Energetics of Amino Acid Uptake by Vicia faba Leaf Tissues

Abstract

The uptake of IU-14Clthreonine and of by Viiafaba leaf discs is strongly pH dependent (optimum: pH 4.0) and exhibits biphasic saturation kinetics. Kinetics of a-AIB uptake at different pH values indicate that acidic pH values decrease the K., of the carriers while the maximal velocity remains nearly unaffected. Similar results were obtained for both system 1 (from 0.5 to 5 milmar) and system 2p(from 20 to 100 miDllma).After addtion of amino acids to a medium containng leaf fragments, alkalinizations depending both on the amino acid added and on its concentration have been recorded.The effects of compounds which increase (fusicoccin) or decrease (uncouplers, ATPase inhibitors, high KCI concentrations) the protonmotive force were studied both on the acidification of the medium and on amino acid uptake by the tissues. There is a close relationship between the time required for the effect of these compounds on the acidification and that needed for inhibition of uptake.Studies with thiol inhibitors show that 0.1 millmolar N-ethylmaleimide preferentially inhibits uptake by the mesophyll whereas 0.1 milimlar parachioromercuribenzenesulfonate affects rather uptake by the veins.New evidence was found which added to the electrophysiological data already supporting the occurrence of proton amino acid symport in leaf tissues, particularly in the veins.Mature leaves export sugars (mainly as sucrose) but also a broad spectrum of amino acids which may be translocated in substantial amounts. Although the mechanism of phloem loading of sucrose has been studied in some detail (7-10, 15, 16), little is known about the uptake of amino acid in leaf cells and particularly in phloem (3,12,25). Until now, most of the evidence suggesting that amino acids are co-transported with protons (and/or ions) comes from electrophysiological studies. For example, a transient depolarization of the membrane potential is recorded upon addition of amino acids to Lemna gibba fronds (14) and Avena coleoptiles (13,18). In the present paper, we have further tested the amino acid proton symport hypothesis by studying the effects of external pH and of various effectors of the protonmotive force on amino acid uptake and by recording proton fluxes linked to amino acid transport. Viciafaba (broadbean) leaves were selected because the lower epidermis can be removed easily, resulting in excellent contact and exchange between the apoplast and the incubation medium. The heterogeneity of the tissues under investigation necessitated measurements of net uptake of label as well as study of its distribution by autoradiography. In several cases, autoradiographs showed that the mesophyll and the veins ex-'This work was supported by the Centre National de la Recherche Scientifique (ERA 701).hibited a different sensitivity. a-AIB2 and threonine were used because preliminary studies revealed only a slight metabolism of these compounds in our material. MATERIALS AND METHODSPlant Material. Non-nodulated broadbean (Vicia faba cv Aguadulce), were grown under conditions...

Evidence for a Specific Glutamate/H+ Cotransport in Isolated Mesophyll Cells

“…2 and 8) indicates that alkalinization results from an influx of H+ and not L-Glu metabolism. Fusicoccin stimulation of H+ efflux and labeled LGlu uptake (data not reported) represents further evidence for the L-Glu/proton co-transport origin of alkalinization (6,13).…”

“…Transient alkalinization and the resumption of acidification is predicted by an amino acid/proton cotransport mechanism of uptake (I13), and has been observed when amino acids are added to various plant tissues (6,14,23). Proton co-transport requires that ATP driven proton effiux continues to maintain the proton electrochemical gradient required for uptake.…”

“…Similarly, alkalinization was observed on the addition of L-Thr and aamino isobutyrate to Vicia faba leaf tissue (6). Inhibitor studies demonstrated a parallel inhibition of proton and amino acid uptake (6,14). Work (8).…”

Primary Action of Indole-3-acetic Acid in Crown Gall Tumors

“…However, it may be assumed that in tumor tissue LAA causes an increased electrochemical proton gradient across the plasmalemma in a similar way as it does in the classical coleoptile system (7) and thus stimulates the symport of organic solutes with protonsintothe cells (8,10,12,14,15,17,21). An enhanced uptake of organic nutrients certainly is a prerequisite for the establishment ofan efficient metabolic sink ifthe tumor develops on a host plant.…”

“…The uptalke of the amino acids lencin methionie tryptophan, lysine, and aspartic acid is also inhibited by 2-naphtlcetic acid to sidr degees. The auxis 1-naphthaleneacetic acid and 2-aphthoxyacetic acid at co tri betwee 10 and 100 micromolars inhibit solute uptake only slhtly (hibition less than 20%). The Impact of the results on the ptted role of indole-3-acetic acid as a modifier of the electrochemical proton gradient acoss the pmlemma in crown pHl tumor tisue discd.…”