THE BIOGENESIS OF NATURAL AUXINS 



S. A. Gordon 

 Division of Biological and Medical Research, Argonne National Laboratory, Lemont, Illinois 



In attempting to organize material for this symposium, I realised that 

 extremely little is known about how auxin is synthesized in the plant. 

 Biochemical pathways and intermediates are easily formulated, and, 

 unfortunately, much too easily accepted. It is indeed fitting to bring to an 

 institution such as Wye College, whose tradition verges into the mythological 

 past, a topic so rich in mythology. In this paper I will attempt to summarize 

 current ideas of auxin biosynthesis and point out a few of the uncertainties. 



Most of us accept the pragmatic definition of auxins : growth substances 

 that, though of multiple effect, have the specific ability to stimidate cell 

 enlargement in shoots. Essentially it is a definition resting ultimately on 

 bio-assay. With this biological criterion, what is the native auxin of higher 

 plants ? 



Two decades ago this question could be readily answered — auxins a and b. 

 Since then a number of attempts to repeat the original classical work have 

 not succeeded, and many experiments indicated that the auxin in plant 

 extracts and diffusates was indoleacetic acid (lAA). Several workers then 

 took the position that lAA was the only auxin of higher plants. This position 

 was equally unwarranted, since it was based almost entirely on indirect tests of 

 doubtful validity: stability of activity in acid and alkali, molecular weight 

 by diffusion velocity, dose-response curves, biological responses elicited by 

 natural extracts as compared to pure auxins, colour tests, and destruction 

 by TAA oxidase'. 



Auxin is not a chemically specific term; it encompasses a large number of 

 compounds having certain structural similarities. In the past few years less 

 equivocal techniques have identified chemically, or indicated chromato- 

 graphically, the presence in plants of ethyl indoleacetate, indoleacetonitrile, 

 indoleacetaldehyde, and indolepyruvic acid. These compounds are all 

 biologically active and may be classified as auxins since at low concentrations 

 they stimulate cell elongation. I believe it is profitable, however, to consider 

 that they are inactive per se, and exhibit activity only after being converted 

 to lAA. Though this generalization cannot be rigorously defended, it seems 

 particvdarly appropriate to the identified indolyl substances. Many of the 

 responses these substances elicit are concomitantly accompanied by the 

 formadon of lAA. Indeed, indoleacetonitrile appears active only in those 

 plant species or organs possessing an active mechanism for hydrolysing this 

 nitrile to the acid. Thus, it does not seem unreasonable, at the moment, to 

 assume that the native auxin of hormonal function in the plant is lAA. Pure 

 lAA has the characteristics of polar transport, correlative action, and histo- 

 genicity as the native auxin formed at production loci. On this basis, our 

 title of natural auxin biogenesis may be altered to the more specific topic of 

 lAA biosynthesis. 



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