48 THE PLANT CELL WALL 



lignin. Coniferin is converted to lignin only after a lag period 

 while coniferyl alcohol is released from the glucoside. 



Although lignin polymers can be formed by several oxidizing 

 enzymes, peroxidases are the most widespread hence must figure 

 most importantly in plants as a whole. In addition to slow poly- 

 merization by cell-free preparations, crude or partly purified 

 enzymes, both the rate and specificity are subject to modification 

 by the presence of polysaccharide surfaces (matrix effects discussed 

 in Chapter 7). 



We have now examined a number of aromatic precursors of 

 lignin and their behavior, but have not given consideration to the 

 obviously important pre-aromatic substances which are interme- 

 diates in the conversion of carbohydrates into phenylpropanes. 

 Although it has long been appreciated that the C7 sugar sedoheptu- 

 lose played a part in the synthesis of aromatic substances, the main 

 lines of biosynthesis have only recently received some clarification. 

 The biosynthetic pathway proceeds from sedoheptulose-l,7-di- 

 phosphate through 2-keto-3-deoxy-7-phospho-D-glucoheptonic acid 

 to a series of alicyclic derivatives. Of particular note are shikimic 

 acid, a trihydroxylated cyclohexene-carboxylic acid and prephenic 

 acid, l-carboxy-4-hydroxy-cyclohexadienyl-pyruvic acid. 



The biogenetic scheme provides for several aromatic compounds: 



protocatechuic acid 



T 

 5-dehydroquinic acid-»5-dehydroshikimic acid 



I 

 tyrosine*- prephenic acid<-shikimic acid 



I 

 phenyl pyruvic acid->phenylalanine 



The biochemistry of cell wall synthesis is obviously far from 

 complete, but the present state of knowledge is sufficient to indicate 

 some of the major pathways and processes involved. 



Changes in the Walls of Growing Cells 



In the primary walls of the Allium rootcap, pentoses and hexo- 

 ses are high whereas the hexuronic acid content is low. During 

 the radial (isodiametric) enlargement, which is concurrent with 



