De novo transcriptome analyses reveal multiple metabolic pathways for L-ascorbic acid biosynthesis under drought stress in Pugionium cornutum (L.) Gaertn

Pugionium cornutum (L.) Gaertn., a member of the Brassicaceae family, is a plant that has been used as traditional Chinese medicine and is consumed as a vegetable rich in L-ascorbic acid(AsA). To identify candidate genes involved in AsA metabolism, we analysed transcriptome data from leaves of P. cornutum under drought stress using Illumina sequencing technology. A total of 144 unique genes were annotated as encoding enzymes involved in AsA metabolism. Compared with that for the control (CK), the numbers of differentially expressed genes were 7 and 22 under moderate and severe drought stress, respectively. The enzyme sites involved in AsA biosynthesis were GDP-D-mannose 3', 5'- epimerase, GDP-L-galactose phosphorylase, inositol-phosphate phosphatase/L-galactose 1-phosphate phosphatase, D-threo-aldose 1-dehydrogenase, L-galactono-1,4-lactone dehydrogenase, UDP-glucose 6-dehydrogenase, aldehyde dehydrogenase (NAD+), inositol oxygenase, L-ascorbate peroxidase, monodehydroascorbate reductase (NADH), L-ascorbate oxidase, and L-galactono-1,4-lactone dehydrogenase. ASA biosynthesis was mainly mediated by the L-galactose pathway in the leaves of P. cornutum. However, enzyme genes involved in the L-gulose pathway were not detected.