Lignin precursors

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. 

Figure 3.5 The primary pathway to the biosynthesis of the lignin precursors L-tyrosine and L-phenylalanine.

Another very important finding was the observation of the inability of species other than Gramineae to utilize tyrosine tis a lignin precursor [cf. 24, 112, 113). This is now known to be due to the presence or absence of the enzyme tyxase [cf. (81, 82)]. 

The Canadian school has recently shown that the production of the higher lignin precursors, i.e. the /i-coumaryl alcohols or their glucosides, does not proceed via the simple acids shown in Fig. 2 but actually via insoluble esters of same. The esters arc probably activated esters of coenzyme A, but esters of quinic acid analogous to chlorogenic acid arc also feasible in this role 35). 

Several groups have identified non-radioactive metabolic intermediates of lignification in cambial sap or sapwood extracts 54, 58 b, 68). The compounds detected agree well with the schemes for the biosynthesis of lignin precursors set forth in Figs. 1 and 2 and with the data described later for the third stretch of lignification. 

Nevertheless, it had to be known beforehand that lignin is in fact derived from the -coumaryl alcohols and this information was not in fact accrued from the biochemical work on lignin precursors. Actually... 

In gymnosperms, the primary lignin precursors are the two monolignols coniferyl and /7-coumaryl alcohols, while in angiosperms sinapyl alcohol is also a participant in... 

The phenol, the cresol isomers, and the dimethylphenols, major pyrolysis products in e Moscow wood sample, are probably also derived frt>m lignin precursors that have been altered through coalification reactions. Hatcher [fr] have shown that an increase is observed in the relative proportion of phenols and cresols as rank of coaHfred wood samples increases to subbituminous coal. Comparing the distribution of pyrolysis products from the Moscow wood to that of other coalified wood samples of Hatcher allows us to deduce that the... 

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

Table I. Use of Doubly labeled Guaiacyl Lignin Precursors in Lignin Biosynthesis Studies...

When radioactive lignin precursors are applied to resistant host plants infected with an avirulent pathogen, the autoradiographic localization of radioactivity in resistant reacting host cells may help to corroborate the participation of lignification in the resistance response. Thorough extraction of non-polymerized precursor with organic solvents and the removal of esterified phenolics by alkaline hydrolysis are important steps in these experiments (25,28,30,31). 

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