CHAPTER VIII 



Theories of the Mechanism 

 of Auxin Action 



It is clear that many functions are involved in growth, and to ascribe 

 the full responsibility for growth to any one function would appear 

 to be an over-simplification. Similarly the action of the growth hor- 

 mone in controlling growth appears to be a complex of many func- 

 tions. Although the mechanism of growth hormone action may be 

 partly explained by each of these functions, it seems that none of them 

 known today can entirely account for the effects of auxin on growth. 



In the last twenty-five years many extremely interesting facts have 

 been set forth bearing on the mechanism of auxin action and many of 

 these have contributed materially to our understanding of the physiol- 

 ogy of growth and the growth hormone. 



For convenience, discussion of the theories concerning the mech- 

 anism of auxin action will be grouped into five sections. First those 

 theories concerning the molecular behavior of auxins will be discussed, 

 then those concerning enzymatic effects, osmotic phenomena, cell wall 

 effects, and finally theories of toxic metabolism. 



MOLECULAR REACTION THEORIES 



The first specific suggestion of a molecular reaction into which 

 auxin might enter in causing growth was made by Skoog et al (1942). 

 They postulated that auxin may act as a sort of coenzyme, serving as 

 a point of attachment for some substrate onto an enzyme controlling 

 growth. The molecular configuration and reactivity of auxins would 

 affect activity through altering the fit and the functioning of this 

 molecular union. The anti-auxin effects of very weak auxins such as 

 y-phenylbutyric acid (discussed in the preceding chapter) could be 

 explained as resulting from proper molecular configuration for attach- 

 ment but insufficient reactivity. The inhibition of growth by high 

 auxin levels would be owing to separate molecules combining with 

 the enzyme and with the substrate: thus the proper union of the two 

 by an auxin molecule would be prevented. 

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