Metabolic engineering of plant secondary products: which way forward?

Secondary products are small molecular weight compounds produced by secondary metabolic pathways in plants. They are re- garded as non-essential for normal growth and development but often confer benefits such as defense against pathogens, pests an d herbi- vores or the attraction of pollinators. Many secondary products affect the survival and/or behavior of microbes, insects and ma mmals and they often have useful pharmacological effects in humans. Most secondary products can only be obtained as extracts from medicin al plants, many of which grow slowly and are difficult to cultivate. Chemical synthesis, although possible in principle, is often impractical or uneconomical due to the complexity of their molecular structures. The large scale production of secondary products by metabo lic en- gineering has therefore been investigated in a number of heterologous systems including microbes, plant cell/organ cultures, an d intact plants. In this critical review of production platforms for plant secondary products, we discuss the advantages and constraints of different approaches and the impact of post-genomics technologies on gene discovery and metabolite analysis. We highlight bottlenecks that re- main to be overcome before the routine exploitation of secondary products can be achieved for the benefit of mankind.

Research is supported by MICINN, Spain (BFU2007-61413; BIO2011-23324; BIO02011-22525; PIM2010PKB-00746); Euro- pean Union Framework 7 Program-SmartCell Integrated Project 222716; European Union Framework 7 European Research Council IDEAS Advanced Grant (to PC) Program-BIOFORCE; COST Ac- tion FA0804: Molecular farming: plants as a production platform for high value proteins; COST Action FA1006: Plant metabolic engineering for high value products; Centre CONSOLIDER on Agrigenomics funded by MICINN, Spain.