Plant Physiol. Journal of Pharmacology and Experimental Therapeutics
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Plant Physiology 94:1143-1148 (1990)
© 1990 American Society of Plant Biologists

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Metabolism and Enzymology

Effect of 2,4-Dichlorophenoxyacetic Acid on Endogenous Cyanide, {beta}-Cyanoalanine Synthase Activity, and Ethylene Evolution in Seedlings of Soybean and Barley 1

Forrest L. Tittle2, J. Stephen Goudey3 and Mary S. Spencer

Department of Plant Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5

Treatment of etiolated seedlings of barley (Hordeum vulgare) and soybean (Glycine max) with 1 millimolar 2,4-dichlorophenoxyacetic acid (2,4-D) resulted in a 14-fold and greater than 100-fold increase in ethylene production, respectively. Simultaneous monitoring of endogenous cyanide and {beta}-cyanoalanine synthase ({beta}-CAS) (EC 4.4. 1.9) activity was also performed. Endogenous levels of cyanide did not change in barley. In soybean, endogenous cyanide increased within 3 hours, increased again 6 hours after exposure to 2,4-D, and continued to increase throughout the experimental period. The activity of {beta}-CAS increased in both barley and soybean 9 hours after herbicide treatment. The increase in cyanide preceded the increase in {beta}-CAS activity by 3 to 6 hours in soybean. The steady-state concentration of endogenous cyanide in soybean was 1 micromolar, based on rates of ethylene production and cyanide metabolism by {beta}-CAS. This agreed with the determination of endogenous cyanide by both distillation and isotope dilution. Given the apparent compartmentalization of {beta}-CAS in mitochondria and the localization of ethylene/HCN production at the plasmalemma and/or tonoplast, our results suggest that extra-mitochondrial accumulation of cyanide in the cytoplasm may occur. If so, the activity of cyanide-sensitive cytoplasmic enzymes could be adversely affected, thus possibly contributing to the toxicity of 2,4-D.

2 Present address: Department of Biological Sciences, Northern Alberta Institute of Technology, Edmonton, Alberta, T5G 2R1.

3 Present address: HydroQual Canada Ltd., #380, 4500-16th Ave. N.W., Calgary, Alberta, T3B OM6.

1 Financial assistance from the Natural Sciences and Engineering Research Council of Canada (grant OPG 1451 to M.S.S.) is gratefully acknowledged.






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