Lignin biosynthesis, precursors

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

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. 

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

Goujon, T., Sibout, R., Eudes, A., MacKay, J., and Jouanin, L., 2003, Genes involved in the biosynthesis of lignin precursors in Arabidopsis thaliana, Plant Physiol. Biochem. 41 677-687. 

Hydrogenolysis has also found use in a lignin biosynthesis study in which 14C-labeled lignin precursors were administered to wheat plants and maple twigs and ultimately recovered in the form of the typical lignin hydrogenolysis products, dihydroconiferyl and dihydrosinapyl alcohols (Brown and Neish 1959). 

Such products implicate the biosynthesis of sinapyl y-acetate as a precursor of Kenaf lignin (see Novel Mono-tetrahydrofurans Provide Evidence that Acylated Monolignols are True Lignin Precursors section) [101,260]. The functional role of such high levels of acetates in kenaf lignins awaits explanation. Although present at much lower levels, except in palms where they may be substantial [323], acetates on the y-position have been confirmed in aspen by a modified DFRC analysis [312]. Detailed NMR studies elucidating the nature of these minor acetate levels have not been carried out. 

Fig. 2.29 Biosynthesis of major lignin precursors sinapyl, coniferyl and coumaryl alcohols (after MacGregor Greenwod 1980).

Further studies about the biosynthesis of SA have been carried out in rice (Oryza sativa). In agreement with the SA biosynthetic pathway in tobacco, SA is synthesized from CA via BA. In addition to the conversion of [ C]-CA into SA, it is also transformed to the lignin precursors /7-coumaric acid and ferulic acid. [" C]-benzoic acid was readily converted to SA [32]. 

Biosynthesis of Lignin.—The mechanism of biosynthetic differentiation between the different lignins produced in gymnosperms and angiosperms has been explored by Shimada et al.f employing both enzymic and tracer procedures. The pathway of synthesis of the —C3 lignin precursors appears to... 

Figure 49 The precursors of lignin biosynthesis, p-coumaryl alcohol (I), coniferyl alcohol (II) and sinapyl alcohol (III). [65]...

Capsaicinoids are synthesized by the condensation of vanillylamine with a short chain branched fatty acyl CoA. A schematic of this pathway is presented in Fig. 8.4. Evidence to support this pathway includes radiotracer studies, determination of enzyme activities, and the abundance of intermediates as a function of fruit development [51, 52, 57-63], Differential expression approaches have been used to isolate cDNA forms of biosynthetic genes [64-66], As this approach worked to corroborate several steps on the pathway, Mazourek et al. [67] used Arabidopsis sequences to design primers to clone the missing steps from a cDNA library. They have expanded the schema to include the biosynthesis of the key precursors phenylalanine and leucine, valine and isoleucine. Prior to this study it was not clear how the vanillin was produced, and thus the identification of candidate transcripts on the lignin pathway for the conversion of coumarate to feruloyl-CoA and the subsequent conversion to vanillin provide key tools to further test this proposed pathway. 

Validation of the role of femloyl-CoA in the synthesis of the vanillin precursor will be detection of the appropriate intermediates and/or enzyme activities in placental extracts that could account for the production of the predicted levels of capsaicinoids. The presence of low levels of monolignol intermediates could be explained by lignin biosynthesis. An alternate route from phenylalanine to vanillin has been considered by some investigators Orlova et al. [68] demonstrated the role of the benzenoid pathway in petunia flowers for the biosynthesis of phenylpropanoid/benzenoid volatiles. 

Noguchi et al. (8) synthesized precursors of lignin biosynthesis (i.e., coniferyl and sinapyl alcohols) in which the side chain and methoxy groups were doubly labeled by and H, prepared the DHP (synthetic lignin) from them, and treated the DHP with the laccase. These results showed that about 25% of Hs of the a- and -positions were removed in a ratio of one to one, respectively, and that the methoxyl group of a syringyl unit was de-methylated easier than that of a guaiacyl. 

Proanthocyanidins and Procyanidins - In a classical study Bate-Smith ( ) used the patterns of distribution of the three principal classes of phenolic metabolites, which are found in the leaves of plants, as a basis for classification. The biosynthesis of these phenols - (i) proanthocyanidins (ii) glycosylated flavonols and (iii) hydroxycinnamoyl esters - is believed to be associated with the development in plants of the capacity to synthesise the structural polymer lignin by the diversion from protein synthesis of the amino-acids L-phenylalanine and L-tyro-sine. Vascular plants thus employ one or more of the p-hydroxy-cinnarayl alcohols (2,3, and 4), which are derived by enzymic reduction (NADH) of the coenzyme A esters of the corresponding hydroxycinnamic acids, as precursors to lignin. The same coenzyme A esters also form the points of biosynthetic departure for the three groups of phenolic metabolites (i, ii, iii), Figure 1. 

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. 

Davin, L. B., and Lewis, N. G., 2000, Dirigent proteins and dirigent sites explain the mystery of specificity of radical precursor coupling in lignan and lignin biosynthesis, Plant Physiol. 123 453-461. 

---