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 K+ content on K+ uptake from low-K+ solutions and plasma membrane (PM) H+-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 K+ concentration. In contrast, the translocation rate of K+ to the shoot was not altered. Depletion assays showed that plants grown with NH4+ more efficiently depleted the external K+ and reached higher initial rates of low-K+ uptake than plants grown with NO3–. One possible influence of K+ content of shoot, but not of roots, on K+ uptake was evidenced. Enhanced K+-uptake capacity was correlated with the induction of H+ extrusion by PM H+-ATPase. In plants grown in high K+ solutions, the increase in the active H+ gradient was associated with an increase of the PM H+-ATPase protein concentration. In contrast, in plants grown in solutions containing 0.2 mM K+, 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 H+-ATPase.