The use of RNAi to elucidate and manipulate secondary metabolite synthesis in plants

Abstract

RNA interference technology (RNAi, dsRNA-mediated gene silencing) has already had a major impact on the study and manipulation of plant secondary metabolites. To date RNAi has mainly been used as a readily available, rapid, reverse genetics tool to create plants with novel chemical phenotypes, and to determine the phenotypes of genes responsible for the synthesis of many different secondary metabolites. These manipulations have also greatly facilitated the identification and improvement of specific plant-insect and plant-pathogen interactions, and have set the stage for greater exploitation of plants to produce commercially-valuable, plant-derived drugs, flavoring agents, perfumes, etc. RNAi has been used to study and manipulate products that are representatives of all three main groups of plant secondary metabolites, the phenylpropanoids (and allied phenolics), alkaloids, and terpenoids. We predict that because there exists so much diversity in chemical structure among plant secondary metabolites, and RNAi is highly efficient, foreign gene expression together with RNAi will undoubtedly play an increasingly important role in enabling plants to produce renewable chemical feed stocks now obtained from petroleum. Thus, various forms of RNAi will be very important in the post peak oil future of agriculture, worldwide. After a brief general introduction to plant secondary metabolites, we will survey recent studies that have used RNAi for phenotyping of secondary metabolite related genes, and to generating novel chemical phenotypes, pointing out the advantages gained. A few examples will be highlighted to exemplify the powerful approach of gene knockdown combined with foreign gene overexpression as a means for creating new secondary-product-based phenotypes (e.g., the blue rose). Where data is available, we will compare knockdown efficiency and stability of antisense, co-suppression and dsRNAi. As a case study, we will describe in more detail how RNAi has been used to chart carbon flow among branch pathways of diterpenoid biosynthesis in trichome glands. Finally we will discuss the potential of artificial miRNAi for enhancing the RNA-mediated gene silencing approach (particularly selectivity) in plants, and as a mutational tool.