The more active members of a proposed class of auxin transport inhibitors have been shown to have the ability to inhibit the active movement of auxin at concentrations where they have Uttle effect on auxin action and no significant auxin activity. They have also been shown to give rise to characteristic biphasic dose-response curves on cress root growth. Based on these physiological similarities and other common physiological properties, it is concluded that they may achieve their effects by a common mode of action which differs from that of other known auxin transport inhibitors. It is suggested that the name "phytotropins" be given to the class of auxin transport inhibitors now defined by a similar mode of action and common chemical properties.It has been shown that a group of chemicals which were known to affect the geotropic response can be defined by a common set of chemical requirements (16) and that these requirements are at least similar to those which can give rise to inhibition of auxin transport ( 18). The group can be divided into eight chemical types, and these are shown in Figure 1. Rules which define the chemical requirements so far known have been formulated (18). In summary, a 2-carboxyphenyl group separated by a conjugated system of atoms from a second aromatic ring may be necessary for a molecule to have high activity.At least one member of each type has been established to be an auxin transport inhibitor, as well as having the additional physiological properties of being able to abolish the root and stem geotropic responses, the phototropic response, and the apical dominance effect. From the close chemical similarity between compounds within each type, it can be inferred that members within each type may act by a similar mode of action. The broader structure-activity correlation, however, implies a relationship between the types (18). It also implies that the mode of action of the group as a whole may be different from other auxin transport inhibitors which do not conform to the chemical parameters. An investigation of this relationship, therefore, would seem warranted, especially since some compounds from within the group, e.g.NPA' III-1; CPD IV-I, and DPX 1840 VIII-I (Fig. 2), have found 'Abbreviations: CFM, methyl-2-chloro-9-hydroxyfluorene-9-carboxylate; CPD, l-(2'-carboxyphenyl)-3-phenylpropane-1,3-dione; CPP, 5-(2'-carboxyphenyl)-3-phenylpyrazole; DPX 1840, 3,3a-dihydro-2-(p-methoxyphenyl)-8H -pyrazolo(5 -1a)isoindol-8-one; NPA, N-(naphth-1-yl)-phthalamic acid; PBA, 2-(1-pyrenoyl)benzoic acid; PCIB, p-chlorophenoxyisobutyric acid; TCBA, 2,3,6-trichlorobenzoic acid; TIBA, 2,3,5-triiodobenzoic acid. use in plant physiological research, together with other auxin transport inhibitors which do not belong to the group, e.g. TIBA IX and the morphactins X (Fig. 2).To enable a class of compounds having similar physiological and chemical properties to be defined, chemicals of known high auxin transport inhibiting activity from within types II through VII were comparatively assessed for re...

Two properties of phytotropins, their ability to bind to 1-N-naphthylphthalamic acid (NPA) receptors located on microsomal vesicles isolated from Cucurbita pepo L. hypocotyls, and to stimulate auxin (indol-3-yl acetic acid, IAA) accumulation into such vesicles by blocking its efflux from them, were assessed in double labelling experiments using [2,3,4,5-(3)H]1-N-naphthylphthalamic acid and 3-indolyl-[2-(14)C]acetic acid. Two sites of differing affinities and activities on IAA accumulation were found. 1-N-Naphthylphthalamic acid was found to have high affinity (KD at 10(-8)mol·l(-1)) for one site and low affinity (KD at 10(-6) mol·l(-1)) for the other, whereas 2-(1-pyrenoyl)benzoic acid displaced NPA with high efficiency (KD below 10(-8) mol·l(-1)) from both sites. Other phytotropins had intermediate affinities for either site. Occupation of the site with low affinity for NPA stimulated auxin accumulation, while occupation of the high-affinity site with a phytotropin did not interfere with auxin accumulation into vesicles.

The structural requirements of a proposed class of auxin transport inhibitors have been shown to be very similar to those required to inhibit the cress (Lepidium sadvum) root geotropic response. A 2-carboxyphenyl group separated by a conjugated system of atoms from a second aromatic ring appears to be necessary for a molecule to have high activity.A variety of substances which can prevent the active movement of auxin in plants is now known (17,18,20). It is at least possible that some of them may act by a common mechanism, and those that do would therefore be expected to have some similarities with respect to physical and chemical properties, enabling them to act at the same active site. It is known that many compounds which inhibit auxin transport also have the ability to abolish the root geotropic response (13,18) although the nature of the relationship between these two processes has yet to be defined. The structural requirements of auxin transport-inhibiting compounds are of interest because such knowledge should assist in defining the physiological processes which are involved and lead to possible mechanisms by which they act. Recently it has been shown that a class of compounds which affect the root geotropic response can be defined by a common set of chemical and physical parameters (13), and since it may be that some of the chemicals which affect both processes achieve their result by a common mechanism, the extent to which the same parameters were required for activity with respect to auxin transport was investigated. MATERIALS AND METHODSChemicals. 2-(1-Pyrenoyl)benzoic acid I (8) and 2-(1-naphthoyl)benzoic acid Id (12) were prepared by the literature methods. Preparation of 1-(2'-carboxyphenyl)-3-phenylpropane-1, 3-dione IV (6) 3,3a-dihydro-2-phenyl-8H-pyrazolo- [5,la]isoindole-8-one V; R=H (10) 2-phenyl-8H-pyrazolo- [5 ,la]isoindol-8-one VI (10) 5-phenacylidine-2(5H)-furanone IVb (6), 1-(2'-carboxyphenyl)-butane-1,3-dione IVc (6), 2-phenyl-8H-pyrazolo[5-la]isoindol-8-one VI, and 5-(2-carboxyphenyl)-3-phenylpyrazole VII (10) has been previously The general technique based on that described by Keitt and Baker (16) was used. This technique was used because it was designed to measure the effect of compounds on transport, rather than uptake of IAA. Twenty petiole segments 10 mm in length were cut and placed in Perspex holders with the basal end resting on receiver planchets of 1.5 ml of 1.5% agar, containing the required concentrations of chemicals under test. Donor blocks of agar gel were prepared containing 1 ,ug/ml of IAA-2-'4C (2.5 cm diameter, 1.5 ml volume, 1.5% agar, IAA activity 30 mCi/mmol). These were placed on the apical ends of the segments, which were incubated in a humid environment for 4 hr under laboratory lighting conditions. The radioactivity transported to treated blocks after this period was determined by a gas flow counter. Acropetal movement of IAA in experiments where receiver and donor blocks were reversed was 5% of basipetal movement in untreated controls, and was not sign...

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