22 PLANT GROWTH SUBSTANCES 



be a substrate it must act catalytically and thus in some way become a 

 part of some enzyme system. Tliis of course raises the question as to 

 what enzyme systems are involved in growth. In addition, however, 

 the participation of a small molecule in an enzyme system presupposes 

 special properties in that molecule. Hence this idea focuses attention 

 on the nature of the auxin molecule itself; what are the properties of the 

 substance — a relatively simple molecule — which might enable it to act 

 in an enzyme system? We are in a position to attempt an answer to this 

 question now because of the large number of synthetic auxins that have 

 been prepared. 



In other comparable cases, like those of the coenzymes and prosthetic 

 groups, there are strict limitations of structure, and quite small changes 

 in the molecule such as, for example, the substitution of two ethyl for 

 two methyl groups in pyridoxine, make the compound inactive. In 

 other cases, such as the replacement of the thiazole ring by a pyridine 

 ring in thiamin, a relatively small change may make the compound a 

 competitive inhibitor. The case of the auxins is somewhat diflerent since 

 even the first known compounds, the naturally occurring auxins, show 

 wide differences in structure and appear virtually unrelated. However, 

 if we consider auxin-a (I) and indoleacetic acid (II) more carefully it 

 will be seen that in fact they have many properties in common. In the 



HN 



first place, both have an unsaturated 5-membered ring with an acid 

 group situated at a distance from the ring. Second, the presence of the 

 N atom in the ring of one compound but not in that of the other indicates 

 that this atom is not essential for activity. A study was therefore made 

 of the activity of compounds resembHng indoleacetic acid but without 

 the nitrogen. It was found at once (9) that indeneacetic acid, (III) is 

 highly active; the ring N atom is therefore not important for activity. 

 However, III shows some curious properties. In the Avena test it gives 



u 



CH^COOH ^.^^ CHjCOOH 





