In situ and de novo biosynthesis of vitamin C in wild type and transgenic tomato hairy roots: A precursor feeding study

The biosynthesis of l-ascorbic acid (AsA) in tomato (Solanum lycopersicum cv. Money Maker) hairy roots (HR) was studied, by quantification of total intracellular AsA during 35 days of batch culture and by evaluation of the effects of potential biosynthetic precursors on AsA concentrations. Two types of tomato HR were used in order to asses the possible functional pathways of vitamin C biosynthesis in isolated roots: wild type (WT) and transgenic HR overexpressing GalUR gene. GalUR codifies a d-galacturonic acid reductase (EC 1.1.1.19) from strawberry fruit, involved in the galacturonate pathway for AsA biosynthesis. Our results show that completely isolated tomato roots are able to synthesize AsA in situ and de novo during 35 days, and that AsA accumulates in the roots in a non-growth-associated manner. Feeding the roots with d-galactose (d-Gal) or d-galacturonic acid (d-GalUA) produced a time- and dose-dependent increase in total AsA content in both types of tomato HR clones. An increase of 2.5-fold in total AsA content was observed in a transgenic HR clone treated with d-GalUA compared with the basal AsA content of the WT one. These results show evidences that at least the galacturonate pathway could operate in isolated wild type and transgenic tomato roots, and that bioavailability of such precursors in roots could be limiting AsA biosynthesis.