Plant Waxes

Abstract

Waxes, found primarily in the cuticle of vascular plants, prevent uncontrolled water loss. They comprise a diverse mixture of aliphatics, triterpenoids, flavonoids and/or phenolic lipids, such as, alkylresorcinols. Aliphatic carbon skeletons are fatty acid synthase (FAS) products extended by fatty acid elongase (FAE) enzyme complexes and type III polyketide synthases (PKSs) to 20–34 carbons ± keto groups that serve as substrates for associated reductive, decarb and enoic pathways plus variants thereof. Study of eceriferum (cer) mutants, reverse genetic molecular approaches and biochemistry have led to increasingly detailed biosynthetic pathways in Arabidopsis and the Gramineae. Nevertheless, many enzymes remain unidentified. How many FAEs and type III PKSs are specific for wax, that is, not contributing to other pathways such as sphingolipid biosynthesis, is unknown. Our knowledge is rudimentary concerning regulation of biosynthesis or translocation of aliphatics during synthesis and thereafter from the endoplasmic reticulum into and onto the cuticle's aerial surface.

Key Concepts

  • Epidermal cells synthesise secondary metabolites called waxes localised in and on the cuticle surface, which protect against water loss.
  • Waxes include a very diverse collection of aliphatic compounds.
  • Waxes may also include other compounds with long carbon skeletons such as phenolic lipids, for example, alkylresorcinols, that function in defence against bacteria and fungi.
  • Primer substrates for waxes are synthesised by fatty acid synthase (FAS) in plastids.
  • FAS products are extended by fatty acid elongases (FAEs) and type III polyketide synthases (PKSs) to give skeletons with as many as 32 carbons.
  • Enzymes in associated pathways localised in the endoplasmic reticulum convert the long carbon skeletons into a broad range of compounds.
  • A handful of genes participating in biosynthesis of the waxes and their translocation within the epidermal cells have been cloned and characterised.
  • The Cer‐cqu gene cluster in barley encodes three enzymes in the β‐diketone synthase (DKS) polyketide pathway.
  • Some of the enzymes may also participate in the synthesis of related aliphatics found in cutin, suberin and sphingolipids, for example.

Keywords: cuticle; eceriferum (CER\Cer) genes; β‐diketone synthase (DKS); epidermal cells; β‐ketoacyl‐CoA synthases (KCS); type III polyketide synthases (pkPKS); fatty acid elongase (FAE); alkylresorcinol (AR); alkanes; β‐diketones; decarbonylation

Figure 1. Common phenotypes of epicuticular waxes in the Gramineae. (a) Leaf sheaths and leaves of field grown wild type Bonus and cer mutant barley. (b, c) A dense coating of small lobed plates on wild type leaves, which shed water, is characteristic for waxes with high concentrations of primary alcohols. (d, e) The small mounds of wax on leaves of cer‐j.59, blocked in synthesis of primary alcohols, retain water drops enabling germination of spores and leaching of nutrients. (f) A dense coating of long, hollow tubes (0.1–0.2 µm in diameter, up to 5 µm long) attributed to high concentrations of β‐diketones covers wild type leaf sheaths that, in addition to the above‐mentioned functions, can protect against frost damage (Barber and Jackson, ). (g) Only thin plates appressed to the surface occur on cer‐a.8 leaf sheaths lacking β‐diketones. (h) Two of the three types of wax structures that occur on leaf sheaths of sugar cane (Haas et al., ); massive compound rodlets attributed to aldehydes and the small‐lobed plates characteristic of primary alcohols (see b). Transmission electron microscopy of preshadowed carbon replicas (b, d and g). Scanning electron microscopy (f and h).
Figure 2. Phenotypes of waxes. (a) The epicuticular wax on Arabidopsis stems with major amounts of alkanes, secondary alcohols and ketones displays a variety of shapes. (b) A transverse section of Arabidopsis cer5 mutant stems with a nonfunctional ABCG12 protein reveals accumulation of cytoplasmic lipid inclusions (arrows) unable to enter the plasmalemma. (c) A preshadowed carbon replica of the wax secreted onto the cuticle surface of a leaf sheath of cer‐zw.286 barley, having reduced amounts of β‐diketones and shorter tubes, discloses formation of thin plates/films (arrows) before self‐assembly (Koch et al., ) into tubes characteristic of β‐diketones. Visualised by cryo‐scanning (a), transmission (b) and scanning electron microscopy (c). (a) and (b) Courtesy of Lacey Samuels, University of British Columbia, Canada.
Figure 3. Assembly of wax carbon skeletons is accomplished by several enzyme complexes: fatty acid synthase (FAS), fatty acid elongase [FAE; originally designated elongase (ELS)] and a type III polyketide synthase (PKS), the β‐ketoacyl‐coenzyme A (CoA) synthase (pkKCS). Plastid localised FAS (left) carries out a reiterated cycle of four reactions in which a joining enzyme first decarboxylates an activated (star) C3‐substrate to give the activated C2 donor (boxed) which is added to the initial acceptor, acetyl‐CoA. Otherwise, acyl carrier protein (ACP) serves as activator for FAS. The β‐keto (O) group of the C4 condensation product is reduced to a hydroxy (OH) group, then dehydrated and finally removed by another reduction yielding an elongated acyl chain serving as a primer for the next extension. Repeated cycles yield chains with 16 and 18 carbons. After removal of the ACP, transport to the endoplasmic reticulum and activation with CoA, the 16 and 18 acyl chains serve as primers for FAE complexes (left, light blue) that construct chains with 20–34 carbons activated with CoA. In A. thaliana CER2s (CER2, CER2‐like 1 and CER2‐like 2) are required for elongation beyond C28 (dark blue) via a deduced interaction with a KCS, e.g., CER6. Type I and II PKS complexes (centre), which use ACP activated acyl chains differ from FAEs in lacking one or more of the four enzymes participating in specific cycles so that the resulting carbon skeletons are decorated with Os, OHs and/or double bonds. pkKCSs (centre, red) lack both reductases and the dehydratase and therefore each elongation introduces an O into the growing chain. This is illustrated on the right where two pkKCS condensations yield a triketide intermediate with O groups on alternating carbons, that is, β to one another. During subsequent cycles in which all four enzymes participate (pink), the number of carbons between the initial two O groups and the most recently added one increases from three to y resulting in acyl chains with internal β‐diketo groups. The resulting pkKCS derived skeletons range from 10 to 34 carbons in length and are designated keto‐CoAs. In A. thaliana the FAE enzymes are ketoacyl‐CoA synthase (KCS), ketoacyl‐CoA reductase (KCR), hydroxyacyl‐CoA dehydratase (HCD) and enoyl‐CoA reductase (ECR) that are encoded, for example, by CER6, KCR1, PAS2 and CER10, respectively. R = one carbon in the first FAS cycle of elongation and increases by two in every successive round thereafter. Two systems for naming the individual carbons in an acyl chain starting from the carboxy carbon (α, β and 1, 2, 3) are included.
Figure 4. Origin of Arabidopsis wax aliphatics from the very long chain, acyl‐CoAs (C26–C30) constructed by fatty acid elongase (FAE) enzyme complexes (see Figure, blue). The decarb pathway (blue arrows) gives rise to aliphatic wax classes (blue) with odd carbon numbers (C29). The OH is on carbon 14 or 15, the O on carbon 15. The reductive pathway (green arrows) yields even carbon (C26–C30) primary alcohols (green). Fatty acid moieties of the esters are predominantly C16. The steps in which the CER1, CER3, CER4, CER8, MAH1 and WSD genes function or preliminary results infer function (enclosed in parentheses) are shown. MAH ? denotes apparent synthesis by MAH1 of ketones and many structurally related ketols and diols (see text). Green ? denotes that synthesis of primary alcohols potentially also occurs via an aldehyde intermediate (see text). In other plants the chain lengths of the FAE products entering the decarb and reductive pathways can differ; for example, in barley leaf waxes the former use primarily C32 and the latter primarily C26. Moreover, the chain length distributions of ester alcohols often match that of the primary alcohols.
Figure 5. The pkKCSs (red), diketone synthase (DKS) and alkylresorcinol synthase (ARS) play significant roles in constructing the carbon skeletons of aliphatics in many Gramineae and other epicuticular waxes. The polyketide pathway to the esterified alkan‐2‐ols (green, left side) and β‐diketone aliphatics (blue, right side) is initiated by CER‐Q, annotated as a lipase, which conceivably selects specific acyl‐CoAs from a glycerolipid (Schneider et al., ). R is 6 carbons for the C13‐2‐ol and 10 for the 14,16‐β‐diketone. Decarboxylation of a just elongated acyl‐CoA gives methylketones that are reduced to alkan‐2‐ols (primarily C13 and C15), which are esterified with fatty acids from an FAE complex (Figure) to give alkan‐2‐ol esters. Alternatively acyl‐CoAs selected by CER‐Q are substrates for the Cer‐c encoded DKS, which carries out two sequential reactions leading to internal β‐Os (see Figure for details). Subsequent loss of a carbon yields β‐diketones (primarily 14,16‐C31, dominating many waxes) into which an OH can be introduced (primarily on carbon 25) by the cytochrome P450 enzyme determined by Cer‐u, and then oxidised to give O derivatives (right side, blue). The Cer‐c, ‐q and ‐u genes belong to the Cer‐cqu gene cluster (Figure). ARS carries out three sequential extensions in the terminal elongation cycles giving tetraketide‐CoA intermediates (C20–C34) that are precursors of the minor aliphatic wax components, 5‐alkylresorcinols (pink, top). The alkyl side chain on 1,3‐dihydroxybenzene ranges from 13 to 27 carbons. Carbons are numbered according to IUPAC nomenclature rules. () enclose intermediates not present in the waxes.
Figure 6. Schematic diagram of the circa 181 kb BAC HVVMRXALLrA0066C06 (black) from barley chromosome arm 2HS encoding the Cer‐c, ‐q and ‐u genes (blue) spanning 101 kb plus a novel haemolysin III family gene (HlyIII, grey), which is not part of the Cer‐cqu gene cluster (Schneider et al., ). Distance between genes as well as to the BAC ends in kilobases are shown above pink arrows. Approximately 122 kb of the BAC sequence consists of transposable elements. The structure of the Cer‐cqu gene cluster is analogous to that of other recently described clusters in plant secondary metabolic pathways all involved in (a)biotic defence (Boycheva et al., ).
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von Wettstein‐Knowles, Penny(Jun 2016) Plant Waxes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001919.pub3]