Occurrence and Role of Growth Hormones 67 



in the same form or state. Although some auxin becomes available 

 immediately either through diffusion or extraction, an additional 

 amount of auxin can be usually obtained if the treatment is extended 

 over a longer period of time. These are called "free" auxin and 

 "bound" auxin respectively. 



Free auxin is capable of moving freely in the polar transport 

 system and is apparently immediately effective in growth. Bound 

 auxin on the other hand is sometimes active in growth and sometimes 

 not. It is not so unrestricted in movement as free auxin. Larsen (1951) 

 recognizes four different types of bound auxin: an auxin-protein com- 

 plex, neutral precursors of auxin, precursor complexes, and structural 

 protein sources of auxin. 



The existence of the first of Larsen's categories, auxin-protein 

 complexes, was first specifically demonstrated by Wildman and Gordon 

 (1942). It appears that auxin molecules, presumably indoleacetic acid, 

 are loosely adsorbed on the surface of various protein fractions. These 

 auxin molecules are active in growth but, as already mentioned above, 

 they are incapable of moving about freely. Generally they may be 

 removed from the adsorbed condition by treatment with mildly alkaline 

 solutions or by gentle proteolysis (Gordon, 1946). A more stable complex 

 has been found in pea roots, however (Siegel and Galston, 1953). The 

 formation of this auxin-protein complex utilizes high-energy phosphate 

 bonds and is inhibited by several metabolic poisons such as iodoacetate. 

 The auxin is not removed by either boiling or strong acid or alkaline 

 conditions. 



A neutral precursor of auxin was first separated from plants by 

 Larsen (1944). This can be considered as bound auxin, since it is 

 not immediately available for growth but can be converted into auxin 

 very easily. These precursors may be aldehydes which become actual 

 auxins upon conversion to free acids. Tests of the growth activity 

 of indoleacetaldehyde have indicated that it promotes growth by 

 reason of its conversion to indoleacetic acid in the test plant (Lar- 

 sen, 1949). Indoleacetonitrile may be another neutral precursor. 



Precursor complexes have been demonstrated by Berger and 

 Avery (1944) and re-examined by Funke and Soding (1948). These 

 complexes appear to be close associations between auxin precursors 

 and certain growth inhibitors. When the complex is broken two frac- 

 tions generally appear, one a neutral precursor (or sometimes an 

 auxin) and the other a growth inhibitor. These precursor complexes 

 have been found mainly in storage tissues such as seeds and tubers. 



Structural proteins containing tryptophan were first recognized 

 as potential sources of auxin by Skoog and Thimann (1940). They 



