Cell wall polymer phenolic cross-linking

In order to think about the nature and consequences of cell wall polymer phenolic cross-linking, we need a working model of the mode of assembly and the final structure of the primary cell wall. Unfortunately, there is no universally acceptable model that proposed by Albersheim and co-workers (3) is not now widely accepted because the postulated interpolysaccharide glycosidic bonds have not been demonstrated (4) and the warp-weft model of Lamport (5) rests on the assumptions that extensin (i) forms a defined-porosity network (not proven) (ii) is orientated anti-clinally to the cell surface [some evidence against (6)] and (iii) is a major component of all primary cell walls (not true). 

Efficient cross-link formation by a small number of wall polymer-bound phenolics requires great precision in the metabolic reactions involved. It is not sufficient to form cross-links the cross-links need to be formed in the proper place within the polymer molecule and within the cell wall. Evidence that cross-links form at all [albeit sometimes as a low percentage of the total wall phenolics] is presented elsewhere (1,2,13,16,30-32). Here we present evidence that sufficient molecular specificity exists to be compatible with useful cross-link formation. 

Lignin is a complex phenolic cell wall polymer that is chemically cross-linked with hemicellulose and cell wall proteins. Most of the methods to determine lignin content are based on the removal of all other cell wall constituents, typically through acid hydrolysis, which will readily remove hemicellulose under mild conditions, and non-crystalline cellulose under more severe conditions. Several different methods will be discussed below. The different methods have also been extensively reviewed and compared by Hatfield et al. (1994), Brinkmann et al. (2002), Fukushima and Hatfield (2004), and Hatfield and Fukushima (2005). 

In conclusion, it seems fair to say that specificity exists in both the biosynthesis and in the oxidative coupling of polymer-bound phenols in the growing cell wall, (a) Tyrosine residues are placed at specific sites along the extensin molecule by genetically-encoded information, (b) Tyrosine cross-linking in vivo is a very specific, carefully steered process in that it occurs... 

Mechanical strength is a property generally associated with secondary walls these walls contain substantial deposits of cellulose microfibrils and lignin, a highly cross-linked phenolic polymer that is synthesized and covalently linked into the wall at the cessation of cell expansion. Secondary walls allow plants to exploit the aerial environment and account for the ability of eucalyptus and redwood trees to reach heights of more than 100 m. 

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