Timothy J Mulkey

Inhibition of polar calcium movement and gravitropism in roots treated with auxin-transport inhibitors.

Primary roots of maize (Zea mays L.) and pea (Pisum sativum L.) exhibit strong positive gravitropism. In both species, gravistimulation induces polar movement of calcium across the root tip from the upper side to the lower side. Roots of onion (Allium cepa L.) are not responsive to gravity and gravistimulation induces little or no polar movement of calcium across the root tip. Treatment of maize or pea roots with inhibitors of auxin transport (morphactin, naphthylphthalamic acid, 2,3,5-triiodobenzoic acid) prevents both gravitropism and gravity-induced polar movement of calcium across the root tip. The results indicate that calcium movement and auxin movement are closely linked in roots and that gravity-induced redistribution of calcium across the root cap may play an important role in the development of gravitropic curvature.

Auxin action on proton influx in corn roots and its correlation with growth

At concentrations inhibitory to the elongation of corn (Zea mays L.) roots, the auxins, indole-3-acetic acid (IAA) and α-naphthaleneacetic acid (α-NAA), cause an increase in the pH of the bathing medium; this increase occurs with an average latent period shorter than the latent period for the inhibitory effect of these auxins on elongation. Indole-2-carboxylic acid, an inactive structural analogue of IAA, and β-naphthaleneacetic acid, an inactive analogue of α-NAA, affect neither growth nor the pH of the medium. Since acid pH is known to promote and basic pH to inhibit root elongation, the data are consistent with the hypothesis that hormone-induced modification of cell-wall pH plays a role in the control of elongation of roots, as has been proposed for elongation of stems and coleoptiles.

The kinetics of abscisic acid action on root growth and gravitropism

Using an auxanometer and time-lapse cinematography we have studied the timing of abscisic acid (ABA) effects on elongation, gravitropic curvature, and hydrogen-ion efflux in several cultivars of maize (Zea mays L.). The effect of high concentrations (e.g. 0.1 mM) of ABA on root elongation is triphasic, including 1) a period of promotion lasting approximately 12 h, 2) a subsequent period of increasing inhibition lasting approximately 12h, and 3) gradual recovery to a rate within approximately 80% of the control rate. With lower concentrations of ABA (e.g. 0.1 μM) only the transient promotive phase is seen. Abscisic acid enhances ethylene biosynthesis in roots of maize but suppression of ethylene biosynthesis does not prevent the long-term inhibitory action of ABA on growth. Application of ABA (0.1 mM) to the upper surface of horizontally placed roots accelerates positive gravitropism. Application of ABA to the lower surface retards gravitropism and in some cases causes the roots to curve upward against the direction of gravity. These observations are consistent with our finding that the initial effect of ABA on root elongation is stimulatory. Since root gravitropism is rapid enough to be completed within the stimulatory phase of ABA action, the data argue against hypotheses of gravitropism based upon accumulation of ABA to inhibitory levels on the lower side of a hirizontal root.

Promotion of growth and shift in the auxin dose/response relationship in maize roots treated with the ethylene biosynthesis inhibitors aminoethoxyvinylglycine and cobalt☆

Abstract Indole-3-acetic acid (IAA) in concentrations from 10 −12 M to 10 −9 M does not stimulate elongation when applied to intact roots of maize ( Zea mays L Bear Hybrid WF 9 × 38). Higher concentrations are inhibitory. In roots pretreated with the ethylene biosynthesis inhibitors, cobalt and aminoethoxyvinylglycine (AVG), IAA from 10 −10 M to 10 −8 M promotes growth. High concentrations of IAA (eg. 10 −6 M) strongly inhibit growth in either pretreated or non-pretreated roots. The data indicate that low concentrations of auxin are capable of stimulating the growth of intact roots in which ethylene biosynthesis is suppressed. This suggests that auxin-induced ethylene biosynthesis may account, at least in part, for auxin inhibition of root growth at high concentrations and for the failure of auxin to stimulate intact root growth at low concentrations.

Correlations between proton-efflux patterns and growth patterns during geotropism and phototropism in maize and sunflower

By placing seedlings of sunflower (Helianthus annuus L.) or maize (Zea mays L.) on agar plates containing a pH indicator dye it is possible to observe surface pH patterns along the growing seedling by observing color changes of the indicator dye. Using this method we find that in geotropically stimulated sunflower hypocotyls or maize coleoptiles there is enhanced proton efflux on the lower surface of the organ prior to the initiation of curvature. As curvature develops the pattern of differential acid efflux becomes more intense. A similar phenomenon is observed when these organs are exposed to unilateral illumination, i.e. enhanced acid efflux occurs on the dark side of the organ prior to the initiation of phototropic curvature and the pattern of differential acid efflux intensifies as phototropic curvature develops. These observations indicate that differential acid efflux occurs in response to tropistic stimuli and that the acid efflux pattern may mediate the development of tropistic curvatures.

THE OCCURRENCE OF CHLOROPHYLLS C1 AND C2 IN THE CHRYSOPHYCEAE1

We analyzed 34 strains representing 25 species of Chrysophyceae for chlorophylls c1 and c2 using thin‐layer chromatography. Most organisms had both chlorophylls c1 and c2 in addition to chlorophyll a but 17 strains of 9 species of Synura and Mallomonas possessed only chlorophylls a and c1. These are the first chlorophyll c‐bearing algae which lack chlorophyll c2. We postulate that at least some of the silica‐scaled algae including Mallomonas and Synura may be distinct from other Chrysophyceae based upon pigmentation and other characters described in the literature.

Effect of ethylene antagonists on auxin-induced inhibition of intact primary root elongation in maize (Zeamays L.)

Root elongation was measured in intact primary roots of maize(Zea mays L.) using a computerized root auxanometer. We examined the fact that root elongation was inhibited by auxin-induced ethylene production. Inhibition of root elongation was closely correlated with the concentrations of the exogenously applied auxin. Auxin-induced inhibition of root elongation was reversed by pretreatment or posttreatment of ethylene biosynthesis antagonists such as aminoethoxyvinylglycine (AVG) and silver ions (Ag2+). The magnitude of recovery effect was dependent on auxin concentrations. Root elongation was inhibited by adding ethylene producing agents such as 1-aminocyclopropane-l-carboxylic acid (ACC) and Ethephon. ACC- and Ethephon-induced inhibition of root elongation was reversed by blocking ethylene biosynthesis or activity. These data suggested that ethylene was involved in auxin-induced inhibition of root elongation and inhibition of root elongation by applied auxin, at least in part, was a reversible process.

Effects of abscisic acid and xanthoxin on elongation and gravitropism in primary roots of zea mays

We examined the involvement of abscisic acid (ABA) and xanthoxin (Xan) in maize root gravitropism by (1) testing the ability of ABA to allow positive gravitropism in dark-grown seedlings of the maize cultivar LG11, a cultivar known to require light for positive gravitropism of the primary root, (2) comparing curvature in roots in which half of the cap had been excised and replaced with agar containing either ABA or indole-3-acetic acid (IAA), (3) measuring gravitropism in roots of seedlings submerged in oxygenated solutions of ABA or IAA and (4) testing the effect of Xan on root elongation. Using a variety of methods of applying ABA to the root, we found that ABA did not cause horizontally-oriented primary roots of dark-grown seedlings to become positively gravitropic. Replacing half of the root cap of vertically oriented roots with an agar block containing ABA had little or no effect on curvature relative to that of controls in which the half cap was replaced by a plain agar block. Replacement of the removed half cap with IAA either canceled or reversed the curvature displayed by controls. When light-grown seedlings were submerged in ABA they responded strongly to gravistimulation while those in IAA did not. Xan (up to 0.1 mM) did not affect root elongation. The results indicate that ABA is not a likely mediator of root gravitropism and that the putative ABA precursor, Xan, lacks the appropriate growth-inhibiting properties to serve as a mediator of root gravitropism.

Auxin and Root Gravitropism: The State of Our Knowledge

During the past 10 years, increasing emphasis has been placed on the study of the involvement of hormones and calcium in growth and gravitropic response of roots. The establishment of gradients of calcium ions across the tip of roots appears to be essential for the initiation of asymmetric growth associated with the gravitropic response [7, 8, 10] and may be the signal transducer which links the root cap-associated sensor of gravitational orientation [9] and the hormonal control of asymmetric growth, which occurs during gravicurvature (GC).

Effect of auxin and ethylene on elongation of intact primary roots of maize (Zeamays L.)

We tested that the hypothesis that root elongation might be controlled by altering the level of ethylene in intact primary roots of maize(Zea mays L.). We measured root elongation in a short period using a computerized root auxanometer. Compounds which regulate ethylene production were applied to intact primary roots in different time periods. Root elongation was stimulated by the treatment with ethylene antagonists such as Co2+, aminoethoxyvinylglycine (AVG) and L-canaline. This result suggested that root elongation was closely related to ethylene level of intact primary roots. Furthermore, IAA- and 1-aminocyclopropane-1-carboxylic acid (ACC)-induced inhibition of root elongation was reversed by treatment with Co2+. The application of ACC to roots which have been exposed to IAA and Co2+ have no significant effect on root elongation. However, the inhibition of root elongation by ACC in roots previously treated with IAA and AVG became manifest when the applied IAA concentrations were lower. These results were consistent with the hypothesis that the level of ethylene in intact roots functions to moderate root elongation, and suggested that auxin-induced inhibition of root elongation results from auxin induced promotion of ethylene production.