Cutin, a biopolyester composed of hydroxy and epoxy fatty acids, is the barrier between the aerial parts of higher plants and their environment. Suberin a polymer containing aromatics and polyesters, functions as a barrier in underground parts, wound surfaces, and a variety of internal organs. The composition and probable structure of these polymers are discussed. The biosynthesis of the hydroxy, epoxy, and dicarboxylic acids of the polyesters from the common cellular fatty acids is elucidated. An extracellular enzyme transfers the hydroxy and epoxyacyl moieties from their coenzyme A derivatives to the growing polyester. The enzymes acting in the biodegradation of the polyesters have been isolated from fungi, pollen, and mammals and characterized. The function and possible practical implications of these polyester barriers are briefly discussed.

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Cutin, the structural component of plant cuticle, is a biopolyester composed of hydroxy- and hydroxyepoxy-fatty acids. The major monomers are a 16-hydroxy C16 acid, a 10,16-dihydroxy C16 acid together with its positional isomers, 18-hydroxy C18 acids, 18-hydroxy-9,10-epoxy C18 acids, and 9,10,18-trihydroxy C18 acids. The hydroxylation, epoxidation, and epoxide hydration reactions postulated to be involved in the biosynthesis of these monomers have been demonstrated in tissue slices and in cell-free preparations. The synthesis of the polymer occurs by the enzymatic transfer of the hydroxyacyl groups from CoA to the free hydroxyl groups in cutin primer. Natural and wound periderms and a variety of internal barrier layers contain a somewhat analogous polymer called suberin. This polymer is probably composed of aromatic domains somewhat similar to those found in lignin and aliphatic polyester domains somewhat similar to cutin. The chemical composition and biosynthesis of this polymer is discussed. Pathogenic fungi use a hydrolytic enzyme, cutinase, to gain entry into the plant through the cuticle. The fungal cutinase has been isolated from a variety of pathogenic fungi and characterized. This enzyme is a "serine hydrolase" containing the characteristic catalytic triad. The primary structure of this enzyme has been determined using both amino acid and nucleotide sequencing of the cloned copy DNA. Inhibition of cutinase was shown to prevent fungal infection of plants. This novel approach to fungal control is described.

Potato tuber skin (suberin), isolated enzymatically, was depolymerized with BF3‐CH3OH, and the structure and composition of the aliphatic monomers were determined by combined gas chromatography‐mass spectrometry. 18‐Hydroxyoctadec‐9‐enoic acid and octadec‐9‐ene‐1,18‐dioic acid were the major components. Products of epoxidation and subsequent hydration of the Δ9 double bond of these compounds, 10,16‐dihydroxy hexadecanoic acid, and much smaller quantities of 9,16‐dihydroxyhexadecanoic acid and 8,16‐dihydroxyhexadecanoic acid also were present. The other significant feature of the monomer composition of potato skin was that it contained substantial quantities of C20−C28 fatty acids, fatty alcohols, and ω‐hydroxy acids. Based upon these studies, a method of distinguishing between suberin and cutin and a biosynthetic pathway for suberin monomers are suggested.

l h e surface wax of the host, avocado (Persea americana) fruit, induced germination and appressorium formation in the spores of Colletotrichum gloeosporioides. Waxes from nonhost plants did not induce appressorium formation in this fungus, and avocado wax did not induce appressorium formation in most Colletotrichum species that infect other hosts. Bioassays of the thin-layer chromatographic fractions of the avocado wax showed that the fatty alcohol fraction was the main appressorium-inducing component. Testing of authentic n-C, to n-Cs2 fatty alcohols revealed that C,, and longer-chain alcohols induced appressorium formation. Casliquid chromatographylmass spectrometry analysis of free fatty alcohols revealed that avocado wax contains a high content of very long chains. Waxes from nonhost plants containing an even higher content of the very long-chain alcohols did not induce appressorium formation. Waxes from nonhost plants strongly inhibited appressorium induction by avocado wax. Thus, a favorable balance between appressorium-inducing very iong-chain fatty alcohols and the absence of inhibitors allows the fungus to use the host surface wax to trigger germination and differentiation of infection structures in the pathogen.