# Molecular Regulatory Mechanisms Affecting Fruit Aroma
## 3 Main Synthetic Pathways
### Terpenoid Synthesis Pathway
Finally, HMBPP forms isopentenyl diphosphate (IPP) under the catalysis of 4-hydroxy-3-methylbuten-2-en-1-yl diphosphate reductase (HDR). It is interesting that IPP is partially converted into the double-bond isomer 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate (DMAPP) under the action of isopentyl diphosphate isomerase (IDI), which are the C5 precursor substances of all terpenoid compounds. In the fruit, IPP is mainly synthesized through the MEP pathway. The C5 precursor IPP and DMAPP further generate the direct precursors of terpenoids under the catalysis of isopentenyltransferase: GPP, FPP, and GGPP. The direct precursor geranyl diphosphate (GPP) of monoterenes is synthesized from one molecule of IPP and one molecule of DMAPP under the catalysis of geranyl diphosphate synthase (GPPS). The direct precursor of sesquiterpenenes, FPP, is generated through a two-step condensation reaction of two molecules of IPP and one molecule of DMAPP catalyzed by farnesyl diphosphate synthase (FPPS). The direct precursor of diterpenenes, GGPP, is generated through a three-step condensation reaction of three molecules of IPP and one molecule of  
Figure 2: Synthetic pathways of terpenoids (**a**), phenylpropane (**b**), and fatty acid derivatives (**c**) related to the formation of aroma components. AACT, acetyl-CoA C-acetyltransferase; HMGCS, hydroxymethylglutaryl-CoA synthase; HMGCR, hydroxymethylglutaryl-CoA reductase; MVK, mevalonate kinase; PMK, phosphohexionate kinase; MVD, mevalonate diphosphohexrolase; DXS, 1-deoxy-D-xylulose-5-phosphate synthase; DXR, 1-deoxy-D-xylulose-5-phosphate reductioisomerase; HDS, 4-hydroxy-3-methylbut-2-enyl-diphosphate synthase; HDR, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase; IDI, isopentenyl-diphosphate delta-isomerase; GPP, geranyl diphosphate synthase; GGPP, geranylgeranyl diphosphate synthase; DMAPP, dimethylallyl diphosphate; IPP, isopentenyl diphosphate; AADC, aromatic L-amino acid decarboxylase; PAL, phenylalanine-nialaxine>; EOMT, eugenol O-methyltransferase; PAR, phenylacetaldehyde reductase; 13-LOX, 13-lipoxygenase; 13-HPL, 13-hydroperoxidase lyase; ADH, ethanol dehydrogenase; AAT, alcohol acyltransferase. DMAPP catalyzed by geranylgeranyl diphosphate synthase (GGPPS). Methyltransferase is one of the important enzymes in this pathway, helping to catalyze and generate more aroma substances or further enhancing the expression of aroma compounds based on the original aroma of aromatic plants. Some studies found that the key enzyme involved in volatile terpenoid synthesis is terpenoid synthetase (TPS), and TPS is an important terminal enzyme that catalyzes the formation of inalisol by GPP as a substrate [23]. Linalsol is a linear monoterpene alcohol widely present in more than 200 plants and is a key odor substance affecting fruit aroma [24; 25]. The energy flow and distribution of MVA in different fruit trees or organs are different under the two pathways, and the two pathways produce different precursor substances (GPP or FPP), subsequently resulting in the diverse structures of terpenes [26; 27]. Molecular breeding technology can enhance plants by regulating the content of terpenoids in transgenic plants. From the perspective of biosynthesis and transcriptional regulation, there is competition between the different branches of the metabolism for the same precursor [28]. For example, _GGPPS_ genes are located at the intersection of the terpene metabolic pathways. They not only participate in the synthesis of monoterenes and sesquiterpenes, but also promote the accumulation of aroma compounds in fruits and catalyze the formation of carotenoid precursors. By regulating the _GGPPS_ gene, it may be possible to simultaneously regulate the production of products from both pathways and co-ordinate the metabolite flux between the monoterene and carotenoid biosynthesis pathways, thereby affecting the formation of fruit color and aroma [29].