Introduction Terpenoids also known as isoprenoids constitute
Introduction Terpenoids, also known as isoprenoids, constitute a large family of natural products comprising at least 22,000 compounds and play diverse and important roles in plants as hormones, phytosynthetic pigments, and electron carriers , . In addition, volatilized terpenoids contribute to many plant-environment interactions and adaptations , including the attraction of pollinators , plant-plant communication , and plant defense responses , , . Clearly, terpenoids are essential to plant biology and physiology . Despite their diversity, all terpenoids are synthesized from two common isomeric five-carbon building blocks, dimethylallyl diphosphate (DMAPP) and isopentenyl diphosphate (IPP). DMAPP and IPP are synthesized via two highly conserved but independent metabolic pathways, the mevalonic SR 57227 hydrochloride australia (MVA) pathway in the cytosol and the 2C-methyl-D-erythritol-4-phosphate (MEP) pathway in the plastids . These common C5 precursors are the substrates for the biosynthesis of geranyl diphosphate (GPP, C10), farnesyl diphosphate (FPP, C15), and geranylgeranyl diphosphate (GGPP, C20) by sequential head-to-tail condensation mediated by short-chain prenyltransferases, such as geranyl diphosphate synthase, farnesyl diphosphate synthase (FPS), and geranylgeranyl diphosphate synthase (GGPS). These short-chain polyisoprenyl diphosphates provide a platform for the biosynthesis of diverse groups of mono-, sesqui-, and diterpenes. Therefore, the biosynthesis of these short-chain polyprenyl diphosphates is critical for the production of secondary metabolites and for controlling metabolism. Among these three short-chain prenyltransferases, FPS is the best characterized and its importance in plant biology has been extensively analyzed. FPS (EC 188.8.131.52/EC 184.108.40.206) is a homodimeric protein and in vivo functions of FPS have been characterized , . FPS catalyzes the formation of FPP by sequential condensation of DMAPP and two molecules of IPP. FPP is an essential precursor for the biosynthesis of GGPP, most linear isoprenoids, sterols, carotenoids, dolichols, sesquiterpens and brassinosteroids , , as well as for the farnesylation of proteins  and also regulates the release of the farnesylated protein from protein farnesyltransferase . The dephosphorylated form of FPP, farnesol, inhibits the activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR), an important enzyme in the MVA pathway . Furthermore, plant volatile sesquiterpenes synthesized from FPP help plants communicate with their environment, suggesting that FPP are essential not only for the biosynthesis of secondary metabolites, but also as fundamental cellular regulators. FPS genes have been isolated and some plants have redundant FPS genes , , , , , , perhaps to compensate for the potential dysfunction of a single gene, and also to allow the expression of genes with different specificities at different times. In Arabidopsis, for example, the two FPS gene homologues (AtFPS1, At5g47770 and AtFPS2, At4g17190) encode three isozymes, AtFPS1L, AtFPS1S, and AtFPS2 , , . AtFPS1S is a truncated form of the N-terminal signal sequence of AtFPL1L and is predicted to localize in the cytosol, whereas AtFPS1L localizes in the mitochondria . AtFPS1S plays a major role during plant growth and development, whereas AtFPS2 plays important roles in embryogenesis and the early stages of seedling development , , suggesting that the expression of plant FPS genes is regulated differently depending on the organ and developmental stage. Tomato FPS appears to be ubiquitously expressed but its expression level is also regulated during cell division and growth . Furthermore, environmental conditions, e.g., light conditions or the position of the leaf induce FPS expression in rice leaves and in tomato, respectively , . Loss of FPS activity in Arabidopsis leads to the up-regulation of HMGR in seeds . An overflow of FPP caused by the overexpression of FPS or the downregulation/knockout of FPS has negative effects on plant growth , , , . Knockout of a single AtFPS results in no obvious growth phenotype change, suggesting that a single functional AtFPS is sufficient for cell growth and development. On the other hand, knockout of both AtFPS genes leads to embryonic lethality. Thus, FPS and FPP are essential for cell viability, and the cellular functions of FPSs are not completely redundant, and that only FPS can synthesize FPP.