Theories of Auxin Action 181 



phate can increase auxin-induced growth in the pea straight-growth 

 test, and that the greatest enzymatic activity for this reaction appears 

 to be associated with tissues in the most rapid state of growth (Leo- 

 pold and Guernsey, 1954). The theory may be criticized on the basis 

 that the amount of auxin added is considerably less than the amount 

 of CoA which disappears. The isolation of the proposed ester will be 

 critical to proving the scheme. 



Several interesting points are brought up by the auxin-CoA 

 theory. The release of bound auxin by alkali has been known for 

 many years (e.g. Berger and Avery, 1944), and the breaking of thiol 

 esters by alkali is similarly well established (Lynen et al, 1952). The 

 proposed ester would be a high-energy bond, which suggests that 

 auxin may control a high-energy bond system in plants. This has al- 

 ready been suggested by Rhodes and Ashworth (1952) as possibly oc- 

 curring through the formation of high-energy phosphate bonds on the 

 auxin itself. The ability of CoA to remove the auxin from the pro- 

 tein complex of Siegel and Galston (1953) is also suggestive of an 

 auxinyl-CoA product. 



Because CoA is a key compound in the metabolism as well as in 

 the synthesis of organic acids, fatty acids and steroids (Lipmann, 

 1951), auxin may control these diverse processes of metabolism and 

 hence of growth itself through its influence on CoA. This would be a 

 case of a hormone having its action through a coenzyme. 



THEORIES OF ENZYME EFFECTS 



The fact that auxins could inhibit enzyme activity was known for 

 many years, but the realization that specific enzymes could be stimu- 

 lated by auxins was not entirely appreciated until 1942 when Berger 

 and Avery demonstrated that under certain circumstances some dehy- 

 drogenases could be stimulated by auxin. Treatment of growing tis- 

 sues with auxin has been found to increase directly or indirectly the 

 activity of a variety of enzymes as described in chapter V. 



A provocative theory to explain auxin action has been proposed 

 by Northen (1942). Having observed that auxins cause decreases in 

 cytoplasmic viscosity, he suggested that auxins bring about dissocia- 

 tion of the protein constituents of the cytoplasm. The dissociation 

 effect would increase water permeability, increase the osmotic value 

 of the cytoplasm, and possibly also bring about increased enzymatic 

 activity. It has been shown that gentle protein dissociation can some- 

 times activate enzymes (Hand, 1939), and such dissociation might 

 increase the availability of substrates for the enzymes as well. Conse- 

 tjuently one would expect that increased respiratory activity and 



