Vascular plants, polysaccharides

The word lignin is derived from the Latin word lignum meaning wood. It is a main component of vascular plants. Indeed, lignin is second only to polysaccharides in natural abundance, contributing 24—33% and 19—28%, respectively, to dry wood weights of normal softwoods and temperate-2one hardwoods. 

Figure 9.26 The major sugars divided as unsubstituted aldoses and ketoses. The aldoses or neutral sugars (rhamnose, fucose, lyxose, ribose, arabinose, xylose, mannose, galactose, and glucose) are the dominant monomeric units of the dominant structural polysaccharides of vascular plants (e.g., cellulose and hemicelluloses).

NMR spectra of humin from three major types of depositional environments, aerobic soils, peats, and marine sediments, show significant variations that delineate structural compositions. In aerobic soils, the spectra of humin show the presence of polysaccharides and aromatic structures most likely derived from the lignin of vascular plants. However, another major component of humin is one that contains paraffinic carbons and is thought to be derived from algal or microbial sources. Hydrolysis of the humin effectively removes polysaccharides, but the paraffinic structures survive, indicating that they are not proteinaceous in nature. The spectra of humin differ dramatically from that of their respective humic acids, suggesting that humin is not a clay-humic acid complex. 

On the basis of these studies on woody tissues, it seems that lignin from vascular plants can be selectively preserved compared to biologically degradable polysaccharides when buried. The same can be expected for the lignin in humin from peat the spectra shown in Figure 2 consistently demonstrate this selective preservation with increasing depth. 

Cellulose is a major constituent in vascular plants and is generally absent in algal and microbial tissue. Cellulose is fibrous, water insoluble, and usually present in protective cell walls of plants, such as stalks, stems, and all the woody portion of the plant. Cellulose constitutes 45-90% of woody tissue, and 15-30% of herbaceous plant tissue. Cellulose consists of a linear, unbranched polysaccharide of 10,000 or more glucose units connected by P(l, 4) linkages (Figure 5.12). 

The results of numerous studies have established that RG-II is present in the primary wall predominantly as a dimer that is cross-linked by a 1 2 borate-diol ester [11, 12]. A single borate ester cross-links two of the four apiosyl residues present in the dimer ([52] see Figure 5B). In vitro studies have shown that in the presence of boric acid and certain cations, two RG-II monomers rapidly self-assemble to form a dimer [52]. Moreover, the structure of RG-II itself may determine the location of the borate ester, since the location of the cross-link is the same in naturally occurring and in vitro-formed dimers. Thus, RG-II is the first example of a plant cell wall pectic polysaccharide that self-assembles to form structurally identical dimers [52]. The specificity and cation-dependence of this cross-linking suggest that there are distinct structural requirements for dimer formation and this may explain why the structure of RG-II is highly conserved in vascular plants [12, 53]. It is not known whether dimer formation in planta results from spontaneous self-assembly or is an enzymically catalyzed process. 

Xanthomonas campestris causes leaf blight in cabbage, and, generally, Xanthomonas species are plant pathogens. In this invasion of the plant, the viscous, extracellular polysaccharides may bind strongly to the cell-wall polysaccharides of the vascular system, and... 

Lignins are complex, cell wall-bound, optically inactive phenolic heteropolymers covalently associated with both polysaccharides and proteins [102]. They are mainly localized in the impermeable water transport conduits of the xylem and other supporting tissues of all the vascular terrestrial plants, and result mainly from the oxidative... 

---