The monomer composition of the esterified part of suberin can be determined using gas chromatography-mass spectroscopy technology and is accordingly believed to be well known. However, evidence was presented recently indicating that the suberin of green cotton (Gossypium hirsutum cv Green Lint) fibers contains substantial amounts of esterified glycerol. This observation is confirmed in the present report by a sodium dodecyl sulfate extraction of membrane lipids and by a developmental study, demonstrating the correlated accumulation of glycerol and established suberin monomers. Corresponding amounts of glycerol also occur in the suberin of the periderm of cotton stems and potato (Solanum tuberosum) tubers. A periderm preparation of wound-healing potato tuber storage parenchyma was further purified by different treatments. As the purification proceeded, the concentration of glycerol increased at about the same rate as that of ␣, -alkanedioic acids, the most diagnostic suberin monomers. Therefore, it is proposed that glycerol is a monomer of suberins in general and can cross-link aliphatic and aromatic suberin domains, corresponding to the electrontranslucent and electron-opaque suberin lamellae, respectively. This proposal is consistent with the reported dimensions of the electron-translucent suberin lamellae.Suberin and cutin are insoluble, lipophilic biopolymers. Together with complex mixtures of soluble lipids, they form the protective layers of higher plants: the cuticle of the epidermis and the suberin layers of the periderm and the exodermis. These cell layers are diffusion barriers for water and other

The fibres of the green-lint mutant (Lg) of cotton (Gossypium hirsutum L.) are suberized and contain a large proportion of wax. The unidentified components of the wax were separated into a colourless fluorescent fraction and a yellow pigmented fraction. Using ultraviolet spectroscopy and nuclear-magneticresonance ((1)H-NMR) spectroscopy, esterified trans-caffeic acid was identified as the only phenolic component in the colourless fraction. This fraction was further purified and was shown to contain caffeic acid esterified to fatty acids (mainly ω-hydroxy fatty acids), and glycerol in molar ratios of 4∶5∶5. When 2-aminoindan-2-phosphonic acid (AIP), an inhibitor of phenylalanine ammonia-lyase (EC 4. 3. 1. 5.) was added to ovules cultured in vitro, at the beginning of secondary wall formation, the fibres remained white and the colourless caffeic-acid derivative and the yellow compounds could no longer be detected by ultraviolet spectroscopy. Fibres grown in the presence of AIP were also examined in the electron microscope. Secondary cell walls were present in the treated fibres, but the electron-opaque suberin layers were replaced by apparently empty spaces. This result indicates that cinnamic-acid derivatives are covalently linked to suberin and have a structural role within the polymer or are involved in anchoring the polymer to the cellulosic secondary wall. Purified cell walls of green cotton fibres contained about 1% (of the dry weight) of bound glycerol, 0.9% of the glycerol being extractable with the wax fraction and 0.1% remaining in the cell-wall residue. The corresponding values for white fibres were 0.03% (total), 0.02% (wax), and 0.01% (cell-wall residue). Fibres synthesizing their secondary walls in the presence of AIP contained about normal amounts of bound glycerol in the wax fraction, but glycerol accumulation in the cell-wall residue was inhibited by about 95%. These observations indicate that glycerol is an important constituent of cotton-fibre suberin. Considerable amounts of bound glycerol could also be determined in exhaustively extracted cell walls of the cork layer of potato periderm (1.2%) and smaller amounts in the outer epidermal cell wall of Agave americana L. leaf (0.1%) indicating that the presence of glycerol in suberins and possibly also in cutins may be more widespread than previously thought.

The fibers of the green lint mutant of cotton (Cossypiom hirsutom L.) contain large amounts of wax and are suberized. More than 96% of the bifunctional aliphatic suberin monomers (qw-alkanedioic acids and w-hydroxyalkanoic acids) have chain lengths of C,, and C,, in green cotton fiber suberin. In fibers grown in the presence of S-ethyl-N,N-dipropylthiocarbamate (EPTC), a specific inhibitor of the endoplasmic reticulum-associated fatty acid elongases, the aliphatic suberin monomers were shortened to chain lengths of C,, and C,8. Whereas the amounts of most suberin monomers were not negatively affected by the inhibitor treatment, the amounts of a,walkanedioic acids and of glycerol were reduced by more than 80%. Analysis in the transmission electron microscope showed a reduction in suberin content after EPTC treatment. The suberin layers were discontinuous and consisted of fewer lamellae than in the controls. A small proportion (up to 22%) of the electron-translucent suberin lamellae were thinner after EPTC treatment, probably because of the shortening of the aliphatic suberin monomers. A larger proportion of the electron-translucent lamellae were thicker than the lamellae in the controls. Possible explanations for this observation are discussed.Color mutants of cotton (Gossypium hirsutum L.) fibers have been known for a long time, and their genetics are well established (Endrizzi et al., 1985). In the last few years, colored cotton fibers have also been used to a small extent for the production of textiles. The fibers of the Lg mutant contain high amounts of wax (Conrad, 1941), and their secondary walls are suberized (Ryser et al., 1983; Yatsu et al., 1983; Ryser and Holloway, 1985). The suberin is deposited in severa1 concentric cell-wall layers alternating with cellulose. The cell walls of cotton fibers and other seed coat epidermal cells are, in addition to suberin, covered by a thin cuticle (Yatsu et al., 1983; Ryser, 1985). White cotton fibers may be suberized to varying degrees at the fiber base (Ryser, 1992). The epidermal cells of the seed coat not forming fibers are suberized in a11 cotton species examined so far (Ryser and Holloway, 1985), and suberin and associated wax probably function as constitutive defensive layers against soil-borne pathogens (Schmutz et al., 1994a(Schmutz et al., , 1994b. 403Suberin is an important but poorly understood biopolymer. Despite the well-known suberin model of Kolattukudy (1980), the monomer composition of suberin and interunit-bonding patterns remain essentially unknown (Davin and Lewis, 1992). In particular, little evidence has been found for the postulated lignin-like, polyaromatic suberin domain (Zimmermann et al., 1985; Monties, 1989,1993). Bernards et al. (1995) have shown that the polyaromatic domain of suberin is composed mainly of hydroxycinnamic acids rather than of the corresponding alcohols as in lignins. Recently, it was shown that glycerol could be liberated from the cell walls of exhaustively extracted green cotton fibers by treatments cleaving ester ...

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