Neiv Theory on Primary Mode of Auxin Action 451 



mode of action of auxin must be of a physico-chemical rather than of 

 a chemical nature. 



All attempts to activate an enzyme system in vitro with auxin 

 have failed, as far as is known. However, the activity of many en- 

 zyme systems is affected after a plant has been treated with auxin. 

 This would lead one to conclude that the primary auxin reaction 

 does not involve a single key enzyme, but that the auxin acts on a 

 number of enzyme systems simultaneously — perhaps via the cyto- 

 skeleton, a membrane system upon which or in which enzymes are 

 located (6). If one accepts this view, one will see that the auxin mole- 

 cule is not a direct participant in the enzyme reactions affected by 

 auxins. One might visualize this by imagining that sorption of an 

 auxin molecule in the lipoprotein membrane of the cytoskeleton may 

 lead, for instance, to a local change in the hydration of the membrane. 

 Such a hydration, in turn, would change the relative distance between 

 the enzyme components on or in the membrane. This would change 

 the relative reaction rates between these enzymes, which ultimately 

 would lead to a changed ratio of metabolites. Such a changed ratio 

 of metabolites is the basis for a changed physiological pattern, as the 

 researches of Skoog et al. have shown. 



Let us assume that the auxin molecule moves with its ring into a 

 cavity of the membrane of the cytoskeleton, and that the polar side- 

 chain sticks out. This cavity, of course, is a temporary opening cre- 

 ated by the thermal agitation of the molecules of the membrane. The 

 penetration of the ring into it is simply an aspect of the phenomenon 

 of solubilization in surface chemistry. 



Our next problem is to imagine what the polar side-chain, sticking 

 out of the surface, could accomplish. It has become known that on the 

 surface of polymers, hydrogen bond systems can occur. Under some 

 conditions these hydrogen bonds become coordinated in forming an 

 oscillating system (2). This strengthens the H-bonding capacity of 

 the system. Let us now postulate that the polar group of the auxin 

 molecule becomes part of an H-bond system at the surface of the 

 cytoskeleton, and that this polar group provides the missing link in 

 the system and sets it to oscillating. Providing this missing link and 

 making the H-bond network oscillate would then be the primary 

 auxin function. 



It is not hard to see that such a strengthening of the H-bond sys- 

 tem would affect the structure of the membrane. It might contract it 

 or, via hydration, expand it. 



According to this picture then, the polar group of the auxin is 

 the crucial part. In order for it to activate the H-bond network, this 

 polar group must be placed just right to function as the missing link. 



