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  • Title: Evidence for cotransport of nitrate and protons in maize roots : I. Effects of nitrate on the membrane potential.
    Author: McClure PR, Kochian LV, Spanswick RM, Shaff JE.
    Journal: Plant Physiol; 1990 May; 93(1):281-9. PubMed ID: 16667448.
    Abstract:
    The electrical response of nitrate-grown maize (Zea mays L.) roots to 0.1 millimolar nitrate was comprised of two sequential parts: a rapid and transient depolarization of the membrane potential, followed by a slower, net hyperpolarization to a value more negative than the original resting potential. The magnitude of the response was smaller in roots of seedlings grown in the absence of nitrate, but, within 3 hours of initial exposure to 0.1 millimolar nitrate, increased to that of nitrate-grown roots. Chloride elicited a separate electrical response with a pattern similar to that of the nitrate response. However, the results presented in this study strongly indicate that the electrical response to nitrate reflects the activity of a nitrate-inducible membrane transport system for nitrate which is distinct from that for chloride. Inhibitors of the plasmalemma H(+)-ATPase (vanadate, diethylstilbestrol) completely inhibited both parts of the electrical response to nitrate, as did alkaline external pH. The magnitude of the initial nitrate-dependent, membrane potential depolarization was independent of nitrate concentration, but the subsequent nitrate-dependent hyperpolarization showed saturable dependence with an apparent K(m) of 0.05 millimolar. These results support a model for nitrate uptake in maize roots which includes a depolarizing NO(3) (-)/H(+) symport. The model proposes that the nitrate-dependent membrane potential hyperpolarization is due to the plasma membrane proton pump, which is secondarily stimulated by the operation of the NO(3) (-)/H(+) symport.
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