Characteristics of Sulfate Transport Across Plasmalemma and Tonoplast of Carrot Root Cells

Abstract

Compartmental analysis of "SO4-2 exchange kinetics is used to obtain S042-fluxes and compartment contents in carrot (Daucus carota L.) storage root cells, where 2 to 5% of the S042-taken up is reduced to organic form. The necessary curve fit is verified by (a) consistency between 'content versus time' and 'rate versus time' plots of washout data; (b) agreement between loading and washout kinetics; and (c) correct identification of the fastest exchange phase as being from extracellular spaces.Sulfate is actively transported up an electrochemical potential gradient at both plasmalemma and tonoplast. The plasmalemma influx is from 2 to 10 times higher than the tonoplast Influx, is much greater than the S042 reduction rate, and would not lmit the rate of either. This is consistent with the finding that the plasmie a influx is not regulated by internal S042-or cysteine (Cram 1982 Plant Sci Lett, in press Sulfate is the second most abundant of the elements which are taken up as ions and metabolized (nitrogen being the most abundant) (8). Understanding the mechanism and control of its uptake in relation to long distance transport, storage and mobility (20, 28), nutrient balance (11,24), and metabolism (1, 13, 14, 24) in plants demands that fluxes of S042-and of organic S compounds across individual cell membranes and between metabolites should be measured. In particular, in the vacuolated higher plant cell, the kinetics and regulation of s042-flows across the plasmalemma and subsequently either to the vacuole or to reductive assimilation products can only be studied using such measurements. This paper provides an experimental justification of the use of compartmental analysis of 'S exchange for estimating fluxes in carrot storage root cells. The method is then used to obtain thermodynamic and kinetic characteristics of S042-transport in these cells, and the implications for the rate limitation and control of So02-accumulation and metabolism are discussed.The first destination of s042-after uptake across the plasmalemma is reductive assimilation. In nonvacuolated bacteria, as in vacuolated fungi and higher plants, it has been shown that reductive assimilation is self-regulated by negative feedback from products of s042-metabolism. Cysteine, for instance, feeds back to the first enzyme of s042-metabolism, ATP sulfurylase (which catalyzes the production of 'activated SO2-,' which in tUn is reduced in a sequence of reactions to S2-and then converted to cysteine, etc.) (1, 21). In bacteria, the study of mutants has shown, further, that this negative feedback also extends to the uptake of S042-across the plasmalemma, so that the uptake and reduction of S042-are to some extent matched (1,7,21). Similar control of s042-influx across the plasmalemma in higher plant cells has also been claimed (15,23,24), although without supporting measurements of the plasmalemma influx.A priori, such a tight regulation of s042 influx across the plasmalemma in vacuolated cells seems unlikely. If s042-uptake were matched to S042 reduct...

Sulfur Transfer through an Arbuscular Mycorrhiza

“…1B), as has been reported for tomato (Solanum lycopersicum) seedlings and carrot (Daucus carota) storage root sections (Cram, 1983;Lopez et al, 2002). In carrot, Cram et al (1983) showed a proportional increase up to an external concentration of 50 mM, leading the authors to conclude that feedback inhibition of sulfate uptake is absent. Vegetative root growth was less affected by alterations in available sulfate concentrations than was uptake or reduction (Fig.…”

Characterization of Anion Channels in the Plasma Membrane of Arabidopsis Epidermal Root Cells and the Identification of a Citrate-Permeable Channel Induced by Phosphate Starvation

“…However, in the absence of measurable ARAC-mediated citrate or malate efflux in this study and the absence of ARAC regulation by phosphate supply (see below) or by extracellular Cu 21 (data not shown), it is unlikely that ARAC is responsible for organic-acid efflux from Arabidopsis roots. Although it has received far less attention than organic-acid anions, inorganic anion efflux has also been recorded from higher plant roots, for example Cl 2 efflux from barley (Jackson and Edwards, 1966) and Arabidopsis (Lorenzen et al, 2004) and SO 4 22 efflux from carrot (Cram, 1983) and tomato roots (Lopez et al, 2002). The physiological significance of these fluxes is not always clear, but they may be important in preventing the toxic accumulation of anions in the cytosol of root cells, particularly when extracellular anion concentration is high.…”

Analysis of 13NH4+ Efflux in Spruce Roots (A Test Case for Phase Identification in Compartmental Analysis)

“…For use with intact plant material, a detailed treatise on parameter extraction was presented by Jeschke and Jambor (1981) and Jeschke (1982). More recently, compartmental analysis has also been applied to metabolized ions, including S0,2p (Thoiron et al, 1981;Cram, 1983; Bell et al, 1994), Pi (Lefebvre and Clarkson, 1984;Macklon and Sim, 1992), NO,-(Presland and McNaughton, 1984 1986; Macklon et al, 1990;Siddiqi et al, 1991;Devienne et al, 1994;Kronzucker et al, 1995a Kronzucker et al, , 1995b, and NH,+ (Presland and McNaughton, 1986;Cooper et al, 1989;Macklon et al, 1990;Wang et al, 1993a;Kronzucker et al, 1995~). Despite this widespread use of the technique, workers have typically neglected to conduct physiological tests to verify the subcellular identities of the phases revealed in efflux data.…”

“…For use with intact plant material, a detailed treatise on parameter extraction was presented by Jeschke and Jambor (1981) and Jeschke (1982). More recently, compartmental analysis has also been applied to metabolized ions, including S0,2p (Thoiron et al, 1981;Cram, 1983;Bell et al, 1994), Pi (Lefebvre and Clarkson, 1984;Macklon and Sim, 1992), NO,- (Presland and McNaughton, 1984;Lee and Clarkson, The work reported in this paper was supported by a National Sciences and Engineering Research Council of Canada grant to A.D.M.G. and by a University of British Columbia Graduate Fellowship to H.J.K.…”