Engineering ascorbic acid biosynthetic pathway in Arabidopsis leaves by single and double gene transformation

Six genes, which encode enzymes involved in ascorbic acid (AsA) biosynthesis, including guanosine diphosphate (GDP)-mannose pyrophosphorylase (GMP), GDP-mannose-3',5'-epimerase (GME), GDP-galactose guanylyltransferase (GGT), L-galactose-1-phosphate phosphatase (GPP), L-galactose dehydrogenase (GDH) and L-galactono-1,4-lactone dehydrogenase (GLDH) were transformed into Arabidopsis thaliana, to evaluate the contribution of each gene to AsA accumulation. Additionally, two combinations, GGT-GPP and GGT-GLDH, were co-transformed into Arabidopsis with a reliable double-gene transformation system. AsA content of GGT transgenic lines was 2.9-fold higher as compared to the control, and co-transformation led up to 4.1-fold AsA enhancement. These results provided further evidence that GGT is the key enzyme in plant AsA biosynthesis.