166 



Fundamentals of Auxin Action 



auxin concentrations. A summary of the large number of anti-auxins 

 reported in the following three years is given by McRae and Bonner 

 (1953). 



Modes of Action. The evidence concerning the molecular require- 

 ments for auxin activity has led to the general conclusion that auxins 

 act by becoming attached to other materials at two points. Thus an 

 auxin must have an unsaturated ring which can be attached to some 

 receptor (at the ortho position in the case of phenoxyacetic acids), 

 and there must be an acid group which can combine with some recep- 

 tor. By assuming that the ring and the acid attach to the same re- 

 ceptor, Foster et al (1952) can account for the spatial configuration 

 requirement as well. 



A provocative theory of the means of anti-auxin actions has been 

 proposed by McRae and Bonner (1953). In their theory, anti-auxins 

 are compounds which have some of the three requirements for auxin 

 activity but fall short of having all three. Three types of anti-auxins 



O 

 or 



CD 



I M6/L 



CONC. OF AUXIN 



Fig. 74. An interaction between an auxin (naphthaleneacetic acid) and an anti- 

 auxin (trans-cinnamic acid) in the pea straight-growth test, showing the inhibition 

 of growth at low auxin concentrations by the anti-auxin (15 mg./l.), and the reversal 

 of this inhibition by the presence of larger amounts of the auxin (van Overbeek 

 et al, 1951). 



