Thiocarbamate Action on very Long Chain Fatty Acid Synthesis in Plants

Abstract

Thiocarbamate herbicides have been reported to affect the biosynthesis of epicuticular lipids of leaves from pea [1]. These alterations may result in an increased vulnerability to environmental factors and enhanced levels of transpiration. Ultimately the death of susceptible plants may ensue. It has been postulated that, once transported into the plant, the thiocarbamate compound is oxidised by a cytochrome P450 mixed function oxidase to its sulphoxide derivative [2]. It is the latter which is believed to be the active herbicidal form. This sulphoxide metabolite is believed to have an inhibitory affect on the various elongases involved in very long chain fatty acid synthesis (precursors of wax and suberin). This inhibition is thought to be selective, no effect having been reported on de novo synthesis of palniitate and stearate [3]. The effects of the thiocarbamate herbicide, pebulate (S-propyl butylethylthiocarbamate) and its sulphoxide on very long chain fatty acid (VLCFA) synthesis were investigated. Experimental data showed, both in vivo and in microsomal fractions from germinating pea seeds, that the sulphoxide caused a much greater inhibition of VLCFA synthesis, than the parent molecule. This suggested that the sulphoxide was the active herbicidal form. Subcellular fractionation studies indicated that the elongases are associated with the endoplasmic reticulum. The secondary effects on the wax quantity and quality due to this direct inhibition of the various elongases were confirmed by scanning electron microscopy.