Auxin Transport Inhibitors

Abstract

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...

Insensitivity of the Diageotropica Tomato Mutant to Auxin

“…The responses of dgt and VFN8 hypocotyls to active and inactive IAA analogs or to inhibitors ofauxin transport were also determined (Table II) (7,19), neither TIBA or NPA significantly affected IAA-induced elongation of VFN8 hypocotyls (Table II).…”

Reorientation of Seedlings in the Earth's Gravitational Field Induces Cytosolic Calcium Transients

“…4). If the specificity of these two inhibitors NPA and TIBA can be believed (Katekar and Geissler, 1980), then the [Ca 2ϩ ] c signal underpinning the spike may be related to auxin movement. Does this indicate that changes in auxin flux are more rapid than previously thought?…”

“…Hence, it is conceivable that treatment with either inhibitor eventually damages sensitive cells so they are no longer able to respond to gravity. Moreover, TIBA supposedly acts as a weak auxin (Katekar and Geissler, 1980), but treatment often reduces the total auxin present in the tissue (Goldsmith, 1969). Finally, TIBA reacts with sulfhydryl groups so side effects are likely, complicating interpretation.…”

“…N-(1-Naphthyl) phthalamic Acid (NPA) and 2,3,5-tri-iodo benzoic acid (TIBA) are inhibitors that have been used for many years to influence the polar transport of auxin and inhibit gravitropic responses (Katekar and Geissler, 1980). Reorientation of growth induced by asymmetric gravity stimulation is in part the result of modified auxin movement and normally commences with a lag period of about 15 to 20 min.…”

Identification and Characterization of Linoleic Acid as an Endogenous Modulator of in Vitro N-1-Naphthylphthalamic Acid Binding

“…Although exact molecular details are lacking, it is clear that the phytotropin receptor is intimately involved in the polar transport of the auxin IAA (Lomax et al, 1995). Using a variety of synthetic compounds, a reasonably good correlation between in vitro binding activity and in vivo inhibition of polar auxin transport has been established (Katekar and Geissler, 1980).…”