## 4 Common Strategies for Optimizing Sesquietpenol Biosynthesis
### Toxicity Tolerance of Terenoids  
Microbial cell factories encounter complex physiological and non-physiological challenges during terpenoid biosynthesis, such as high temperatures, high ethanol concentrations, high osmolarity, high viscosity, and substrate toxicity [127]. Collectively, these extracellular stressors impair normal protein activity and compromise membrane integrity. These issues trigger excessive reactive oxygen species (ROS) generation, primarily through protein denaturation in the electron transport chain. They also cause organelle dysfunction and metabolic disturbances [128]. Strengthening chassis resilience against terpene toxicity is crucial for enhancing sesquiterpenal yields (Figure 5d, Table 2). Antioxidant system engineering is a widely adopted strategic approach to mitigate these effects. Overexpression antioxidant enzymes, such as catalase and superoxide dismutase, can reduce intracellular reactive oxygen species (ROS) levels. [PERSON] et al. achieved a 4.1-fold increase in patchoul production through the overexpression of cytosolic CTT1 and peroxisomal CTA1 [77]. In a previous study, [PERSON] et al. demonstrated that the unfolded protein response (UPR) is triggered in yeast strains producing nerolidol, primarily due to endoplasmic reticulum stress. To address this issue, they introduced the human anti-apoptotic protein Bcl-2 into the yeast, which significantly enhanced nerolidol synthesis [129]. This research offers an effective, straightforward strategy for boosting sesquiterpenal production in engineered yeast. Enhancing the supply of NADPH can also strengthen cellular redox balance, thereby bolstering chassis resilience. [PERSON] et al. increased farnesol production in _Rhodobacter shareoides_ by overexpressing glucose-6-phosphate dehydrogenase (zwf) and 6-phosphogluconate dehydrogenase (gnd), thereby increasing NADPH levels [130]. Modulating efflux pumps to enhance cellular export and tolerance shows great potential for boosting the production of heterologous terpenoids in _E. coli_[131]. [PERSON] et al. explored efflux pumps to enhance patchoul production by evaluating various transmembrane transporters. They overexpressed and modulated proteins such as TolC, ABC transporters (MacAB, MsbA), LptABCDFG, and YadGH in _E. coli_. To mitigate cytotoxicity caused by overexpression of membrane proteins, they used a subtilin-inducible promoter to control the expression of efflux transporters. The results showed that overexpressing MacAB and MsbA significantly increased patchoul production by optimizing hydrophobic compound extrusion, reducing intracellular metabolite accumulation, and maintaining cellular viability [74]. Another approach to mitigating the toxicity of terpenoids is bioderivatization, which transforms the target terpenoid molecule into a less toxic derivative. This derivative can revert to the original terpenoid, when required, following the synthesis process. Terpene glycosylation can reduce cytotoxicity while maintaining recoverable products. Nerolidol glucoside exhibits superior ROS scavenging versus free nerolidol, thereby mitigating oxidative damage in plants [132]. Heterologous expression of glucosyltransferaseterase ugt91q2 in _S. cerevisiae_ or _E. coli_ enables in situ nerolidol glycosylation, thereby enhancing production capacity [133].  
These strategies establish toxicity tolerance as a cornerstone for industrial sesquiterpenal production. Future directions include CRISPR-engineered dynamic regulation of efflux systems and machine learning-guided design of glycosylation enzymes for tailored bioderivatization.