Alkylguanidines as Inhibitors of K⁺ Transport in Isolated Barley Roots

Abstract

It has beeii shown that plants can accunmulate K+ through an energy-dependent process. The effect of alkylguanidines, in particular octylguanidine on the uptake of 86Rb+ by excised barley roots (Hordeum vulgare var. Apizaco LV-72), has been studied. s6Rb+ was used as tracer of K+. The uptake of 86Rb+ which is linear with time and shows saturation kinetics is inhibited by octylguanidine. Half-maximal (5,6,14,15).Little is known with respect to the molecular events that occur during K+ translocation. It has been suggested that a membranebound ATPase participates in the process (8,9,18,20,22,25) and that the uptake of K+ involves the combination of a hypothetical carrier with the ion in such a way that the intermediate ion-carrier complex that is formed on the outer part of the membrane breaks down irreversibly on the internal side of the membrane releasing the ion inside the cell (3,6,10,11). In this work, a possible action of various alkylguanidines on the absorption of K+ by excised barley roots has been explored. The effect of alkylguanidines on K+ uptake in plants was studied because these lipophilic cations (their pKa is around 12) have been shown to inhibit numerous metal ion-dependent systems (1,2,12,13,16,23,24,(27)(28)(29) tinuously aerated distilled H20, and rinsed again 3 times with distilled H20. Seedlings were grown essentially as described by Epstein (4). The seeds were placed on a plastic screen covered with cheesecloth which was supported by a 7-liter plastic tank containing 5 liters of 0.5 mm CaSO4; the corners of the cheesecloth were dipped into the solution. The tank was placed in the dark and covered with a plastic cover. The temperature was 24 C + 3 and vigorous aeration was provided.Roots from 6-day-old seedlings were used for the absorption experiments. The roots were cut in 2-cm segments (the 3-cm basal portions and 5-cm apical portions of the roots were discarded) and washed 3 times with ice-cold distilled H20. The tissue prepared in this form is a highly homogeneous material. Batches of 16 segments were maintained in distilled H20 until the experiment was initiated. The uptake experiments were carried out by incubating barley roots in 4 ml of a mixture that contained 20 mm maleic buffer (pH 6), 0.05 mm CaSO4, and different concentrations of the indicated alkylguanidines and RbCl according to the experiment. Experiments on the ion absorption by excised tissues of higher plants have shown that potassium and rubidium behave virtually as isotopes of the same element (21). Therefore, l6Rb+ was used as a radioactive analog of K+. The incubation medium was maintained at 30 C and continuously shaken in a Dubnoff incubator. The solution was equilibrated for a 10-min period prior to the addition of the roots. At the times indicated in the respective figures (see under "Results"), the solution was decanted, and the roots were rinsed for 30 sec with a mixture of 30 ml of ice-cold 0.5 mm CaSO4 and 5 mm RbCl to eliminate adsorbed lGRb+. Subsequently the roots were washed for 30 min in 15 ml...

Purification and characterization of a potassium ion stimulated adenosine triphosphate phosphohydrolyase from Zea mays roots

“…Alkylguanidines have been shown to be inhibitors of uptake in barley roots (68). Of the alkylguanidines tested the octyl and hexyl homologues were the best inhibitors.…”