Metabolic Engineering of the Mevalonate and Non-Mevalonate Pathways in Tomato

Abstract

In higher plants isoprenoids have many essential roles e.g. in membrane structure (phytosterols), redox chemistry (plastoquinone) and as antioxidants (carotenoids). Despite their functional and chemical diversity all isoprenoids are biosynthetically related from a common precursor, isopentenyl pyrophosphate (IPP). In higher plants two pathways exist for the formation of IPP, the mevalonate pathway where IPP is formed from acetyl-CoA via mevalonic acid (MVA) and a second, recently proven, pathway where IPP is formed from D-glyceraldehyde-3-phosphate and pyruvate via 1deoxy-D-xylulose 5-phosphate (DXP) (Rohmer, 1999). In the cytosol/endoplasmic reticulum (ER) and mitochondria the mevalonate pathway is responsible for the synthesis of phytosterols and ubiquinone. 3-Hydroxy-3-methylglutaryl Co-enzyme A reductase (HMGR) is a key regulatory enzyme in the mevalonate pathway and catalyses the formation of MVA from 3-hydroxy-3-methylglutaryl Co-A (HMG-CoA). Plastid isoprenoids such as carotenoids and tocopherols are formed via the non-MVA pathway. The formation of DXP, catalysed by 1-deoxy-D-xylulose 5-phosphate synthase (DXS), has been shown to be a regulator of flux through this pathway to carotenoids (Lois et al., 2000).