Plant secondary metabolism and challenges in modifying its operation: an overview

Plants have metabolic pathways leading to tens of thousands of secondary products capable of effectively responding to stress situations imposed by biotic and abiotic factors. These pathways, often recruited from essential primary metabolism pathways upon initial gene duplication, are frequently restricted to specific taxonomic groups and play a major role in the plant x environment interaction. A strict spatial and temporal control of gene expression ensures the correct accumulation pattern of various secondary products. The required transport of metabolic intermediates constitutes an additional level of regulation. The induction of secondary metabolism gene expression by wounding, herbivore-derived molecules, pathogen elicitors, and oxidative stress caused by heat, drought, flooding, UV light, or temperature extremes is often mediated by integrating signaling molecules such as jasmonate, salicylic acid, and their derivatives. Ontogeny and circadian clock-controlled gene expression are also important features of plant secondary metabolism, as are master regulatory transcription factors. These regulators are attractive targets for engineering secondary metabolic pathways. In spite of the complexity of secondary metabolism, important advances have been achieved, leading to success stories in engineering this diverse reservoir of useful molecules.