NH4+-stimulated low-K+ uptake is associated with the induction of H+ extrusion by the plasma membrane H+-ATPase in sorghum roots under K+ deficiency

The effect of external inorganic nitrogen and le content on le uptake from low-le solutions and plasma membrane (PM) W-ATPase activity of sorghum roots was studied. Plants were grown for 15 days in full-nutrient solutions containing 0.2 or 1.4 mM K+ and inorganic nitrogen as NO3-, NO3-/NH4+ or NH4+ and then starved of K+ for 24,48 and 72 h. NH4+ in full nutrient solution significantly affected the uptake efficiency and accumulation of K. and this effect was less pronounced at the high le concentration. In contrast, the translocation rate of le to the shoot was not altered. Depletion assays showed that plants grown with NH4+ more efficiently depleted the external le and reached higher initial rates of low-le uptake than plants grown with NO3-. One possible influence of le content of shoot, but not of roots. on K+ uptake was evidenced. Enhanced le-uptake capacity was correlated with the induction of H+ extrusion by PM H+-ATPase. In plants grown in high le solutions, the increase in the active H+ gradient was associated with an increase of the PM W-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2 mM le, only the initial rate of H+-pumping and ATP hydrolysis were affected. Under these conditions, two specific isoforms of PM H+-ATPase were detected, independent of the nitrogen source and deficiency period. No change in enzyme activity was observed in NO3--grown plants. The results suggest that K+ homeostasis in NH4+-grown sorghum plants may be regulated by a high capacity for K+ uptake, which is dependent upon the H+-pumping activity of PM FP-ATPase. (C) 2011 Elsevier GmbH. All rights reserved.