CONTENTS INTRODUCTION "." '" ." ."."."."." .. "".".,,,.,, .,,.,,.,,.,,.,, . . ,,.,,,, .. ,,., ,.,,.,,." .. "."" .. "." . . . . " ....... , ..... 393 AUXIN-BINDING PROTEINS IN THE ENDOMEMBRANE SYSTEM .. " ...... " ... " ...... "..... 394 ASP i identification and Puri/ication ..... " ...... "." . . . . . . . . . .... " " . . . . . . . . . . " .. " "..... 394 ASP 1 Structure ... " . . " ... " ..... "." ""." "." .. "."."."." ..... , , ."."." .... "." .. "." ....... " .. 398 Auxin-binding Protein Localization in Organs, Tissues, and Cells"."" .. " 401 ASP J Function .... ... .... " "." ....... "." "." ....... "." "." " ....... "." ..... 403 AUXIN-BINDING PROTEINS IN THE PLASMA MEMBRANE" .. """" .. "" ".... 40 6 AUXIN-BINDING PROTEINS IN THE NUCLEUS " .... " ... """"""""""""",,.,,"",,.,,""""",, 408 SOLURLE AUXJN-BINDING PROTEINS THAT ARE NOT YET LOCALIZED""."."."" 410 Enzymes Labeled by 5-Azidoindole-3-acetic acid""."""""."""""."."."""""""""."."."." 410 Soluble Auxin-binding Sites"."." ....... " .. " ....... "."."."."."." .. "." .... "." "."." .... " .. "" .. 411 CONCLUDING REMARKS .. "." " " "."." ..... " .... " "."." 412

Auxin-Binding Proteins

We have reviewed recent progress in research on plant growth regulator (PGR) receptors. For some growth regulators, no receptor protein has yet been identified, but promising new approaches are discussed. For other receptors, specific and sensitive probes have been developed and, in one case, the membrane-associated auxin-binding protein of maize, these have been used to study the function of the receptor. The maize receptor has been cloned and sequenced; cDNA probes will allow the expression of receptor genes in normal and transformed plants to be studied. PGR sensitivity mutants have been described and, in conjunction with biochemical probes, should prove valuable in elucidating the functions of receptors and the nature of subsequent signal transduction events.

Receptors for plant growth regulators: Recent advances

Rapid advances have been made in the study of auxin binding proteins (ABPs) in the last five years. In particular, an ABP in maize membranes has been cloned, sequenced and both monoclonal and polyclonal antibodies to this ABP have been developed. Structural and functional analysis has begun and there is good electrophysiological evidence that ABP in the plasma membrane functions as a receptor, probably involved in auxin-induced cell expansion. The role of the large amount of ABP in the endoplasmic reticulum is less clear, as is the relationship to soluble ABPs. At present there is only some circumstantial evidence relating any ABP to cell division. Receptors for synthetic inhibitors of auxin transport (phytotropins) are also of interest in relation to auxin action, but are less well characterised. Identification of new naturally-occurring phytotropins could lead to novel plant growth regulators.

Auxin receptors: recent developments

Rapidly growing cell suspensions of soybean were analyzed for the presence of cytoplasmic high-affinity binding sites for auxin. Cytosol preparations were studied in lag, log and early stationary phase of the growth cycle. Two binding sites were detected, which show some similarities with binding sites previously reported from etiolated pea epicotyls. While the number of both sites declined in the cytoplasm during the growth cycle, the number of one of the two sites increased at the onset of rapid cell divisions. In parallel, both sites exhibited an increase in binding affinity during the growth cycle. The data will be discussed in relation to other reports on soluble auxin binding as well as to possible physiological functions.

Modulation of soluble auxin-binding proteins in soybean cell suspensions

In the study of auxin perception and transduction we have initially focused on auxin perception. In this respect we have tried to identify soluble IAA-binding proteins which, in analogy with the model of steroid hormone action, may interact with gene expression at the transcriptional level. The main characteristics of such binding sites from tobacco have been described [10] and it was found that addition of partially purified binding proteins to isolated nuclei from tobacco callus gave an IAA-dependent stimulation of transcription [15]. Such reconstitution experiments are very important in the study of transduction of the auxin signal. However, this system [15] was too crude to draw definite conclusions. One of the first steps in improving the system should be to better specify auxin-dependent stimulation of nuclear transcription in vivo. For this purpose we used an auxin-dependent batch-cultured cell line from tobacco as auxin-target system [16]. We were able to demonstrate that readdition of auxin to auxin-starved cells rapidly induced at least 7 classes of mRNAs.

IAA Perception and Auxin-Regulated Gene-Expression

It is a general observation in plant tissue culture experiments that explants, depending on type, age, physiological state or genotype show specific and different reactions to the application of phytohormones in the medium: Tissues seem to possess different competence to react to the phytohormones applied, in most cases auxins and/or cytokinins. In large seeded legumes, for instance, high concentrations of NAA, 2,4-D or Picloram induce the formation of somatic embryos from immature zygotic embryos (Lazzari et al., 1985, Barwale et al. 1985, Ranch et al. 1986, Kysely et al. 1986), while the same concentrations applied to leaf - or epicotyl explants will lead to poor or nearly no callus formation. From numerous physiological experiments on the action of phytohormones the concept has been evolved that there might be changes in the “growth substance sensitivity” (see Firn 1986 and the literature cited in this paper). It was argued by Trewavas (1981 a,b, 1982, 1983) that the number of receptors may be altered thus causing changes in the sensitivity. Basing the discussion on our own data on soluble auxin binding proteins in pea and soybean cell suspensions as well as on the observations mentioned above, we will discuss the possibility that, besides changes in the number of receptors or modifications of the affinity of the receptor, also differences in the expression of different receptor sites may contribute to the observed phenomena of differential phytohormone sensitivity.

Soluble Auxin-Binding: Is there a Correlation between Growth-Stage Dependent High-Affinity Auxin-Binding and Auxin Competence?

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Soluble Auxin-Binding: Is there a Correlation between Growth-Stage Dependent High-Affinity Auxin-Binding and Auxin Competence?