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.