed (Lemna gibba G1) proved to be energy dependent. At high internal ATP levels of 74 to 105 nmol ATP g(-1) FW, pd was between -175 and -265 mV. At low ATP levels of 23 to 46 nmol ATP g(-1) FW, pd was low, about -90 to -120 mV at pH 5.7, but -180 mV at pH 8. Upon addition of glucose in the dark or by light energy the low pd recovered to the high values. The active component of the pd was depolarized by the addition of hexoses in the dark and in the light. Hexose-dependent depolarization of the pd (=Δ pd) followed a saturation curve similar to active hexose influx kinetics. Depolarization of the pd recovered in the dark even in the presence of the hexoses and with a 10fold enhancement in the light. Depolarization and recovery could be repeated several times with the same cell. Glucose uptake caused a maximum depolarization of 133 mV, fructose uptake half that amount, sucrose had the same effect as glucose. During 3-O-methylglucose and 2-deoxyglucose uptake the depolarizing effect was only slightly lower. The pd remained unchanged in the presence of mannitol. The glucose dependent Δ pd and especially the rate of pd recovery proved to be pH-dependent between pH 4 and pH 8. It was independent of the presence of 1 mM KCl. Although no Δ pH could be measured in the incubation medium, these results can be best explained by a H(+)-hexose cotransport mechanism powered by active H(+) extrusion at the plasmalemma.
Sulfate uptake into duckweed (Lemna gibba G1) was studied by means of [(35)S]sulfate influx and measurements of electrical membrane potential. Uptake was strongly regulated by the intracellular content of soluble sulfate. At the onset of sulfate uptake the membrane potential was transiently depolarized. Fusicoccin stimulated uptake up to 165% of the control even at pH 8. It is suggested that sulfate uptake is energized in the whole pH range by a 3H(+)/sulfate cotransport mechanism. Kinetics of sulfate uptake and sulfate-induced membrane depolarization in the concentration range of 5 μM to 1 mM sulfate at pH 5.7 was best described by two Michaelis-Menten terms without any linear component. The second system had a lower affinity for sulfate and was fully active only at sufficiently high proton concentrations.
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