## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses  
### Protective Roles and Mechanisms of Volatile Terpenoids Under Abiotic Stress  
Under abiotic stresses such as drought, high temperatures, and ozone exposure, plants rapidly initiate early signaling cascades involving the burst of ROS and transient calcium fluxes (Ca\({}^{2+}\)). ROS are primarily generated by photosystem II electron leakage in chloroplasts, disruptions in mitochondrial electron transport chains, and peroxisomal \(\beta\)-oxidation [117, 118]. The excessive accumulation of ROS--particularly superoxide anion \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Transcription Factor** & **Gene** & **Species** & **Regulatory Substance** & **Reference** \\ \hline \multirow{4}{*}{MYB} & _OfMYB1R114_, 70, 201_ & _Osmanthus fragrans_ & Promotes \(\beta\)-ionone biosynthesis & [65] \\  & _CsMYB68, 147,_ 148, 193_ & _Camellia sinensis_ & Promotes monoterpene and & [100] \\  & _MYB24_ & _Vitis vinifera_ cv. _’Bequigrol’_ & Involved in & [101] \\  & _MYB5_ & _Rosa rugosa_ & \begin{tabular}{} \end{tabular} & [102] \\ \hline \multirow{4}{*}{bHLH} & _MYC2_ & _Arabidopsis thaliana_ & Promotes sesquiterpene & [69] \\  & _PbbHLH4_ & _Phalenopsis orchids_ & biosynthesis & [103] \\  & _CpMYC2_, & _Climonantthus praecox_ L. & Promotes \(\beta\)-caryophyllene and & [74] \\  & _PbHLH13_ & _Osmanthus fragrans_ & Promoteslinalool and linalool & [104] \\  & _AbHLH2, 3_ & _Artemisia annua_ L. & Promotes sesquiterpene & [105] \\  & _LibHLH22, 63_ & _Lilium ‘Siberia’_ & Promotes linalool and & [106] \\  & & & & \\  & _SIMYC1_ & _Solanum Lycopersicum_ & \
\begin{tabular}{} \end{tabular} & [107] \\  & _PbbHLH1_ & _Prunus persica_ L. & Promotes linalool biosynthesis & [108] \\  & _LaMYC4_ & _L angusifolia_ & Promotes volatile organic & [109] \\ \hline \multirow{4}{*}{AP2/ERF} & _MdERF3_ & _Malus domestica_ & Promotes \(\alpha\)-farnesene biosynthesis & [110] \\  & _CitERF71_ & _Citrus sinensis Ospeck_ & Promotes E-geraniol biosynthesis & [59] \\  & _PpERF5,7_ & _Prunus persica_ & Promotes linalool biosynthesis & [111] \\ \hline \multirow{4}{*}{WRKY} & _AaWRRKY40_ & _Artemisia annua_ & Promotes terpene biosynthesis & [112] \\  & _CrWRKY1_ & _Catharantthus roseus_ & Promotes terpene indole & [79] \\  & _OfWRKY139_ & _Sweet Osmanthus_ &
\begin{tabular}{} \end{tabular} & [113] \\ \hline \multirow{4}{*}{NAC} & _AaNAC2, 3, 4_ & _Actinidia chinensis Planch_ & Promotes monoterpene biosynthesis & [114] \\  & _GoNAC42_ & _Gossypium hirsutum_ & Promotes monoterpene biosynthesis & [115] \\  & _NAC-NOR_ & _Solanum Lycopersicum_ & Promotes volatile organic compound biosynthesis & [116] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. (O\({}_{2}\)\({}^{-}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), and hydroxyl radicals (-OH) [119]--results in oxidative damage, including lipid peroxidation, protein denaturation, and ultimately, programmed cell death (PCD) [117]. In parallel, stress signals activate calcium channels on plasma membranes and organelles, triggering Ca\({}^{2+}\) release from extracellular reservoirs or intracellular stores such as the vacuole and endoplasmic reticulum, leading to a 'calcium burst.' Elevated cytosolic Ca\({}^{2+}\) binds to calmodulin (CaM) and calcium-dependent protein kinases (CDPKs), propagating the signal to downstream modules, including mitogen-activated protein kinase (MAPK) cascades, hormonal pathways (e.g., JA, ABA, SA, and ET), and ROS-producing or scavenging systems. These interactions ultimately activate stress-responsive gene expression. Moreover, a positive feedback loop exists between ROS and Ca\({}^{2+}\): ROS promote Ca\({}^{2+}\) influx, while Ca\({}^{2+}\) in turn stimulates NADPH oxidases to generate more ROS, forming a self-amplifying signal network that rapidly escalates and disseminates stress cues throughout plant tissues [120; 121; 122]. To mitigate oxidative damage, plants must promptly activate antioxidant defenses to maintain redox homeostasis.