On the Ethylene-Auxin Interaction in Growth ControlAuthor(s): C. L. MerSource: New Phytologist, Vol. 73, No. 4 (Jul., 1974), pp. 653-655Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2431234 .

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New Phytol. (I974) 73, 653-655.

ON THE ETHYLENE-AUXIN INTERACTION IN GROWTH CONTROL

BY C. L. MER

Botany Department (ARC Staff), Imperial College of Science and Technology, South Kensington, London, S.W.7

(Received io February I974)

SUMMARY

The evidence on which this proposal was made has been examined and found unconvincing. An alternative interpretation of the data is suggested, which is amenable to a simple experimental test.

The idea that growth control results from an interaction between inhibition and promo- tion, respectively by ethylene and auxin, is one of the more interesting hypotheses to emerge from the study of ethylene metabolism during the last decade. This proposal (Burg and Burg, I966) has gained some measure of acceptance, but before it becomes fully established its validity might be carefully scrutinized.

The suggestion arises from data, reproduced here for convenience (Fig. i), which have been interpreted (with added italics) as follows. 'Those concentrations of IAA which inhibit elongation also stimulate the tissue to produce ethylene (Figs. I, 2), and since there is a close correlation between the intensity of ethylene production and the extent of inhibition, it is natural to wonder whether the ethylene might be contributing to the auxin response.'

These remarks appertaining to the data of Fig. i(b) at once strike a discordant note. The curve is a standard calibration-type response, the like of which has been obtained on many previous occasions to demonstrate, and to establish, that IAA is a growth-promot- ing substance. Why, then, an analysis in terms of inhibitions?

The data convey information as follows: with the control solution allowing an incre- ment of 3300, the lowest concentration of IAA (io8 M) slightly promoted elongation to 43 %; maximum stimulation was given by io-6 M, and thereafter the promotion de- clined to about 480% at the highest concentration of iO - 3 M. This concentration therefore induced a nett promotion of about I5%. The observations of ethylene production (Fig. ic) between the concentrations of io-6 M and iO-3 M identifies them as the ones which Burg and Burg described as inhibitory, despite the I5% promotion.

The only means whereby a promotion of this order can be turned into an 'inhibition' is by making the peak point of 80% into an artificial 'control'; an increment of 480 then becomes less than the 'control'. There is, however, no justification for making such a change (Mer, i968). But if this is the way in which the data are to be interpreted, then the 48% point to the left of the peak, caused by an IAA concentration of about 3 x IO-3

M, is also an 'inhibition' and with an increment of only 33%0 the control solution is evidently the most powerful 'inhibitor' of the entire sequence. These two 'inhibitions', moreover, are not associated with ethylene production. Furthermore, with the 'control'

653

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654 C. L. MER

at the peak, it becomes impossible to show that IAA is a growth-promoting substance. These conclusions, so much at variance with all the well-established views of IAA

activity, raise legitimate doubts about the cogency of their interpretation. An alternative explanation for the decline in promotion caused by these high, and

physiologically supra-optimal, concentrations of IAA can be derived from Burg and Burg's data without any need to invoke an inhibition. At the levels of IAA in question the increment in fresh weight, essentially a measure of volume, remained virtually unchanged while that of length declined sharply. From these two circumstances taken together it

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-log IAA concentration (M)

Fig. I. Effects of IAA on the increments of fresh weight (a), length (b), and production of ethylene (c). Data of Burg and Burg (I966, Fig. I). (a), (b). After 3 h: A, control in sealed flasks; after i8 h: o, control in Petri dishes; 0, control in sealed flasks; A, treated with IO ppm C2H2. All flasks contained 2% sucrose, 5 X IO-2 M potassium phosphate buffer (pH 6.8), and 5 x IO 2 M CoCl2-

may be inferred that the diameter must have increased; that is, the cells were swelling and this is an effect known to result from treatment with ethylene. In this event, incre- ments of length are unsatisfactory as indicators of auxin activity. Although it is possible to attribute this response to the ethylene released in these experiments, there is some evidence that IAA itself can give rise to a swelling. In some published data (Mer, I957) the mesocotyls of etiolated oat seedlings were cut across just above the grain and dipped into a solution of IAA. After 3 days the base of the mesocotyl had swollen to twice its normal diameter. The concentration of IAA was i ppm (about 5.7 x Io_6 M) which, according to Burg and Burg's observations, produces little or no ethylene.

These considerations suggest that acceptance of the idea of an auxin-ethylene inter- action in growth control be withheld temporarily, at least until data of cell dimensions have been obtained.

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Ethylene-auxin interaction 655

REFERENCES

BURG, S. P. & BURG, E. A. (I966). The interaction between auxin and ethylene and its role in plant growth. Proc. nat. Acad. Sci. U.S.A., 55, 262.

MER, C. L. (I957). A re-examination of the supposed effect of riboflavin on growth. Pl. Physiol., Lancaster, 32, I 75.

MER, C. L. (I968). What is an inhibitor? Z. PflPhysiol., 59, 4I5.

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