Chelation in Auxin Action

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Abstract
Factorial experiments have been carried out on the effects, upon growth of roots of intact wheat seedlings and growth of wheat coleoptile sections, of different concentrations of 3-indolylacetic acid (IAA) and various known chelating agents. These have demonstrated a similar mutual antagonism between pairs of agents whether these are IAA and a single known chelating agent or two known chelating agents. This interaction takes the form that either agent alone in ‘high’ concentration severely inhibits growth but this inhibitory effect is almost or entirely removed by the presence of one-millionth the concentration of the other agent; when both agents are present in ‘high’ concentration the inhibition is again severe. The substitution of a non-chelating analogue for one of the agents either destroys the mutual character of the antagonism or entirely prevents either agent at low concentration from reducing measurably the inhibition caused by high concentration of the other. The fact that IAA interacts with known chelating agents, in controlling the growth both of roots and coleoptile sections, in the same unexpected and symmetrical way that these interact with each other, is held strongly to support the hypothesis that it is here itself acting as a chelating or complexing agent; the absence of such interactions with a non-chelating analogue makes this the more convincing. These results are concerned with the removal of growth inhibition, due to supra-optimal concentrations of one agent, by minute proportions of another; it cannot be regarded as proven that the promotion of growth by IAA in the absence of another agent is also due to chelation or complex formation. This seems probable, however, when the findings here presented are taken in conjunction with the accumulating evidence that IAA and other auxins can form complexes or chelates with metals in vitro, and with the finding already published in detail that the eight chelating agents tried promoted growth in the wheat coleoptile test. The main criticisms to which this hypothesis has been subjected have been concerned with the relative magnitudes of effects of IAA and chelating agents upon growth, with the low stability constants of metal complexes with IAA and other auxins, with the lack of parallelism between stability constants and growth-promoting activity, and with the fact that one chelating agent (ethylenediamine-tetraacetic acid; EDTA) has been found inactive in certain growth tests. A series of factorial experiments comparing the authors' techniques (which are here described in detail), chemicals, and strain of wheat with those used by Fawcett et al. (1956) demonstrate that the discrepancies found, both as regards magnitudes of effects of IAA and EDTA and optimal concentrations, were partly due to differences in strain but mainly to differences of technique. It is considered that ‘foreign’ molecules such as EDTA are likely to have side effects, which may well differ in different strains or tests; competition with internal chelators (Burstrom and Tullin, 1957) is also likely to differ; differences in rate of penetration and steric hindrance may also be involved. For these reasons effective chelating activity in vivo may be very different from that in vitro and in the first instance the magnitudes of growth-promoting effects of chelating agents (which may indeed be the net result of stimulatory and inhibitory processes) seem less important than the fact that they are found in so many instances. Possible ways in which IAA and other growth substances may regulate growth by chelation or complex-formation are discussed.

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