Suberin structure

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. 

It therefore follows that when isolated lignins (and suberins) are examined and subsequent structural representations are proposed, critical information on native structure has already been lost, e.g., as regards the extent of polymer modification during removal from the cell wall, and the effect of mixing polymers from the various cell wall layers from which they originated. For these reasons, all current representations of native lignin (and suberin) structure should be viewed with caution until such questions are satisfactorily resolved. 

In this chapter, we review our recent progress in establishing the exact bonding patterns of lignin in situ in intact plants. Presumably, similar strategies can be employed to study suberin structure. 

Graca J., Pereira H., Suberin structure in potato periderm Glycerol, long-chain monomers, rmd glyceryl and feruloyl dimers, J. Agric. Food Chem., 48(11), 2000, 5476-5483. 

Kolattukudy PE (1980) Cutin, suberin and waxes. In Stumpf PK (ed) The biochemistry of plants vol 4 - lipids structure and function. Academic Press, London, p 571... 

Kolattukudy PE (1981) Structure, biosynthesis and biodegradation of cutin and suberin. In Briggs WR (ed) Annual reviews of plant physiol, vol. 32. Annual Reviews, Palo Alto CA, p 539... 

The functions of phenylpropanoid derivatives are as diverse as their structural variations. Phenylpropanoids serve as phytoalexins, UV protectants, insect repellents, flower pigments, and signal molecules for plant-microbe interactions. They also function as polymeric constituents of support and surface structures such as lignins and suberins [1]. Therefore, biosynthesis of phenylpropanoids has received much interest in relation to these functions. In addition, the biosynthesis of these compounds has been intensively studied because they are often chiral, and naturally occurring samples of these compounds are usually optically active. Elucidation of these enantioselective mechanisms may contribute to the development of novel biomimetic systems for enantioselective organic synthesis. 

Suberin is a composite of polymeric phenylpropanoids and ester-linked long chain fatty acids and alcohols and consists of a hydrophobic layer attached to the cell walls of roots, bark and the vascular system (8,10). The phenylpropanoid portion of suberin purportedly has a lignin-like structure to which both aliphatic domains and hydroxycinnamic acids are esterified. 

It should now be self-evident that substantial progress has been made in developing methodology to probe lignin structure in situ. Obviously similar strategies for suberin also could be developed. The following points can now be made ... 

High-resolution 13C NMR studies have been conducted on intact cuticles from limes, suberized cell walls from potatoes, and insoluble residues that remain after chemical depolymerization treatments of these materials. Identification and quantitation of the major functional moieties in cutin and suberin have been accomplished with cross-polarization magic-angle spinning as well as direct polarization methods. Evidence for polyester crosslinks and details of the interactions among polyester, wax, and cell-wall components have come from a variety of spin-relaxation measurements. Structural models for these protective plant biopolymers have been evaluated in light of the NMR results. 

Solid-state 13C NMR was employed to characterize intact samples of cutin and suberin biopolyesters. Although a considerable degree of structural heterogeneity was observed for both materials, it was possible nonetheless to resolve and assign many NMR peaks, even when the polyesters were accompanied by waxes or cell walls. Quantitative estimates for the various aliphatic, aromatic, and carbonyl carbon types indicated that cutin was primarily aliphatic in composition, whereas suberin had more aromatic and olefinic moieties. Additional analysis should be facilitated by the biosynthetic incorporation of selectively 13C-enriched precursors (26,27). 

For the study of complex cuticular mixtures, measurements of cross-polarization dynamics proved to be especially informative. The equality of Ti/>(H) values in cutin-wax assemblies demonstrated that these cuticular materials were mixed intimately. By contrast, Ti >(H) measurements showed that the polymeric components of suberized cell walls were present in distinct domains, suggesting that suberin was attached at a few structural sites rather than being embedded in the polysaccharide wall. 

Preliminary structural studies of cutin and suberin breakdown involved examination of 13C NMR spectra for insoluble residues that were resistant to chemical depolymerization. In cutin samples, flexible CH2 moieties in particular were removed by such treatments, but CHOCOR crosslinks and polysaccharide impurities were retained preferentially. A concomitant narrowing of NMR spectral lines suggested that the treatments produced more homogeneous polyester structures in both cases. Our current studies of cu-ticular breakdown also employ selective depolymerization strategies with appropriate enzymes (1,28). 

These results demonstrated the usefulness of 13C NMR in studies of molecular structure and dynamics for the polymeric constituents of plant cuticle. Although these materials are insoluble and sometimes present as interpenetrating phases, CPMAS and spin relaxation techniques helped identify important carbon types and provided structural clues to the protective functions of cutin and suberin in terrestrial plants. 

DC046 Kolattukudy, P. E., K. Kronman, and A. ]. Poulose. Determination of structure and composition of suberin from the roots of carrot, parsnip, rutabaga, turnip, red beet and sweet potato by combined gas-liquid chromatography and mass spectrometry. Plant Physiol 1975 55 567. 

This allows branching of the polymer. Monomers of other chain lengths as well as aromatic components related to lignin are also present and polymerized into a high molecular mass branched structure. Suberin is a more complex ligninlike polymer with a high content of phenolic constituents135 such as vanillin (Fig. 25-8). 

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