### Tocopherol Synthesis
#### 2.1.3 Genetic Approaches for Boosting Isoprenoid Production in Plants  
The biosynthetic pathway of isoprenoid synthesis involves numerous enzymes (see Table 1 and Figure 2). For many of these enzymes, the impact of overexpression or knockout on the content of various isoprenoids and the characteristics of the mutant plants have been studied. The regulation of isoprenoid biosynthesis is a complex, multi-level process,including transcriptional, epigenetic, and post-translational control. At each of these stages, it is possible to interfere with genetic engineering methods. The content of isoprenoids in plants can be changed using different approaches, such as introducing additional gene cassettes, which affect the intensity of enzyme biosynthesis or by introducing targeted mutations into the genes of these enzymes. One of the approaches for the elevation of the level of various isoprenoids is upregulation of the expression of the genes encoding the enzymes of the initial steps of the isoprenoid biosynthetic pathway. However, such an approach may be associated with certain challenges, since the correspondent metabolites are used in various branches of the biosynthesis of antioxidants further downstream. For instance, Arabidopsis plants overexpressing the FPPS encoding gene (see Figure 2) exhibited lower levels of endogenous isoprenoids compared to the wild-type plants [106]. Moreover, these plants displayed necrotic damage accompanied by an increase in hydrogen peroxide accumulation, especially under constant illumination. It appears that elevation of _FPPS_ expression leads to metabolic disruptions, possibly due to the rapid depletion of IPP (isopentenyl diphosphate), a precursor for all isoprenoids. Another enzyme, HPPD, plays a crucial role in the metabolic pathway leading to the synthesis of tocopherols, PQ, and plastochromanol. Studies involving overexpression of the _HPPD_ gene yielded contradictory results. For example, overexpression of _HPPD_ in Arabidopsis led to an approximately 40% increase intocopherol accumulation in leaves [107]. In tobacco plants with overexpression of the barley _HPPD_ gene under the 35S promoter, there was no increase in tocopherol content in the leaves; however,tocopherol levels increased in seeds [108]. Interestingly, these tobacco plants exhibited a significant (up to 50% of wild-type levels) increase in PQ content in leaves [107]. Transgenic sweet potato plants overexpressing HPPD were found to be more resistant to drought (cessation of watering for 14 days), salinity (watering with 200 mM NaCl), and oxidative stress (induced by incubating leaf discs in 5 \(\upmu\)M methyl viologen) compared to non-transgenic plants [109]. Similar to the findings in [PERSON] et al. [108], the authors of that study did not observe an increase in tocopherol content in the leaves of transgenic sweet potatoes. Unfortunately, the authors did not analyze the content of plastoquinones and plastochromanol, so the mechanism of resistance of these transgenic plants remains unclear. Nevertheless, when enzymes are exclusively involved in the biosynthesis of specific types of isoprenoids, successful genetic transformations have been reported. For example, in Arabidopsis, the overexpression of the gene encoding peroxisomal 4 CL involved in the oxidation of p-coumaric acid for the subsequent biosynthesis of UQ (Figure 2) led to an approximately 1.5- to 2-fold increase in UQ accumulation [16, 17]. The most promising approaches to enhance isoprenoid content involve genetic engineering manipulations with enzymes catalyzing the last steps of biosynthesis of certain antioxidants of interest. Overexpression of _VTE1_, which catalyzes both the penultimate step intocopherol biosynthesis and the conversion of PQ to plastochromanol (Figure 2), resulted in a seven-fold increase in tocopherol accumulation in Arabidopsis leaves [110].