34 PLANT GROWTH SUBSTANCES 



SO only one or two points need be mentioned here. The growth process 

 involves the metabolism of the organic acids. One of the enzymes in 

 this system, which has a sulfliydryl structure, is a critical link; sulfhydryl 

 reagents, such as iodoacetate, inhibit growth strongly and specifically. 

 Malate, pyruvate, succinate, and other acids promote growth; they are 

 used up during growth, and malate or citrate may accumulate when 

 growth is inhibited. Malate and other acids, including malonate, protect 

 growth against the inhibiting influence of iodoacetate. Thus enzymes 

 acting upon the 4-carbon acids play a major role in growth. \'iewed in 

 this light the structural requirements for auxin activity take on a strik- 

 ing significance. The protecting action of malonate against inhibition 

 by iodoacetate is only part of a general phenomenon in which substrates 

 and interfering substances or inhibitors compete for the active locations 

 on catalyst molecules, as for example, the relation between O2 and CO 

 with activated charcoal, or between succinic and malonic or maleic 

 acids with succinic dehydrogenase. In this case malonic acid will protect 

 the enzyme against inactivation by oxidized glutathione. The parallel 

 with succinic dehydrogenase is indeed very close, for Quastel and 

 Wooldridge (7) showed long ago that besides malonic acid and related 

 substances, phenylpropionic acid inhibits this enzyme. Thus the phenyl 

 group can substitute on the enzyme for one of the carboxyl groups of 

 succinic acid. If now w^e view the auxins as protective substances, we see 

 that the ring takes the place of a carboxyl. The requirement for one car- 

 bon atom between the ring and carboxyl means that the auxin is really a 

 malonic acid derivative. Suppose the tissue contains a natural inhibitor of 

 succinic dehydrogenase. (The argument may apply equally to certain 

 other enzymes of the 4-carbon acid series.) For the enzyme to have 

 activity and thus to allow growth it must be protected against this 

 inhibitor; the best protector is indoleacetic acid. 



This line of thought at once brings many observations together: 

 i) Succinic dehydrogenase is inhibited not only by malonic acid but 

 by maleic acid. If auxins are envisaged as malonic acid derivatives, the 

 corresponding maleic acid derivatives should have the same action. 

 These, of course, are the aromatic acids of aV-configuration like cis- 

 cinnamic acid. If this is true it is the c/V-structure itself, rather than the 

 angle of the carboxyl, which is important. We should note, too, that 

 the active naphthalene- 1 -acetic acid and the almost inactive naphthalene- 

 2-acetic acid can be considered as analogous to cis- and trans-ioxms,. 



