Plant Lipids and Their Interactions - ACS Symposium Series (ACS

Chapter 1

Plant Lipids and Their Interactions

Downloaded by KAOHSIUNG MEDICAL UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: December 24, 1987 | doi: 10.1021/bk-1987-0325.ch001

Glenn Fuller and W. David Nes Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Albany, CA 94710

Lipids are distinguished by their high solubility in non-polar organic solvents and their low solubility in water. Lipids can be classified as derivatives of fatty acids or other compounds not containing a fatty acid moiety. Quantitatively, the acyl glycerides, polar and non-polar, make up the bulk of plant lipids. These include the neutral triglycerides, found in seed storage lipids, and the polar acyl glycerides such as the phosphatides, glycolipids and sulfolipids. There are a variety of non-glyceride lipids which embrace waxes, sterols, terpenes and their derivatives, hydrocarbons, and even some phenolics. Many organisms do not synthesize all their required lipids de novo, but obtain them from other species. Some plants are able to synthesize lipids which modify behavior in and help to protect against pests and pathogenic organisms. Hence, a variety of interactions have evolved which are the subject matter of this volume. The symposium leading to t h i s book was designed to bring together s c i e n t i s t s working i n the f i e l d of interactions between various species based on l i p i d biochemistry. This area of research i s important to a g r i c u l t u r e because i t can lead to b i o l o g i c a l control of b e n e f i c i a l or deleterious species. Many organisms have evolved requiring l i p i d nutrients from other organisms, e.g., c e r t a i n insects do not synthesize cholesterol de novo, but they can e i t h e r use plant steroids without modification or convert these steroids to cholesterol (Svoboda et a l . , Chap. 11). Other species have evolved protective compounds. Harborne (8) has c l a s s i f i e d the plant protective compounds as p r o h i b i t i n s , i n h i b i t i n s (pre-infect i o n a l ) , p o s t - i n h i b i t i n s , and phytoalexins ( p o s t - i n f e c t i o n a l ) . A high proportion of these compounds are l i p i d s , often functioning by changing membrane permeability i n the invading species. Although there i s overlap, the chapters of the book have been loosely grouped to cover general l i p i d metabolism and function, plant-plant This chapter not subject to U.S. copyright. Published 1987, American Chemical Society

Fuller and Nes; Ecology and Metabolism of Plant Lipids ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


1.

FULLER AND NES


3

interactions, plant-insect interactions and plant-nematode i n t e r a c t i o n s , and e f f e c t s of microbes and plants on one another. Since the book i s a symposium volume, there i s heavy emphasis on plant-microbial r e l a t i o n s h i p s and somewhat less coverage of other areas. However, i t i s representative of current research i n the field. L i p i d s are one of the four major categories of compounds which are involved with growth and reproduction of crop plants and t h e i r pathogens, the others being carbohydrates, proteins and nucleic acids. Bioregulatory processes of l i p i d s have been l a r g e l y ignored. Acyl l i p i d s and s t e r o l s were assumed to play a nonmetabolic r o l e i n the maintenance of c e l l membrane physicochemical properties, while t r i g l y c e r i d e s were important i n energy reserve and isopentenoid hormones influenced reproduction. However, l a t e r chapters of t h i s volume show that l i p i d s may have acted as e c o l o g i c a l determinants of plant interactions with other organisms. Sterols have multiple non-metabolic roles during the l i f e h i s t o r y of fungi and yeasts (W. R. Nes, Chapter 16), each of which i s f a m i l i a l l y and temporarily expressed. In nature t h e i r influence i s dependent on a v a i l a b i l i t y of s t e r o l s from the host plant i n which structure and quantity of s t e r o l s are important to the host-pathogen r e l a t i o n s h i p . Fatty acids or a c y l l i p i d s play p h y s i o l o g i c a l roles which are s i m i l a r l y related to t h e i r structures and t h e i r compartmentalization within the c e l l (Mudd, Chapter 2; F u l l e r and Stumpf, Chapter 4 ). The aim of t h i s monograph i s to attempt to cover f o r the f i r s t time t h i s diverse area of research on l i p i d i n t e r a c t i o n s . We have emphasized the s t e r o l and f a t t y acid f i e l d s and plant i n t e r a c t i o n s , rather than those of mammals since t h i s r e f l e c t s the e d i t o r s ' i n t e r e s t i n these subjects. In addition, a few chapters are devoted to structure-occurrence and structure-biosynthesis of l i p i d s since physiology i s the basis f o r the interactions described. L i p i d s are distinguished from other classes of b i o l o g i c a l l y important compounds by the f a c t that they contain large non-polar moieties, which make them poorly soluble i n aqueous media but soluble i n organic solvents such as chloroform, a l c o h o l , hexane or mixtures of these solvents. This c h a r a c t e r i s t i c enables one to extract l i p i d s from fresh t i s s u e ; mixtures of chloroform - methanol or hexane - isopropanol are the most commonly used solvent systems. A f t e r e x t r a c t i o n , the l i p i d s may be separated by t h e i r chemical properties. The techniques of l i q u i d chromatography, gas chromatography and t h i n - l a y e r chromatography have been e s p e c i a l l y useful i n separating classes of l i p i d s for a n a l y t i c a l purposes. I t i s now generally accepted that two major l i p i d biosynthetic pathways e x i s t - the so-called isopentenoid and f a t t y acid pathways. While the two pathways have been assumed to be biochemically independent of one another, carbon-flow v i a the mevalonic acid shunt into the f a t t y acid pathway has been demonstrated i n a crop plant and insect (2, 3). Because some l i p i d s are very h y d r o p h i l l i c and remain i n aqueous media, we sometimes group l i p i d s according to t h e i r biosynthetic rather than chemical r e l a t i o n s h i p s .


4

ECOLOGY AND METABOLISM OF PLANT LIPIDS


L i p i d Classes Fatty acids and t h e i r d e r i v a t i v e s . Fatty acids are characterized by the presence of a carboxylic acid function attached to a hydrocarbon chain. Because the biosynthesis of f a t t y acids involves the combination of a series of two-carbon fragments, the common f a t t y acids are unbranched chains with even numbers of carbon atoms. Many are saturated, but biochemical interest centers p r i n c i p a l l y around the unsaturated f a t t y acids containing up to f i v e double bonds. Common unsaturated acids have t h e i r double bonds i n the c i s - c o n f i g u r a t i o n rather than the thermodynamically more stable trans form. M u l t i p l e double bonds are u s u a l l y methylene-interrupted, rather than conjugated. Table I indicates the common names and structures of some of the p r i n c i p a l f a t t y acids found i n plant and animal l i p i d s . Though acyl glycerides of the r e l a t i v e l y few acids l i s t e d make up the bulk of l i p i d s i n l i v i n g organisms, small amounts of many other acids of unusual structure are found i n nature. These include branched acids, long chain (>C ) saturated and unsaturated acids, ones with very high degrees of polyunsaturation (found i n marine o i l s ) , epoxy and hydroxy f a t t y acids and acids with cyclopropane moieties i n the chain. The chemistry and biochemistry of the f a t t y acids and t h e i r derived l i p i d s have been reviewed by Gurr and James (4). lg

Table I.

Major Fatty Acids i n Plants and Animals

Chemical Structure

Common Name Saturated Acids Laurie Acid

CH (CH ) COOH

M y r i s t i c Acid

CH (CH ) COOH

P a l m i t i c Acid

CH (CH ) COOH

Stearic Acid

CH (CH ) COOH

3

2

3

1()

2

3

2

3

2

12

14

16

Unsaturated Acids Oleic Acid

CH (CH ) CH=CH(CH ) COOH 3

2

?

2

7

cis L i n o l e i c Acid

CH (CH ) CH=CHCH CH=CH(CH ) COOH 3

2

4

2

cis a-Linolenic Acid

2

?

cis

CH CH CH=CHCH CH=CHCH CH=CH(CH ) COOH 3

2

2

cis

2

cis

2

7

cis


1.

FULLER AND NES

Plant Lipids and

Their Interactions

5


Fatty Acid Derived L i p i d s The f a t t y acids occur most commonly i n nature as a c y l glycerides (Table I I ) . T r i g l y c e r i d e s are the predominant neutral l i p i d s i n most l i v i n g organisms. T r i g l y c e r i d e s are the storage l i p i d s i n animal f a t and i n plant seeds, and because of t h e i r p h y s i c a l properties as w e l l as high energy content, they are components of many food products. They are also the raw materials f o r making soaps and other surface a c t i v e compounds. Animal t r i g l y c e r i d e s are made up of s i g n i f i c a n t amounts of saturated f a t t y acids and thus tend to be s o l i d at ambient temperature, while vegetable o i l s are u s u a l l y l i q u i d s and have e i t h e r shorter chain saturated acids or acids with higher polyunsaturation. Waxes have s i m i l a r p h y s i c a l properties to those of t r i g l y c e r i d e s , but they occur as saturated acids e s t e r i f i e d to long chain monohydric alcohols rather than to g l y c e r o l , and as minor components, long chain alcohols and alkanes. L i p i d s with high l e v e l s of polyunsaturated f a t t y acids are considered desirable by many n u t r i t i o n i s t s because these l i p i d s help to maintain low l e v e l s of blood c h o l e s t e r o l and favorable l e v e l s of serum high density l i p o p r o t e i n s . Many e f f o r t s are now directed at modification of the f a t t y acid composition i n plants, e s p e c i a l l y the composition of seed o i l s (5). Genetic improvement of soybean o i l i s an e s p e c i a l l y desirable goal since the small amount of ct-linolenic acid present i n the o i l causes f l a v o r instability. Polar L i p i d s . The polar l i p i d s (Table I I ) are extremely important to the l i f e processes of l i v i n g organisms, since glycerophosphol i p i d s are p r i n c i p a l components of membranes. These membranes are for the most part l i p i d b i l a y e r s i n which the nonpolar hydrocarbon t a i l s point toward one another and the polar groups are on the outside, i n t e r a c t i n g with the aqueous phases inside and outside the region enclosed by the membrane. Various g l y c o l i p i d s , s t e r o l s , proteins, lipopolysaccharides and other compounds are also incorporated i n the membranes and influence t h e i r s e l e c t i v e properties. A s i g n i f i c a n t proportion of cell enzymes are membrane-bound and hence are difficult to isolate and characterize. The biosynthesis and r o l e of phospholipids has been reviewed by Mudd (6). Although g a l a c t o l i p i d s (Table I I ) are found i n the nervous systems of animals, they are present i n very few other animal tissues. On the other hand, g a l a c t o l i p i d s and s u l f o l i p i d s are prominent i n green plants as important constituents of the chloroplast photosynthetic membranes. The g a l a c t o l i p i d f a t t y acids of the chloroplast lamallae are highly polyunsaturated with a - l i n o l e n i c acid making up ca. 90% of the f a t t y acid content (7,). S u l f o l i p i d s are also found i n the photosynthetic tissues of the chloroplast. S u l f o l i p i d s are s i m i l a r to the phospholipids i n f a t t y acid composition, i . e . , they contain s i g n i f i c a n t amounts of p a l m i t i c , o l e i c and l i n o l e i c acids, as w e l l as l i n o l e n i c acid.



6

TABLE IE PRINCIPAL

LIPID

GROUPS

Siructure:

Name: A.

Fatty

Acids

RCOOH CH OOCR, 2

B.

Triglycerides

R COOCH 2

CH OOCR 2

R COOCH R


1

D.

Waxes

E.

Glycerophospholipids

2

3

2

CH,OOCRi I R,COOCH _ CHoOP-OX

A. • Phosphatidyl choline

X= C H C H N ( C H )

•Phosphatidyl ethanolamine

X= C H C H N H

• Phosphatidyl serine

X= C H C H N H

2

2

2

3

2

2

3

2

2

COOH • Phosphatidyl glycerol

X= CH CHOHCH OH 2

2

O C H ( C H ) C H = CHCH-CHCH OPOX 3

F.

2

12

2

Sphingophospholipids

H. Galactolipids • Monogalactosyl diacylglycerol (MGDG)

OH NH I COR H Q J — O . O- -CCIH

O.

2

HOOCRi CH OOCR 2

2

CH OH 2

• Digalactosyl diacylglycerol (DGDG)

\qhJ

CHOC CH

H.

Sulfolipids • Plant Sulfolipid (Sulfoquinovosyl diacyl glycerol)

-CH

I

2°

2

CHOOCRi

I

CH OOCR 2

J.

2

Sterols

• Cholesterol

• Fucosterol


FULLER AND NES

1.


7

L i p i d s not derived from f a t t y acids


Sterols. Sterols and terpenes are both isopentenoid compounds. Like the f a t t y acids, t h e i r biosynthesis begins with acetate, which undergoes a s e r i e s of reactions forming acetoacetate, hydroxymethylglutarate and f i n a l l y mevalonate. Mevalonic acid i s the precursor to a l l the isopentenoid compounds. Through a further series of reactions mevalonic acid i s converted to isopentenyl pyro-

HOOC

3R-Mevalonic acid phosphate and then through successive condensations to squalene, a 30 ^ isopentenoid. I n nonphotosynthetic organisms squalene forms an epoxide which c y c l i z e s to l a n o s t e r o l , a precursor of s t e r o l s (8). The p r i n c i p a l s t e r o l synthesized by mammals and red algae i s c h o l e s t e r o l , from which a number of important s t e r o i d a l hormones are derived. A s t e r o i d synthesized by one organism may not possess an analogous r o l e i n another organism i n which i t i s synthesized. I f present a t a l l , c h o l e s t e r o l i s only formed i n minute amounts by crop plants; however, plants synthesize several important s t e r o l s , most of which are characterized by an a l k y l o r alkenyl group a t the p o s i t i o n of the s t e r o l side chain. The 24-alkylated s t e r o l s may be metabolized to hormones f o r which cholesterol cannot serve as a precursor, e.g., a n t h e r i d i o l . I n addition to appearance as free s t e r o l s , these compounds are often found as esters or as glycosides. S t e r o i d a l a l k a l o i d s or azasteroids are nitrogen d e r i v a t i v e s which may be important i n the defense mechanisms of plants (9). C

o p e n

c n a

n

Other isopentenoids. Many l i p i d s other than steroids are formed v i a the isopentenoid pathway. Terpenes and t h e i r d e r i v a t i v e s are very important i n i n t e r a c t i o n s of plants with other organisms. Kuc and coworkers have proposed that fungal elicitors modify isopentenoid pathways i n potato, s h i f t i n g biosynthesis from triterpene a l k a l o i d s which are p r e - i n f e c t i o n i n h i b i t o r s to sesquiterpene lactone stress metabolites (9). A v a r i e t y of insect attractants, insect j u v e n i l e hormones, i n h i b i t o r s and plant hormones are terpene d e r i v a t i v e s . Other l i p i d s . Waxes are major l i p i d s i n a few organisms (e.g., jojoba, sperm whale). Cutins (condensation polymers of hydroxy f a t t y acids) are discussed i n a l a t e r chapter (Kolattukudy et a l . , Chap. 10). Hydrocarbons other than isopentenoid compounds occur i n a v a r i e t y of species.


8



Interactions of L i p i d s The ecology of plants includes t h e i r interactions with b e n e f i c i a l and harmful organisms—human beings, animals, insects, b a c t e r i a , yeasts and fungi. In many of these interactions l i p i d s are produced which are e i t h e r rquired nutrients f o r other organisms or which are part of the defense mechanisms of plants. Isopentenoid compounds produced by one plant may be harmful to another (Elakovich, Chapter 7), while steroids may i n h i b i t plant growth by exerting or modifying a regulatory function (Roddick, Chapter 18, W. D. Nes, Chapter 19). Some of the most i n t e r e s t i n g defenses of plants are those against insects, including physical b a r r i e r s (Kolattukudy, Chapter 10) and regulatory compounds f o r insect development (Svoboda, Chapter 11, Thompson, Chapter 12). The interactions which have evolved i n nature suggest protective strategies i n which molecular biology and other biotechnological approaches may be used to protect crop plants. We expect to see a number of such solutions achieved from the types of work reported i n t h i s volume.

Literature Cited 1. Harborne, J. B., "Introduction to Ecological Biochemistry", 2d Edition; Academic Press: London, 1982, p. 230. 2. Nes, W. D.; Bach, T. J. Proc. R. Soc. Lond. B225. 1975, p. 425-444. 3. Nes, W. D.; Campbell, B. C.; Stafford, A. E.; Haddon, W. F.; Benson, M. Biochem. Biophys. Res. Commun., 1982, 108, 1258-1263. 4. Gurr, M. I.; James, A. T., "Lipid Biochemistry: An Intro duction", 3d Edition; Chapman and Hall: New York, 1980, Chap. 2. 5. Ratledge, C.; Dawson, P.; and Rattray, J . , "Biotechnology for the Fats and Oils Industry"; American Oil Chemists Socieity, Champaign, IL., 1984. 6. Mudd, J. B. in "The Biochemistry of Plants, Vol. 4, Lipids: Structure and Function"; Stumpf, P. K., Ed.; Academic Press: New York, 1980; Chap. 9. 7. Harwood, J. L., Ibid., Chapter 1. 8. Nes, W. R.; McKean, M. L., "Biochemistry of Steroids and Other Isopentenoids"; University Park Press: Baltimore, 1977; Chapters 4-7. 9. Kúc, J . ; Tjamos, E.; Bostock, R., in "Isopentenoids in Plants"; Nes, W. D., Fuller, G. and Tsai, L.-S., Eds.; Marcel Dekker: New York; 1984. pp. 103-123. RECEIVED September 5, 1986


Plant Lipids and Their Interactions - ACS Symposium Series (ACS

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