Biochemistry and function of cutin and suberin

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Abstract
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.

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