Spermidine inhibits K+ uptake without interfering with the basal activity of plasma membrane proton pump

The effect of spermidine (SPD) on the activity of plasma membrane proton pump of maize roots was studied both in an in vitro and in vivo systems. The phosphohydrolysing activity of plasma membrane. ATPase measured on isolated vesicles was not affected by the presence of SPD in a wide range of concentrations and assay conditions. Also the rate of ATP-dependent H+ accumulation into inside-out vesicles was not influenced by 0.5 mM SPD both in the presence and absence of 100 mM K+. In in vivo condition the already observed inhibition of SPD on K+ uptake (M. de Agazio et al. Plant Physiol., 87 (1988) 176–178) was confirmed at external K+ concentrations ranging from 0.1–2.0 mM without a simultaneous effect on the activity of proton pump measured as [14C]leucine accumulation and H+ extrusion. The mode of inhibition of K+ uptake exerted by SPD seems not to be related to an effect on the basal activity of plasma membrane proton pump but linked to a specific interaction with the structure(s) responsible for K+ transport through the plasma membrane.