Membranes and Transport Systems in Plants: An Overview

Abstract

Membranes define the outer boundary of the living protoplast and the internal compartmentation of plant cells. From a structural point of view, membranes consist of a lipid bilayer and proteins essential for functions such as solute transport, signal transduction, and numerous metabolic reactions. While membranes can represent a significant barrier to the free movement of many solutes, those with sufficient lipid solubility may move across membranes by dissolving into the lipid bilayer. However, selective membrane transport is generally observed for hydrophilic solutes such as mineral nutrients and cell metabolites. Such selective transport requires an input of metabolic energy, and in plants this occurs via the production of proton electrochemical gradients across the membrane by substrate- (ex. ATP, PPi) driven proton pumps. Selective solute transport is then mediated by membrane-associated secondary transport systems which utilize the proton electrochemical gradient to drive the transport process. This review of membrane structure and transport system function provides a background for a further examination of herbicide interactions with plant membranes.

Metabolism of the herbicide glufosinate-ammonium in plant cell cultures of transgenic (rhizomania-resistant) and non-transgenic sugarbeet (Beta vulgaris), carrot (Daucus carota), purple foxglove (Digitalis purpurea) and thorn apple (Datura stramonium)

“…It is generally accepted that passive diffusion of polar, especially ionized, molecules through the plasma membrane is low 21. 22 At physiological pH, glufosinate (pK a < 2, 2.9 and 9.8) is prevailingly completely ionized 3. These facts additionally argue against intracellular accumulation of glufosinate by means of the ion‐trapping mechanism 21.…”

“…These facts additionally argue against intracellular accumulation of glufosinate by means of the ion‐trapping mechanism 21. 22 In general, the cellular uptake of pesticides is considerably influenced by the intracellular metabolism of the compounds. A rapid internal transformation usually leads to an enhanced uptake, with movement away from the higher external concentration.…”

Mechanisms of Herbicide Absorption Across Plant Membranes and Accumulation in Plant Cells

“…Energy-dependent absorption is most likely due to the cell expending metabolic energy to maintain a pH gradient across the plasma membrane and the tonoplast; membrane-bound ATPases, which pump H + out of the cy tosol, require ATP as their substrate (10). Ammonium sulfate-stimulated imazethapyr uptake was sensitive to the plasma membrane-ATPase (PM-ATPase) inhibi tors sodium orthovanadate and diethylstilbestrol (DES), to the uncoupler CCCP, and to metabolic inhibitors sodium azide and N2> suggesting that uptake is dependent on ATP production and a functioning PM-ATPase (34).…”

“…An understanding of the principles of solute transport in plants is essential for describing mechanisms of herbicide ab sorption by plant cells; these principles are discussed by Briskin (10) in this review series. Physicochemical characteristics of the herbicide molecule, the plant cell membrane, and the electro chemical potential across the plant cell membrane control herbi cide absorption across plant membranes and subsequent accumulation in the cell.…”

Comparison of physiological effects of fluazifop‐butyl and sethoxydim on oat (Avena sativa L.)

“…The depolarization of plasma membrane actually perturbed the transmembrane proton gradient. This gradient is maintained by a proton pump to continuously keep a higher pH value (pH 7.2) within the cell membrane and a lower value (pH 5.5) outside the membrane (Briskin 1994). During their growth, roots also emit protons resulting in the acidification of ambient solution.…”