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

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Abstract
Compartmental analysis of 35SO42- exchange kinetics is used to obtain SO42- fluxes and compartment contents in carrot (D. carota L.) storage root cells, where 2 to 5% of the SO42- taken up is reduced to organic form. The necessary curve fitting is verified by consistency between content vs. time and rate vs. time plots of washout data, agreement between loading and washout kinetics, and 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 2-10 times higher than the tonoplast influx, is much greater than the SO42- reduction rate, and would not limit the rate of either. This is consistent with the finding that the plasmalemma influx is not regulated by internal SO42- or cysteine. Both SO42- influxes rise with only limited saturation as the external SO42- concentration increases up to 50 mM. Both effluxes appear to be passive, with extensive recycling in the plasmalemma influx pump. SO42- permeability is about 10-11 m/s at both membranes. The high, nonlimiting fluxes of SO42- at the plasmalemma relative to the tonoplast contrasts with SO42- fluxes in bacteria and with Cl- fluxes in plant cells. Their implications for work on characteristics and regulation of SO42- uptake in roots and tissue cultures are discussed.