## 2 Biosynthesis and Key Regulatory Networks of Volatile Terpenoids  
### Biosynthetic Pathways of Volatile Terpenoids  
As the predominant constituents of plant volatile organic compounds (VOCs), VTs constitute the largest and most structurally diverse class, exhibiting a broad range of physiological activities, including antioxidant, antimicrobial, and anti-inflammatory effects [13]. These compounds play pivotal roles in plant growth and development, environmental adaptation, and responses to both biotic and abiotic stressors. Common VTs include monoterpenes, sesquiterpenes, and diterpenes. Structurally, most VTs are biosynthesized from the C\({}_{5}\) precursors isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), following the [(C\({}_{5}\)H\({}_{8}\))\({}_{n}\)] isoprene rule. Hemiterpenes (C\({}_{5}\)H\({}_{10}\)), exemplified by photosynthesis-derived isoprene, represent the simplest category. Monoterpenes (C\({}_{10}\)H\({}_{16}\)), formed via the condensation of one IPP and one DMAPP unit, include confer-dominant compounds such as \(\alpha\)-pinene, limonene, and myrcene [14]. Sesquiterpenes (C\({}_{15}\)H\({}_{24}\)), synthesized from two IPP units and one DMAPP unit, encompass ecologically significant molecules like \(\beta\)-farresene and the phytoalexin gossypol. At higher molecular weights, diterpenes (C\({}_{20}\)H\({}_{32}\)) such as phytol (a chlorophyll-associated alcohol) and gibberellins (key regulators of plant growth) arise from three IPP units and one DMAPP unit. This structural hierarchy--from C\({}_{5}\) to C\({}_{20}\) frameworks--directly correlates with their functional versatility in plant survival strategies. While terpenoid biosynthesis involves intricate biochemical networks, it is predominantly governed by two evolutionarily conserved pathways: MVA and MEP pathway (Figure 1). The MVA pathway, localized in the cytosol [15], facilitates the biosynthesis of terpenoid precursors through a series of enzymatic reactions. The pathway is initiated with acetyl-CoA (acetyl-coenzyme A) serving as the primary substrate. Acetoacetyl-CoA thiolase (ATOT) catalyzes the condensation of two acetyl-CoA molecules to form acetoacetyl-CoA. Hydroymethylglutaryl-CoA synthase (HMGS) then facilitates the synthesis of HMG-CoA, marking the formation of a key early intermediate. HMG-CoA is subsequently reduced by hydroxymethylglutaryl-CoA reductase (HMGR), yielding mevalonic acid (MVA). Mevalonate kinase (MK) phosphorylates MVA to form mevalonate-5-phosphate (MVAP), which is further phosphorylated by phosphomevalonate kinase (PMK), producing mevalonate-5-diphosphate (MVAPP). Mevalonate diphosphate decarboxylase (MDP) subsequently catalyzes the decarboxylation of MVAPP, yielding IPP, the key C\({}_{5}\) building block of terpenoid biosynthesis. IPP is isomerized to DMAPP by IPP isomerase (IPI), completing the MVA-derived isopenoid precursor pool [16; 17]. Farnesyl diphosphate synthase (FPPs) catalyzes the head-to-tail condensation of these C\({}_{5}\) units, leading to the production of farnesyl diphosphate (FPP, C\({}_{15}\))--the central precursor for volatile sesquiterpenes and their derivatives. The 2-C-methyl-D-erythnitol 4-phosphate (MEP) pathway operates exclusively within plastids and executes terpenoid precursor biosynthesis through a conserved enzymatic cascade. The MEP pathway is initiated by the condensation of pyruvate and D-glyceraldehyde-3-phosphate (GA-3P), catalyzed by 1-deoxy-D-xylulose-5-phosphate synthase (DXS), yielding 1-deoxy-D-xylulose-5-phosphate (DXP). In the presence of NADPH, 1-deoxy-D-xylulose 5-phosphate reducto isomerase (DXR) catalyzes the reduction and rearrangement of DXP, resulting in the formation of 2-C-methyl-D-erythnitol 4-phosphate (MEP). 2-C-methyl-D-erythnitol 4-phosphate cytidylyltransferase (MCT) then transfers a cytidyllyl group from cytidine triphosphate (CTP) to MEP, generating 4-(cytidine 5'-diphospho)-2-C-methyl-D-erythnitol (CDP-ME).