Genetic diversity and engineering of selected B vitamins in staple crops

Vitamin B1, in the form thiamin diphosphate (ThDP), serves as an enzymatic cofactor for several enzymes involved in glucose metabolism, the Krebs cycle, and branched-chain amino acid biosynthesis. Vitamin B6 in its form as pyridoxal 5'-phosphate (PLP) is required as a cofactor for more than 140 enzymatic reactions in cells, primarily in amino acid, sugar and fatty acid metabolism. Mammals lack the ability to biosynthesize both vitamins B1 and B6 de novo, and plants represent one of their major sources. Symptoms of severe vitamin B1 deficiency include degeneration in the nerves and heart, a disease known as beriberi, which can be fatal. Acute vitamin B6 deficiency can lead to various chronic diseases in humans, including neurological disorders, cardiovascular disease and diabetes. Due to their importance in regions where vitamins B1 and B6 deficiencies are reported, we identified cassava and rice as good target crops for biofortification.
In the present work, we performed a screening for B1 and B6 accumulation in selected varieties and landraces representative of the rice and cassava diversity. HPLC analysis allowed precise quantification of vitamin accumulation in selected genotypes contrasting for vitamin accumulation. We also report the first profiling of B1 and B6 vitamers in cassava storage roots and rice endosperm. This work is of primary importance given the reported varying degree of bioavailability for certain vitamers.
Identification and expression analysis of B1 and B6 biosynthetic genes from rice and cassava was instrumental to find correlation between expression of selected biosynthetic genes and vitamin accumulation.
Genetic engineering of vitamin biosynthetic pathways led to the generation of transgenic rice and cassava lines with high vitamin B6 content. Importantly, our Caco-2 cell assay demonstrated that field-grown storage roots from transgenic cassava lines accumulate high levels of bioavailable vitamin B6.
Despite their key role in plant metabolism and their antioxidant activities, increased accumulation of vitamin B6 in transgenic rice and cassava lines could not be associated with improved tolerance to selected biotic or abiotic stresses under lab conditions.