### Tocopherol Synthesis
#### 2.1.3 Genetic Approaches for Boosting Isoprenoid Production in Plants
However, despite the similarly increased plastochromanol content, the plants overexpressing _VTE1_ did not differ from wild-type plants in their resistance to high light conditions [48]. Furthermore, plants with a _VTE1_ knockout, which were highly sensitive to increased light conditions, regained their resistance to light stress when _SPS1_ was overexpressed in these plants [120]. Therefore, increasing PQ content in leaves by genetic engineering approaches could be a promising way to enhance plant sustainability under photoinhibitory conditions. As detailed above, carotenoids also play an important antioxidant role. Furthermore, \(\beta\)-carotene serves as a precursor to vitamin A, which humans cannot synthesize on his own and must obtain it from food. Both of these factors explain the numerous attempts to increase the content of carotenoids in the leaves and fruits of plants. One of the best-known examples of plants with increased \(\beta\)-carotene content is \"Golden Rice\", created through the simultaneous overexpression of the narcissus gene encoding PSY (Figure 2) and a bacterial caroten desaturase [121]. Subsequently, \"Golden Rice II\" was developed, using the _PSY_ gene from maize, resulting in a 23-fold increase in carotenoid content compared to the original \"Golden Rice\" (Table 1) [122]. Seeds of _Brassica napus_, known as \"Golden Canola\", were generated through the overexpression of a bacterial _phytoene synthase_ gene, leading to carotenoid levels in these seeds approximately 50 times higher than in non-transgenic seeds [123]. Similar approaches were used to produce \"Golden Maize\" seeds [124]. In transgenic bananas, the highest accumulation of carotenoids was detected in lines carrying the banana _PSY_ gene rather than the maize _PSY_. In these bananas, the carotenoid content reached 55 \(\upmu\)g/g, exceeding that in \"Golden Rice II\" seeds [125]. An alternative approach for obtaining plants with increased \(\upbeta\)-carotene content involves inhibiting the activity of enzymes that use \(\upbeta\)-carotene as a substrate. Orange plants with a significant (up to 36 times) increase in \(\upbeta\)-carotene content were generated through the suppression of \(\upbeta\)-carotene hydroxylase (\(\upbeta\)-_OHase_) expression using RNA interference (Figure 2, Table 1) [126]. The antioxidant properties of these oranges were tested on a model organism, a small nematode worm _Caenorhabditis elegans_. The study revealed that the survival rate of worms under oxidative stress conditions (hydrogen peroxide supplementation) increased by 20% when they were fed oranges with enhanced \(\upbeta\)-carotene content [126]. To increase the \(\upbeta\)-carotene content in sweet potatoes, RNA interference of \(\upbeta\)-_OHase_ was also employed to suppress its expression level [127]. The total carotenoid content in these transgenic plants increased by 10 to 18 times, \(\upbeta\)-carotene content increased by 16 to 35 times, and the amount of zeaxanthin increased by 5 to 15 times. The resulting plants demonstrated increased resistance to salinity (150 mM NaCl) and reduced accumulation of ROS under stress conditions [127]. On the other hand, overexpression of \(\upbeta\)-_OHase_ in Arabidopsis only led to a slight reduction in the amount of \(\upbeta\)-carotene due to its conversion into xanthophylls, along with a significant accumulation of xanthophyll carotenoids, reaching up to 40% of the total carotenoid content under moderate light conditions [128]. These transgenic Arabidopsis plants showed increased resistance to higher light intensity and elevated temperatures (1000 \(\upmu\)mol photons/m\({}^{2}\) s, 40 \({}^{\circ}\)C) [128], which is in line with the role of xanthophylls in protecting plants against photoinhibition. Additionally, the transgenic Arabidopsis plants accumulated significantly lower amount of anthocyanin (see below), which also serves as a stress indicator for Arabidopsis.