The kinetics of auxin-induced growth 



discussion above has shown, however, that the apphcation of the MichaeHs- 

 Menten equation to the coleoptile-auxin system makes no assumption about 

 and imphes nothing concerning the enzymatic nature of auxin-induced 

 growth. 



Formulation of the auxin-induced growth response in the terms of equation 

 (3) has been useful. Thus it has been shown 



(i) that chemically different auxins react within the plant as would be 

 expected on the basis that they compete for a common site; 



(ii) that certain substances related in structure to auxins but themselves 

 inactive influence growth as would be expected on the basis that they compete 

 with active auxins for the receptive sites. 



Point (i) above has been established (McRae, Foster, and Bonner, 1953) 

 by experiments with mixtures of active auxins. The growth rate of coleoptile 

 sections in such a mixture has been shown with certain restrictions to be that 

 expected on the basis of competition for a common receptive site. Point (ii) 

 above, which involves the determination of which inhibitors of auxin- 

 induced growth rigorously compete with auxin for the receptive sites of the 

 plant, leads us to the concept of the multifunctionality of the auxin molecule 



It has been shown by McRae and Bonner (1952, 1953) that among the 

 substances structurally related to 2:4-D, for example, there are several which 

 are devoid of growth-promoting activity, but which competitively inhibit the 

 action of 2:4-D and of other active auxins. Compedtive inhibition is used 

 here to signify that the substance in question interacts with auxin in the 

 coleoptile in accordance with the formulation 



E+S ^ ES -> growth, .... (9) 



£+/ ^£7 (inactive). (10) 



In this formulation we adopt the notation of enzyme kinetics. The auxin- 

 receptive sites of the coleoptile are now designated as E, the auxin as .S', and 

 the inhibitor of auxin action as 7. By considerations similar to those used in 

 the development of equation (8), it can be shown that growth rate in the 

 presence of auxin competitor 7 may be expected to follow the relation 



1 



V V 



' '^ max 



Ks 





-, + — ...•(ii; 



'-' ' max 



in which Fj^ax replaces the term (growth rate),„ax- Substances which 

 interact with auxin in the coleoptile in accordance with equation (11) are 

 then substances which compete with the auxin for a common receptor site. 

 The 2:4-D related inhibitors which fulfil the criteria of equation (11) include 

 three broad classes, of which two are of particular interest here. These are 

 typified by 2:4-dichloroanisole and related substances on the one hand, and 

 by the d'wrtho substituted phenoxy acetic acids, 2:6-dichloro and 2:4:6- 

 trichlorophenoxy acetic acid, on the other. The first class of compounds 

 includes 2:4-D derivatives which lack the carboxyl group, a group which we 

 know from other work to be essential to auxin activity. The second class 

 includes those which lack a suitably reactive ortho position in the aromatic 

 nucleus, a feature shown by Muir and Hansch (1951) to be essential to 



V 299 



