On form and function of plant growth substances 



interaction at a functioning macromolecular surface and/or at interfaces in 

 the cell, in our opinion is the most plausible starting point. 



We had previously, therefore, considered the possibility that auxins might 

 'start' activity of potential, but 'masked' enzymes (Veldstra and Booij, 1949) 

 (cf. also the discussion by Soding (1953), a theme further elaborated by 



Booij (1953)). 



While at the moment no direct evidence in support of these views can be 

 given and work in this domain is still in a preliminary stage, it may be useful 

 to focus attention on other possibilities of a physico-chemical action. 



Since auxins, in addition to promoting growth, are acting also in differen- 

 tiation (initiation of root formation, control of flower initiation) and are 

 morphological determinants too, it may well be that, apart from actions at 

 protein surfaces, the site of action will have to be looked for 'higher up' in 

 the hierarchy of the cell, viz. with the nucleic acids. 



In fact, Skoog (1950; 1953) already concluded that the action of auxins in 

 growth is intimately concerned with nucleic acid metabolism (DNA/RNA 

 balance), on account of evidence for lAA-adenine interactions in growth and 

 in organ formation and of lAA effects on nucleic acid content of the cells 

 (cf. also Galston et al., 1953, and Ber, 1953). 



In this connection we should like to point out the well-known facts that 

 in certain cases the deformations in plants, caused by an over-dosage of 

 growth substances (e.g. by 2:4-D), hardly can be distinguished from the 

 symptoms of a virus infection and that growth substances are able to influence 

 tiie course of virus infection in plants. Here one gets the impression that 

 influencing of synthesis (and/or functioning) of nucleic acids by growth 

 substances are involved or, perhaps, nucleic acid dynamics are interwoven 

 with auxin balances. 



Moreover, in recent work on animal cell physiology and embryonic 

 development, the possibility emerges that the action of oestrogens may be 

 directly connected with nucleic acid synthesis (Tondury, 1955). Since there 

 are connections between oestrogens and plant growth substances, with 

 regard to type of compounds and importance of spatial structure (cf. the 

 doisynolic acids (Miescher, 1946)) as well as to action (see, for example, 

 Helmkamp and Bonner, 1953), all possibilities of putting the pieces of this 

 jigsaw puzzle together should be sought. 



While expecting that they may be put together, we are convinced that 

 progress in our understanding of auxin action not only will imply a contri- 

 budon to the solution of one of the main problems of plant physiology, 

 but also to our insight into mechanisms which govern development and 

 differentiation in general. 



REFERENCES 



Ber, A. (1953). Studien iiber Auxinwirkung. Endokrinologie, 30, 329. 



Bonner, J. (1954). The hormonol control of plant growth. The Harvey Lectures, 



Series XLVIII, I. 

 Bonner, J., and Bandurski, R. S. (1952). Studies of the physiology, pharmacology, 



and biochemistry of the auxins. Annu. Rev. PL Physiol. 3, 59. 

 Booij, H. L. (1953). Lecture given at Plant Growth Substance Conference, Lund; 



see review by R. L. Wain, Nature, 172, 710. 



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