size of end products indicates increase and decrease in the content, respectively.

The small subunit of geranyl diphosphate synthase: a tool to improve aroma and fl avour by metabolic engineering Monoterpenes represent a major class of volatile terpenes produced by plants and play important roles in pollination, defense and seed dispersal (Dudareva et al. 2006). Also, plant-derived terpenoids, including monoterpenes are used as natural fl avour and aroma compounds, and have benefi cial impact on humans as health promoting compounds (Wagner and Elmadfa 2003). All monoterpenes are derived from the common precursor, geranyl diphosphate (GPP, C10), which is synthesized by a head-to-tail condensation of one molecule each of isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP) in a reaction catalyzed by GPP synthase (GPPS, EC 2.5.1.1) (fi gure 1) (Ogura and Koyama 1998). GPPSs have been isolated and functionally characterized in a limited number of plant species and are known to exist as homodimeric and heterodimeric structures. Heterodimeric GPPSs have been described in peppermint (Mentha piperita), snapdragon (Antirrhinum majus), Clarkia (Clarkia breweri) and hop (Humulus lupulus), which produce large amounts of monoterpenes in specifi c organs (Burke et al. 1999; Tholl et al. 2004; Wang and Dixon 2009). Structurally, the heterodimeric GPPS constitutes a smal