J. VAN OVERBEEK 



Shell Development Company 

 Modesto, California 



New Theory on the Primary Mode of 



Auxin Action 



The search for the primary action of auxin has so far only given 

 negative results. It appears unlikely that in the primary auxin re- 

 action the molecule undergoes covalent bonding, and it appears also 

 unlikely that the primary auxin reaction involves any one specific 

 enzyme system. 



Previously it had been assumed that auxin combines with some 

 entity, perhaps a protein, both through an ortlio position on the ring, 

 and through the carboxyl group. This idea was developed with the 

 phenoxyacetic acids in mind. The highly active ones, such as 2,4-D 

 (Figure 1, I) all have at least one unsubstituted ortJio position. When 

 both ortho positions are substituted (II) activity is lost. However, 

 at present we realize that this is not generally true. The di-ortho sub- 

 stituted phenylacetic acid (VI) is highly active, and so is 2,6-dichloro- 

 benzoic acid (IV). On the other hand, 2,4-dichlorobenzoic acid (III) 

 is inactive, while 2,4-dichlorophenylacetic acid (V) is rather weakly 

 active (3, 4, 5). Even if one would suppose that the halogenated ortho 

 position were reactive, this is made entirely unlikely by the fact that 

 2,6 methyl substituted acids (VII, VIII) show considerable activity 

 (7). The chlorine atom is electro-negative, and the methyl group is 

 electro-positive. For this reason, covalent bonding at the ortho posi- 

 tion of these halogenated or methylated benzoic acids (IV, VII, VllI) 

 would appear impossible. 



As far as the carboxyl group is concerned, Veldstra et al. (7, 9, 10) 

 have shown that this could be replaced by a number of other acidic 

 groupings including tetrazole (X, XII). Therefore, it also seems im- 

 possible that the carboxyl group could be involved in covalent bonding 

 in the auxin reaction. We are, therefore, forced to conclude that the 



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