ENERGIZATION OF POTASSIUM UPTAKE IN ARABIDOPSIS-THALIANA

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 (E(m)). To address this question unequivocally, we measured E(m), 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 muM). In these conditions, net K+-uptake was about 0.1 mumol . (g FW)-1 . h-1, E. 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 E(m) 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 E(m) - 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.