Curling leaves challenge cherished assumptions
It is certain that, like me, many of our readers have used 2,4-D in place of the naturally occurring auxin, IAA. In making this substitution we assume that 2,4-D acts in the same way as IAA. In many situations this is undoubtedly correct, but the work of Kawano et al. (Champenoux, France and Hiroshima, Japan, pp. 465-471) shows that it is not always so. Confirming the results of previous authors, they find that IAA causes a marked epinastic curvature of tobacco leaf strips. However, application of 2,4-D instead of IAA caused only a very weak curvature. This in itself is interesting, implying that in this system there is a significant quantitative difference between the effects of the two hormones. This contrasts with many other epinastic responses where IAA and 2,4-D have the same effect. A further factor in this situation is that many epinastic responses to auxin are actually mediated by ethylene, the synthesis of which is stimulated by the auxin whether it is IAA or 2,4-D. However, the epinastic response in tobacco leaf strips is ethylene-independent. Even more intriguing is the finding that when 2,4-D is applied to tobacco leaf strips together with IAA at optimal concentration, the 2,4-D inhibits the IAA-induced epinastic response. Further evidence for a different mode of action of the two hormones in tobacco leaf strips comes from a kinetic analysis of the interaction, using the Lineweaver-Burke plot. This clearly shows that inhibition by the synthetic auxin of the action of the natural auxin is not competitive. It is thus very unlikely that in this ethylene-independent response, they are acting via the same receptor, and the discovery in another laboratory of a 2,4-D-binding protein adds weight to this suggestion. We await with great interest the results of further research on these unexpected findings.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
