Lipid Metabolism in Plants

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Lipid metabolism in plants I [edited by] Thomas S. Moore, Jr. ...... L. 72. Neither genes nor inhibitors have been identified that modify the chain ...... Holloway, P. J., Structure and histochemistry of plant cuticular membranes: an overview, ... Sutton, J. C., Rowell, P. M., and James, T. D., Effects of leaf wax, wetness duration.
Lipid Metabolism in Plants Edited by

Thomas S. Moore, Jr., Ph.D. Professor Department of Botany and Adjunct Professor Department of Biochemistry Louisiana State University Baton Rouge, Louisiana

CRC Press Boca Raton Ann Arbor London Tokyo

Library of Congress Cataloging-in-Publication Data

Lipid metabolism in plants I [edited by] Thomas S. Moore, Jr. p. em. Includes bibliographical references and index. ISBN 0-8493-4907-9 I. Plant lipids- Metabolism I. Moore, Thomas S. QK898.L56L55 1993 581.1'3346--dc20

92-46760 CIP

This book represents information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Every reasonable effort has been made to give reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this bk nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical , including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. All rights reserved. Authorization to photocopy items for internal or personal u e, or the personal or internal use of specific clients, is granted by CRC Press, Inc., provided that $.50 per page photocopied is paid directly to Copyright Clearance Center, 27 Congress Street, Salem, MA 01970 USA. The fee code for users of the Transactional Reporting Service is ISBN 0-8493-49079/93/$0.00+$.50. The fee is subject to change without notice. For organizations that have been granted a photocopy license by the CCC, a eparate system of payment has been arranged. The copyright owner's consent does not extend to copying for general distribution , for promotion, for creating new works, or for resale. Specific permission must be obtained from CRC Press for such copying. Direct all inquiries to CRC Press, Inc. , 2000 Corporate Blvd. , N.W. , Boca Raton , Florida 33431.

© 1993 by CRC Press, Inc. International Standard Book Number 0-8493-4907-9 Library of Congress Card Number 92-46760 Printed in the United States I 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

Chapter 4

WAXES, CUTIN, AND SUBERIN Penny M. von Wettstein-Knowles

TABLE OF CONTENTS I.

Scope ............................................................................ .. ....... ............. 128

II.

Nature and Function of Plant Waxes, Cutin, and Suberin .............. 128

III.

Synthesis of the Carbon Skeletons ................................................... 130 A. The Condensation Elongation Mechanism, Elongases and Polyketide Synthases ......................................................... 130 1. Straight, Branched, and Unsaturated Chains ................. 133 2. f3-Diketo Chains .............................................................. 134 B. Dissecting the Elongation Condensation Pathways with Mutants and Inhibitors ............................................................. 134 1. Sequential Systems ......................................................... 134 2. Parallel Systems .............................................................. 137 C. Membrane Elongases ............................................................... 140 1. Progress Toward Isolation .............................................. 140 2. Molecular Organization and Programming ................... 141

IV.

Reactions of the Associated Pathways ............................................. 142 A. Reduction .................................................................................. 143 B. Decarbonylation and Decarboxylation .................................... 144 C. Hydroxylation ........................................................................... 141 1. Midchain ......................................................................... 147 2. Omega ............................................................................. 149 3. Epoxidation + Hydration ................................................ 149 D. Oxidation .................................................................................. 152 E. Transacylation ........................................................................... 152

V.

Subcellular Localization of the Synthesizing Machinery ................ 153 A. Cell Fractionation ..................................................................... 154 B. The cer-cqu Gene ..................................................................... 156

References .................................................................................................... 158

0-R493-4907-9/93/$0.00+$.50 ~ 1993 by CRC Press Inc.

127

Lipid Metabolism in Plants

128

I. SCOPE Following a very brief introduction to the location, structure, composition, and function of the three groups of plant lipids, this chapter will delve into the mechanisms whereby they are synthesized. Although they are generally treated as three quite distinct groups of lipids, they will be seen to be very closely related. That is, in each case, they are synthesized by a set of membranelocalized enzymes each belonging to a small family, with different members of each family participating in various combinations. The result i three groups of overlapping lipid classes present in different proportions. Moreover, other members of the pertinent gene families encode isoenzymes which function in synthesis of the membrane and seed storage lipids. The synthetic reactions are divided into two groups, those producing the carbon skeletons and those modifying them.

II. NATURE AND FUNCTION OF PLANT WAXES, CUTIN, AND SUBERIN Plant waxes consisting of very long-chain, relatively nonpolar lipid molecules are associated primarily with the cuticle which extends in a continuous sheet exterior to the walls of the epidermal cells of aerial tissues. Two subgroups of these lipids soluble in organic solvents are recognized. (1) The epicuticular waxes forming the outermost layer of the cuticle exist as an amorphous lipid film on which may be found a wide variety of structured bodies. (2) The intracuticular waxes embed cutin, a rather insoluble lipid polymer which is the main structural component of the cuticular membrane. The relatively thin outer layer of the cuticular membrane, or primary cuticle, generally appears amorphous, but sometimes has distinct lamellae. The much thicker inner portion of the cuticular membrane, or secondary cuticle, also containing wax and cutin often displays a reticulate appearance. It is deposited between the primary cuticle and wall after cell expansion is finished. In underground tissues (roots, storage organs), stems undergoing secondary growth, and wound healing sites, waxes are associated with the suberin matrix, a polymer related to cutin which has in addition to an aliphatic domain also an aromatic one. Suberin is located between the plasma lemma and walls of the outermost one or two cell layers. Microscopic examination of the suberized regions reveals a lamellar structure interpreted to represent alternating layers of wax and polymer. While numerous observations support thi contention, 1 it has not been definitively demonstrated. 2 Waxes are also found at internal sites having specialized functions where they can be recognized by their characteristic lamellate structure. 3 An exception to the generalization that internal waxes are associated with suberin occurs in the seeds of some plants which store their major energy reserves as waxes rather than as triacylglycerols.4-6 An extraordinary diversity of aliphatic lipid classes has been identified in epicuticular waxe , including numerous types of hydrocarbons, ketones, ~-

Waxes, Cutin, and Suberin

129

diketones, esters, estolides (polyesters), alcohols, aldehydes, and free fatty acids. 7 Generally, a series of homologs encompassing ten carbons occurs dominated by either the even or odd members. While chains 20-35 carbons long are most frequently encountered, fatty acids and hydrocarbons with fewer than 20 and esters with more than 60 are known. The waxes associated with cutin and suberin polymers, by comparison, are much less variable, being limited to n-alkane , fatty acids, primary alcohols, and esters. Usually, the chain length distributions are broader, have a greater preponderance of horter members, and the relative prominence of either even or odd chain lengths is less marked than in the epicuticular waxes. Only esters and hydrocarbons have been reported in wax-storing seeds. Other lipids such as terpenoids and sterols which occur in epicuticular waxes are the subjects of Chapter 11 and 12. The building blocks of cutin and the aliphatic domain of suberin consist of fatty acids with one or more additional substitutions either at the opposite that is w-end or internally circa midchain. 7 Cutin monomers, which are derivatives of C 16 and C 111: 1, normally have midchain substitutions (predominantly hydroxy and epoxy). By comparison, those of suberin, which generally range from C 16-C 24 including C 18: 1, but may be as long as C 30 , are more likely to have w-substitutions (hydroxy and oic). Since terminal groups are used in construction of linear polymers, while midchain groups participate in crosslinking, the resulting cutin polymer is the more rigid one. Monomers of suberin's aromatic domain include the phenolic aldehydes p-hydroxybenzaldehyde and vanillin with small amounts of syringaldehyde. The latter is an important constituent of the lignin polymer. A structure for the aromatic domain of suberin similar to that of lignin has been envisaged 8 which is covalently attached on one side to the cell wall and on the other to its aliphatic domain. The tentative nature of the model has been emphasized. 3·7·9 The primary function of the polymer-associated waxes is to prohibit loss of water and/or other molecules by diffusion through the cutin and suberin matrices. 10•11 The plant or a tissue thereof is not only protected against desiccation, but can enclose specified regions to shield against the internal movement of unwanted solutes or to prevent decay. 3· 12 In addition to providing a structural matrix in and on which the waxes are located, the polymers have been implicated as a line of defense against pathogens which invade by direct penetration of the cuticle. For fungi with an active extracellular cutinase, that is an esterase belonging to the class of serine hydrolases, penetration of the cutin polymer presents no difficulties. 13 That this enzyme is required in some cases has been well-documented, for example, by inhibiting penetration using specific antibodies and restoring penetration to cutinase defective mutants by adding cutinase. Thus, a few molecules of the cutinase are released in a fluid of unknown composition by the fungal spore when it lands on the plant. The enzyme attacks the polymer, releasing by hydrolysis a few monomers which have been shown to induce transcription of the enzyme in the fungus , thereby ensuring an adequate supply of cutinase for complete penetration. The mechanism whereby the cutin monomers on the plant surface trigger transcription

130

Lipid Metabolism in Plants

in fungal nuclei is at present an enigma, but with the progress being made in dissecting the promoter region of the cutinase structural gene, 14 an answer should be forthcoming. Another interesting question is why the epicuticular wax structures apparently dissolve when the fluid is released by the conidium upon landing, ince most of the wax lipids, as exemplified by barley leaves, lack an ester bond, the substrate for the cutinase. 15 · 16 To what extent a battery of cutinase enzymes with different properties can be used to clarify the detailed structure of the polymers remains to be determined. Becau e of their location on the outer surface of the cuticle in direct contact with the environment, the epicuticular waxes provide a diversity of other protective functions for the plant, many of which are analogous to those assigned to surface waxes of insects. 17 Wax bodies exist in a fascinating array of forms, and in a limited number of cases a direct correlation between structure and chemical composition has been established. Each aerial organ, tissue, or even cell type has its own peculiar morphology plus density of wax bodies which helps determine how effective the protection is. For example, the wax coat is a major factor in determining whether or not moisture is trapped 111•19 and/or retained on a cuticle surface as illustrated in Figure 1. Water on the surface for extended periods of time provides a suitable atmosphere for germination of fungal spores 20-22 and leads to leaching of nutrients. 23 Some wax coats result in an airspace above the epidermal cells which has been implicated as a factor contributing to frost hardiness, since nucleating ice crystals are not in direct contact with the cuticular membrane. 24-26 The nature of the wax layer also determines what type of insects as adults or larvae can adhere to or move on the surface 27 -29 and can even assist a plant in trapping insects. 30 Additional potential interactions between plant epicuticular waxes and insects have been described. 3 u 2 Finally, light reflection and refraction from the cuticle surface is affected by the structure of the wax bodies. While plant temperature may be influenced, 33 and perhaps also photosynthetic efficiency ,34 the phenotypic modification resulting from the changed wax morphology permitted the isolation of numerous mutants with altered wax composition 35 (Figure 1). The mutations all affect the specific lipids making major contributions to the structure of the wax coats. Studies of a number of them have contributed to unraveling various facets of the wax biosynthetic pathways. Moreover, a few have been designated regulatory genes. 35 -37 In most cases, no indication as to the nature of the mutated gene could be discerned.

III. SYNTHESIS OF THE CARBON SKELETONS A. THE CONDENSATION ELONGATION MECHANISM, ELONGASES, AND POLYKETIDE SYNTHASES Elongases are enzyme complexes which repetitively condense short activated carbon chains to an activated primer and prepare the growing chain for the next addition. Included in this general definition is the soluble fatty acid

Waxes, Cutin, and Suberin

131

FIGURE 1. Epicuticular waxes function as raincoats. Circa half an hour after raining, water drops are gone from the leaf blades of the wild type Bonus barley (Hordeum vulgare), although they still hang from the plastic divider (top left) and adhere to the leaf surfaces on cer-j mutants (top right). Transmission electron micrographs of shadowed carbon replicas of the cuticle surfaces reveal that this difference is due to the presence of small lobed plates, diagnostic for predominating amounts of primary alcohols in the wax, on the wild type (bottom left) and their absence on cerj mutants (bottom right) which have few, small smooth plates scattered among thin plates appressed to the surface. Bar - I ).!In.

synthetase (FAS) complex, which joins eight activated C 2 units together to give 16-carbon chains. An additional elongation step carried out by another soluble complex known as C 16 elongase gives 18-carbon acyl chains, which are in tum desaturated by the soluble stearoyl desaturase giving cl 8: 1• This plastid-localized pathway supplying the 16- and 18-carbon acyl chains characterizing plant membranes is the subject of Chapter 1. The same or an analogous condensation elongation pathway presumably provides the carbon chains for the cl 6 and c lll:l families of cutin monomers and the substrates for the membrane-localized elongases whose coordinated action results in the longer carbon skeletons characterizing the wax lipids and many of the suberin monomers. It should be noted that C 111 rather than C 111: 1 chains generally serve as the primer for these

132

Lipid Metabolism in Plants 0II

OH-c-~-

~ c -5-x

+ (Q]

reo· ·-· II

R-C~-C-5-X

Acyl elongases

0

·----I I I I

I I I I

II

C----;-5-CoA+~

~-ketoacylelongasel

I I

I

0

I

II

I

*

I

I I I I I

0

I I

II

---e--

0

~ R

II

0

:

II

__ _;

-C~-CH2-©-i2- ©-5 - X

0 II

I

0 II

0 II

~

0 II

C ---- C-C- C -C- C=C- C- 5-CoA

* 0

II

O~OH

*

I

I I

-~- 5- X

0

II

C - - - - C - C - C - C- C - C - C - 5- CoA

1 I I

R- Q-f- a-f=@-i2

0

C - -- - C- C - C - C - C - C - C - 5 - CoA

I

L

0 II

0 II

~

0 II

C- --- C-C-C-C-C- C- C- 5-CoA

*

FIGURE 2. The basic condensation elongation mechanism synthesizes the carbon skeletons of wax acyl lipids and cutin plus suberin aliphatic monomers. In syntheses carried out by acyl elongases including FAS, the 3-oxo group introduced by the condensation reaction is removed in three steps by the reactions shown before the next addition takes place (far left). Polyketide synthesis is characterized by the omission of one or more of these three reactions in specified cycles of elongation (center), with the solid arrow illustrating the ~-ketoacyl elongase functioning in the formation of wax acyl lipids. Three successive condensations carried out by the ~-ketoacyl elongase result in a tetraoxo acyl chain from which the 3-oxo group is then removed in the same manner as in elongation carried out by acyl elongases (right). The primer and donor units used by the microsomal acyl and ~-ketoacy l elongases are activated (X) by coenzyme A (CoA), whereas those used by plastid-localized complexes and most polyketide synthases are activated by an acyl carrier protein (ACP), with the exception that in the first condensation carried out by FAS, a CoA primer serves.

elongases. Whether one considers the wax lipids or the suberin monomers, the primary elongated products of the elongases in the form of free fatty acids are usually minor components, since most of them serve as substrates for the associated enzymes discussed in Section IV. The total length attained during elongation, however, can be deduced from the chain lengths of the members of the various lipid classes. As illustrated in Figure 2 (left), each condensation carried out by an elongase introduces a ~-keto group into the growing chain. This keto group is normally removed by a series of three reactions: a ~-keto reduction, a ~-hydroxy dehydration, and an enoyl reduction. Thus, an elongase requires a minimum of four activities to carry out each elongation step, if one excludes from consideration the enzyme providing the donor unit. Variations of this basic

Waxes, Cutin, and Suberin

133

condensation elongation biosynthetic mechanism are well-known, however, which give rise to compounds classified as polyketides. 3x Their modified carbon chains synthe ized by polyketide synthase enzyme complexes can be recognized by the presence of keto groups, hydroxy groups, or double bonds that were not removed before the next condensation took place (Figure 2, center). For example, pla tid-localized 6-methylsalicylic acid syntase (6-MSA) joins four activated C 2 units, leaving keto groups after the first and third rounds of elongation and a double bond after the second. That polyketide synthases, as well as elongases, participate in construction of the wax and polymer acyl chains increases the number of possible carbon skeleton structures. Use of different primer and donor units expands the repetoire still further as illu trated below. 1. Straight, Branched, and Unsaturated Chains

Kolattukudy 39 -42 obtained the original data supporting the elongation thesis by analyzing the alkanes which are major components of the Brassica, pea (Pisum sarivum), spinach (Spinacia oleracea), Senecio, and tobacco (Nicotiana tobacum) waxes. Single- and double-labeled C 12-C 1R fatty acids presented to whole leaf and/or tissue slices were recovered intact in the expected C 29 , C 31 , and/or C 33 alkanes. Tissue slices of leek (Alliun porrum) leaves were later shown to incorporate lignoceric acid into C 26-C 32 fatty acids and the corresponding C 25-C 31 alkanes. 43 That elongation was by C 2 units was indicated by feeding 1- and 2-labeled acetate to Brassica leaves, followed by isolation and degradation of nonacosan-15-one. The data revealed that the methyl group of acetate preferentially became the oxygen-bearing carbon. 44 Similar radio tracer experiments demonstrated that use of different primers and/or donor units by the elongases gave rise to other carbon skeletons. Primer units derived from valine and isoleucine led to the synthesis of the most frequently encountered branched plant wax lipids. That is, the former amino acid was incorporated by tobacco leaves into 2-methyl C 15-C 25 fatty acids and 2-methyl C2R-C32 alkanes, whereas the latter gave rise tO 3-methyl C16-C26 fatty acids and 3-methyl C 29-C 31 alkanes. 40 More recently, detailed investigations of the synthesis of branched hydrocarbons in insect waxes pinpointed activated methylmalonic acid as a primary source of methyl branches. The latter compound functioned both as an initial primer unit giving 3-methyl branches and/or as a donor unit yielding other internal methyl branches. While the same role for methylmalonic acid is well-documented for given vertebrate tissues 45 and in prokaryotes such as Mycobacterium tuberculosis, 46 its potential contribution to the synthesis of 3-methyl and internal methyl branches in plant wax lipids has not been investigated. These experiments established the origin of the major homo logs in the lipid classes and inferred that the minor ones aro e from a c3 primer unit, presumably activated propionic acid. The prominence of the minor homologs would depend on the ratio of c2 to c3primer units used. Some of the double bonds in the wax and monomer lipids arise from use of an unsaturated primer by a membrane elongase. For example, adding C 2

134

Lipid Metabolism in Plants

units to CIK:I chains results in c 20: 1' c 22:1' and c 24: 1 monounsaturated chains stored as the acyl moieties of seed ester and triacylglycerols and would explain the origin of the up to C 37 alkenes in waxesY

2.

~Diketo

Chains

To ascertain the origin of the ~-diketone carbon skeletons, acetate was fed to barley spikes and hentriacontan-14, 16-dione isolated. The cl 4 and cl 6 fatty . _acids derived from the two ends of the molecule by hydrolysis were subjected to a.-oxidation and the specific activities of the individual carbons determined by radio-GC. The results implied elongation proceeding from C 31 to C 1.4K When exposure to arsenite preceeded the acetate, label was found solely in the C 1_14 end, indicating that only endogenous precursors of 18 or more carbons could be elongated 49 in accord with the known inhibitory effect of arsenite on C 16 elongase. Stearic acid did not function as a ~-diketone precursor. 49 Feeding pentadecanoic acid resulted in a novel C 30 ~-diketone with the label in the C 16- 30 end. 50 These results confirmed the proposaJ5 1 that the two oxygens were built into the carbon chain during elongation (Figure 2, right). In other words, ~-diketones can be classified as polyketides. The steps at which the introduction of the oxygen occurs is quite variable considering the known range of ~-diketones. For example, synthesis of hentriacontan-8, 10-dione in Buxus sempervirens52 would require retention of the 3-oxo group in either a CIO or c24 acyl chain depending on the direction of elongation vs. a ell! acyl chain in barley. The condensation elongation system responsible for synthesis of the ~-diketone carbon skeletons was named a ~-ketoacyl elongase after the 3-oxoacyl primers it uses to distinguish it from the elongases using acyl primers in constructing the other wax and polymer acyl chains (Figure 2).

B. DISSECTING THE ELONGATION CONDENSATION PATHWAYS WITH MUTANTS AND INHIBITORS The term elongation system as used herein encompasses not only FAS, elongase, and polyketide synthase complexes, but also complementary factors such as substrate availability and compartmentalization, while excluding the associated enzyme systems receiving the acyl chains. Mutations and inhibitors (photoperiodic and chemical) are unlikely to alter the composition of the epicuticular waxes and acyl chains of seed esters and triacylglycerols plus cutin and suberin monomers in a manner allowing easy recognition of the primaryinduced change. Nevertheless, analyses of differential effects on waxes and seeds often in conjunction with radiotracer experiments have uncovered a surprising number of parallel and sequential elongation systems.

1. Sequential Systems One of the earliest indications for the existence of sequential systems came from pea experiments in which the photoperiod was varied. 53 The results together with those from a later study in barley 16 revealed that in epicuticular waxes on light-grown seedling leaves one chain length or group thereof predominated, whereas on dark-grown leaves an additional shorter prominent

Waxes, Cutin, and Suberin

135

chain length or group thereof was also present. An analogous differential in the ability of the yellow vs. green segments of developing maize leaves to incorporate acetate into waxes also results in a uni- vs. bimodal distribution of homo logs. 5455 Mutations in genes whose products function in elongation can potentially be recognized by an increase in the amount of shorter homo logs. Table 1 lists those meeting this criterium. The four maize genes and five Bras sica mutations each distinguish a minimum of three systems; in the Brassica genus, for example, elongation up to C 26 , from C 26-C 28 , and from C2x-C 30 . Interestingly, of the nine gl genes which have been mapped in maize (Zea mays) and their effect on the wax composition determined, the four involved in elongation are located in chromosome 2 (gl 2 and gl 11 ) and chromosome 4 (gl3 and gl4 ) , while the others are scattered among five different chromosomes. 56 Isolation and characterization of the four pertinent genes by chromosome tagging is feasible and would reveal whether any of them represent duplicated genes. 57 The wa gene in peas also acts in the terminal step of elongation. 58 Mutations in the FAEJ gene in Arabidopsis thaliana inhibit three elongation systems, namely c I!I:O-C20:0• c Iil: I-C20:I' and c 20:I-C22: )• 59 The gene has been mapped on chromosome 4 with respect to restriction fragment length polymorphism (RFLP) markers, and walking is in progress60 ·61 with the goal of isolating it. Determining if FAEJ or any of the four maize gl genes codes for an elongase component should be possible by comparison to the relevant gene sequences becoming available for FAS components. 62-65 Of the numerous reports describing inhibitory effects of selected chemicals on wax synthesis, relatively few present detailed analyses of the wax composition. Trichloroacetic acid and various thiocarbamates were not only among the earliest studied (see, for example, References 44 and 66), but are also those which have received most attention. While the results are most readily interpreted as an inhibition of elongation starting with the terminal step and progressing toward shorter chain lengths, a closer examination reveals inconsistencies (References 54 and 67 and references therein). The difficulty in interpreting the data reflects the facts that the lipid classes occur in markedly different proportions and that the degree of elongation differs from one class to the other. 68 Thus, neither trichloroacetic acid nor the various thiocarbamates have been useful in discriminating among elongation systems. By comparison, arsenite, cyanide, and 2-mercaptoethanol identified a minimum of four sequential systems contributing to elongation of C 18-C 32 chains present in barley waxes69 (Table 1). Extrapolation from the known effect of arsenite 70 infers that the condensing moiety (a ~-ketoacyl-CoA synthase) of the C 20-C 22 system is more closely related to that of C 16 elongase than to those of FAS or C 18 elongase. The effects of arsenite and cerulenin are reciprocal with respect to the plastid acyl carrier protein (ACP) condensing enzymes ~ketoacyl -ACP synthase I and II (KAS I, II) (Table 1). Given the differences in sensitivity to arsenite of the CoA elongases, it would be of interest to carry out parallel experiments with cerulenin. Should one of the acyl-CoA elongases be very sensitive,

I-'

TABLE 1 Sites of Blocks in Elongation Induced by Gene Mutations and Chemical Inhibitors

~

0\

Mutations Blocked in addition to carbon8

Chemicatse

Epicuticular waxes

Zea mays

Pisum sativum

BrassicasC Hordeum vulgare

Seeds Arabidopsisd

Arsenite

Mercaptoethanol

Cyanide

4-14

~

KASI

cer-q cer-q cer-q

12 14 16

KASI

-

~

KAS II 18, 18: 1 20, 20: 1 22, 22: I

fael fael gill

26 28

30 32

*

g/J K g/ 1 BS g/2 BS g/3 BS g/5 c

g/J

g/2, g/4

wa

*"

*

*

~

KAS II ~

~

t""'

"6'

~

~ ~

*

Unless specified, applies to saturated acyl chain. Longest significant chain in vivo. c B. o/eracea; K kale; BS - brussel sprouts; C - cauliflower. d A. thaliana, impaired in elongation of 18:0, 18: I, and 20: I. ~ ._..-somewhat sensitive; ~-very sensitive; KAS I and II - ~-ketoacyl-ACP synthase I and II, respectively. a

" *-

Cerulenin

~