L. J. AUDUS 



and 

 J. K. BAKHSH 



Bedford College 

 University of London 



On the Adaptation of Pea Roots 

 to Auxins and Auxin Homologues 



The phenomenon of adaptation whereby enzyme balance in an 

 actively functioning cell may be modified or even transformed by a 

 change of exogenous metabolite or the administration of a growth 

 repressant has been widely studied in microorganisms. It now seems 

 likely that enzyme induction by endogenous substrates or by struc- 

 turally related molecules in the cell is a universal phenomenon 

 whereby the adjustment of constitutive enzyme levels is normally 

 accomplished (5). The fundamental significance of these new con- 

 cepts for the biochemical, and hence structural, differentiation of the 

 organism needs no stressing. Recently, these ideas from microbial 

 behavior have been applied to higher plants in which, it has been 

 suggested by Galston (6) , indole-3-acetic acid-oxidase may be induced 

 to form in this way by its own substrate. A certain amount of ex- 

 perimental evidence supports this claim, and the implications, in 

 terms of the auxin control of plant growth, are, as Galston has 

 pointed out, far-reaching. But Burstrom (4), who has demonstrated 

 that without doubt growing root cells show progressively adaptive 

 changes in their response to indole-3-acetic acid (lAA) when grown 

 continuously in dilute solutions of that substance, does not think 

 that the induced augmentation of lAA-oxidase activity can explain 

 those changes. He visualized a switch in the mechanism of cell ex- 

 tension involving, as we interpret his meaning, other enzyme sys- 

 tems, probably those concerned in the incorporation of cellulose into 

 the growing wall. This being so, it might be expected that changes 

 in growth response to other homologous plant growth regulators, 

 which could be expected to act in the same cell wall system, but 

 which are not metabolized by lAA-oxidase, might result from lAA 

 adaptation. Furthermore, the system might also be expected to adapt 



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