The kinetics of auxin-induced growth 



auxins are characterized by even lower values of F^^ax i^ the coleoptile 

 system. Thus the F^ax for j&flrachlorophenoxyacetic acid is about 0-4, that of 

 or/Aochlorophenoxyacetic acid about 0-14 of that found for the case of lAA. 

 We can approach this matter as indicated in Figure 4. 



Suppose that auxin, as we have already discussed, approaches the coleoptile 

 and interacts, binds, with it. This initial binding may occur either through 

 the carboxyl group (point 1) or the ortho position (point 2). Once the 

 initial binding is consummated, the complex thus formed can proceed to 

 consummate binding at the second position to form growth-active two-point 

 attached auxin-receptor complex, or alternatively it may react with a second 

 molecule o^ S to form inactive bimolecular complex ES^S^. We have already 

 seen that the interaction of coleoptile with auxin to form growth-active 



Assumptions 



Figure 4. Equilibria relating the 

 formation of growth-active auxin- 

 receptor complex {ESi 2) fo formation 

 of other related complexes in the Avena 

 coleoptile. After Foster, AicRae, and 

 Bonner {\952). 



E^S 



Growth = ?:fES^ ^J 



complex is a reversible reaction, implying then that all of the component 

 reactions are similarly reversible. Appropriate experiments (removal of 

 coleoptiles from high to low auxin concentration) reveal that the formation 

 of the ES^S.2 complex must be similarly reversible. In the steady-state 

 growth condition then the auxin receptor sites of the coleoptile would be 

 expected according to our kinetic hypotheses to be partitioned between the 

 various forms illustrated in Figure 4. The exact proportion of receptor in each 

 form will be determined by the equilibrium constants which describe the 

 tendency to formation of each complex. We will now show that auxins of 

 low Fjnax ^re those auxins for which the tendency to formation of growth- 

 active ES-^.2 is low relative to the tendency to formation of the growth- 

 inactive precomplexes such as ES-^ so that at any given instant many of the 

 total receptor sites of the coleoptile are tied up as inactive precomplexes. 



It has been indicated above that the curves which relate growth rate to 

 concentration of auxin are hyperbolae and that one parameter of such an 

 hyperbola is the concentration. Kg, of auxin which elicits half maximum 

 growth rate. This parameter signifies in our formulation that concentration 

 of auxin which is needed to assure formation of one half as many ES-^^ 

 complexes as would be formed at substrate saturation. We have similarly 

 seen that we can calculate for each inhibitor from our kinetic data a value 



V_ 



301 



