Phototropic stimulation does not induce unequal distribution of indole-3-acetic acid in maize coleoptiles

Distribution of endogenous diffusible auxin into agar blocks from phototropically stimulated maize coleoptile tips was studied using a bioassay and a physicochemical assay, to clarify whether phototropism in maize coleoptiles involves a lateral gradient in the amount of auxin. At 50 min after the onset of phototropic stimulation, when the phototropic response was still developing, direct assay of the blocks with the Avena curvature test showed that the auxin activity in the blocks from the shaded half-tips was twice that of the lighted side, at both the first and second positive phototropic curvatures. However, physicochemical determination following purification showed that the amount of indole-3-acetic acid (IAA) was evenly distributed in the blocks from lighted and shaded coleoptile half-tips at both the first and second positive phototropic curvatures. The even distribution of the IAA was also confirmed with the Avena curvature test following purification by HPLC. These results indicate that phototropism in maize coleoptiles is not caused by a lateral gradient of IAA itself and thus cannot be described by the Cholodny-Went theory. Furthermore, the lower auxin activity in the blocks from the lighted half-tips suggests the presence of inhibitor(s) interfering with the action of auxin and their significant diffusion from unilaterally illuminated coleoptile tips.     

Inhibition of auxin-induced elongation and xyloglucan breakdown in azuki bean epicotyl segments by fucose-binding lectins

Auxin-induced elongation of epicotyl segments of azuki bean ( Vigna angularis Ohwi and Ohashi cv. Takara) was suppressed by fucose-binding lectins from Tetragonolobus purpureus Moench and Ulex europaeus L. These lectins also inhibited auxin-induced cell wall loosening (decrease in the minimum stress-relaxation time of the cell walls) of segments. Auxin caused a decrease in molecular mass of xyloglucans extracted with 24% KOH from the cell walls. The lectins inhibited auxin-induced changes in molecular mass of the xyloglucans. The autolytic release of xylose-containing products from the pectinase-treated cell walls was also suppressed by the lectins. Fucose-binding lectins pretreated with fucose exhibited little or no inhibitory effect on auxin-induced elongation, cell wall loosning, or breakdown of xyloglucans. These results support the view that the breakdown of xyloglucans is involved in the cell wall loosening responsible for auxin-induced elongation in dicotyledons.     

The changing sensitivity to auxin of the plasma-membrane H+-ATPase: Relationship between plant development and ATPase content of membranes

The auxin sensitivity of the plasma-membrane H⁺-ATPase from tobacco leaves (Nicotiana tabacum L. cv. Xanthi) depends on the physiological state of the plant (Santoni et al., 1990, Plant Sci. 68, 33–38). Results based on the study of auxin sensitivity according to culture conditions which accelerate or delay tobacco development demonstrate that the highest auxin sensitivity is always associated with the end of the period of induction to flowering. Auxin stimulation of H⁺-translocation activity corresponds to an increase of the apparent ATPase affinity for ATP. The plasma-membrane H⁺-ATPase content, measured with an enzyme-linked immunosorbent assay using a specific anti-H⁺-ATPase antibody, varies according to plant development, and was found to increase by 100% during floral induction. The specific molecular ATPase activity also changes according to plant development; more particularly, the decrease in molecular ATPase activity upto and during the floral-induction period parallels the increase of sensitivity to indole-3-acetic acid.Abbreviations ELISA enzyme-linked immunosorbent assay - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate Authors are grateful to Mrs. Grosclaude (Lab. Virologie, INRA, Jouy-en-Josas, France) and Mrs. Boudon (Lab. Mycoplasmes, INRA, Dijon, France) for support and advice in the preparation of antibodies. This work was supported by grants No. 89/512/6 from the E.P.R of Bourgogne and No. 89 C 0662 from M.R.T.     

Stimulation by auxin of phospholipase A in membrane vesicles from an auxin-sensitive tissue is mediated by an auxin receptor

Microsomal vesicles were prepared from zucchini (Cucurbita pepo L.) hypocotyls containing radioactive phosphatidylethanolamine or phosphatidylcholine, and these lipids were used as substrates by phospholipase A which is activated by auxins. Phospholipase D and phospholipase C hydrolysed the same substrates but were not influenced by auxin. Phospholipase A was activated by the auxins indolyl-3-acetic acid, 2,4-dichlorophenoxyacetic acid and, to a lesser extent, by -naphthaleneacetic acid whereas the weak auxins 2,3-dichlorophenoxyacetic acid and -naphthaleneacetic acid were almost inactive. This hormone specificity was also found in growth tests with etiolated zucchini hypocotyls. Phospholipase A activation by auxin was blocked by a polyclonal antibody against the maize auxin-binding protein. We propose that phospholipase A activation is a primary reaction in the signal transduction leading from hormone-binding to the growth response.Abbreviations IAA indolyl-3-acetic acid - 2,3-D, 2,4-D 2,3- and 2,4-dichlorophenoxyacetic acid - -NAA; -NAA - and -naphthaleneacetic acid This work was supported by the Deutsche Forchungsgemeinschaft. We thank D. Klämbt (Botanical Institute, University of Bonn, FRG) for a generous gift of polyclonal antibody (IgG fraction) against auxin-binding protein and U. Kutschera (Botanical Institute, University of Bonn, FRG) for advice with the growth tests.     

The place of brassinolide in the sequential response to plant growth regulators in elongating tissue

In the sequential response to plant growth regulators in young elongating tissue from peas and wheat the peak of sensitivity to 24-epi-brussinolide (1 μM) occurs after those of gibberellin and cytokinin and begins before that of auxin in isolated wheat ( Triticum vulgare L. ev. Egret) coleoptiles aged from 21-96 h. In dwarf pea ( Pisum sativum L. cv. Greenfeast) segments, the peak of sensitivity also lies between those of gibberellin and auxin, and it also occurs before sensitivity to auxin in sections from first leaves of wheat. All the leaf sections and all but the most mature coleoptiles and pea segments were sensitive to fusicocein (1 μM).     

Acetic acid esters and permeable weak acids induce active proton extrusion and extension growth of coleoptile segments by lowering the cytoplasmic pH

In Avena coleoptile segments a decrease of cytoplasmic pH activates energy-dependent H⁺ extrusion into the apoplast, thereby triggering extension growth. This sequence of events cannot be inhibited by cycloheximide and is induced by the following conditions and compounds. (i) A short anaerobic treatment of coleoptile segments results in the formation of lactic acid and an intracellular decrease of pH. For a period of 20 min after transfer to normal air, the growth rate is up to six times higher than the rate before anaerobiosis. (ii) Similarly, incubation of segments with CN^– (0.1 mM) in the presence of oxygen causes and accumulation of lactic acid and a fall in cell-sap pH. After removing CN^– a growth burst occurs. (iii) Higher concentrations of permeable acids (10 mM in buffer pH 5.8) induce extension growth. This growth is O₂-dependent and therefore differs from the acid growth, which can be triggered under anaerobic conditions by acid buffers of pH5 via the direct increase of cell-wall plasticity. (iv) A short application of CO₂-saturated buffer (pH 5.8) causes CO₂-induced elongation growth; after a 3-min pulse the growth rate is enhanced for about 15 min. (v) Lipophilic esters of acetic acid or propionic acid, such as naphthylacetate, naphthylpropionate, phenylacetate, benzylacetate induce elongation growth. These compounds, when taken up into the cell, are hydrolized by esterases; the acids released lower the cytoplasmic pH (shown by the pH indicator, fluorescein). The highest esterase activity was found in a microsomal membrane fraction of coleoptiles. While the carboxyester-induced extension growth is completely inhibited under anoxia, the initial acidification of the bathing solution can still be observed. This decrease in external pH is obviously the result of ester hydrolysis, caused by damaged cells, and is not the result of pH changes within the cell-wall compartment. It is suggested that a fast uptake of carboxyesters and the shift in equilibrium caused by their internal hydrolysis leads to a continuous formation of acids which lowers the cytoplasmic pH and activates the ATP-dependent H⁺ extrusion. In most experiments fusicoccin (a diacetic acid ester) acts similarly to naphthylacetate and the other carboxyesters, although quantitative differences exist. Therefore, it is possible that fusicoccin is effective partly on the basis of its ester characteristic. The effects observed are discussed with regard to the very narrow pH optimum of plasma-membrane H⁺-ATPases exhibiting their highest levels of activity at pH 6.5 (Hager and Biber 1984, Z. Naturforsch. C 39, 927–937).Abbreviations CHM cycloheximide - DMO dimethadione (5.5-dimethyl-2,4-oxazolidinedione) - FC fusicoccin - IAA indole-3-acetic acid - Mes 2-(N-morpholino)ethanesulfonic acid - NA (or )-naphthylacetate (acetic acid-1(or-2-)naphthylester) - NAA (or )-naphthaleneacetic acid - PA phenylacetate (acetic acid phenylester)     

Effect of applied and endogenous indol-3-yl-acetic acid on maize root growth

The level of endogenous Indol-3-yl-acetic acid (IAA) measured by gas chromatography-mass spectrometry in the elongating zone of intact primary roots of Zea mays showed a good linear correlation with the growth rate of these roots. When they were treated with IAA, their relative elongation decreased; this indicates a supraoptimal content of endogenous IAA. However, the growth of some of the relatively rapidly extending roots was enhanced by such treatment. Interactions between endogenous and applied IAA in the control of root growth are discussed.Abbreviations GC-MS gas chromatography-mass spectrometry - IAA Indol-3-yl-acetic acid     

A mechanism of respiration-dependent water uptake enhanced by auxin

Summary There are many contradictory observations on the mechanohydraulic relation of growing higher plant cells and tissues. Graphical analysis of the simultaneous equations which govern irreversible wall yielding and water absorption has made more comprehensive the understanding of this relation when relative growth rate is plotted against turgor pressure. It suggests that some respiration-dependent and auxin sensitive process might regulate the difference of osmotic potential between cells and water source. Based on anatomical and electrophysiological knowledge of the pea stem xylem, we propose the wall canal system as the mechanism of respiration-dependent water uptake which is sensitive to auxin. This system consists of the xylem apoplastic walls, the xylem proton pumps, active solute uptake system and cell membranes. In the simplest case, third-order simultaneous differential equations are involved. Numerical analysis showed that net uptake of solutes enables water to be taken up against an opposing gradient of water potential. The behaviour of this wall canal system describes well the mechano-hydraulic relation of enlarging plant cells and tissues. Recent typical, but incompatible, interpretations of this relation are critically discussed based on our model.Abbreviations V the volume of enlarging symplast - the average extensibility of the wall - Pⁱ turgor pressure - Y the yield threshold of the wall - L the relative hydraulic conductance - the solute reflection coefficient of the plasmamembrane - Cⁱ the osmotic concentration of the symplast cells - C^x the osmotic concentration of the xylem vessels - P^x hydrostatic pressure in the xylem vessels - R the gas constant - T absolute temperature - ^o water potential of xylem fluid - ⁱ water potential of symplast cells     

Some factors affecting spore replication of Nosema algerae (Microspora,Nosematidae) in Pieris brassicae (Lepidoptera)

The production of Nosema algerae spores was examined in Pieris brassicae. Spore replication in the insect host followed a logistic pattern of development. The factors studied which affected spore production and replication were dose level (5 × 10², 5 × 10³, and 5 × 10⁴ spores per insect), larval instar (fourth and fifth), and cool pretreatment of the insects at 20°C prior to inoculation compared with a constant temperature of 26°C. A three-way analysis showed the interactions between these factors. The logistic pattern of spore replication was used to explain the results.