Interactions of Indoleacetic Acid and Gibberellic Acid in Leaf Abscission Control

A 1-hour application of indole-3-acetic acid to bean (Phaseolus vulgaris L. cv. Red Kidney) explants inhibited abscission for an 8-hour aging period. Use of indole-3-acetic acid-'4C showed that the applied indole-3-acetic acid was conjugated within explant tissue and that this conjugation mechanism accounts for loss of effectiveness of indole-3-acetic acid in inhibiting abscission after 8 hours. Reapplication of indole-3-acetic acid to an explant at a later time, before the induced aging requirement was completed reinhibited abscission. 2,4-Dichlorophenoxyacetic acid, which is not destroyed or conjugated by this system, did not lose its ability to inhibit abscission. It was concluded that indole-3-acetic acid destruction is one of the processes involved in the aging stage of abscission in explants.Indoleacetic acid has been demonstrated to effectively inhibit abscission in many plant systems if it is applied to the distal cut surface of petioles (10,14,15,17). The applied auxin has been supposed to replace the continuous auxin supply from the leaf (14,20). However, the mechanism by which IAA exerts its inhibition properties on abscission is unknown. Research has been primarily concerned with auxin transport systems, auxin balances, and auxin concentrations. Such work is well summarized in recent review articles (2, 12).There has been no correlation between time of application, location, or form of auxin at the time it functions to inhibit abscission processes. This paper presents a series of experiments designed to determine movement and conjugation of IAA in explants as correlated with initiation of abscission. MATERIALS AND METHODSPlant Material. Bean plants (Phaseolus vulgaris L. cv. Red Kidney), were grown at 24 + 2 C, 64% relative humidity, and a 12-hr photoperiod from fluorescent and incandescent lamps with 1200 ft-c at plant level. Petiole abscission zone explants from the primary leaf were excised from 14-day-old plants and inserted into a 3-mm layer of 1.5% agar contained in 43 + 2-ml gas collection bottles (5, 6). During the experiments, the explants were exposed to 25 C and 400 ft-c of continuous fluorescent light. Unless otherwise noted, each experimental datum represents experi- ments repeated on three different occasions with three sets of 10 explants.Application of Chemicals. Indole-3-acetic acid, the conjugates and derivatives of IAA (indoleacetyl aspartate, indoleacetamide, and ethyl indoleacetate), and 2,4-D were applied by placing a 50-Al drop of 1.5% agar containing the compound on the distal cut surface of the explant. In instances where the agar was subsequently removed, the explant was wiped with a moist tissue (10). The concentration of growth substance supplied in the agar was 50 M for IAA and its derivatives and 500 AM for 2,4-D.These concentrations of IAA and 2,4-D were selected as being effective for inhibition of abscission (1, 8, 10).Radioactive Assays. In studies on the movement of conjugation mechanisms of IAA in explants, IAA-1-14C (specific radioactivity 25.6 mc/mmole) o...

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confidence: 82%

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confidence: 99%

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Abscission

Craker¹,

Chadwick²,

Leather³

1970

“…The potential of hormones such as auxin and gibberell,in to act as effector sulbstances (5,16) th,at conitrol repression and derepression o.f different pants of the genome and their involvement in abscission control (2,12) led us to look at the effect of these hormones on the rate of protein synthesis in the tissue associated with abscission. This paper demonstrates the biochemical distinctiveness with respect to protein synthesis of the abscission zone from the adjacenit tissue.…”

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confidence: 99%

Protein Synthesis in Abscission: The Distinctiveness of the Abscission Zone and its Response to Gibberellic Acid and Indoleacetic Acid

Lewis

Bakhshi

1968

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A bstract. Abscission zone tissue of citrus was shown to have a higher rate of protein synthesis than tissue distal or proximal to it. based on the incorporation of leucine-1-14C. The proximal tissue had the slowest rate of protein synthesis. As the tissue approached abscission, the rate of synthesis in the abscission zone decreased and the differences in rate of protein synthesis between the 3 zones almost disappeared. IAA, which delayed abscission, maintained the protein synthesis gradient between the abscission and proximal zones, but the distal zone tissue was as active in protein synthesis as the abscission zone. Differences in uptake of the leucine were also observed between different zones and treatments.Regardless of the tissue or the treatment, there was a sharp increase in uptake at. the 24 hour point.Direct incubation of abscission zones in IAA and gibberellic acid (GA) indicated that the action of gibberellic acid on abscission is probably through a stimulation of protein synthesis, while IAA seems to act by maintaining the existing rate of protein synthesis in the cells of the abscission zone.The potential of hormones such as auxin and gibberell,in to act as effector sulbstances (5, 16) th,at conitrol repression and derepression o.f different pants of the genome and their involvement in abscission control (2,12) led us to look at the effect of these hormones on the rate of protein synthesis in the tissue associated with abscission. This paper demonstrates the biochemical distinctiveness with respect to protein synthesis of the abscission zone from the adjacenit tissue. Materials and MethodsSeedlings of sweet orange (Citrus sinensis, Osbeck) were the source of ithe plant miaiterial used in these experiments. Planits about 9 months old were selected for uniformity of height, leaf color, and leaf qutaliilty. The growing terminal was removed so that only mature le-aves were tused. 'T'he leaf blade and petiolar winigs were removed, leaving a 4 mm 'porition of the leaf midrib (the debladed midrib) aittached to the petiole (12). Preliminary experiments showed no correlation between the position of the deblialded midrib on the seedling and the order in which they aibscised. Hormones to be tested were dissolved in 0.1 At citrate buffer, pH 6.0, and incorporated into an equlal vollume of 2 % (w/v) agar. A 5 juliter dropleft of the agar gel, containing the test compound was applied to the 1 Present address: Punjal) Agrictlltural University.Hissar, India. distal eni,d of each explaant with a tuberculin syringe. Con'troll plianits were treated with a simil,ar drop of buffered 1 % (w/v) agar. Treated seed,l1ings were reotained in the growth ch,amber in the dark at 250 and a relative hutmidcity of 60 % or higher.Incorporation of L-leucine-1-'AC (specific activiity 180 uc/mg) into the alcohol and oxalate insolluble fraction w'as used ais a meialsure of the ra'te of protein synthesiis associated wilth various stages of abscission. In 1 series of experimnents, abscission zone sections abouirt 1 mm thick were incubate...

“…Usually, however, these data do not show as great a magnitude of stimulation by auxin (6,7,11) as with bean, and in some cases no stimulation was observed (2,5,(9)(10)(11)(12) phase response to auxin in abscission has been conclusively found only in beans. In reviewing the effects of auxin in abscission, Jacobs has questioned the importance of the stimulatory role of auxin (8).…”

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confidence: 96%

The Influence of Auxin, Cacodylic Acid, and Amitrole on the Abscission of Petiole Explants

Burt

Muzik

1971

The influence of indoleacetic acid, cacodylic acid (hydroxydimethylarsine oxide), and amitrole (3-amino-1,2,4-triazole) on the petiole explant abscission rate was studied in three species. Indoleacetic acid increased the abscission rate in both bean (Phaseolus vulgaris L. var. Red Kidney) and Coleus (Coleus blumei Benth) at 10' and 10' M but had no effect on abscission in privet (Ligustrum ovalifolium). Cacodylic acid was found to stimulate abscission in explants of beans and privet, but not in Coleus. Amitrole did not stimulate abscission under any circumstance tested. In no case was the abscission rate dependent on the time at which any of the chemicals was applied. These data do not support the two-phase response of explants to applied auxin.The two-phase theory of petiole explant response to applied auxin was developed with bean petiole explants (14). This theory was developed primarily from experiments using explants treated with auxin at different times after excision. It was found that auxin at high (10-' M) concentrations stimulated abscission if placed on an aged petiole explant but inhibited abscission if placed on an explant freshly excised from the intact plant. Auxin in low concentration (10-5 M), however, stimulated abscission in both aged and fresh explants.Based on these experiments, it was proposed that the petiole exists in two different physiological states which determine the response to the applied auxin. Auxin's accelerating effect on abscission is caused by auxin's ability to increase ethylene evolution (1, 3). The first physiological state is considered insensitive to auxin-mediated ethylene evolution, whereas, in the second state, auxin-mediated ethylene production is effective in accelerating the abscission process. It is assumed that applications of a high concentration of IAA to fresh explants prevent the explants from entering phase 2 and thus block the abscission process.Data supporting thse two-phase theory of auxin action in abscission have been published on other species than bean. Usually, however, these data do not show as great a magnitude of stimulation by auxin (6, 7, 11) as with bean, and in some cases no stimulation was observed (2, 5, 9-12). The two- phase response to auxin in abscission has been conclusively found only in beans. In reviewing the effects of auxin in abscission, Jacobs has questioned the importance of the stimulatory role of auxin (8).This paper describes experiments conducted to re-evaluate the two-phase hypothesis using different species and environmental conditions. MATERIALS AND METHODSTwo basic types of experiments are reported with three species and three different compounds. The first type of experiment was conducted to determine if the compounds could stimulate abscission in explants at low concentration. For these experiments, explants were placed in various concentrations of the compound under study and the abscission rate was determined. The second type of experiment was conducted to determine if the explants have a two-phase reaction to...