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 has lead 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 biosynthesis, that is, they do not also participate in 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 waxes localised in and on the cuticle surface which protect against water loss. Waxes include a very diverse collection of aliphatic compounds. 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 (pkKCSs) 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. Some of the enzymes may also participate in synthesis of related aliphatics found in cutin, suberin and sphingolipids, for example. Waxes may also include other compounds with long carbon skeletons such as phenolic lipids that function in defence against bacteria and fungi.