JAMES BONNER 



California Institute of Technology 



On the Mechanics of Auxin-Induced Growth' 



It is a cornerstone of growth-substance lore that auxins cause an in- 

 creased rate of plant cell elongation. The question of how auxins 

 bring about such auxin-induced cell elongation constitutes a classical 

 problem of auxinology. We shall here consider the state of knowledge 

 on this subject. The present discussion is restricted to information 

 concerning but a single plant tissue, the Avena coleoptile section, 

 about which much is known. The elongation of the Avena coleoptile 

 is normally controlled by auxin produced in the apex of the organ. 

 The native hormone, which appears to be chemically identical with 

 indole-3-acetic acid (I A A) (29), moves by polar transport to the lower 

 regions of the coleoptile and there promotes elongation. Excised sec- 

 tions of the coleoptile, floated in solution, respond to added lAA by 

 increased growth rate, and such sections constitute a convenient sys- 

 tem for the study of auxin-induced growth. 



Two characteristics of the response of Avena coleoptile sections to 

 lAA deserve particular note. The first, as illustrated in Figure 1, is 

 that the response is large. The rate of elongation of the section in the 

 presence of added lAA (at optimal concentration) is seven- to tenfold 

 greater than the rate of elongation in the absence of the growth sub- 

 stance. The second noteworthy characteristic is that elongation in the 

 absence of added lAA is not controlled by endogenous growth sub- 

 stance. This conclusion is based upon the fact that the rate of elonga- 

 tion of sections in the absence of added lAA is not slowed by added 

 competitive inhibitors of auxin action. In the response of the Avena 



^ Much of the work here summarized has been made possible by the continued 

 and generous support of the Herman Frasch Foundation. The present discussion 

 is primarily a summary of work done with this support and is not intended as a 

 general revie\v of the entire field. 



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