Sulfoquinovosyl diglyceride

Another important group of membrane lipids consists of sterols, for example ji-sistosterol:

Through their structural role in membranes sterols may indirectly affect transport processes 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 in the tonoplast (Table 2.6). Despite these differences in lipids, the fatty acid composition of the phospholipids is similar in both membranes. The long-chain fatty acids in polar membrane lipids vary in both the length and degree of unsaturation (i.e. number of double bounds) which influence the melting point (Table 2.6).

Lipid composition not only differs characteristically between membranes of individual cells but also between cells of different plant species (Stuiver et al., 1978), it is also strongly affected by environmental factors. In leaves, for example, distinct annual variations in the levels of sterols occur (Westerman and Roddick, 1981) and in roots both phospholipid content and the proportion of highly unsaturated fatty acids decrease

Table 2.6

Lipid and Fatty Acid Composition of Plasma Membranes and Tonoplasts from Mung Bean"

Table 2.6

Lipid and Fatty Acid Composition of Plasma Membranes and Tonoplasts from Mung Bean"

Plasma membrane

Tonoplast

Lipids

¿¿mol mg 1 protein

//mol mg~1 protein

Phospholipids

1.29

1.93

Sterols

1.15

1.05

Glycolipids

0.20

0.80

Fatty acid composition of the phospholipids

Melting

Plasma

Chain

point

membrane

Tonoplast

Fatty acid

length

(°C)

(% of total)

(% of total)

Palmitic acid

Ci6

+62.8

35

39

Stearic acid

Cig

+70.1

6

6

Oleic acid

r b

+13.0

9

9

Linoleic acid

r b »-18:2

-5.5

21

22

Linolenic acid

<-18:3

-11.1

19

20

Others

10

4

"Based on Yoshida and Uemura (1986). Reprinted by permission of the American Society of Plant Physiologists.

^Numeral to the right of the colon indicates the number of double bounds.

"Based on Yoshida and Uemura (1986). Reprinted by permission of the American Society of Plant Physiologists.

^Numeral to the right of the colon indicates the number of double bounds.

under zinc deficiency (Cakmak and Marschner, 1988c). In many instances changes in lipid composition reflect adaptation of a plant to its environment through adjustment of membrane properties. Generally, highly unsaturated fatty acids predominate in plants that grow in cold climates. During acclimatization of plants to low temperatures an increase 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 membranes to a lower temperature and may thus be of importance for maintenance of membrane functions at low temperatures. It is questionable, however, to generalize about the effect of temperature on lipid composition of membranes. In rye, for example, which is a cold-tolerant plant species, the proportion of polyunsaturated fatty acids in the roots decreased 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 considerably (Kinney et al., 1987). Since phospholipids probably act as receptors for phytohor-mones such as gibberellic acid, increasing responsiveness of membranes to gibberellic acid 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 highly correlated 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 and barley differ not only in the fatty acid composition of root membranes (Stuiver et 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 to changes in the mineral nutrient supply or exposure to salinity (Kuiper, 1980). Of the mineral nutrients, calcium plays the most direct role in the maintenance of membrane integrity, a function which is discussed in Section 2.5.2. In soybean roots, changes in calcium and nitrogen supply affect the ratio of saturated to unsaturated fatty acids as well as the uptake rate of certain herbicides (Rivera and Penner, 1978). An increase in membrane permeability can be observed in roots suffering from phosphorus deficiency (Ratnayake et al., 1978) and zinc deficiency (Welch et al., 1982; Cakmak and Marschner, 1988b, 1990). In the case of phosphorus deficiency, a shortage of phospholipids in the membranes has been assumed to be the responsible factor. In the case of zinc deficiency, autoxidation in the membranes of highly unsaturated fatty acids is presumably involved in membrane leakiness (Section 9.4).

The dynamic nature of membranes is clearly demonstrated, for example, by the rapid decrease in efflux of low-molecular-weight solutes (potassium, sugars, amino acids) after resupplying of zinc to zinc-deficient roots (Cakmak and Marschner, 1988b). Another example is the rapid incorporation of externally supplied membrane constituents such as phospholipids into the membrane structure. For the plasma membrane turnover rates seem to be in the order of only a few hours (Steer, 1988a). Such high turnover rates indicate that certain subunits (e.g. with intrinsic proteins; Fig. 2.4) are already synthesized and transported to the plasma membrane via secretory vesicles as for example of the Golgi apparatus (Coleman et al., 1988).

The incorporation also of externally supplied compounds, however, renders membranes more sensitive to injury. The incorporation of antibiotics such as nystatin induces the formation of pores ('holes') in the membranes and a corresponding rapid leakage of low-molecular-weight solutes such as potassium. Monocarboxylic acids such as acetic acid and butyric acid, also induce membrane injury. The undissociated species of these acids are readily taken up and lead to a sharp rise in membrane leakiness, as indicated by the leakage of potassium and nitrate from the root tissue (Lee, 1977). The capacity of monocarboxylic acids to induce membrane leakiness increases with the chain length of the acids [C2 (acetic acid) C8 (caprylic acid)] and hence with increased lipophilic behaviour, as well as with a lowering of the external pH (R • COO- + H+ —» R • COOH). Undissociated monocarboxylic acids may increase membrane leakiness by changing the fatty acid composition of membranes, particularly by decreasing the proportion of polyunsaturated fatty acids such as linolenic acid (Jackson and St. John, 1980). The effect of monocarboxylic acids on the membrane permeability of roots is of considerable ecological importance, since these acids accumulate in waterlogged soils (Section 16.4).

These examples demonstrate that composition, structure and integrity of membranes are affected by a range of environmental factors. In the last decade in particular increasing evidence has accumulated that a range of environmental stress factors such as high light intensity, drought, chilling, air pollutants, and also mineral nutrient deficiencies are harmful to plants by impairment of membrane integrity, and that elevated levels of toxic oxygen species are causally involved in this impairment (Elstner, 1982; Hippeli and Elstner, 1991).

As shown in a model in Fig. 2.5, these toxic oxygen species are either radicals such as superoxide (02 ~) or hydroxyl (OH"), or the molecule hydrogen peroxide (H202). All are formed in various reactions and metabolic processes where oxygen is involved, for example photosynthesis (Asada, 1992) and respiration, including oxidation of NADPH

  • Mitochondrial ^transport
  • Photosynthetic e-transport
  • Membrane-bound, cell wall-

bound NAD(P)H oxidases •Chinones • Redox metals

Defense

• Superoxide dismutases:

scavenger of Oj"

  • Catalase: scavenger of H,02
  • Peroxidases (i.e., ascorbate peroxidase): scavenger of H20:
  • Superoxide dismutases:

scavenger of Oj"

  • Catalase: scavenger of H,02
  • Peroxidases (i.e., ascorbate peroxidase): scavenger of H20:

Vitamin C: scavenger of OH'

and ; substrate for peroxidases Vitamin E: scavenger of OH' and peroxyl radicals and inhibits chain reactions of lipid peroxidation Glutathione: protects SH-enzymes fl-carotene: scavenger of 1Q>

Membrane damage (lipid peroxidation, SH-oxidation, enzyme inactivation, leakage —»loss of membrane function)

Defense

Vitamin C: scavenger of OH'

and ; substrate for peroxidases Vitamin E: scavenger of OH' and peroxyl radicals and inhibits chain reactions of lipid peroxidation Glutathione: protects SH-enzymes fl-carotene: scavenger of 1Q>

Membrane damage (lipid peroxidation, SH-oxidation, enzyme inactivation, leakage —»loss of membrane function)

Fig. 2.5 Model of generation of, and membrane lipid peroxidation by, oxygen radicals and hydrogen peroxide, and systems for scavenging and detoxification. (I. Cakmak, unpublished.)

or NADH at the plasma membrane-cell wall interface (Werf et al., 1991; Vianello and Macri, 1991). Toxicity by activated oxygen species and its derivates is caused, for example by oxidation of thiol groups (-SH) of enzymes and peroxidation of polyunsaturated fatty acids in membranes (Halliwell, 1978; Pukacka and Kuiper, 1988). Aerobic organisms including plants possess a range of defense systems (Fig. 2.5) for detoxification of oxygen radicals and hydrogen peroxide, including superoxide dismutase (02'~ —» H202) and peroxidases/catalase (H202 —> H20).

Mineral nutrition of plants may affect at various levels both, generation of toxic oxygen species and hydrogen peroxide, and mechanisms for detoxification. These may be summarized as follows:

  1. As a component of detoxifying enzymes (e.g. zinc, copper, manganese or iron in superoxide dismutases; iron in peroxidases and catalase);
  2. By the accumulation of precursors for radical formation (e.g. phenols and quinones) under nutrient deficiency (e.g. boron deficiency);
  3. Through a decrease in sink activity (i.e. demand) and accumulation of photosyn-thates and correspondingly elevated levels of toxic oxygen species in source leaves under mineral nutrient deficiencies (e.g. potassium and magnesium).

These various aspects are discussed in more detail in the relevant sections on photosynthesis (Chapter 5) and functions of mineral nutrients (Chapters 8 and 9).

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