Introducing synthetic genes into organisms offers enormous practical opportunities. Systems of metabolic genes can be arranged to produce useful biomass such as fuels, medicines, and plastics. Crop plants are attractive targets as such Genetically Modified Organisms because of the enormous agricultural infrastructure we have constructed for producing food–and crop plants only require water, land, sunlight, atmospheric carbon dioxide, and some source of nitrogen, phosphorus, and potassium in order to to create useful metabolic products.
However, there are social and technical difficulties for such transgenic crops. First, there is concern that ‘cash crop’ GMOs might be grown in the place of foodstuffs in areas where humans are undernourished, creating a conflict of interest for farmers who must decide each season which crop to plant. Second, the expression of many foreign genes within a plant leads to what is called ‘growth inhibition’: when the genes or their metabolic products interfere with the growth of the plant. Genes which cause even a slight reduction in plant growth can significantly reduce product yields.
Both of these difficulties might be overcome by precise control of gene expression. ‘Double crops’ could be engineered to express useful metabolites in a separate tissue than the food (such as the plant stalk), and/or to express them only after the food tissue has been harvested. How to maximize yield while avoiding growth inhibition is a general problem for expressing engineered metabolic pathways. To enable useful expression of such genes in plants, we want to control when, where, and how much of each gene is expressed. By using an appropriate genetic control system to allow expression only of pathway genes in only at a specific time and/or plant tissue, we can limit the overall toxicity.
The region up to 500 base-pairs from the start of transcription is known to contain most of the position specific transcriptional regulation in plants, due to transcription factor protein binding. Enhancers, non position specific factors often occurring even further from the promoter, are generally still effective when synthetically introduced into this region. Though many natural tissue specific promoters are known, the contribution of each transcription factor and enhancer to tissue/temporal specificity is an open research topic. A set of compact synthetic promoters with defined interactions would make a useful toolkit–both for probing spatial/temporal gene regulation, and for engineering plant metabolism. Additional background and reference information about eukaryotic promoters and synthetic promoter design may be found in the Promoter Design Manuals.