Dieter Klämbt

Bio: Dieter Klämbt is an academic researcher from University of Bonn. The author has contributed to research in topics: Auxin & Coleoptile. The author has an hindex of 15, co-authored 38 publications receiving 1100 citations.

Functional evidence for an auxin receptor at the plasmalemma of tobacco mesophyll protoplasts.

Abstract: Tobacco mesophyll protoplasts were previously shown to respond to naphthaleneacetic acid by modifying their transmembrane potential difference. In the present work, evacuolated protoplasts were used to show that this response resides only at the plasmalemma. This electrical response was investigated by using polyclonal antibodies directed against plasma membrane antigens presumably involved in the reception and transduction of the auxin signal. An IgG fraction from an antiserum directed against the membrane auxin-binding protein from maize coleoptile completely inhibited the naphthaleneacetic acid-induced response of tobacco protoplasts. The suppression of the auxin-induced variation in the transmembrane potential difference by an IgG preparation directed against the plasmalemma ATPase from yeast demonstrated the involvement of the ATPase in the electrical response. Variation induced by fusicoccin in the transmembrane potential difference of tobacco protoplasts was unaffected by the anti-auxin-binding protein IgG fraction but was completely suppressed by the anti-ATPase IgG preparation. These results demonstrate the presence of a membrane receptor for auxin at the plasmalemma, the binding of the hormone to this receptor leading to the activation of the proton-pumping ATPase. They also show that at least the primary steps of activation by naphthaleneacetic acid are distinct from those of the fusicoccin-induced response.

Perception of the auxin signal at the plasma membrane of tobacco mesophyll protoplasts

Abstract: Summary Auxin-induced variations of transmembrane potential difference have been shown to be a useful tool for analyzing hormone sensitivity in tobacco protoplasts. Using this technique, we demonstrated that protoplasts derived from wild-type, an auxin-resistant mutant and Agrobacterium-rhizogenes transformed plants differed widely in the sensitivity of their electrical response to naphthalene acetic acid. We have used different antibodies, raised to auxin binding proteins (ABP) from maize coleoptiles, or to the axr1 gene product (ABP1), to test whether changes in auxin sensitivity can be correlated with the presence of tobacco proteins immunologically related to this ABP. Titrations indicated that 0.4 nM anti-ABP IgG inhibited 50% of the auxin-specific response of wild-type protoplasts, whereas 0.04 nM or 4 nM anti-ABP IgG were necessary to inhibit the response of mutant and transformed protoplasts, respectively, to the same extent. On wild-type protoplasts, blocking part of the immunoreactive sites with anti-ABP antibodies resulted in a decrease in auxin sensitivity of the electrical response (0.4 nM anti-ABP IgG inducing a 10–fold decrease), whereas addition of maize ABP increased this auxin sensitivity (1 pM ABP1 raised the sensitivity more than 1000–fold). The results obtained suggest that the auxin sensitivity detected by our assay system correlates with the amount of tobacco proteins immunologically related to the axr1 gene product from maize. A hypothesis accounting for the presence of these proteins at the external surface of tobacco protoplasts and for the effects of hetero-logous maize ABP on auxin sensitivity is proposed.

cDNA clones of the auxin-binding protein from corn coleoptiles (Zea mays L.): isolation and characterization by immunological methods.

Abstract: An auxin-binding protein (ABP) cDNA clone was selected from a lambda gt11 cDNA library from corn coleoptiles with highly purified IgGanti ABP The sequence of 794 bp contains an open reading frame (ORF) of 603 bp, coding for a 22 kd protein There are indications of a signal peptide of 38 amino acids (von Heijne, G 1983, Eur J Biochem, 133, 17-21) A N-glycosylation site can be deduced and a C-terminal KDEL amino acid sequence is detected An EcoRI fragment containing the beginning portion of the cDNA with about three quarters of the ORF was used to select cDNA clones from an independently produced lambda gt11 cDNA library of corn coleoptiles Northern blot analysis with in vitro transcribed biotinylated RNA showed a single band of not more than 850 bases The full-length in vitro transcript directed the in vitro synthesis of a protein which is precipitated by IgGanti ABP Rabbit antibodies raised against a fusion protein detect the ABP as a double band on Western blots Only the smaller of the two ABP bands is labeled by two different KDEL-specific IgG preparations

A view about the function of auxin-binding proteins at plasma membranes

Abstract: The auxin-binding protein isolated from maize coleoptiles and characterized in detail describes an auxin recognition protein at the outer surface of the plasmalemma which mediates the auxin effect on cell elongation in maize coleoptiles. Its homologue in tobacco mesophyll protoplasts mediates the auxin effect on secretion. The cDNA clones of the auxin-binding protein independently sequenced in three different laboratories contain one unique open reading frame describing the auxin-binding protein as a nonmembrane-integrated glycoprotein containing the ER-sorting C-terminal tetrapeptide KDEL. There are hints but no hard facts that a membrane-located receptor for the ABP-auxin complex and a G-protein may be included in this signal-transducing pathway.

Structural models of primary cell walls in flowering plants: consistency of molecular structure with the physical properties of the walls during growth

Abstract: Advances in determination of polymer structure and in preservation of structure for electron microscopy provide the best view to date of how polysaccharides and structural proteins are organized into plant cell walls. The walls that form and partition dividing cells are modified chemically and structurally from the walls expanding to provide a cell with its functional form. In grasses, the chemical structure of the wall differs from that of all other flowering plant species that have been examined. Nevertheless, both types of wall must conform to the same physical laws. Cell expansion occurs via strictly regulated reorientation of each of the wall's components that first permits the wall to stretch in specific directions and then lock into final shape. This review integrates information on the chemical structure of individual polymers with data obtained from new techniques used to probe the arrangement of the polymers within the walls of individual cells. We provide structural models of two distinct types of walls in flowering plants consistent with the physical properties of the wall and its components.

Plant Growth Substances Produced by Azospirillum brasilense and Their Effect on the Growth of Pearl Millet (Pennisetum americanum L.)

Abstract: Azospirillum brasilense, a nitrogen-fixing bacterium found in the rhizosphere of various grass species, was investigated to establish the effect on plant growth of growth substances produced by the bacteria. Thin-layer chromatography, high-pressure liquid chromatography, and bioassay were used to separate and identify plant growth substances produced by the bacteria in liquid culture. Indole acetic acid and indole lactic acid were produced by A. brasilense from tryptophan. Indole acetic acid production increased with increasing tryptophan concentration from 1 to 100 μg/ml. Indole acetic acid concentration also increased with the age of the culture until bacteria reached the stationary phase. Shaking favored the production of indole acetic acid, especially in a medium containing nitrogen. A small but biologically significant amount of gibberellin was detected in the culture medium. Also at least three cytokinin-like substances, equivalent to about 0.001 μg of kinetin per ml, were present. The morphology of pearl millet roots changed when plants in solution culture were inoculated. The number of lateral roots was increased, and all lateral roots were densely covered with root hairs. Experiments with pure plant hormones showed that gibberellin causes increased production of lateral roots. Cytokinin stimulated root hair formation, but reduced lateral root production and elongation of the main root. Combinations of indole acetic acid, gibberellin, and kinetin produced changes in root morphology of pearl millet similar to those produced by inoculation with A. brasilense. Images

The Five "Classical" Plant Hormones.

Abstract: It is 60 years since Went and Thimann (1937) published their classic book Phytohormones. At that time, the term phytohormone was synonymous with auxin, although the existence of other phytohormones, such as cell division factors, was anticipated on the basis of physiological experiments. It is impressive that aside from some confusion about the structure of auxin, many of the basic phenomena of auxin physiology were already known at that time. It is equally impressive that much auxin biology, including the CholodnyWent hypothesis (Went and Thimann, 1937) regarding the role of auxin in mediating graviand phototropism, the pathway of auxin biosynthesis, and the mechanism by which auxin causes cell wall loosening, remains controversial. Since 1937, gibberellin (GA), ethylene, cytokinin, and abscisic acid (ABA) have joined auxin as phytohormones, and together, they are regarded as the "classical five" (Figure 1). This group is expected to grow as the hormonal functions of other compounds are recognized and as new hormones are discovered (see Creelman and Mullet, 1997, in this issue). As is evident from this short review, recent progress on hormone biosynthesis and on hormonal transduction pathways has been impressive. Also evident is that there are many blanks still to be filled in. With the application of the powerful new techniques of chemical analysis and molecular genetics, the rate at which new discoveries are made will continue to accelerate. It's a great time to be a plant hormonologist!

Cellular Responses to Auxin: Division versus Expansion

Abstract: The phytohormone auxin is a major regulator of plant growth and development. Many aspects of these processes depend on the multiple controls exerted by auxin on cell division and cell expansion. The detailed mechanisms by which auxin controls these essential cellular responses are still poorly understood, despite recent progress in the identification of auxin receptors and components of auxin signaling pathways. The purpose of this review is to provide an overview of the present knowledge of the molecular mechanisms involved in the auxin control of cell division and cell expansion. In both cases, the involvement of at least two signaling pathways and of multiple targets of auxin action reflects the complexity of the subtle regulation of auxin-mediated cellular responses. In addition, it offers the necessary flexibility for generating differential responses within a given cell depending on its developmental context.