Tocopherol biosynthesis is enhanced in photomixotrophic sunflower cell cultures

Alpha-tocopherol is the most biologically active component of vitamin E and is synthesized only by photosynthetic organisms. Two heterotrophic cell lines of sunflower (Helianthus annuus L.) of differing alpha-tocopherol biosynthetic capability, three-fold higher in the high synthesizing cell line, HT, than in the low synthesizing one, LT, were previously identified. To investigate the relationship between alpha-tocopherol biosynthesis and photomixotrophic culture conditions, a new photomixotrophic sunflower cell line HS3 was established by selecting HT cells able to grow in the presence of a ten-fold reduced sucrose concentration in the culture medium. The photosynthetic properties of HS3 cells were characterized in comparison with HT and LT cells, revealing an increase in chlorophyll content, chloroplast number, and level of the photosynthesis related enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Furthermore, an enhanced expression of the gene encoding for the tocopherol biosynthetic enzyme geranyl-geranylpyrophosphate synthase (GGPPS) was observed in HS3 cells. HS3 cells also revealed a 25% and a more than three-fold higher tocopherol level than HT and LT, respectively, indicating a positive correlation between alpha-tocopherol biosynthesis of sunflower cell cultures and their photosynthetic properties. These findings can be useful for improving the tocopherol yields of the sunflower in vitro production system.