Lignins formation

Hatfield, R. D. Vermerris, W. Lignin formation in plants. The dilemma of linkage specificity. Plant Physiol. 2001, 126, 1351-1357. 

Hatfield, R. D. Ralph, J. Grabber, J. H. A potential role of sinapyl p-coumarate as a radical transfer mechanism in grass lignin formation. Planta 2008, 228, 919-928. 

The data presented in a recent communication by Freudenberg et al. (32) show that the methoxyl content of the dehydrogenation polymers of coniferyl alcohol do not change with condensation time. However, their reference to p-hydroxycinnamyl alcohols seems to indicate their appreciation of the significance of a p-hydroxyphenylpropane unit in the mechanism of lignin formation. 

Hi — — Investigations on Lignin and Lignification. XII. A Study of Lignin Formation Based on the Oxidation of Native and Enzymatically Liberated Lignins. Proc. nat. Acad. Sci. USA. 39, 80 (1953). 

Biochemical Studies of Lignin Formation. Physiologia Plantamm 10. 633-648 (1957). 

The PAL activity that is necessary for lignin formation occurs in the cytoplasm or bound to the cytoplasmic surface of the endoplasmic reticulum membranes. The cinnamic acid produced is probably carried on the lipid surface of the membranes, since it is lipophilic, and it is sequentially hydroxylated by the membrane-bound hydroxylases (47,50). In this way there is the possibility of at least a two-step channeling route from phenylalanine to p-coumaric acid. The transmethylases then direct the methyl groups to the meta positions. There is a difference between the transmethylases from angiosperms and those from gymnosperms, since with the latter... 

In order to prove that specific isozymes are, in fact, involved in lignin formation, each must first be purified to homogeneity and subjected to appropriate immunochemical and kinetic investigations. Some cautious optimism for the resolution of this problem can be hoped for, since an antibody of the isozyme from Nicotiana tabacum has been obtained and its cDNA cloned (75). It is to be hoped that the following can now be determined ... 

Figure 1. Among the five resonance structures shown, Ra, Rb and Rc will undergo random coupling reactions more readily than Rd and Re, since the radicals Rd and Re are sterically hindered. Indeed, this is what is found experimentally, since only minor amounts of 0-1 structures (derived from Rd) have been found in milled wood lignin (22,23), and demethoxylation does not occur to any appreciable extent during lignin formation (21). The reactivity of the phenoxy radical, Ra, will be affected greatly by pH since it can be masked by protons under acidic conditions, and its reactivity will thus be diminished. On the other hand, the reactivity of Rb and Rc will not be affected appreciably by pH. Thus more frequent coupling of Rb-Rc, Rb-Rb and Rc-Rc can be expected at low pH, and more of Ra-Rb and Ra-Rc coupling at high pH.

L.) was also found, and was readily distinguishable by a brittleness of the culm which appeared only after maturity of the plant. This mutant had a lower cellulose content, and this difference was assumed to be related to the brittleness of the culm (24). Significant differences were also found in the extractability of the lignin fractions and associated phenolic acids (25-26), suggesting that lignin formation was also affected. 

Firstly, we studied possible relationships between lignin variation and brittleness of plant organs, using two ecotypes of tall fescue grass (Festuca arundinacea Schreb). Thus, both normal fescue and a brittle ecotype (discovered by Jadas-Hecart (27)), characterized by a brittleness of leaves, sheath and stem, were compared. Possible environmental effects on the biochemistry of lignin formation were estimated by comparison of several parallel crops from two locations. 

The application of radioactive phenolic precursors—quinic acid and shikimic acid (52), phenylalanine (30,53), tyrosine (53), and cinnamic acid (30,31,53)—to infected wheat leaves led to a solvent- and alkali-resistant incorporation of radioactivity into hypersensitively reacting host cells suggesting lignin formation had occurred. 

Woody plants can synthesize anywhere from 15-30% of all biomass as lignin. Hence, equivalent amounts of phenylalanine are required at some point, i.e., during lignin formation, large amounts of ammonia are recycled as a consequence of PAL activity. While plants have no serious problems in obtaining glucose as a carbon source, supplied abundantly through photosynthesis,... 

Over recent years, considerable progress has been made in elucidating the biochemical processes of lignin formation (biosynthesis) and lignin biodegradation. Table I summarizes, and compares, the main reactions involved in both processes. In our opinion, the overall chemical principles leading to both formation and degradation are similar. 

The wall material of plant cells is one of their distinguishing characteristics. As a result, lignin, cellulose, and other wall constituents have been studied in many plant tissue cultures. Phenylpropanoids. for example, have been shown lo be precursors of lignin formation in while pine. Set/noiii. lilac, rose, carrot, and geranium tissue cultures. Moreover, the biosynthesis of lignin has been shown to be alTeeted by kinetin. boron, and major elements, such as calcium. 

T ignin is one of the most abundant natural products constituting about one-fourth of the woody tissue in plants. Nature has chosen a unique synthetic technique to prepare this cross-linked polymeric material from coniferyl alcohol and related substances. The mechanism of lignin formation is not completely known yet, and the structural characterization of lignin has been only partially successful despite considerable research. 

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