Energization of potassium uptake in Arabidopsis thaliana
1993
Maathuis, F.J.M. | Sanders, D.
Plant roots accumulate K+ from micromolar external concentrations. However, the absence of a firm determination of the trans-plasma-membrane electrochemical gradient for K+ in these conditions has precluded an assessment of whether K+-accumulation requires energization in addition to the driving force provided by the inside-negative membrane electrical potential (Em). To address this question unequivocally, we measured Em, and the cytosolic and external K+-activities in root cells of Arabidopsis thaliana (L.) Heynh. cv. Columbia in conditions in which net K+-accumulation occurs at low external K+ (10 micromolar). In these conditions, net K+- uptake was about 0.1 micromole.(g FW)-1.h-1, Em varied between -153 and -129 mV and the cytosolic K+-activity, determined with K+-selective electrodes, was 83 +/- 4 mM. These values yield an outwardly-directed driving force on K+ of at least 6.5 kJ.mol-1. Only if external potassium is raised to the region of 1 mM does Em become sufficient to drive net K+-accumulation. It is therefore concluded that at micromolar external K+-activities which prevail in most soils, K+-uptake cannot be solely energized by Em--as exemplified by a channel-mediated mechanism. The nature of the energization mechanism is discussed in relation to processes operating in fungal and algal cells.
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