[PERSON]  
2 Guangzhou Flower Research Center, Guangzhou 510360, China; [EMAIL_ADDRESS] 2  
[PERSON]  
1 Guangdongdong Key Laboratory for Innovative Development and Utilization of Forest Plant Germplasm, College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 5106 cytosolic mevalonate acid (MVA) pathway and the plastidial methylerythntyl phosphate (MEP) pathway [15]. The MVA pathway begins with the condensation of three molecules of acetyl-CoA, whereas the MEP pathway starts with the condensation of D-glyceraldehyde 3-phosphate and pyruvate. Both pathways generate isopentenyl diphosphate (IPP) and its homologous isomer dimethylallyl pyrophosphate (DMAPP) through a series of enzymatic reactions [16]. The sequential head-to-tail condensation of IPP and DMAPP leads to the formation of FPP in the cytosol, as well as pyrophosphate (GPP) and geranylgeranyl diphosphate (GGPP) in plastids [17]. Subsequently, different terpene synthases (TPSs) convert GPP, GGPP, and FPP into structurally diverse monoterpenes, diterpenes, and sesquiterpenenes, respectively [18]. The monoterpenes are mainly synthesized by the MEP pathway, in which nine key enzymes catalyze a series of successive reactions, including 1-deoxy-\(D\)-xylulose 5-phosphate synthase (DXS), 1-deoxy-\(D\)-xylulose 5-phosphate reductio isomerase (DXR), 2-C-methyl-\(D\)-erythntitol 4-phosphate cytidylyl transferase (MCT), 4-(cytidine 5\({}^{\prime}\)-diphospho)-2-C-methyl-\(D\)-erythntitol kinase (CMK), 2-C-methyl-Derythntitol-2,4-cyclodiphosphate synthase (MDS), 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase (HDS), 4-hydroxy-3-methylbut-2-en-1-yl diphosphate reductase (HDR), geranyl pyrophosphate synthase (GPPS), and TPS [19]. Phenylpropanoids and benzenoids, the second largest class of plant VOCs, are divided into three subclasses depending on their carbon skeleton: phenylpropanoids (with a C6-C3 backbone), benzenoids (with a C6-C1 backbone), and phenylpropanol-related compounds (with a C6-C2 backbone) [14]. All three subclasses are derived from the aromatic amino acid (AAAS) phenylalanine (Phe). Aromatic amino acid aminotransferase (AAAT) and phenylacetaldehyde synthase (PAAS) are key enzymes in phenylpropanol-related compound biosynthesis, and phenylalanine ammonia-lyase (PAL) is the key enzyme in benzenoid and phenylpropanol biosynthesis, which deaminates Phe to _trans_-cinnamic acid (CA) and competes with PAAS for Phe utilization. Phe is derived from chorismate, the final product of the shikimate pathway [20]. Many studies have demonstrated that the level of Phe is greatly regulated by the activity of key enzymes in the shikimate and arogenate pathways (Figure 1), such as 3-deoxy-di-arabino-heptulosonate 7-phosphate synthase (DAHPS), 5-enolpyruvateylshitz-3-phosphate synthase (EPSPS), chorismate mutase (CM), prephenate dehydratase (PDT), aproenate dehydratase (ADT), and S-adenosylmethionine synthetase (SAMS) [21; 22]. Fatty acid derivatives, the third class of plant VOCs, are derived from unsaturated C18 fatty acids, namely linelonic or linoleic acids [23]. Lipoxygenase (LOX) directly leads to the formation of 9- and 13-hydropyre intermediates via two branches: the allene oxide synthase (AOS) branch, which gives rise to jasmonic acid (JA), and the hydroperoxide lyase branch, which leads to the formation of volatile alcohols and their esters [24]. Many transcription factor (TF) families are involved in regulating the biosynthesis of VOCs. The _TPS_ gene family, responsible for the formation of terpen, has been characterized in many plants--however, the regulatory network that controls _TPS_ expression is still unclear. In _Arabidopsis thaliana_ (Arabidopsis), myeloblastosis protein 21 (MYB21) and myelocytomatosis protein 2 (MYC2) regulate the expression of _TPS11_ and _TPS21_ via the gibberellic and JA pathways [25; 26]. In Arabidopsis, auxin response factors 6 and 8 (ARF6 and ARF8) can bind to the promoters of _TPS11_ and _TPS21_ to regulate the synthesis of sesquiterpenes [25].