Detoxification of cyanide by plants and hormone action

In higher plants cyanide is a co-product of ethylene synthesis. The increase in ethylene production that occurs during the senescence of certain flowers and the ripening of climacteric fruit is accompanied by a rise in beta-cyanoalanine synthase activity. Although these events correlate temporally and spatially, the potential for cyanide detoxification in these tissues is high compared with the expected rate of cyanide formation from the ethylene pathway. However, in stigmas and styles of Petunia flowers a semi-quantitative relationship exists between the activity of beta-cyanoalanine synthase and the activity of ACC (1-aminocyclopropane-1-carboxylic acid) oxidase, the last enzyme in the ethylene pathway. To account for these observations it is proposed that ACC oxidase can react with other amino acids by a general mechanism that liberates cyanide. This hypothesis could also account for the substrate stereospecificity of ACC oxidase, for the extreme lability of this enzyme and for the high accumulation of asparagine in some tissues.