Natural auxins 



pertinent also is the observation by Nance that acetaldehyde is liberated 

 from some tissues by the addition of chlorophenol or indolephenol. This 

 suggests that free aldehyde liberation depends both upon the particular redox 

 system as well as upon the aldehyde diversion reactions present in tissues or 

 heterogeneous tissue preparations. It would be of considerable interest to 

 establish whether the mechanism indicated in Figure 9 operates with the 

 indolyl analogue of pyruvic acid, and whether the sequence can be blocked 

 by dimedone or bisulphite. Either of these carbonyl reagents will completely 

 inhibit the conversion of tryptophan to lAA. 



The recent work of Leopold and Guernsey (1953), supported in part by 

 that of Siegel and Galston (1953), is appropriate in this connection. It may 

 be recalled from the former work that auxins enhanced sulphydryl oxidation 

 in CoA mixtures; the latter work indicated that esterification of auxin with 

 CoA does occur. Moreover, the results of Leopold and Guernsey show a 

 fascinating correlation between the physiological activity of various auxins 

 and their ability to enhance what is presumably CoA esterification in vitro. 

 Additional significance may therefore be attached to the hypothetical scheme. 

 It implies that (1) the process of auxin formation may be channeled directly 

 into the auxin action mechanism without necessarily going to free lAA; 

 (2) administered lAA and other auxins function by backing into the mechan- 

 isms of auxin action (as well as auxin 'inactivation') as the acyl ester; (3) the 

 auxin which functions as a hormone, i.e. as a translocated free acid, is the 

 residual of concomitant biosynthetic and depletion mechanisms. Attractive 

 as such a hypothesis may be, the operation of the pyruvate dehydrogenase 

 mechanism upon which it is based is difficult to reconcile with the inhibitions 

 occurring after irradiation by X-rays. The enzymatic conversions of trypto- 

 phan and indoleacetaldehyde are highly radiosensitive, whereas very large 

 doses of radiation have no observable effect on the enzymatic transformation 

 of indolepyruvate to lAA in the same plant material. 



Finally, we may glance again at Figure 1 and comment briefly about the 

 nitrile (IAN). Although the existence of IAN in one family of plants may be 

 considered as established, the mechanisms of nitrile biosynthesis and of 

 hydrolysis to lAA indicated in Figure 1 are very hypothetical. There is no 

 evidence that the nitrile arises from tryptophan in vivo, or even that a 

 mechanism for the transformation of tryptophan to IAN exists in plant 

 tissue. While hydrolysis of the nitrile to lAA in a number of plant tissues 

 does occur, apparently the amide is not an intermediate as suggested by 

 Jones et al. (1952). In the chromatographic examination of IAN hydrolysed 

 by plant preparations, Stowe and Thimann (1954) could find no trace of the 

 amide, though they did identify I AA. The lack or low activity of IAN as an 

 auxin in a number of plant tissues where lAA is active, and the absence of a 

 mechanism converting the nitrile to an acid in many tissues able to utilize 

 tryptophan, makes it rather unlikely that IAN is a normal intermediate in 

 the conversion of tryptophan to lAA. 



In brief, at this time it can be said that tryptophan quite likely is the primary 

 precursor of lAA, that indoleacetaldehyde probably is involved either as a 

 free or complexed carbonyl, that indolepyruvate may be involved, and that 

 tryptamine and indoleacetonitrile probably are not involved in normal 

 auxin production (see also Gordon, 1954). If the keto-acid does participate, 



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