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Loading of nitrate into the xylem: apoplastic nitrate controls the voltage dependence of X‐QUAC, the main anion conductance in xylem‐parenchyma cells of barley roots
Author(s) -
Köhler Barbara,
Wegner Lars H.,
Osipov Viktor,
Raschke Klaus
Publication year - 2002
Publication title -
the plant journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.058
H-Index - 269
eISSN - 1365-313X
pISSN - 0960-7412
DOI - 10.1046/j.1365-313x.2002.01269.x
Subject(s) - xylem , apoplast , nitrate , parenchyma , conductance , chemistry , ion , biophysics , botany , biology , cell wall , biochemistry , physics , organic chemistry , condensed matter physics
Summary We report here that NO 3 – in the xylem exerts positive feedback on its loading into the xylem through a change in the voltage dependence of the Qu ickly Activating A nion C onductance, X‐QUAC. Properties of this conductance were investigated on xylem‐parenchyma protoplasts prepared from roots of Hordeum vulgare by applying the patch‐clamp technique. Chord conductances were minimal around − 40 mV and increased with plasma membrane depolarisation as well as with hyperpolarisation. Two gates with opposite voltage dependences were postulated. When 30 m m Cl – in the bath was replaced by NO 3 – , a shift in the midpoint potential of the depolarisation‐activated gate by about − 60 mV from 43 to − 16 mV occurred (K m = 3.4 m m ). No such effect was seen when chloride was replaced by malate. Addition of 10 m m NO 3 – to the pipette solution and reduction of [Cl – ] from 124 to 4 m m (to simulate cytoplasmic concentrations) did not interfere with the voltage dependence of X‐QUAC activation, nor was it affected by changes in external [K + ]. If only the NO 3 – effect on gating was considered, an increase of the NO 3 – concentration in the xylem sap to 5 m m would result in an enhancement of NO 3 – efflux by about 30%. Although the driving force for NO 3 – efflux would be reduced simultaneously, NO 3 – efflux into the xylem through X‐QUAC would be maintained with high NO 3 – concentrations in the xylem sap; a situation which occurs for instance during the night.