Natural auxins 



Schematic pathways of lAA biogenesis are familiar to most of you. 

 Figure 1 indicates a recent version. 



The concept that tryptophan was the primary precursor of lAA in plants 

 arose from the works of Dolk, Nielson, and Thimann in the early nineteen- 

 thirties, culminating in the isolation and characterization of lAA from 

 Rhizopus cultures by Thimann (1935). Since the yield of auxin in cultures of 



^^^"^ 



-CH2-CH-COOH 

 NH, 



CH, — C — COOH 



^ II 

 NH 



Indolylimmopnopionic acid 



CH2-C — COOH 



II 

 



NH'"" 

 Indolylpyruvic acid 



^^ 



NH 



Indolylethylidenimine 



CH, — C 



.^ 



=N 



NH 

 Indoljlacetaldehyde 



NH 



Indolylacetonitrile 



f?^^>^ ^CH2 — CNHz 



,^ 







NH' 



Indoljlacetamide 



NH' 



Indolylacetic acid 

 Figure 1. Potential pathways of tryptophan conversion to indoleacetic acid {after Reinert, 1954). 



the micro-organism was dependent upon the amount of tryptophan present 

 in the medium, and roughly proportional to the extent of aeration, the 

 oxidative deamination of tryptophan to indolepyruvic acid (IPyA) followed 

 by oxidative decarboxylation of the keto acid to lAA was postulated. This 

 picture was strengthened subsequently by Wildman, Ferri, and Bonner ( 1 947) , 

 who showed that leaf enzyme preparations could convert tryptophan to 

 auxin or lAA, and that IPyA could also function as a source of auxin. Since 

 Skoog (1937) found that tryptamine as well as tryptophan was converted 

 slowly to auxin when applied to coleoptiles, the alternative pathway of 

 tryptamine oxidation to lAA via indoleacetaldehyde was proposed. The 

 existence of this dual pathway was also suggested by the results with whole 

 tissue and enzyme preparations of the pineapple leaf. With these materials, 

 the tryptophan to indoleacetaldehyde to lAA reactions apparently took 



66 



