Active Transport of Choline Sulfate by Barley Roots

Abstract

Choline sulfate is a major product of sulfate metabolism in plants (9,13). This sulfate ester accounts for up to one-third of the radioactivity in the soluble organic compounds fornmed after uptake of radliosulfate by plant roots. The analogous choline phosphate occurs in higher plants and is involved in phosphate transport (8). No similar role for choline sulfate as a carrier for sulfate could be detected, but the uptake of choline sulfate was found to be the result of active transport.Studies on the active transport of choline sulfate by excised barley roots are described in this paper. Barley roots have been widely used in active transport studies and form considerable amounts of choline sulfate. The uptake of this zwitterion was studied over a wide range of concentrations in an attempt to elucidate the nature of its dual uptake mechanism. A number of structural analogues of choline sulfate were synthesized and tested as competitive inhibitors of the active transport of choline sulfate. These inhibitor studies were designed to utilize sonme of the concepts developed in the course of the intensive investigation of the active surface of acetylcholinesterase (6, 18). A preliminary report has appeared (10). Materials and MethodsBarley seels (Hordeumii vulgare L.) were germinate(l and seedlings grown in aerate(d solutions of 2 X 10' -is Ca(NO)42under the condlitions described by Epstein (3). After 7 days the roots were excised, rinsed, and used in the uptake experiments. Samples of roots weighing 1 to 2 g (fresh weight) were transferred to test tubes with 50 ml of aerate(l experimental solutions containing 5 X 10 AI Ca(NO:) 2. After 3 hours the experiment was termlinated by decanting the experimental solutions and rinsing twice with distillecl water. The roots were then placedI in a desorption solution of 5 X 10 AI Ca(NOj)2 for 1 hour (7). For assay of the radioactivity in the roots we (levelopecl a procedure for liquid scintillation counting of the intact, dried roots (11). "Carrier-free" choline sulfate-S35 was synthesizedI according to the method of Segel and Johnson (12). For the uptake experinsents sufficient activity of the "carrier-free" choline sulfate was added to solutions of nonradioactive choline sulfate to give less than 3 % counting error in the final assay of the roots. Choline sulfate and its sulfate ester analogues were made by esterification of the corresponding amino alcohols with sulfuric acid (1, 14). The esters were recrystallized from aqueous ethanol. The infrared spectra corresponded to spectra of other nmonoalkyl sulfates. The other inhibitors used xvere ohtained comnmercially. ResultsThe uptake of choline sulfate was first studlied over a range of concentrations, from 10-7 AI to 10-l'I. The results for the range 3 X 10-6 WI to 10-1 NM are shown in figure 1. Choline sulfate is apparently taken up by 2 mechanisnms. One mechanisns accounts for the uptake of choline sulfate at low substrate concentrations, while the other dominates at higher coIncentrations. The kinetics of the 2 mechanismls ...

Absorption of 2,4-Dichlorophenoxyacetic Acid and 3-(p-Chlorophenyl)-1, 1-dimethylurea (Monuron) by Barley Roots

“…the diseased tissue by an active transport system. Two mechanisms of absorption of choline sulfate by excised barley roots have been demonstrated (15), one being the result of active transport, while the nature of the other could not be determined.…”

Identification and Determination by Fast Atom Bombardment Mass Spectrometry of the Compatible Solute Choline-O-sulfate in Limonium Species and Other Halophytes

“…It is at first sight surprising that the mangrove Avicennia germinans accumulated rather little choline-0-sulfate, given that esterification to choline was by far the major fate of tracer 3 5~0 $in this species (Benson and Atkinson 1967). Similarly, there was no detectable choline-0sulfate in salinised maize or barley, even though both plants readily produced labelled choline-0-sulfate when supplied with 3 5~0 $ - (Nissen and Benson 1961). One way to reconcile these observations is to suppose (a) that Avicennia, maize, barley and most other plants have small, metabolically active pools of choline-0-sulfate, perhaps with some function in uptake or long-distance transport of SO$- (Nissen andBenson 1961, 1964); and (b) that Limonium species are exceptional because they have recruited choline-0-sulfate for a novel function in, say, cytoplasmic osmoregulation.…”

“…Similarly, there was no detectable choline-0sulfate in salinised maize or barley, even though both plants readily produced labelled choline-0-sulfate when supplied with 3 5~0 $ - (Nissen and Benson 1961). One way to reconcile these observations is to suppose (a) that Avicennia, maize, barley and most other plants have small, metabolically active pools of choline-0-sulfate, perhaps with some function in uptake or long-distance transport of SO$- (Nissen andBenson 1961, 1964); and (b) that Limonium species are exceptional because they have recruited choline-0-sulfate for a novel function in, say, cytoplasmic osmoregulation. Such an evolutionary departure would presumably entail large changes in the expression of the enzymes responsible for choline-0-sulfate synthesis and breakdown.…”

“…Because they have been reported to form choline-0-sulfate from 3 5~~; - (Nissen and Benson 1961), barley and maize were analysed for choline-0-sulfate when grown on NaCl/ Na2S04 levels close to the upper limit of their salt tolerance (Table 2). Neither species accumulated a measurable amount.…”

“…Choline-0-sulfate has been reported as a significant metabolite of 35~024in several mesophytic higher plants (Nissen and Benson 1961) and as the predominant (90%) 3 5~~zmetabolite in the salt-secreting mangroves Avicennia gerininans ( = A . nitida) and Aegialitis annulata (Benson and Atkinson 1967).…”