Terpenoids, also known as isoprenoids, are one of the largest and most diverse classes of naturally occurring products . In animals, terpenoids play crucial roles as membrane constituents (e.g., cholesterol) and components of the respiratory electron transport chain (e.g., ubiquinone) , whereas in microbes and plants, they are found as secondary metabolites that have been used as pharmaceuticals (e.g., paclitaxel and artemisinin), flavors, and fragrances (e.g., menthol and patchoulol) . Terpenoids, including monoterpenes, sesquiterpenes, and diterpenes, are synthesized by terpene synthases (TPSs) with universal five-carbon building units, isopentenyl diphosphate (IPP) and its isomer dimethyl allyldiphosphate (DMAPP). IPP and DMAPP are synthesized in two different pathways: the mevalonate (MVA) pathway , and the 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway  (Fig. 1a). In general, Gram-negative bacteria, including Escherichia coli, employ the MEP pathway, whereas eukaryotes, archaea, humans, and Gram-positive cocci use the MVA pathway . Prenyltransferases assemble the IPP and DMAPP units to produce the prenyl diphosphates, which are terpenoid precursors, including geranyl diphosphate (GPP, C10), farnesyl diphosphate (FPP, C15), and geranyl geranyl diphosphate (GGPP, C20). These prenyl diphosphate molecules are converted into a large variety of terpenoids by TPSs [7, 8].Fig. 1Biosynthesis of (−)-α-bisabolol in engineered E. coli. a Schematic representation of (−)-α-bisabolol production in an engineered E. coli harboring endogenous 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway genes and exogenous mevalonate (MVA) pathway genes. The native MEP pathway consists of: DXS deoxyxylulose 5-phosphate synthase; DXR deoxyxylulose 5-phosphate reductoisomerase; CMS 2-C-methylerythritol 4-phosphate cytidyl transferase; CMK 4-(cytidine 5′-diphospho)-2-C-methylerythritol kinase; MDS 2-C-methylerythritol 2,4-cyclodiphosphate synthase; HDS (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase; and HDR hydroxymethylbutenyl diphosphate reductase. The engineered mevalonate pathway consists of seven enzymes: MvaE, acetoacetyl-CoA thiolase (GenBank accession number: AF290092); MvaS, 3-hydroxy-3-methylglutaryl- CoA synthase (GenBank accession number: AF290092); MvaA, 3-hydroxy-3-methylglutaryl-CoA reductase (GenBank accession number: AF290092); MvaK1, mevalonate kinase (GenBank accession number: AF290099); MvaK2, phosphomevalonate kinase (GenBank accession number: AF290099); MvaD, mevalonate 5-diphosphate decarboxylase (GenBank accession number: AF290099); Idi, isopentenyl diphosphate isomerase (GenBank accession number: AF119715); and IspA, geranyl diphosphate synthase or farnesyl diphosphate synthase (GenBank accession number: AAC73524). MrBBS; E. coli codon-optimized (−)-α-bisabolol synthase of German chamomile (GenBank accession number: KU680479). b Plasmid constructs used for (−)-α-bisabolol production in E. coli. pTSN-MrBBS, pTrc99A with MrBBS gene from German chamomile; pSSN12Didi, pSTV28 containing mvaK1, mvaD, mvaK2 of Streptococcus pneumoniae, and idi of Escherichia coli; pSNA-MrBBS, pSTV28 containing mvaK1, mvaK2, and mvaD of S. pneumoniae, idi of E. coli, mvaE and mvaS of Enterococcus faecalis, and MrBBS of German chamomile; pSNA-MrBBS-IspA, pSNA-MrBBS containing ispA of E. coli