### Tocopherol Synthesis  
VTE3 and VTE1 are also involved in the biosynthesis of \(\gamma\)- and \(\delta\)-tocomberols (Figure 2). Phytyl-PP for tocopheroids synthesis originates from GGDP by phosphorylation of its free phytol with VTE5 or VTE6. Both of these enzymes are important for vitamin E synthesis:tocopherol levels reduced to 20% in _VTE5_ knockout Arabidopsis plants, compared to the wild-type plants [32]. Phytol phosphorylation, catalyzed by VTE6 was also shown to be important for phylloquinone biosynthesis, which in turn is required for Photosystem I (PS I) complex functioning and stability [33]. The VTE6 knockout Arabidopsis mutants exhibited an impaired function of PS I due to a higher rate of PS I subunits degradation and increased PS I susceptibility to photodamage, compared to the wild-type plants. Phytyl-PP molecules are condensed with HGA by homogentiaste prenyl transferase (HPT, VTE2) to yield 2-methyl-6-phytylplastoquinol (MPBQ), which is methylated by VTE3 to form 2,3-dimethyl-5-phytyl-1, 4-benzoquinone (DMPBQ). MPBQ and DMPBQ are substrates for VTE1 to yield \(\delta\)- and \(\gamma\)-tocpherols (Figure 2). Finally, \(\gamma\)-tocopherol methyl-transferase ( VTE4) converts \(\delta\)- and \(\gamma\)-tocpherols (and toctorienols) to \(\beta\)- and \(\alpha\)-tocpherols. HPT is the enzyme which catalyzes the limiting step in tocopherols biosynthesis [49]. Carotenoid synthesis is also a branch of terpenoid plastid biosynthesis from the same precursors as for tocopherols and PQ (Figure 2). Carotenoid synthesis starts from the condensation by the enzyme phytoene synthase (PSY) of two GGDP molecules to produce phytoene. Two desaturases, phytoene desaturase (PDS) and \(\zeta\)-carotene desaturase (ZDS), catalyze similar dehydrogenation reactions by introducing four double bonds to form tetra-cis-lycopene. The knockout of the _PDS_ gene in many crop species resulted in a phenotype with pigmentation loss. Albino regenerants of cells with edited _PDS_ gene have been obtained for _Malus domestica_ plants, for diploid and octoploid strawberries, yams, and onion [50; 51; 52; 53]. The regenerants of carrot cells with mutations in the region of the _DcPDS_ and _DcMYB113_-like genes were depignmented [54]. Desaturation requires a plastid terminal oxidase and plastoquinone in photosynthetic tissues [55; 56; 57]. In the next step, the carotenoid isomerase (CRTISO) catalyzes cis-trans isomerization resulting in all-trans-lycopene. This enzyme is important for optimal carotenoid synthesis in etioplasts, chromoplasts, and chloroplasts [58]. Mutant plants deficient in CRTISO activity accumulate various cis-isomer biosynthetic intermediates when grown in the dark, but these intermediates can be photoisomerized in the light and yield viable plants, albeit with reduced lutein levels [24]. The next steps of carotenoid biosynthesis split into two main branches differing by cyclic end-groups. One branch is responsible for the synthesis of \(\alpha\)-carotene and its derivatives by lycopene epsilon cyclase (\(\varepsilon\)LCY) and lycopene beta cyclase (\(\beta\)LCY). Their altered expression in mutants resulted in lutein levels ranging from 10% to 180% of those in the wild-type plants [59]. With participation of \(\beta\)LCY, lycopene is cyclized to introduce \(\beta\)-ionone, which leads to the other \(\beta\),\(\beta\) branch of caroten synthesis. The functioning of this branch leads to the synthesis of \(\beta\)-carotene and its derivatives, provitamin A and the components of the xanthophyll cycle, violaxanthin, anthraxanthin, and zeaxanthin. The _Orange genes_ (_Or_), which were found in many plant species, are a class of regulatory genes that mediate carotenoid accumulation [60]. It has been shown that, in sweet potato, the product of _lbOr_ can protect PSY and the oxygen-evolving enhancer protein PsbP of photosystem II (PS II) [61; 62]. The product of this gene does not directly participate in carotenoid biosynthesis but is involved in chromoplast differentiation, creating a storage site for carotenoids [63].