Futile transmembrane NH
4
+
cycling: A cellular hypothesis to explain ammonium toxicity in plants
Authors: Britto Dev T. , Siddiqi M. Yaeesh , Glass Anthony D. M. , Kronzucker Herbert J. 
Published: July 2002

Most higher plants develop severe toxicity symptoms when grown on
ammonium (NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

) as the sole nitrogen source. Recently,
NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

toxicity has been implicated as a cause of forest
decline and even species extinction. Although mechanisms underlying
NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

toxicity have been extensively sought, the primary
events conferring it at the cellular level are not understood. Using a
high-precision positron tracing technique, we here present a
cell-physiological characterization of NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

acquisition
in two major cereals, barley (
Hordeum vulgare
), known to
be susceptible to toxicity, and rice (
Oryza sativa
),
known for its exceptional tolerance to even high levels of
NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

. We show that, at high external
NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

concentration
([NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

]
o
), barley root cells experience a
breakdown in the regulation of NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

influx, leading to
the accumulation of excessive amounts of NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

in the
cytosol. Measurements of NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

efflux, combined with a
thermodynamic analysis of the transmembrane electrochemical potential
for NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

, reveal that, at elevated
[NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

]
o
, barley cells engage a
high-capacity NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

-efflux system that supports outward
NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

fluxes against a sizable gradient. Ammonium efflux
is shown to constitute as much as 80% of primary influx, resulting in
a never-before-documented futile cycling of nitrogen across the plasma
membrane of root cells. This futile cycling carries a high energetic
cost (we record a 40% increase in root respiration) that is
independent of N metabolism and is accompanied by a decline in growth.
In rice, by contrast, a cellular defense strategy has evolved that is
characterized by an energetically neutral, near-Nernstian,
equilibration of NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

at high
[NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

]
o
. Thus our study has characterized
the primary events in NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

nutrition at the cellular
level that may constitute the fundamental cause of NH

\documentclass[12pt]{minimal}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{amsbsy}
\usepackage{mathrsfs}
\setlength{\oddsidemargin}{-69pt}
\begin{document}
\begin{equation*}{\mathrm{_{4}^{+}}}\end{equation*}\end{document}

toxicity in plants.