Fluoroacetate metabolism of Dichapetalum Cymosum

Abstract

A fast and sensitive method was developed for the determination of fluoroacetate in Dichapetalum cymosum using high-performance liquid chromatography. The highest concentrations of fluoroacetate were found in the immature seeds, flowers and young leaves of the plant. The young leaves are more toxic than the older leaves, probably because the rate of fluoroacetate degradation is higher in old leaves than in younger leaves. Foliarly applied fluoroacetate is also more readily accumulated by the young than by the older leaves of D. cymosum. Fluoroacetate can be taken up by the roots of D. cymosum and be transported to the leaves. Whether this happens to a significant extent under natural conditions is unknown. It was, however, demonstrated in this study that aseptically grown D. cymosum seedlings, as well as an aseptic callus culture of the plant, is capable of producing fluoroacetate. A pseudomonad was isolated from D. cymosum and identified as Pseudomonas cepacia. It was established that an isolate of this bacterium could grow in fluoroacetate enriched solutions without any reduction in growth rate. This bacterium is capable of defluorinating fluoroacetate and also of liberating CO2 from fluoroacetate. It seems as though the synthesis of N-methyl alanine and the occurrence of N-methyl serine in D. cymosum, is a result of the symbiosis between the plant and its endophyte. An aseptic callus culture of D. cymoswa is capable of degrading fluoroacetate albeit at a much lower rate than the leaves of the plant. By contaminating the callus with P. cepacia, isolated from the plant, the rate of CO2 release from fluoroacetate was increased about five fold. A D. cymosum crude mitochondrial enzyme extract can release CoASH from fluoroacetyl-CoA at a rate of 29.5 nmoles/min/mg protein, indicating the presence in the extract of an enzyme which is probably fluoroacetyl-CoA hydrolase. This enzyme could not use acetyl-CoA as a substrate. The presence of the fluoroacetyl-CoA hydrolase-like enzyme in D. cymosum together with the ability of the plant and its endophyte to degrade the fluoroacetate, helps to explain why D. cymosum is not poisoned by the high fluoroacetate concentrations which occur in the plant at times.