Art E. Geissler

Structure-activity differences between indoleacetic acid auxins on pea and wheat

Abstract A series of halogenated indoleacetic acids was assessed for auxin activity on pea stem and wheat coleoptile sections. Activity differences between the two species were found. These are discussed, in terms of differences in receptor models for pea and wheat, with the models differing in the areas covered by the 6- and 7-substituted compounds.

Phytotropin-binding sites and auxin transport in Cucurbita pepo: evidence for two recognition sites

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 [2,3,4,5-3H]1-N-naphthylphthalamic acid and 3-indolyl-[2-14C]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 10-8mol·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.

Auxins II: The effect of chlorinated indolylacetic acids on pea stems

Abstract A series of chlorinated indolylacetic acids was assessed for auxin activity on pea stem sections. It is suggested that the activities shown are reasonably consistent with a receptor site theory of structure-activity previously proposed[1].

Recognition of phytotropins by the receptor for 1-N-naphthylphthalamic acid

The structure requirements for phytotropin activity and receptor binding are expressed in terms of a recognition site on the receptor with which phytotropins, including 1-N-naphthylphthalamic acid, interact. It is postulated that the site can be represented by a large region which accepts planar molecules, and is possibly electrophilic in nature. A second area is also postulated which may be lipophilic or electrophilic, together with a carboxyl acceptor. It is suggested that if the requirements of the carboxyl acceptor and adjacent area are met, then phytotropin activity will result if the candidate molecule has a configuration, or can adopt a configuration, such that a conjugated portion of the molecule can interact with the larger area. It is argued that the close relationship observed between receptor binding and effect on the gravitropic response implies that the receptors may be directly involved in the gravitropic response mechanism.

Auxin Transport Inhibitors: Fluorescein and Related Compounds

Fluoresceins are shown to be effective inhibitors of indoleacetic acid transport as measured by the receiver agar block technique, eosin having the same order of activity as 2,3,5-triiodobenzoic acid and N-1-naphthylphthalamic acid, with fluorescein less effective. It is suggested that many of their characteristic effects on plants, especially those which involve auxin, are at least partially due to their effects on auxin transport.

A conformational study of the topographical requirements of a phytotropin recognition site on the naphthylphthalamic acid receptor

Abstract Phytotropins are a group of chemicals which have the ability to abolish the gravitropic response in plants by inhibiting the polar movement of auxin in the plant. They have other physiological properties such as inhibiting the phototropic response of stems. They bind to the naphthylphthalamic acid receptor and may elicit their physiological responses by this means. Most phytotropins consist of a benzoic acid moiety substituted at the ortho position by a bridging group connected to a second aryl group. Conformational energy calculations were performed on a subset of phytotropins. The calculations yielded a single, low energy conformation common to each molecule and thus identified three dimensional requirements for binding to the receptor. Electrostatic potential calculations, in the vicinity of the benzoic acid moiety, identified recognition and binding requirements for this group. Similar calculations for the second aryl group indicated that some similarity exists between the electrostatic potentials of molecules which bind most tightly to the receptor. The revised binding model was assessed by consideration of a second series of molecules showing phytotropic activity. The model was consistent with the biological activities of these molecules.

Phytotropins: Conformational requirements for the abolition of the root geotropic response

Abstract Compounds of the phytotropin class have been assessed for possible conformational requirements with respect to their ability to aftect the root geotropic response. It is shown that part of the molecule may need to adopt a planar configuration, and evidence is adduced which indicates that the remainder of the molecule may also have conformational requirements which need further definition. It is suggested that molecules which favour a conformation such that the aromatic ring which bears the carboxyl function is out of plane with the remainder of the molecule may have activity. Coplanarity of the carboxyl group with the ring to which it is attached is not a prerequisite for activity.

Binding of lunularic acid, hydrangeic acid and related compounds to the receptor for 1-N-naphthylphthalamic acid

Abstract Lunularic acid and hydrangeic acid have been shown to bind with moderate affinity to the receptor for 1- N -naphthylphthalamic acid in maize coleoptiles and zucchini hypocotyls. While they can inhibit ???uxin egress from membrane vesicles, unlike non-phenolic phytotropins, they do not prevent the gravitropic response. Analogues of lunularic acid with the carboxylic acid function in the meta position had identical binding, while the carboxyl in the para position decreased binding.

On the role of the NPA receptor in the root gravitropic response mechanism

In a recent review of root gravitropism, attention has once again been focused upon auxin as the controlling substance emanating from the root cap. Changes in auxin sensitivity were proposed to account for the observed concentrations being insufficient to account for gravitropic bending (Evans 1991, and references cited therein). It remains possible, however, that a different substance, either unknown or unrecognized as a hormone, could control the response (Jackson and Barlow, 1981). This includes the possibility that such a substance may be a natural ligand of the NPA receptor. The suggestion that there may be such a natural ligand is not new (Thomson, 1972). Exploratory experiments aimed at determining the feasibility of such a postulate, are described.

Hormone Recognition in Plants

It is axiomatic that for a substance to produce an effect in a biological organism, there must be some interaction between its molecules and certain counterparts in the organism. In the case of a hormone, the substance is made by the organism itself, and it is there to control specific functions. To have any value, therefore, these counterparts must be able to interact with the hormone only, and not with other substances, including other hormones, which may be present. In other words, the counterparts - receptors - have a recognition characteristic. This would hold regardless of how the effect is achieved, or the nature of the effect.