WILLIAM S. HILLMAN 



and 

 WILLIAM K. PURVES 



Yale University 



Does Gibber eltin. Act Through an 

 Auxin- mediated Mechanism? 



Probably most studies of the chemical control of plant growth have 

 dealt with the action of auxins. Recently, however, an increasing 

 number of other growth regulators have claimed the attention of 

 plant physiologists. These include the gibberellins (2), various sub- 

 stituted purines (13), and compounds obtained from endosperm tis- 

 sue (12). Since the mechanisms of action of all such compounds, in- 

 cluding auxins, are unknown, the question as to whether the gib- 

 berellins act through an auxin-mediated mechanism cannot be an- 

 swered conclusively. The chief motive for this attempt is that, for 

 various reasons, the working hypothesis implied by the question ap- 

 pears to have been somewhat uncritically accepted as established in 

 a few papers. In our view, the evidence is largely negative, and such 

 acceptance unjustified. Our first task will be to define the question 

 more precisely. 



DEFINING THE QUESTION 



The term auxin will be used to mean indole-3-acetic acid (lAA) or 

 closely related natural or synthetic compounds presumed to have 

 similar physiological action. This is similar to the definition given by 

 Tukey et al. (14) except that their definition could also be used to 

 include the gibberellins, which would merely confuse the issue here. 

 For gibberellin see the review by Brian (2). Gibberellic acid (GA) was 

 used in the experiments reported. 



An affirmative answer to the question would mean that the physio- 

 logical action of gibberellin is due to a primary effect on a bio- 

 chemical system participating directly in auxin synthesis, transport, 

 action, or inactivation. The term auxin-mediated thus implies an in- 

 timate relation between gibberellin action and auxin, and the ques- 



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