 The discovery of RNAi was preceded first by observations of transcriptional inhibition by antisense RNA expressed in transgenic plants , and more directly by reports of unexpected outcomes in experiments performed by plant scientists in the United States and the Netherlands in the early 1990s . In an attempt to alter flower colors in petunias , researchers introduced additional copies of a gene encoding <unk> synthase , a key enzyme for flower pigmentation into petunia plants of normally pink or violet flower color . The overexpressed gene was expected to result in darker flowers , but instead produced less pigmented , fully or partially white flowers , indicating that the activity of <unk> synthase had been substantially decreased ; in fact , both the endogenous genes and the transgenes were downregulated in the white flowers . Soon after , a related event termed quelling was noted in the fungus Neurospora crassa , although it was not immediately recognized as related . Further investigation of the phenomenon in plants indicated that the downregulation was due to post @-@ transcriptional inhibition of gene expression via an increased rate of mRNA degradation . This phenomenon was called co @-@ suppression of gene expression , but the molecular mechanism remained unknown . 
