The biogenesis of natural auxins 



place; either the amine or keto-acid could act as potential precursors of the 

 aldehyde and auxin. 



Many experimental results indirectly support the concept of tryptophan 

 as the primary precursor. There is the almost ubiquitous occurrence of the 

 tryptophan-IAA enzyme system in metabolically active tissues. There is 

 also a parallelism in the relative enzyme activity and substrate concentration 

 with known auxin production loci, and a parallelism between endogenous 

 auxin level and tryptophan availability. 



Further support for the thesis that native auxin or lAA is formed from 

 tryptophan can be adduced from experiments dealing with the responses of 



Figure 2. Relative free auxin 

 levels in kidney bean plants follow- 

 ing X-irradiation. The values shown 

 were derived from the ^ig free auxin 

 found per plant [bio-assay). 



plants to ionizing radiation (Gordon, 1955). Low doses of ionizing radiation 

 cause an immediate depression of free auxin levels in various plant species. 

 For example, Figure 2 indicates the relative auxin levels in kidney bean 

 plants at various intervals after exposure to X-i-adiation. It may be observed 

 that all the dosages employed, beginning at 25 r, caused an immediate 

 depression of free auxin level. With the lower radiation dosages apparent 

 recovery to control level was attained within one or two weeks. Inhibition 

 and no recovery resulted after irradiations of 5 and 10 kr. The assumption 

 that the lowering of auxin levels was caused bv reduced rates of auxin 

 production was substantiated by a number of studies on the effect of radia- 

 tion on morphological phenomena known to depend on continued auxin 

 production. 



This sensitivity to irradiation and the pattern of temporary inhibition and 

 recovery is closely paralleled by the effect of radiation on the enzymes 

 converting tryptophan to lAA. Figures 3, 4, and 5 indicate that the enzyme 

 system in mung bean seedlings may be examined under conditions where 

 the conversion of tryptophan to auxin as a function of time is approximately 

 linear. Substrate saturation and linear conversion may be attained in vivo by 



67 



V 



