Lignins secondary cell walls

All secondary cell walls develop from primary cell walls. Cells no longer grow once lignin is added to their wails. Lignification, which is a key step in the conversion of a primary cell wall into a secondary cell wall, results in terminal differentiation of the encased cell. Indeed, many cells with lignified walls die. The totipotency of plant cells is limited to cells enveloped in primary walls. 

A clear understanding of lignin deposition in the cell wall is not yet possible, but a number of facts are known. Lignin precursors of the phenylglucoside type are formed either in the region of the cambium (the zone of new cell synthesis) or within the lignifying cell itself. Lignification is thus initiated in the differentiated wood cells from the primary walls adjacent to the cell corners and then extends into the inter-cellular area, the lamella, and thereafter to the primary and secondary cell walls. 

The secondary cell wall, produced after the cell has completed growing, also contains polysaccharides and is strengthened by polymeric lignin covalently crosslinking to hemicellulose.6... 

In both primary and secondary cell walls, cellulose is mixed with other polysaccharides (O Table 1) and lignin [20,21,22,23,24,25,26]. Historically, the other polysaccharides have been called hemicelluloses. The term hemicellulose indicates a polysaccharide closely associated with cellulose in cell walls. The relationship is physical, not stmctural. These polysaccharides are almost always heteroglycans 8-glucans and some polysaccharides that are composed almost exclusively of D-xylose are among the exceptions. Stmctures of hemicelluloses vary from linear to highly branched and bushlike. 

JG Taylor, TP Owen Jr, LT Koonce, and CH Haigler. Dispersed Lignin in Tracheary Elements Treated with CeUnlose Synthesis Inhibitors Provides Evidence that Molecules of the Secondary Cell Wall Mediate Wall Patterning. The Plant Journal 2 959-970, 1992. 

Along the degradation front, there is a substantial breakdown and dissolution of carbohydrates of both Sg and S1. About 90% of the carbohydrates still present in the slightly degraded tissue are dissolved here. The residual lignin skeletons of the secondary cell walls appear granular and are detached from the compound middle lamellae. In SEM preparations they often appear as a very flimsy network (Figure 20). 

In this transition zone we find secondary cell walls that take up blue stain, which indicates that they are strongly swollen. But evidently this state does not last very long. Breakdown of carbohydrates seems to set in immediately, with a detached granular lignin skeleton left behind (Figure 27). This thorough degradation starts at the lumen and comprises the whole secondary cell wall (S2 and Sj). The residual system of compound middle lamellae is very frail and brittle and is easily ruptured. 

Then the cellulose in the inner layers of the secondary cell walls, the S2 and S3, is attacked either from the lumen side or from the Sj-Sg border. Its crystalline structure is broken down, and the chain macromolecules are degraded and dissolved. The residual lignin skeleton degenerates to a granular debris that might shrink and come loose from the S, which is still intact and adheres to the middle lamellae. The thin Si is then degraded, until finally only the system of compound middle lamellae remains. 

Lignin is mainly concentrated in the lamellae of plants, and lignin formation proceeds steadily from here into the primary and secondary cell walls. Individual plants have varying quantities of lignin (Table 24-14). 

The secondary cell wall contains about 94% cellulose and only grows when the primary cell wall is formed. In it, the cellulose molecules are highly oriented. With decreasing diameter, the following structural types can be distinguished fibers (0.06-0.28 mm), cell walls, macrofibrils (400 nm), microfibrils (20-30 nm), and elementary fibrils (3.5 nm). Lignin deposits as a cement in interfibrillar spaces of 5-10 nm in width. In fact, the inter-micellar spaces of 1 nm in width that occur between the elementary fibrils are accessible to H2O, ZnCl2, or I2, but not to dyestuffs. 

In plants, lignin is concentrated mainly in the lamellae, and from there the lignin production gradually proceeds into the primary and secondary cell walls, strengthening the wood. Thus, individual plants contain different amounts of lignin (Table 32-1). 

Cellulose microfibrils are usually embedded in a continuous phase of lignin, pectin, and hemicellulose hemicellulose usually predominates (Whistler and Richards, 1970). Most of the hemicellulose is found mixed with cellulose in both the primary and secondary cell walls. Hemicellulose is now known not be a precursor to cellulose. In general, hemicellu-loses are the cell wall polysaccharides other than cellulose and pectin. These polymers are classified on the basis of the type of sugar residues present. For example, o-xylan is made up of D-xylose, D-mannan of D-mannose, and D-galactan of D-galactose units (Whistler and Richards, 1970). In practice, few hemicelluloses contain only one sugar, most contain two to four. Furthermore most hemicelluloses have branched structures. The major types of hemicellulose and their distribution have been reviewed (Whistler and Richards, 1970). Methods for the analysis of mannans have been reviewed (Matheson, 1990). 

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