The present invention relates to the field of plant molecular biology, more particularly Jatropha curcas casbene synthase genes whose encoded proteins are involved in phorbol ester biosynthesis. The present invention also relates to methods of down regulating casbene synthase expression in Jatropha and other plants. The present invention further relates to methods for decreasing the amount of phorbol esters in the seeds of Jatropha and other plants.