196 BIOCHEMICAL SYSTEMATICS 



have shown, for example, that acetate is preferentially incorporated 

 into the A ring of phloretin by apple leaf tissue. 



phloretin 



The Ce — C3 compounds which apparently are synthesized by 

 way of the shikimic acid pathway are important units in the forma- 

 tion of lignin in addition to their roles in amino acid and flavonoid bio- 

 synthesis. While it is not appropriate to include herein a detailed dis- 

 cussion of lignin biosynthesis and lignin chemistry, it is pertinent to 

 note that, although the exact structure of hgnin is not known, it 

 is believed to be a phenylpropane polymer. There is reason to be- 

 lieve that many different kinds of lignin exist, and a specific lignin 

 may characterize a particular taxonomic group. The systematic im- 

 plications of Hgnin chemistry are discussed in Chapter 14. 



Coumarins comprise a particularly interesting group of 

 phenohc compounds. There are a number of different coumarin 

 derivatives of widespread occurrence, and some, such as scopoletin, 

 affect plant growth. From the formula of coumarin itself, one un- 

 famihar with phenol chemistry may conclude that the coumarin 

 structure is homologous with the A ring plus the heterocycHc ring of 

 the flavonoid nucleus. 



HO 



0-^0 



coumarin 



In such case the benzene ring of coumarin would be expected 

 to come directly from acetate. Coumarin, however, is a lactone of 

 o-hydroxycinnamic acid, and tracer studies support the view that 

 coumarin synthesis follows the shikimic acid pathway (for example, 

 labelled caffeic acid and labelled scopoletin are formed when 

 labelled phenylalanine is provided to Nicotiana, Reid, 1958). 



The disclosure that isoflavones are formed from phenylalanine 

 by a mechanism which includes an aryl migration (Grisebach, 1961), 

 in addition to relating this flavonoid group to the shikimic acid path- 

 way, suggests that the rotenoids may also be included since rotenoids 

 bear a structural resemblance to isoflavonoids. 



