期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2001
卷号：98
期号：7
页码：4255-4258
DOI：10.1073/pnas.061034698
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Most higher plants develop severe toxicity symptoms when grown on ammonium (NH[IMG]f1.gif" ALT="Formula" BORDER="0">) as the sole nitrogen source. Recently, NH[IMG]f1.gif" ALT="Formula" BORDER="0"> toxicity has been implicated as a cause of forest decline and even species extinction. Although mechanisms underlying NH[IMG]f1.gif" ALT="Formula" BORDER="0"> 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[IMG]f1.gif" ALT="Formula" BORDER="0"> 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[IMG]f1.gif" ALT="Formula" BORDER="0">. We show that, at high external NH[IMG]f1.gif" ALT="Formula" BORDER="0"> concentration ([NH[IMG]f1.gif" ALT="Formula" BORDER="0">]o), barley root cells experience a breakdown in the regulation of NH[IMG]f1.gif" ALT="Formula" BORDER="0"> influx, leading to the accumulation of excessive amounts of NH[IMG]f1.gif" ALT="Formula" BORDER="0"> in the cytosol. Measurements of NH[IMG]f1.gif" ALT="Formula" BORDER="0"> efflux, combined with a thermodynamic analysis of the transmembrane electrochemical potential for NH[IMG]f1.gif" ALT="Formula" BORDER="0">, reveal that, at elevated [NH[IMG]f1.gif" ALT="Formula" BORDER="0">]o, barley cells engage a high-capacity NH[IMG]f1.gif" ALT="Formula" BORDER="0">-efflux system that supports outward NH[IMG]f1.gif" ALT="Formula" BORDER="0"> 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[IMG]f1.gif" ALT="Formula" BORDER="0"> at high [NH[IMG]f1.gif" ALT="Formula" BORDER="0">]o. Thus our study has characterized the primary events in NH[IMG]f1.gif" ALT="Formula" BORDER="0"> nutrition at the cellular level that may constitute the fundamental cause of NH[IMG]f1.gif" ALT="Formula" BORDER="0"> toxicity in plants.
