The Phototropic Responses of Avena Coleoptiles

Macleod, K., Firn, R. D. and Digby, J. 1986. The phototropicresponses of Avena coleoptiles.—J. exp. Bot. 37: 542–548. A number of studies of the elongation rate changes causing phototropismhave been made but the findings of different groups have notbeen entirely Consistent. Studies of oat coleoptile phototropismin response to first-positive and second-positive doses indicatethat no single pattern of elongation rate changes causes phototropismeven in a single species. The relative effect of phototropicstimulation on the elongation rate at the shaded or the illuminatedside of coleoptiles subject to unilateral illumination dependson physiological state of the cell-for instance its positionin the elongation zone or whether it has been given red lightrecently. Models of phototropism will have to account for sucha diversity of phototropic responses. The importance of makingfull elongation rate measurements has once again been demonstrated. Key words: Phototropism, first-positive, second-positive, phytochrome, coleoptile, elongation rate     

Some Responses of Avena Coleoptiles to Ethylene

Ethylene pretreatment of intact Avena seedlings or of excisedcoleoptile sections results in an increased response of thecells to auxin. It is suggested that ethylene brings about theacceleration of hydrolytic reactions controlling the physicalproperties of cell walls and hence increases their capacityfor growth. Coleoptile elongation of intact seedlings is inhibitedby ethylene; this inhibition is concurrent with a lateral expansionof the entire coleoptile. It is suggested that under a givenset of conditions coleoptile cells are capable of attaininga finite volume and that the preferential lateral expansioninduced by ethylene is accomplished at the expense of longitudinalextension. Experiments with intact and deseeded plants indicatethat lateral expansion depends on the supply of some factorfrom the endosperm.     

The Effects of Paraffin Oil on Phototropic and Geotropic Responses in Avena Coleoptiles

Experiments are described which were designed to test the significanceof the coleoptile tip as the site of reception of light stimulusleading to negative photo-tropic response under paraffin oil.The results show clearly that the tip is of paramount importancein this respect. Further experiments in which the coleoptiletips were bisected in a plane at right angles to the light rayslend support to the hypothesis that negative phototropism underoil is related tolateral transport of material in the tip. Lastly,experiments which show that the geotropic response of coleoptilesis not reversed by immersion in oil are described. These findingsare discussed in relation to certain hypotheses concerning themechanism of negative phototropism under oil.     

Effects of Chlorpromazine on Gravitropism in Avena Coleoptiles

Chlorpromazine (CPZ), an inhibitor of the calcium-activatedform of calmodulin, is readily taken up by the roots of intactoat seedlings but poorly translocated from the roots to thecoleoptile of these plants. However, plants repeatedly rotatedthrough solutions containing low concentrations of CPZ (10^–8–10^–5M)are infiltrated, and under these conditions, CPZ significantlyinhibits the negative gravitropic response of the coleoptilewithout retarding elongation growth. This effect is observablein ‘decapitated’ (apical 1–2 mm removed) coleoptilesections and in intact whole coleoptiles. If exogenous auxinis supplied to the decapitated sections, both their growth ratesand gravitropic responsiveness are increased and, under theseconditions, CPZ can reduce the gravitropic curvature withoutreducing the overall growth rate. These results are discussedin relation to the possible role of calmodulin-dependent calcium-ionpumps in gravitropism. chlorpromazine, gravitropism, calmodulin, calcium, oat, Avena sativa     

Growth and Geotropic Responses of Avena Coleoptiles to 4-Amino-3,5,6-Trichloropicolinic Acid

The elongation and geotropic responses of coleoptile sections of Avena sativa L. to various concentrations of 4-amino-3,5,6-trichloropicolinic acid (Tordon) proved to be qualitatively similar to those previously reported for 2,3,6-trichlorobenzoic acid (TCBA). Tordon stimulated growth in a range of concentrations from 1 × 10^?6 to 1 × 10^?4M but higher concentrations were inhibitory. Geotropic curvature was extensively depressed by 1 × 10^?5 and 1 × 10^?4M Tordon, concentrations which accelerated elongation. A similar differential effect has been reported for TCBA and other auxins. Several other picolinic acids and related compounds were tested, but only very slight responses were noted.     

DMPA and Avena Coleoptiles RNA Turn-over

Addition of α-methoxy-(3, 4-dichlorophenyl) acetic acid (DMPA) at different concentrations in the culture medium of sub-apical sections of Avena coleoptile results in an increased growth rate, which correspond to the quantitative measure of the increasing of ribosomal and soluble RNA turn-over. The relative decrease in total RNA synthesis observed for the supraoptimal DMPA concentration of 1000 μg/ml appears to be due to the specifical retardation in the 4 S RNA turn-over.     

Effect of Peeling on IAA-induced Growth in Avena Coleoptiles

The act of peeling removes the epidermis exclusively from Avenacoleoptiles. Peeling inhibits IAA-induced growth, by inhibitingthe growth of segments incubated in the presence of IAA, andpromoting that of those incubated in water. The magnitude ofthe inhibition of IAA-induced growth is proportional to theamount of epidermis removed. It is shown that neither lateralswelling, wounding, anaerobiosis, nor exposure to supraoptimalconcentrations of IAA cause the inhibition. It is concludedthat in Avena coleoptiles the epidermis regulates the rate ofexpansion of the underlying parenchyma cells and is the principaltarget of IAA-action. Avena sativa L., oat, coleoptile, indol-3-ylacetic acid, auxin, extension growth     

Isolation and Purification of an alpha-Mannosidase from Coleoptiles of Avena sativa

An alpha-mannosidase has been purified from the coleoptiles of Avena sativa L. var. Segrehavre. The enzyme, which is tightly associated with the cell wall, was solubilized with 3 m LiCl. The purification involves precipitation with (NH(4))(2)SO(4), gel filtration, ion exchange chromatography, and isoelectric focusing. The enzyme appears homogeneous when chromatographed on disc gels and on isoelectric focusing gels. The enzyme runs as a single protein of constant specific activity when chromatographed on Sephadex G-200. The estimated molecular weight of the enzyme is 630,000. The enzyme appears to have no metal ion cofactor requirement and is insensitive to p-chloromercuribenzoate. The pH optimum for the enzyme with p-nitrophenyl-alpha-d-mannoside as the substrate is 4.5 and the K(m) is 3.2 mm. The enzyme may have some carbohydrate associated with it as indicated by a positive periodate-Schiff reaction on disc gels.     

An Examination of a Method of Auxin Assay using the Growth of Isolated Sections of Avena Coleoptiles in Test Solutions

1. Methods of auxin assay using the Avena coleoptile are discussed.
2. A review is given of experimental procedure and evaluationofresults in the straight-growth method using isolated sectionsof coleoptiles in test solutions.
3. Possible sources of variationin the straight-growth methodare investigated and discussed.
4. A revised experimental procedure for the straight-growth methodis described.

The author wishes to thank Professor E. Ashby for his adviceand encouragement during the course of the experiments. Thanks are also due to Mr. D. Payne, a technical assistant inthe Botany Department, for his assistance in some of the assays.     

Subcellular Location of Phosphoglucomutase and UDP Glucose Pyrophosphorylase in Avena Coleoptiles

Extraction of phosphoglucomutuse and UDP glucose pyrophosphorylase from oat coleoptiles using both aqueous and non-aqueous systems showed that both enzymes are largely soluble. Phosphoglucomutase was completely absent from the cell wall fraction. The inhibition of phosphoglucomutase by peroxyacetyl nitrate was confirmed and the enzyme in the subcellular participate fractions was shown to be more inhibited than that in the soluble fraction. UDP glucose pyrophosphorylase was not inhibited by peroxyacetyl nitrate or ozone in vivo but was inhibited by peroxyacetyl nitrate in vitro. The SH reagents, iodoacetamide and p-chloromercuribenzoate, inhibited phosphoglucomutase severely but UDP glucose pyrophosphorylase moderately or not at all.     

The Effect of Indoleacetic Acid on Cell Wall Extensibility in Avena Coleoptiles

The extensibility of cell walls in segments of living Avenacoleoptiles has been determined after treatment with IAA bythe use of a microextensometer. It has been demonstrated thatIAA has an effect both on the plastic and elastic extensibility,which is more marked if the segments are allowed to grow, exceptat temperatures nearing 0° C. The auxin-induced increasein extensibility is therefore not a direct effect on the walland metabolic factors must be involved. The results are interpreted,along cunent lines of thought, as indicating effects on thelinkage between protopectin or hemicellulose chains themselvesand between these and the microfibrillar network of cellulose.The bearing of these interpretations on the normal growth processin cell walls is briefly discussed.