Aust. J. Plant Physiol., 1993, 20, 527-39 The Route, Control and Compartmentation of Auxin Synthesis H. M. ~onhebel~~, T. P. cooneyAC and R. simpsonA A Department of Biochemistry, University of Auckland, Private Bag, Auckland, New Zealand. Present address: Department of Science, University of Western Sydney, Hawkesbury, Richmond, NSW 2753, Australia. Present address: R. J. Hill Laboratories, PO Box 4048, Hamilton, New Zealand. Abstract The study of indole-3-acetic acid synthesis has undergone something of a revival recently in an attempt to understand the control of IAA levels. Results are, however, contradictory with three separate hypotheses emerging. Our own work supports older evidence for L-tryptophan as the IAA precursor and appears to simplify the metabolism of tryptophan to IAA. Work comparing incorporation of 'H from 'H~O into IAA, tryptophan, tryptamine and indole-3-pyruvate in tomato shoots showed that the indole-3-pyruvate became labelled at a rate compatible with it being the sole intermediate between tryptophan and indole-3-acetaldehyde. Results also showed that tryptamine was not involved in IAA synthesis although it was present. Indole-3-acetaldoxime was not detected in tomato shoots. An aromatic aminotransferase able to catalyse the synthesis of indole-3-pyruvate has been purified from mung beans. This enzyme was separated from aspartate aminotransferase and is fairly specific for aromatic L-amino acids. Other work, however, has implicated D-tryptophan as a more direct precursor than the L-enantiomer. A D-tryptophan aminotransferase has been isolated from dark grown pea seedlings. Finally, other recent work has indicated the existence of an alternative biosynthetic route to IAA which does not involve tryptophan. These results are reviewed in this paper and the apparent contradictions between them discussed. Introduction We are starting to know something of the action of indole-3-acetic acid (IAA) at the gene level, but which genes control IAA? Although auxin was the first hormone to be discovered, uncertainties still remain over its biosynthetic pathway. Most of the work leading to the currently generally accepted path of IAA synthesis was carried out during the late 1960s and early 1970s (see Schneider and Wightman 1974). These studies indicated that IAA is synthesised from tryptophan via three possible intermediates with rather weak evidence favouring the route via indole-3-pyruvate as the major path. However, there are conceptual difficulties in understanding a hormone, whose levels must be controlled, being synthesised via a number of different routes by enzymes, some of which apparently have low specificity and whose activities do not correlate with hormone levels. A desire to understand the control of IAA levels is presently the basis for considerable renewed research activity on IAA biosynthesis. This recent work has yielded important new information on IAA synthesis and has challenged the traditional hypothesis. In this paper the evidence for the traditional hypothesis is outlined and compared with more recent results. The somewhat contradictory observations obtained are discussed.