Theories of Auxin Action 179 



evidence of the nature of auxin action. First, these kinetic analyses 

 are based on the assumption that the velocity of the reaction being 

 measured is dependent upon such an equilibrium as that in the 

 prior equation, whereas in systems as complex as growing cells it is 

 possible that the velocity is controlled by steady state conditions in- 

 stead. Second, the agreement of the kinetic performance with that pre- 

 dicted by the assumption of two-point attachment does not prove the 

 assumptions. Nevertheless these kinetic analyses are certainly the best 

 evidence yet obtained as to how auxin acts. It will be critical for the 

 theory to establish the identity of the receptor complex or complexes. 



Suggested Auxin Receptors 



Two lines of evidence as to the possible nature of substances 

 which may form complexes with auxin have appeared almost simul- 

 taneously. 



In studies of the disappearance of indoleacetic acid in pea brei, 

 Siegel and Galston (1953) observed that some of the auxin was bound 

 to a protein fraction in the brei. This bound auxin was detectable by 

 the ferric chloride color test for indoleacetic acid which gave a red 

 color with precipitated protein. The auxin-protein complex was stable 

 to acid and alkali. In the presence of adenosine-triphosphate the bind- 

 ing reaction was facilitated, suggesting that energy might be con- 

 sumed in the reaction. It is tempting to hope that this auxin-protein 

 complex may be the auxin-receptor complex postulated by Foster et 

 al (1952). 



In studies of the reaction of some organic acids with coenzyme A 

 (CoA) in the presence of tomato mitochondria it was found that the 

 presence of several compounds which are auxins could bring about the 

 enzymatic disappearance of the free sulfhydryl of CoA (Leopold and 

 Guernsey, 1953). An interesting aspect of the study was the finding 

 that the most active auxins were the most effective (e.g. 2,4-D, in- 

 doleacetic acid), weak auxins were less effective (e.g. /?-chlorophe- 

 noxyacetic acid, indolebutyric acid), and compounds closely allied to 

 auxins but lacking auxin activity were ineffective (e.g. cyclohexaneace- 

 tic acid, 2,4-dichlorobenzoic acid). The relative effects of some of 

 these acids in the CoA reaction are shown in figure 80. A further 

 parallelism between the auxin effects on this reaction and on growth 

 is the inhibitions by higher auxin concentrations. It can be seen in 

 figure 80 that at high concentrations each of the auxins inhibited the 

 disappearance of CoA. 



As in the protein reaction of Siegel and Galston (1953), adenosine- 

 triphosphate facilitated the reaction. 



