Cutin acids

O C o fatty acids AWAWC hexadecanoic acid (cutin) hexacosanoic acid (wax)... 

Cutin-insaure, /. cutinic acid. -sMure, /. cutic acid. 

Plants were probably the first to have polyester outerwear, as the aerial parts of higher plants are covered with a cuticle whose structural component is a polyester called cutin. Even plants that live under water in the oceans, such as Zoestra marina, are covered with cutin. This lipid-derived polyester covering is unique to plants, as animals use carbohydrate or protein polymers as their outer covering. Cutin, the insoluble cuticular polymer of plants, is composed of inter-esterified hydroxy and hydroxy epoxy fatty acids derived from the common cellular fatty acids and is attached to the outer epidermal layer of cells by a pectinaceous layer (Fig. 1). The insoluble polymer is embedded in a complex mixture of soluble lipids collectively called waxes [1], Electron microscopic examination of the cuticle usually shows an amorphous appearance but in some plants the cuticle has a lamellar appearance (Fig. 2). 

The most common major components of cutin are derivatives of saturated C16 (palmitic) acid and unsaturated C18 acids (Fig. 4). The major component of the C16 family of acids is 9- or 10,16-dihydroxyhexadecanoic acid (and some mid-chain positional isomers), with less 16-hydroxyhexadecanoic acid and much smaller amounts of hexadecanoic acid. In some cases other derivatives are significant constituents. For example, in citrus cutin 16-hydroxy-10-oxo-C16 acid, and in young Vicia faba leaves 16-oxo-9 or 10-hydroxy C16 acid are significant... 

Table 1. Fatty acids with one or more additional functional groups that have been reported as components of cutin or suberin3. Adapted from [16]...

The composition of cutin shows species specificity although cutin from most plants contains different types of mixtures of the C16 and C18 family of acids. Composition of cutin can vary with the anatomical location. For example, cutin preparations from fruit, leaf, stigma, and flower petal of Malus pumila contain 73%, 35%, 14%, and 12%, respectively, of hydroxy and hydroxy-epoxy C18 monomers [23]. In general, fast-growing plant organs have higher content of C16 family of monomers. 

Leaf discs from rapidly expanding V.faba leaves incorporated 14C-labeled palmitic acid into cutin. After removal of the soluble lipids and other materials, the insoluble residue was subjected to LiAlH4 hydrogenolysis and the labeled reduction products of cutin monomers were identified by chromatography as hexadecane-... 

Based on the composition of the C18 family of cutin monomers we postulated that oleic acid would be > hydroxy la ted first, followed by epoxidation of the double bond at C-9 followed by the hydrolytic cleavage of the oxirane to yield 9,10,18-trihydroxy acid. This postulate was experimentally verified by the demonstration of specific incorporation of exogenous 18-hydroxyoleic acid into 18-hydroxy-9,10-epoxy C18 acid in grape berry skin slices and apple fruit skin disks, and incorporation of exogenous labeled 18-hydroxy-9,10-epoxy C18 acid into 9,10,18-trihydroxy C18 acid of cutin in apple fruit skin slices [61]. 

The biosynthetic origin of the depolymerization-resistant core of cutin (cutan) remains to be established. The early observation that linoleic acid and linolenic acid were preferentially incorporated into the non-depolymerizable core of cutin in apple skin slices suggested that the ether-linked or C-C-linked core might arise preferentially from the czs-l,4-pentadiene system [31]. The insoluble residue, that contained the label from the incorporated polyunsaturated C18 acids, released the label upon treatment with HI, supporting the notion that some of those aliphatic chains were held together by ether bonds. More recently,... 

The unique suberin components that are not found as significant components of cutin are the very long chain molecules and the dicarboxylic acids. Therefore, chain elongation and conversion of co-hydroxy acids to the corresponding dicarboxylic acids constitute two unique biochemical processes involved in the synthesis of suberin. Incorporation of labeled acetate into the very long chain components of suberin was demonstrated and this ability developed during suberization in potato tuber disks [73]. The enzymes involved... 

How the aliphatic monomers are incorporated into the suberin polymer is not known. Presumably, activated co-hydroxy acids and dicarboxylic acids are ester-ified to the hydroxyl groups as found in cutin biosynthesis. The long chain fatty alcohols might be incorporated into suberin via esterification with phenylpro-panoic acids such as ferulic acid, followed by peroxidase-catalyzed polymerization of the phenolic derivative. This suggestion is based on the finding that ferulic acid esters of very long chain fatty alcohols are frequently found in sub-erin-associated waxes. The recently cloned hydroxycinnamoyl-CoA tyramine N-(hydroxycinnamoyl) transferase [77] may produce a tyramide derivative of the phenolic compound that may then be incorporated into the polymer by a peroxidase. The glycerol triester composed of a fatty acid, caffeic acid and a>-hydroxy acid found in the suberin associated wax [40] may also be incorporated into the polymer by a peroxidase. 

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