Occurrence and Role of Growth Hormones 77 



strikingly parallel to the distribution of auxin itself. By far the great- 

 est concentration of enzymatic activity, as measured by the production 

 of auxin from tryptophan, was found at the tip of the coleoptile. The 

 relative activity of the enzyme in successively lower sections of the 

 coleoptile falls off in a pattern essentially like the pattern of auxin 

 concentrations. This enzyme from the Avena coleoptile is destroyed 

 by boiling. Its activity increases at the tip of the decapitated coleop- 

 tile at the same time that regeneration of the physiological tip takes 

 place. Another clear demonstration of an enzyme system which pro- 

 duces auxin has been carried out by Gordon and Nieva (1949) in the 

 pineapple. 



/j iCHgdiCOOH 



NH 

 CH,CHCOOH 



Indolepyruvic X\ nCHoCH 



Acid I I i 



Tryptophan -COz^ / \ n^2 XnHz ,^^^1^ Tryptopho 



' ^ ^ Acefaldehyde ^ , 



CHgCHOH 



H . 



Tryptafnjne 



a— jjCHgCOOH 

 'J 



Indoleacetic 

 Acid 



Fig. 37. The pathways of formation of indoleacetic acid from tryptophan. 



The actual steps by which tryptophan is converted into indole- 

 acetic acid have been followed in considerable detail by Gordon and 

 Nieva. They found that leaf discs or crude enzyme preparations from 

 pineapple leaves were quite active in producing auxin when the leaf 

 discs were incubated with tryptophan, indolepyruvic acid, or trypta- 

 mine. In these same experiments they also found that large amounts 

 of a neutral precursor of auxin were formed from these substrates, and 

 they identified the neutral precursor as being indoleacetaldehyde. They 

 ])roposed then that the formation of auxin might occur through two 

 alternative routes, as shown in figure 37. The tryptophan may be 

 tleaminated to indolepyruvic acid, and thence decarboxylated to indole- 

 acetaldehyde, or alternatively, the tryptophan may be first decarboxyl- 

 ated to tryptamine and then deaminated to indoleacetaldehyde. The 

 aldehyde, then, is the immediate precinsor of indoleacetic a(id I)y 

 either route of formation. 



Larsen first suggested in 1949 that indoleacetaldehyde was a pre- 

 cursor of auxin. He recognized it as a neutral substance which was 

 capable of being converted into auxin. Larsen's concept was graphi- 



cu 



^A «5* : 



v: 



