Abstract
The effect of glutamate on plant plasma membrane cation transport was studied in roots of Arabidopsis thaliana (L.) Heynh. Patch-clamp experiments using root protoplasts, 22Na+ unidirectional fluxes into intact roots and measurements of cytosolic Ca2+ activity using plants expressing cytosolically-targeted aequorin in specific cell types were carried out. It was demonstrated that low-millimolar concentrations of glutamate activate within seconds both Na+ and Ca2+ currents in patch-clamped protoplasts derived from roots. The probability of observing glutamate-activated currents increased with increasing glutamate concentration (up to 29% at 3 mM); half-maximal activation was seen at 0.2–0.5 mM glutamate. Glutamate-activated currents were voltage-insensitive, ‘instantaneous’ (completely activated within 2–3 ms of a change in voltage) and non-selective for monovalent cations (Na+, Cs+ and K+). They also allowed the permeation of Ca2+. Half-maximal Na+ currents occurred at 20–30 mM Na+. Glutamate-activated currents were sensitive to non-specific blockers of cation channels (quinine, La3+, Gd3+). Although low-millimolar concentrations of glutamate did not usually stimulate unidirectional influx of 22Na+ into intact roots, they reliably caused an increase in cytosolic Ca2+ activity in protoplasts isolated from the roots of aequorin-transformed Arabidopsis plants. The response of cytosolic Ca2+ activity revealed a two-phase development, with a rapid large transient increase (lasting minutes) and a prolonged subsequent stage (lasting hours). Use of plants expressing aequorin in specific cell types within the root suggested that the cell types most sensitive to glutamate were in the mature epidermis and cortex. The functional significance of these glutamate-activated currents for both cation uptake into plants and cell signaling remains the subject of speculation, requiring more knowledge about the dynamics of apoplastic glutamate in plants.
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Abbreviations
- GLR :
-
Gene in plants encoding glutamate receptor-like protein
- iGluRs :
-
Ionotropic glutamate receptors
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Acknowledgements
We particularly thank Dr. Julia Davies in our Department for her support of this work in many ways. We also thank other members of our laboratory, Drs. Romola Davenport and Chris Cheffings for helpful advice and comments on the manuscript, Dr. Fouad Lemtiri-Chlieh, Dr. Matt Gilliham and Mr. John Banfield for technical advice, and Dr. Marc Knight (University of Oxford, UK) for supplying the seeds of Arabidopsis expressing aequorin constitutively. Financial support from a Royal Society/NATO Fellowship to V.D.; Churchill College Studentship and an Overseas Research Students award to P.A.E.; BBSRC Research Development Fellowship to M.T. is also gratefully acknowledged.
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Demidchik, V., Essah, P.A. & Tester, M. Glutamate activates cation currents in the plasma membrane of Arabidopsis root cells. Planta 219, 167–175 (2004). https://doi.org/10.1007/s00425-004-1207-8
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DOI: https://doi.org/10.1007/s00425-004-1207-8