Suberin-associated waxes

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. 

Hydrocarbons n-alkanes CH3(CH2) CH3 C21 to C35 Most common wax component. High proportion of even chain length in suberin-associated waxes... 

Secondary alcohols RiCH(OH)R2 Cg to C33 Hydroxyl usually near the middle but toward the end also found 2-ols found in suberin-associated waxes... 

Phenolic esters Ferulic acid esters of primary alcohols Cjg to C28 Commonly found in suberin-associated wax... 

Waxes are usually isolated by extracting the tissue with a nonpolar solvent such as chloroform or hexane. Cuticular waxes can be extracted by a quick dip into the solvent at room temperature, but suberin-associated waxes are more difficult to remove because they are embedded in the suberized cell wall (232). Wax from the suberized cells of bark has to be isolated by Soxhlet extraction of dried and powdered tissue to assure its complete removal. Isolated wax can be subjected to GC/MS analysis either after separation into various classes by thin-layer or column chromatography or directly after derivitization of the functional groups (232, 253, 459, 460). 

Suberin-associated waxes have not been studied in great detail. Since it is impossible to isolate pure suberin, it is difficult to isolate waxes associated with the polymer without contamination from other cellular lipids. This problem is especially relevant in relation to the suberin-associated waxes in the bark because large... 

Wax esters usually constitute a minor component in suberin-associated waxes. In the periderms of underground storage organs they comprised 1% to 7% of the total wax (116), and there were no detectable wax esters in the suberin-associated wax from the periderm surrounding the crystal idioblasts of Agave americana (117). In many cases where wax esters have been reported to be components of bark waxes, they comprised an unknown or low percentage of wax — e.g., Pinus monticola (7%) (82), Pinus contorta (9%) (389) and Pinus banksiana (7%) (387). 

Free fatty alcohols are perhaps the most common components of cuticular waxes and often are also a major constituent of suberin-associated waxes (232). ThQr comprised between 10% and 45% of the waxes from the periderm of underground storage organs (116). The dominant chain lengths of free fatty alcohols of cuticular waxes are C26 and C28 (232, 253, 292, 460), but the free fatty alcohols of suberin-associated waxes usually have a shorter chain length. For example, the dominant alcohols in the leaf cuticular wax of Agave americana were (36% of the alcohols) and C28 (62%), whereas the dominant component in the fatty alcohols of the suberin-associated wax from the periderm layer surrounding the crystal idioblasts within the same leaf was C22 (88%) (117). The most common fatty alcohols reported as components of bark wax are C24 and C22. 

Free fatty acids are major components of many suberin-associated waxes, whereas they are usually minor components of cuticular waxes (245). For example, free fatty acids were the major class of components in the suberin-associated wax from the storage organs of Daucus carota (50% of the wax) and Brassica napobrassica (49%) (116, 219), and the bark waxes of Pinus contorta (43%) (389) and Ailan-thus glandulosa (59%) (76). The major fatty acids in many bark waxes are C20, C22, C24 or a combination of these acids. For example, C22 and C24 were the two dominant components (23% and 34%, respectively) of the free fatty acid fraction that comprised 34% of the wax from the bark of Picea abies (484). In many ana-... 

The time course of deposition of aromatic monomers into the phenolic portion of suberin in wound-healing slices of Solanum tuberosum tubers was determined using the amount of / -hydroxybenzaldehyde and vanillin generated by alkaline nitrobenzene oxidation as a measure of aromatic deposition (85). The deposition of such phenolics into the polymer exhibited a lag period of about three days after wounding followed by several days when the deposition of phenolics increased rapidly, and subsequently the process ceased. Exogenous L-[U- C]-phenylalanine and [U- Clcinnamic acid were incorporated into the insoluble polymeric material by wound-healing slices of S. tuberosum (85). Nitrobenzene oxidation of the polymeric material derived from labeled cinnamic acid released labeled /7-hydroxybenzaldehyde and vanillin. The time course of incorporation of phenolics into the suberin polymer correlated with the time course for the deposition of aliphatic monomers into the polymer, the deposition of suberin-associated waxes into the periderm layer and the development of diffusion resistance (Fig. 6.4.12) (86). 

Espelie K E, Sadek N Z, Kolattukudy P E 1980 Composition of suberin-associated waxes from the subterranean storage organs of seven plants parsnip, carrot, rutabaga, turnip, red beet, sweet potato and potato. Planta 148 468-476... 

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