## 3 Regulatory Mechanisms of Volatile Terpenoids in Abiotic Stress Responses
### Temperature Stress  
As a key environmental regulator, temperature imposes multifaceted effects on plant physiological metabolism and the emission dynamics of VTs. Studies have demonstrated that temperature fluctuations can directly influence both the emission rates and compositional profiles of plant VOCs. Under elevated temperature stress, plants typically increase terpene emissions as an adaptive response [163; 164], with isoprene and monoterenes being the dominant volatile fractions, whereas sesquiterpenes and diterpenes contribute comparatively less. This temperature-dependent emission pattern results from a complex interplay of physiological processes, including stomatal regulation, photosynthetic electron transport efficiency, and the thermal sensitivity of key enzymatic activities [139]. Mechanistically, isoprene biosynthesis occurs via the MEP pathway within chloroplasts, utilizing photosynthetic intermediates as substrates [165]. The rate-limiting step in DMAPP biosynthesis, along with the catalytic efficiency of isoprene synthase (IspS), exhibits pronounced temperature sensitivity, making isoprene emissions particularly responsive to thermal fluctuations [166]. For instance, the heterologous expression of _IspS_ in _Arabidopsis thaliana_ enables non-isoprene-emitting plants to produce temperature-dependent isoprene emissions, with significant increases under thermal stress [167]. This emission pattern closely aligns with the temperature-dependent activity profiles of IspS enzymes observed in _Populus tremula \(\times\) Populus tremuloides_[168]. However, temperature effects exhibit threshold characteristics: _Pueraria lobata_ leaves grown under a constant 19 \({}^{\circ}\)C (day/night) fail to emit isoprene, whereas isoprene biosynthesis becomes inducible above 20 \({}^{\circ}\)C [169]. Notably, studies on temperature responses in _Quercus robur_ and _Populus deltoides_ reveal biphasic  
Figure 3: The signaling cascade of volatile terpenoid biosynthesis and their role in enhancing plant resilience to abiotic stress. The figure illustrates the signaling cascade triggered by abiotic stress in plants, where the accumulation of reactive oxygen species (ROS) generates a ROS wave, which activates calcium (Ca\({}^{2+}\)) flux and initiates early signaling events. These signals subsequently activate a series of plant growth hormones, including auxin (AUX), salicylic acid (SA), abscisic acid (ABA), jasmonic acid (JA), ethylene (ET), and gibberellins (GA). Different hormone signaling pathways then activate corresponding transcription factors (such as MYB, WRKY, and bZIP) and heat shock proteins (IFSFs). For instance, the auxin signaling pathway activates ARF, the ABA pathway activates bZIP and ABF, the ethylene pathway activates ERF, and the JA and SA pathways activate NAC. Finally, these transcription factors upregulate genes involved in terpenoid biosynthesis, including geranyl pyrophosphate synthase (GPPs), famesyl pyrophosphate synthase (FPPS), geranylgeranyl pyrophosphate synthase (GGPS), and terpene synthases (TPSs), leading to enhanced accumulation and release of terpenoids, which in turn improve the plant’s resilience to abiotic stresses. emission patterns--isoprene emissions increase with temperature up to 35-40 \({}^{\circ}\)C, beyond which they decline. This suppression correlates with heat-induced metabolic constraints, including depletion of DMAPP and subsequent limitation of IspS activity [170; 171]. Collectively, these findings underscore the pivotal role of enzymatic thermostability in regulating temperature-dependent terpenoid emissions. Regarding the protective functions of isoprene under high temperatures, existing studies propose multiple complementary mechanisms. In 1995, [PERSON] first proposed membrane stabilization as a mechanistic explanation for the function of isoprene. Due to its lipophilic nature and localization within chloroplasts, isoprene is likely partitioned into the lipid phase of thylakoid membranes.