Suberins walls

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. 

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. 

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

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... 

Kolattukudy PE, Espelie KE (1989) Chemistry, biochemistry and function of suberin and associated waxes. In Rowe J (ed) Natural products of woody plants, chemicals extraneous to the lignocellulosic cell wall. Springer, Berlin Heidelberg New York, p 304... 

The conquest of the land by plants necessitated the development of a coating, the cuticle, that would reduce water loss. Suberin and cutin vary in their proportion of fatty acids, fatty alcohols, hydroxyfatty acids, and dicarboxylic acids. The cuticle is synthesized and excreted by the epidermis of aerial portions of the plant, such as the primary stems, leaves, flower organs, and fruits. The two major hydrophobic layers that contribute to the cuticle are composed of phenolic molecules combined with lipid polymers. Cutin is a polymer found in the outer cell wall of the epidermis, which is... 

Cork cells Tabular with all walls suberized occur in thick layers on the outer surfaces of older stems and roots Secrete a fatty substance, suberin, into the walls, suberin renders cork cells waterproof and helps protect the tissues beneath... 

Many cell walls have layers in the outer regions of the wall that carry lipid material. These are cutin, suberin, and waxes (67). How these are transported to the outside of the cell wall is not known. Pores have not been found, nor has a volatile lipid solvent been detected that would carry the lipid through the hydrophilic wall. 

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

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. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

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. 

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. 

Figure 1. The location of plant polyesters cutin attached to the epidermal wall (top) and suberin within the cell wall of a plant periderm (bottom). Reproduced by permission of the National Research Council of Canada, from Ref. 1.

Suberized Cell Walls. An analogous set of CPMAS experiments is presented for suberin in Figure 6. Because this polymer is an integral part of the plant cell wall, the 13C NMR spectrum had contributions from both polysaccharide and polyester components. Chemical-shift assignments, summarized in Table IV, demonstrated the feasibility of identifying major polyester and sugar moieties despite serious spectral overlap. Semiquantitative estimates for the various carbon types indicated that, as compared with cutin, the suberin polyester had dramatically fewer aliphatic and more aromatic residues. A similar observation was made previously for the soluble depolymerization products of these plant polymers (1,8,11). 

Figure 6. 31.94 MHz 13C NMR spectra for suberized cell walls from potatoes, before (bottom) and after (top) depolymerization treatment. The experimental parameters were as in Figure 4. Chemical-shift assignments and relative numbers of carbons for the untreated material are found in Table IV. Delayed-decoupling experiments left some (CH2) signal intensity in the spectrum of intact suberin, but the analogous signals were drastically attenuated in the NMR spectrum of the depolymerization residue.

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. 

In certain instances, however, factors other than the cell wall polymers of the phellem may be important in the protection provided by the secondary surface. Rosellinia desmazieresii inoculated in a food base onto the underground stems of a resistant Salix repens hybrid (5. x Friesiana) exhibited greatly reduced epiphytic growth and cord formation compared with inoculations onto susceptible S. repens itself. Attempted penetration was not observed on the resistant hybrid (30). This behaviour suggests that diffusible chemical inhibitors at the stem surface may be important in resistance to this pathogen, which has a demonstrated ability to degrade suberin and penetrate the surface periderm (30). 

Cell walls in the necrotic tissue of these wounds were browned. Staining with diazotized Q-tolidine and toluidine blue confirmed the polypheno-lic nature of these brown depositions, which may have resulted from the polymerization of the stilbenes present in large quantities in spruce bark. Phenolic residues were deposited on the walls of certain cells internal to the necrotic tissues by 10 days after wounding. By 36 days these cells had become thick-walled. The precise nature of substances responsible for this thickening has not been determined, variable responses being obtained with histochemical tests for lignin (cf. Table I). Suberin was detectable in cells immediately underlying the thick walled cells, which corresponded to the... 

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