Ethylene Evolution from Mung Bean Hypocotyl Segments

    Abstract

    The effect of the defoliant thidiazuron (N-phenyl-N'1,2,3-thiadiazo15-ylures) on ethylene evolution from etiolated mung bean hypocotyl segments was examined. Treatment of hypocotyl segments with concentrations of thidiazuron equal to or greater than 30 nanomolar stimulated ethylene evolution. Increased rtes of ethylene evolution from thidiazuron-treated tissues could be detected within 90 minutes of treatment and persisted up to 30 hours after treatment. Radioactive methionine was readily taken up by thidiazuron-treated tissues and was converted to ethylene, I-aminocyclopropane-l-carboxylic acid (ACC) and an acidic coiJugate of ACC. Aminoethoxyvinylglycine, aminooxyacetic acid, cobalt chloride, and e-aminoisobutyric acid reduced ethylene evolution from treated tissues. An increase in the endogenous content of free ACC coincided with the increase in ethylene evolution following idiauron treatment. Uptake and conversion of exogenous ACC to ethylene were not affected by thidiazuron treatment. No increases in the extrctable activities ofACC synthase were detected following thidiazuron treatment. Harvest aids such as defoliants or desiccants are currently regarded as essential components of modem cotton production. The use of these plant bioregulators facilitates mechanical harvesting and improves the quality and value of the harvested cotton lint. Defoliating compounds are generally preferred over compounds that desiccate because the abscised leaves do not interfere with mechanical picking machines. TDZ'"2 (Fig. 1) is currently registered for use as a cotton defoliant. The abscission-inducing properties of this compound were first reported by Arndt et al. (4). The physiological basis(es) ofthe defoliating activity ofTDZ are currently unknown. Preliminary experiments in this laboratory have shown that treatment of cotton seedlings with TDZ results in a large and sustained elevation of ethylene evolution (22). Since ethylene is currently regarded as an endogenous regulator ofabscission in many plants (2, 18), it is important to understand the biochemical mechanism(s) by which TDZ enhances its production. Etiolated mung bean hypocotyls have been used extensively as a model system for evaluating the effects ofmany compounds that either enhance ' Mention of trademark or proprietary product does not constitute a guarantee or warranty of the product by the United States Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable. 2 Abbreviations: TDZ, thidiazuron; AVG, aminoethoxyvinylglycine; PCIB, 2-p-chlorophenoxy-2-methyl propionic acid; ACC, I-aminocyclopropane-l-carboxylic acid; AIBA, a-aminoisobutyric acid; AOA, aminooxyacetic acid. or reduce the rate of ethylene production. As a first step toward understanding/defining the physiological basis(es) of the defoliating activity of TDZ, its effect on ethylene evolution was examined. This paper describes the effects of TDZ on ethylene evolution from mung bean hypocotyls and attempts to define the biochemical basis for this interaction. A preliminary report of this research has been presented (22). MATERIALS AND METHODS Plant Material and Experimental Procedure. Mung bean (Vigna radiata L. Wilczek) seeds were surface-sterilized by soaking in 1% (v/v) NaOCI (1:5 dilution of commercial bleach) for 5 min. The seeds were then rinsed in running distilled H20 for 4 h and were sown in flats containing vermiculite. Etiolated seedlings were raised in an incubator (25 ± 1C) for 6 d. Hypocotyl segments (1.5 cm) were prepared by excising a portion of the hypocotyl immediately below the closed hook. All manipulations were conducted under a low fluence, green safelight in a darkroom. All experiments described in this paper were repeated at least three times. Whenever possible, each treatment within an experiment was replicated (n = 3). Due to the nature of some of the experiments, replication within an experiment was not feasible. Data from a typical experiment are presented. Chemicals. Technical-grade TDZ was a gift from E. Pieters of Nor-Am Agricultural Products, Inc. Stock solutions of thidiazuron were prepared in DMSO. A DMSO concentration of 0.1% (v/v) was used in all studies (including controls). AVG was a gift from Dr. R. W. Bagley of HLR Sciences, Inc. AOA, chloramphenicol, a-amino-isobutyric acid, BSA, DL-DTT, Hepes, pyridoxal-5-phosphate, sucrose, and PCIB were purchased from Sigma Chemical Co. ACC and Mes were purchased from Calbiochem. (NH4)2 SO4 was purchased from Schwartz-Mann. Dose-Response Studies. Excised hypocotyl segments were placed in 25 ml flasks that contained 4 ml of treatment solution (10 segments/flask). The treatment solution consisted of 10 mM Mes/KOH buffer (pH 5.7) containing 2% (w/v) sucrose, 5 mM CaCl2, 50 gg/ml chloramphenicol ± various concentrations of TDZ. The flasks were sealed and were incubated in the dark (25C) for 24 h. At that time the headspace was sampled and the ethylene content determined by GC using an alumina column. Time Course Studies. Excised hypocotyl segments were placed in 25-ml flasks (10 segments/flask) that contained 4 ml of the above-mentioned treatment buffer ± 10 gM TDZ. The flasks were sealed and were incubated in the dark (25°C). At various times, up to 30 h after the start of the treatment, the ethylene content of the headspace was determined. Experiments examining the short-term effects of TDZ treatment were conducted as follows: excised hypocotyl segments were floated on 25 ml of buffer ± 100 gM TDZ in 9 cm Petri dishes in the dark (25°C). At various intervals thereafter, groups of 10 segments were placed 902 www.plantphysiol.org on July 22, 2017 Published by Downloaded from Copyright © 1984 American Society of Plant Biologists. All rights reserved. THIDIAZURON AND ETHYLENE EVOLUTION

    10 Figures and Tables

    Cite this paper

    @inproceedings{EthyleneEF, title={Ethylene Evolution from Mung Bean Hypocotyl Segments}, author={} }