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Title: Mechanisms of toxic sodium influx in wheat
Author: Davenport, R.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1998
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Soil salinity constitutes a major limitation to crop productivity in many parts of the world. Wheat is sensitive to high Na+, and this may be because the plant is unable to prevent high Na+ influx into the roots. In this study the crude aspects of Na+ influx were first characterised by measurements of 22Na+ uptake into excised wheat root segments. Transporters responsible for mediating this influx were then screened using the planar lipid bilayer technique as an assay for Na+- permeable ion channels. Four Na+-conducting channels were identified from wheat root plasma membrane preparations. The most abundant channel in the bilayer was a 44 pS channel which was nonselective for monovalent cations and weakly voltage-dependent. Na+ influx through the channel resembled 22Na+ influx into root segments in its sensitivity to inhibition by Ca2+, Mg2+ and Gd3+, and insensitivity to all other inhibitors tested (TEA+, quinine, Cs+, TTX, verapamil, amiloride and flufenamate). The inhibitory effects of multivalent cations were shown to be due to competition for permeation, rather than due to any effects on surface charge. The 44 pS channel also closely resembled an instantaneous current in wheat root protoplasts, thought to represent the main pathway of toxic Na+ influx at the single-cell level (Tyerman et al., 1997). The 44 pS channel and the instantaneous current had the same permeability sequences, same selectivity for K+ over Na+ (≈ 1.25), insensitivity to TEA+, and similar voltage-independence and sensitivity to Ca2+. Thus it appears likely that the 44 pS channel discovered in the bilayer is responsible for the major part of toxic Na+ influx into wheat in saline conditions. The 44 pS channel was not affected by intracellular Ca2+, Mg2+, cyclic nucleotides, ATP, H+, NH4+, or spermine, nor extracellular H+ or glutamate, and therefore did not resemble any nonselective channels identified in other systems. In non-saline conditions the channel could function to catalyse NH4+ uptake for nutrition, and may also play a role in nonselective uptake of cations for osmoregulation. The ability of the channel to catalyse uptake of large cations such as TEA+ suggested that the channel may also have as yet unidentified functions in vivo.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available