CHAPTER I 

 LITERATURE REVIEW AND RATIONALE 



Aromatic Amino Acid Biosynthesis in Plants 

 In higher plants, the three aromatic amino acids (L- 

 phenylalanine, L- tyrosine and L- tryptophan) are not only 

 required for protein synthesis and primary metabolite 

 synthesis (such as the hormone, indole acetic acid and the 

 structural component of woody plants, lignin) , but also must 

 be available as starting substrates for a vast array of 

 secondary metabolites including alkaloids, coumarins, 

 isoflavones, and tannins (2, 78, 79) . Intermediates of the 

 pathway are also precursors for synthesis of other essential 

 metabolites in plant cells. The vitamin-like derivatives, 

 folic acid and ubiquinone, are synthesized from chorismate, 

 the first branch point intermediate in the aromatic pathway. 

 Protocatechuic acid is derived from the common pathway 

 intermediate, dehydroshikimate (Fig. 1) . It has been 

 estimated that up to 60% of plant carbon flows through the 

 aromatic amino acid pathway (40) . Humans as well as other 

 higher forms of life depend heavily on plants to provide the 

 three essential aromatic amino acids, and knowledge of 

 aromatic biosynthesis in higher plants is fundamentally 



