Suberin

It may be that the presence of this isoform promotes the strengthening of calli cell walls through a special mechanism, since cultured cells have mainly undifferentiated cell walls missing lignification and suberin deposition, similar to meristematic cells in plants. 

The above outline of the evolution of the plant kingdom during coal-forming eras has been presented largely in the language of botanical anatomy. However, the alert chemist will note that the anatomical differences imply considerable quantitative and spatial differences in the distribution of the principal plant constituents [cellulose, lignin, cutin, suberin and other waxes, contents of protoplasmic cells, pigments, resins, sporopollenin. 

Certain compositional differences between coals of differing origins can be inferred from available data. Differing anatomical distributions of cellulose, lignin and suberin, with implications for the origins of vitrinites, and differing distribution of phenolic substances in plants of different orders and families, have been referred to above. Some biochemical investigations of modern representatives of ancient plants have been made (e.g., refs. 14,... 

Suberin, being an adcrustation on the cell wall, cannot be separated from cell walls. Instead, suberin-enriched wall preparations can be obtained by digesting away as much carbohydrate polymers as possible using pectinases and cellu-lases [3,7]. Depending on the source of the suberized cell wall preparation, the polyester part may constitute a few percent to 30% of the total mass. 

Table 2. Compositional difference between cutin and suberin ...

Depolymerization techniques that cleave ester bonds release the indicated aliphatic monomers and phenolic components from suberin. 

Treatment of suberin with nitrobenzene generates vanillin, p-hydroxy benz-aldehyde, but not much syringaldehyde that arises mostly from lignin. 

Suberized cell walls stain positively for phenolics with indications that suberin contains monohydroxyphenolic rings and has fewer O-methoxy groups than lignin. 

The inability to solubilize aromatic components of suberin-enriched preparations by the methods used for lignin suggests that suberin structure is distinctly different from that of lignin, probably due to the aliphatic cross-linking and the higher degree of condensation present in suberin. 

Phenolic acids and aliphatic acids are both involved in the biosynthesis of suberin, and phenolic acids are not synthesized in tissue slices that do not undergo suberization. 

Inhibition of synthesis of the aromatic matrix by inhibitors of phenylalanine ammonia lyase causes the inhibition of deposition of aliphatic components and prevents development of diffusion resistance. Inhibition of synthesis of peroxidase, the enzyme involved in the deposition of the polymeric phenolic matrix, caused by iron deficiency, prevents deposition of aliphatic components of suberin. 

The time-course of deposition of aromatic monomers into the polymer laid down by suberizing tissue slices indicates that the phenolic matrix is deposited simultaneously with or slightly before the aliphatic components. The specific anionic peroxidase appeared with a time-course consistent with its involvement in the polymerization and deposition of the phenolic matrix of the suberin. Increase or decrease in suberin content involves similar changes in both the aliphatic and aromatic components and such changes are associated with the expected increase or decrease in the anionic peroxidase activity caused by physical or biological stress. 

Removal of the aliphatic materials by hydrogenolysis leaves a residue that contains low amounts of polymethylenic components, suggesting that the suberized material contains some aliphatic components not susceptible to cleavage by such methods [3]. On the other hand, removal of suberin from cork cell wall preparations was examined by CPMAS and the results showed that the aliphatic components were nearly completely removed from this suberin preparation as the spectra showed that the residual material was virtually devoid of methyl... 

In suberizing potato tuber disks, labeled oleic acid was incorporated into co-hy-droxyoleic acid and the corresponding dicarboxylic acid, the two major aliphatic components of potato suberin [73]. Exogenous labeled acetate was also incorporated into all of the aliphatic components of suberin, including the very long chain acids and alcohols in the wound-healing potato slices. The time-course of incorporation of the labeled precursors into the suberin components was consistent with the time-course of suberization. The biosynthetic pathway for the major aliphatic components of suberin is shown in Fig. 8a. 

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... 

Fig. 8a, b. a Biosynthetic pathways for the major aliphatic components of suberin. b Representation of the active site of co-hydroxy acid dehydrogenase involved in the synthesis of the dicarboxylic acids characteristic of suberin. From [74]... 

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. 

When polyester-hydrolyzing activity was isolated using synthetic polyesters such as polycaprolactone, and the enzyme was examined in detail, it was found that it was a cutinase that was responsible for the hydrolysis [113]. Similarly, the polyester domains of suberin were found to be degraded by cutinase. Cutinase is a polyesterase, and similar enzymes may be widely distributed and can degrade a variety of natural and synthetic polyesters. Microbial polyhydroxy-alkanoic acids that are attracting increasing attention as biodegradable polyesters can be hydrolyzed by bacterial polyesterases that share some common features with cutinases [114] and this area is covered in another chapter [115]. 

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