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12 Mineral Nutrition of Higher Plantsminor fraction of the K+ (42K) is readily exchangeable within this 30-min period, mostof the K+ having already been transported across the membranes into the cytoplasmand vacuoles ('inner space').Although the plasma membrane and the tonoplast are the main biomembranesdirectly involved in solute uptake and transport in roots, it must be borne in mind thatcompartmentation by biomembranes is a general prerequisite for living systems (Leighand Wyn Jones, 1986). Solute transport into organelles such as mitochondria andchloroplasts must also therefore be regulated by membranes which separate theseorganelles from the surrounding cytoplasm. An example of solute transport across theouter chloroplast membrane is given in Section 8.4 for phosphorus and sugars.The capability of biomembranes for solute transport and its regulation is closelyrelated to their chemical composition and molecular structure. Before the mechanismsof solute transport across membranes are discussed in more detail (Sections 2.4and2.5), it is therefore appropriate to consider some fundamental aspects of the co

mpo-sition and structure of biomembranes.2.3 Structure and Composition of MembranesThe capacity of plant cell membranes to regulate solute uptake has fascinated botanistssince the nineteenth century. At that time the experimental techniques available limited the investigation of the process. Nevertheless, even by the early yearsof thetwentieth century some basic facts of solute permeation across the plasma membraneand tonoplast had been established, as for example of the inverse relationship between

membrane permeation and the diameter of uncharged molecules and the rates at whichthey permeate membranes. These ultrafilter-like properties of membranes have beenconfirmed more recently, at least in principle (Table 2.5).Thus, in addition to the cell walls (Section 2.2.1) cell membranes are effective barriers to solutes of high molecular weight. Most synthetic chelators such as EDTA(see also Table 2.4) and microbial siderophores as specific chelators of iron (Section16.5) are of high molecular weight and their rate of permeation is restricted through the

Table 2.5Reflection Coefficient (?) of Some Nonelectrolytesat the Cell Membranes of Valonia utricularisaCompound db Molecule radius (nm)Raffinose 1.00 0.61Sucrose 1.00 0.53Glucose 0.95 0.44Glycerol 0.81 0.27Urea 0.76 0.20



"Based on Zimmermann and Steudle (1970). 61.00 indicates that the membranes are impermeable to thesolute; 0 indicates that the membranes are freely permeableto the solute.Ion Uptake Mechanisms of Individual Cells and Roots 13Fig. 2.4 Model of a biomembrane with polar lipids and with either extrinsic or intrinsic,integrated proteins. The latter can cross the membrane to form 'protein channels'.plasma membrane of root cells. It is possible, therefore, to use high-molecular-weightorganic solutes such as polyethyleneglycol at high external concentrations as effectiveosmotica in order to induce water deficiency (drought stress) in plants.Molecules which are highly soluble in organic solvents, i.e. with lipophilic properties,penetrate membranes much faster than would be predicted on the basis of their size.The solubility of these molecules in the membrane and their ability to diffuse throughthe lipid core of the membranes are presumably the main factors responsible forthefaster permeation.Membranes are typically composed of two main classes of compounds: proteins and

lipids. Carbohydrates comprise only a minor fraction of membranes. The relativeabundance of proteins and lipids can be quite variable depending on whether themembrane is a plasma, mitochondrial, or chloroplast membrane (Clarkson, 1977).Membranes also differ in diameter, for example in spinach from 10.5 nm (plasmamembrane) to 8.1 nm (tonoplast) and 6.3 nm (endoplasmic reticulum; Auderset etal.,1986). However, all biomembranes have some common basic structure as shown in amodel in Fig. 2.4.Polar lipids (e.g. phospholipids) with the hydrophilic, charged head regions (phos-phate, amino, and carboxylic groups) are oriented towards the membrane surface.Protein molecules can be attached (extrinsic proteins), for example, by electrostatic

binding to the surfaces as membrane-bound enzymes. Other proteins may be integratedinto membranes (intrinsic proteins), or traverse the membranes to form 'proteinchannels' (transport proteins) which serve to function in membrane transport ofpolarsolutes such as ions (Section 2.4).Three polar lipids represent the major lipid components of membranes: phospho-lipids, glycolipids, and less abundant, sulfolipids (except in the thylakoid membranes ofchloroplasts, where they occur in substantial amounts). Examples of these polarlipidsare shown below:14 Mineral Nutrition of Higher Plants

QLI -R1 /\/\/\/\A/\/ vO-CH 2-0-P-0-CH2-CH2-N+(CH3)3O"Phosphatidylcholine (lecithin)H n ^ 2¦ 0 J\ H Oh^c



(Long chain polyunsaturatedfatty acids)CH 2 -R2 /Ny^As/V^NyN^O-CH aCH2OHOH/OHOH HMonogalactosyl diglycerideoCH2 -§-0"\H OOH/OHOH HSulfoquinovosyl diglycerideAnother important group of membrane lipids consists of sterols, for example ß-sistosterol:ß-SistosterolThrough their structural role in membranes sterols may indirectly affect transportprocesses such as the activity of the proton pumping ATPase in the plasma membrane(Sandstrom and Cleland, 1989). In agreement with this assumption the sterol content is

very low in endomembranes (e.g. endoplasmic reticulum) but may make up more than 30% of the total lipids in the plasma membrane (Brown and DuPont, 1989) and also inthe tonoplast (Table 2.6). Despite these differences in lipids, the fatty acid compositionof the phospholipids is similar in both membranes. The long-chain fatty acids in polarmembrane lipids vary in both the length and degree of unsaturation (i.e. numberofdouble bounds) which influence the melting point (Table 2.6).Lipid composition not only differs characteristically between membranes of indi-vidual cells but also between cells of different plant species (Stuiver et aL, 1

978), it isalso strongly affected by environmental factors. In leaves, for example, distinct annualvariations in the levels of sterols occur (Westerman and Roddick, 1981) and in rootsboth phospholipid content and the proportion of highly unsaturated fatty acids decreaseIon Uptake Mechanisms of Individual Cells and Roots 15Table 2.6Lipid and Fatty Acid Composition of Plasma Membranes and Tonoplasts from Mung BeanaLipidsPlasma membrane

µt??\ mg- 1 proteinTonoplastµp??? mg- 1 proteinPhospholipidsSterolsGlycolipids1.291.15



0.201.931.050.80Fatty acidFatty acid composition of the phospholipidsMelting PlasmaChain point membranelength (°C) (% of total)Tonoplast(% of total)Palmitic acidStearic acidOleic acidLinoleic acidLinolenic acidOthersCi 6Cl 8G b^-18:1r br b^18:3

+62.8+70.1+ 13.0-5.5-11.1 356921191039

6922204"Based on Yoshida and Uemura (1986). Reprinted by permission of the American Society of PlantPhysiologists.^Numeral to the right of the colon indicates the number of double bounds.under zinc deficiency (Cakmak and Marschner, 1988c). In many instances changes inlipid composition reflect adaptation of a plant to its environment through adjustment of

membrane properties. Generally, highly unsaturated fatty acids predominate in plantsthat grow in cold climates. During acclimatization of plants to low temperatures anincrease in highly unsaturated fatty acids is also often observed (Bulder et al., 1991).Such a change shifts the freezing point (i.e. the transition temperature) of membranesto a lower temperature and may thus be of importance for maintenance of membrane



functions at low temperatures. It is questionable, however, to generalize abouttheeffect of temperature on lipid composition of membranes. In rye, for example, which isa cold-tolerant plant species, the proportion of polyunsaturated fatty acids inthe rootsdecreased rather than increased as the roots were cooled (White et al., 1990b).During acclimatization of roots to low temperatures synthesis of new membraneproteins is also enhanced (Mohapatra et al., 1988) and phospholipids increase consider-ably (Kinney et al., 1987). Since phospholipids probably act as receptors for phytohor-mones such as gibberellic acid, increasing responsiveness of membranes to gibberellicacid at low temperatures may be related to these changes (Singh and Paleg, 1984).The property of membranes in ion selectivity and lipid composition are often highlycorrelated as for example between chloride uptake and sterols (Douglas and Walker,i983) and galactolipids (Section 16.6). Also the crop plant species bean, sugarbeet andbarley differ not only in the fatty acid composition of root membranes (Stuiveret al.,

1978) but also considerably in the uptake of sodium (Section 10.2).Alterations in the lipid composition of root membranes are also typical responses tochanges in the mineral nutrient supply or exposure to salinity (Kuiper, 1980). Of the12 Mineral Nutrition of Higher Plantsminor fraction of the K+ (42K) is readily exchangeable within this 30-min period, mostof the K+ having already been transported across the membranes into the cytoplasmand vacuoles ('inner space').Although the plasma membrane and the tonoplast are the main biomembranes

directly involved in solute uptake and transport in roots, it must be borne in mind thatcompartmentation by biomembranes is a general prerequisite for living systems (Leighand Wyn Jones, 1986). Solute transport into organelles such as mitochondria andchloroplasts must also therefore be regulated by membranes which separate theseorganelles from the surrounding cytoplasm. An example of solute transport across theouter chloroplast membrane is given in Section 8.4 for phosphorus and sugars.The capability of biomembranes for solute transport and its regulation is closelyrelated to their chemical composition and molecular structure. Before the mechanisms

of solute transport across membranes are discussed in more detail (Sections 2.4and2.5), it is therefore appropriate to consider some fundamental aspects of the compo-sition and structure of biomembranes.2.3 Structure and Composition of MembranesThe capacity of plant cell membranes to regulate solute uptake has fascinated botanistssince the nineteenth century. At that time the experimental techniques available



limited the investigation of the process. Nevertheless, even by the early yearsof thetwentieth century some basic facts of solute permeation across the plasma membraneand tonoplast had been established, as for example of the inverse relationship betweenmembrane permeation and the diameter of uncharged molecules and the rates at whichthey permeate membranes. These ultrafilter-like properties of membranes have beenconfirmed more recently, at least in principle (Table 2.5).Thus, in addition to the cell walls (Section 2.2.1) cell membranes are effective barriers to solutes of high molecular weight. Most synthetic chelators such as EDTA(see also Table 2.4) and microbial siderophores as specific chelators of iron (Section16.5) are of high molecular weight and their rate of permeation is restricted through theTable 2.5Reflection Coefficient (?) of Some Nonelectrolytesat the Cell Membranes of Valonia utricularisaCompound db Molecule radius (nm)

Raffinose 1.00 0.61Sucrose 1.00 0.53Glucose 0.95 0.44Glycerol 0.81 0.27Urea 0.76 0.20"Based on Zimmermann and Steudle (1970). 61.00 indicates that the membranes are impermeable to thesolute; 0 indicates that the membranes are freely permeableto the solute.Ion Uptake Mechanisms of Individual Cells and Roots 13Fig. 2.4 Model of a biomembrane with polar lipids and with either extrinsic or intrinsic,integrated proteins. The latter can cross the membrane to form 'protein channels

'.plasma membrane of root cells. It is possible, therefore, to use high-molecular-weightorganic solutes such as polyethyleneglycol at high external concentrations as effectiveosmotica in order to induce water deficiency (drought stress) in plants.Molecules which are highly soluble in organic solvents, i.e. with lipophilic properties,penetrate membranes much faster than would be predicted on the basis of their size.The solubility of these molecules in the membrane and their ability to diffuse throughthe lipid core of the membranes are presumably the main factors responsible for

thefaster permeation.Membranes are typically composed of two main classes of compounds: proteins andlipids. Carbohydrates comprise only a minor fraction of membranes. The relativeabundance of proteins and lipids can be quite variable depending on whether themembrane is a plasma, mitochondrial, or chloroplast membrane (Clarkson, 1977).Membranes also differ in diameter, for example in spinach from 10.5 nm (plasmamembrane) to 8.1 nm (tonoplast) and 6.3 nm (endoplasmic reticulum; Auderset etal.,1986). However, all biomembranes have some common basic structure as shown in a



model in Fig. 2.4.Polar lipids (e.g. phospholipids) with the hydrophilic, charged head regions (phos-phate, amino, and carboxylic groups) are oriented towards the membrane surface.Protein molecules can be attached (extrinsic proteins), for example, by electrostaticbinding to the surfaces as membrane-bound enzymes. Other proteins may be integratedinto membranes (intrinsic proteins), or traverse the membranes to form 'proteinchannels' (transport proteins) which serve to function in membrane transport ofpolarsolutes such as ions (Section 2.4).Three polar lipids represent the major lipid components of membranes: phospho-lipids, glycolipids, and less abundant, sulfolipids (except in the thylakoid membranes ofchloroplasts, where they occur in substantial amounts). Examples of these polarlipidsare shown below:14 Mineral Nutrition of Higher PlantsQLI -R1 /\/\/\/\A/\/ vO-CH 2-0-P-0-CH2-CH2-N+

(CH3)3O"Phosphatidylcholine (lecithin)H n ^ 2¦ 0 J\ H Oh^c(Long chain polyunsaturatedfatty acids)CH 2 -R2 /Ny^As/V^NyN^O-CH aCH2OHOH/OHOH H

Monogalactosyl diglycerideoCH2 -§-0"\H OOH/OHOH HSulfoquinovosyl diglycerideAnother important group of membrane lipids consists of sterols, for example ß-sistosterol:ß-SistosterolThrough their structural role in membranes sterols may indirectly affect transportprocesses such as the activity of the proton pumping ATPase in the plasma membra

ne(Sandstrom and Cleland, 1989). In agreement with this assumption the sterol content isvery low in endomembranes (e.g. endoplasmic reticulum) but may make up more than 30% of the total lipids in the plasma membrane (Brown and DuPont, 1989) and also inthe tonoplast (Table 2.6). Despite these differences in lipids, the fatty acid compositionof the phospholipids is similar in both membranes. The long-chain fatty acids in



polarmembrane lipids vary in both the length and degree of unsaturation (i.e. numberofdouble bounds) which influence the melting point (Table 2.6).Lipid composition not only differs characteristically between membranes of indi-vidual cells but also between cells of different plant species (Stuiver et aL, 1978), it isalso strongly affected by environmental factors. In leaves, for example, distinct annualvariations in the levels of sterols occur (Westerman and Roddick, 1981) and in rootsboth phospholipid content and the proportion of highly unsaturated fatty acids decreaseIon Uptake Mechanisms of Individual Cells and Roots 15Table 2.6Lipid and Fatty Acid Composition of Plasma Membranes and Tonoplasts from Mung BeanaLipidsPlasma membraneµt??\ mg- 1 proteinTonoplastµp??? mg- 1 protein

PhospholipidsSterolsGlycolipids1.291.150.201.931.050.80Fatty acidFatty acid composition of the phospholipidsMelting PlasmaChain point membrane

length (°C) (% of total)Tonoplast(% of total)Palmitic acidStearic acidOleic acidLinoleic acidLinolenic acidOthersCi 6Cl 8G b^-18:1

r br b^18:3+62.8+70.1+ 13.0-5.5-11.1 35



69211910396922204"Based on Yoshida and Uemura (1986). Reprinted by permission of the American Society of PlantPhysiologists.^Numeral to the right of the colon indicates the number of double bounds.under zinc deficiency (Cakmak and Marschner, 1988c). In many instances changes inlipid composition reflect adaptation of a plant to its environment through adjustment ofmembrane properties. Generally, highly unsaturated fatty acids predominate in plantsthat grow in cold climates. During acclimatization of plants to low temperatures anincrease in highly unsaturated fatty acids is also often observed (Bulder et al.

, 1991).Such a change shifts the freezing point (i.e. the transition temperature) of membranesto a lower temperature and may thus be of importance for maintenance of membrane functions at low temperatures. It is questionable, however, to generalize abouttheeffect of temperature on lipid composition of membranes. In rye, for example, which isa cold-tolerant plant species, the proportion of polyunsaturated fatty acids inthe rootsdecreased rather than increased as the roots were cooled (White et al., 1990b).During acclimatization of roots to low temperatures synthesis of new membrane

proteins is also enhanced (Mohapatra et al., 1988) and phospholipids increase consider-ably (Kinney et al., 1987). Since phospholipids probably act as receptors for phytohor-mones such as gibberellic acid, increasing responsiveness of membranes to gibberellicacid at low temperatures may be related to these changes (Singh and Paleg, 1984).The property of membranes in ion selectivity and lipid composition are often highlycorrelated as for example between chloride uptake and sterols (Douglas and Walker,i983) and galactolipids (Section 16.6). Also the crop plant species bean, sugar

beet andbarley differ not only in the fatty acid composition of root membranes (Stuiveret al.,1978) but also considerably in the uptake of sodium (Section 10.2).Alterations in the lipid composition of root membranes are also typical responses tochanges in the mineral nutrient supply or exposure to salinity (Kuiper, 1980). Of the//BIMx//Copyright 2002-2013 Graphisoft SE All rights reserved



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