74 THE PLANT CELL WALL 



including flavonoids with an additional phenolic group; the dimeric 

 lignanes; the polymeric lignins; or various benzoic acid derivatives 

 such as the tannins. Further, compounds bearing Ci and C 3 side 

 chains may be linked biogenetically through acyl group transfer 

 (Ci->C 3 ) or (3-oxidation (C 3 ->Cj). 



Where, in such a scheme can we find the most likely place 

 for the genetic events leading to a supply of lignin precursor? In 

 some forms tyrosine can serve as a precursor, in others it does 

 not seem to be suitable. If we make the reasonable assumption 

 that all of the organisms considered necessarily produce tyrosine 

 as a part of their protein anabolism, it follows that they must 

 have some capacity for making the Cg— C 3 precursors of lignins. 

 Elodea, for example contains tyrosine and flavonoids (as anthocya- 

 nins), but little or no lignin. 



In this case, at least and likely in others as well, we may be 

 confronted with an example of competing metabolic pathways. 

 Thus, the potential precursors are not realized as they are trans- 

 formed into other products more effectively (rapidly) than they 

 can be converted to lignins. 



The origins of lignin synthesis are yet obscured, but it is possible 

 to suggest that the genetic basis for lignification resides in at least 

 four areas: 



(a) Generally, but not exclusively, in the ability to synthesize 

 peroxidase. 



(b) In quantitative factors controlling the rate of phenylpropane 

 synthesis (relative, of course, to the rate of phenylpropane con- 

 sumption). 



(c) In specific factors controlling hydroxylation patterns and 

 methyl transfer. 



(d) In specific factors controlling the composition of the poly- 

 saccharide framework (hence matrix properties) of the cell wall. 



Finally, lignin synthesis is clearly an aerobic process. Although 

 the precursors could be fermentation products, peroxidases are 

 aerobic, H 2 2 is formed by reduction of 2 and, of course, the 

 phenolases utilize 2 directly as an electron accepter. Hence, 

 precursor synthesis may have proceeded in some forms during 

 part of the long phase preceeding formation of an oxidizing atmo- 



