Transport & Distribution of Auxin during Tropistic Response. I. The Lateral Migration of Auxin in Geotropism

A central tenet of the theory of tropisms is that shoots are enabled to bend toward light and away froml the earth by a migration of auxin across the stimiulated plant. Following Cholodny's proposal of such a lateral migration (12), Went in 1928 (31) shoX-ed directly that when the tips of Avena coleoptiles are illuminated from one side, more auxin diffuses out of the darker than out of the lighter side. The evidence was almost simultaneously extended to geotropism by Dolk, whose doctoral thesis (15) provi(lel a careful demonstration that when Avena coleoptile tips are held horizontal more auxin diffuses out of the lower than out of the upper side. Dolk's experiments further showed that when auxin is externally applied to horizontal sections cut from decapitated coleoptiles, it undergoes a similar asymmetric distribution. For both phototropism and geotropism the asymmetric distribution of endogenous auxin was soon confirmed by other workers, and the observations were extended both by diffusion and by extraction to dicotyledonous seedlings as well (see Went & Thimann (33) and Thimann & Curry (27) for reviews). Subsequently a large aggregate of concepts, though actually not many additional data, have crystallized around this simple notion of lateral auxin transport. In recent years, however, the lateral transport hypothesis has been seriously questioned, primarily because a number of workers failed to find any asymmetric distribution within the tissue when radioactive indoleacetic acid was applied to stimulated plant parts, and radioactivity, instead of growthpromoting activity, was the criterion. Negative results with plants placed horizontally or illuminated unilaterally were reported for more than one species by several groups of workers, notably Bunning et al. (10), Gordlon and Eib (19), Reisener (21, 22), Reisener and Simon (23), and Ching and Fang (11). On the other hand the original measurements, based on lioassay of endogenouts auxin diffusing from the basal surface of stimulated plant sections into agar blocks, have been confirmed and extended; Briggs et al. (9) and Gillespie and Briggs (16) have found clear-cut asymmetric distribution. Briggs (8) presents still more evidence for the reproducibility of these results. Much of this work has recently been reviewed by Anker (2). Such striking disagreement between two methods of measurement calls for explanation. If a plant indeed translocates its native indoleacetic acid, it must without discrimination move exogenous indoleacetic acid in the same way, providing that the applied auxin is made accessible to the transport system. It is of fundamental importance that when IAA-C14 is applied to oat coleoptile sections, the radioactivity transported out into agar blocks is indeed biologically active as auxin (18). On the other hand, several interpretations of the discrepancy may be considered. In the first place, the endogenous auxin which becomes asymmetrically distributed might not be indoleacetic acid (IAA). This is, however, made improbable by the repeated demonstrations that IAA is the auxin produced by Avena coleoptile tips (34, 20, 25) and by the fact that it does restore geotropic sensitivity to decapitated Avena coleoptile sections placed horizontally in solutions (1). As a second alternative, the material which becomes asymmetrically distributed might not be the auxin itself, but an auxin precursor or some other factor controlling the synthesis of auxin. Thirdly, it is important to note that the measurements of radioactivity have invariably been made on the tissue itself or on tissue extracts, while most of the measurements of biological activity have been made on agar blocks receiving the transported auxin. Thus gravity or light might conceivably cause the lateral migration of a factor controlling polar transport, so that although the amounts of auxin on the two sides of the plant remained the same, the amounts transported through the tissue into receiver blocks might be different. It would, indeed, be surprising if careful determinations of radioactivity in the tissue failed to reveal some change, but it is not impossible. Mechanisms of this and otlher kinds involving changes in auxin production. release, transport, or utilization on one side (without change in the anmounts present at any moment) have been postulated 1 Received Nov. 5, 1962. 2 This work was supported in part by a grant from the National Science Foundation, No. G9084, to Professor K. V. Thimann, and by predoctoral fellowships to B. Gillespie from the National Science Foundation (1959-61) and from the Public Health Service, Division of General Medical Sciences (No. CPM-8662-C1, 1961-63).