L- ascorbic acid metabolism in two contrasting hardy kiwifruit (Actinidia arguta) cultivars during fruit development

Hardy kiwifruit (Actinidia arguta) has recently been popular in the fresh market due to its edible skin, and higher level of nutrition especially ascorbic acid (AsA) over kiwifruit (A. deliciosa and A. chinensis). On the behalf of screening high AsA content material for further genetic improvements, to gain a better understanding of the molecular mechanism of AsA metabolism, two hardy kiwifruit cultivars (‘Yilv’ and ‘Lvmi-1′) with contrasting AsA content were selected as materials for analysis. The ‘Yilv’ fruit contained 5 folds higher AsA content than ‘Lvmi-1′. AsA and total AsA (T-AsA) content showed a gradually decreasing in ‘Yilv’, while slightly increasing in ‘Lvmi-1′ during fruit development. However, the content of T-AsA increased with fruit weight in both cultivars. In accordance with AsA content, the expression of genes encoding GDP-L-galactose pyrophosphatase (GGP), L-galactose-1-phosphate phosphatase (GPP) and L-galactono-1,4-latone dehydrogenase (GalLDH), as well as the GalLDH activity were higher in ‘Yilv’ than that in ‘Lvmi-1′. During fruit development, no correlation between the biosynthetic genes expression and AsA pool was observed in each cultivar. In ‘Lvmi-1′ fruit, the expression of genes encoding ascorbate oxidase (AO), monodehydroascorbate reductase (MDHAR), the activity of AO, GDP-D-mannose pyrophosphorylase (GMP) and GGP showed significantly correlations with AsA pool level. In ‘Yilv’ fruit, the expression of MDHAR, the activity of GMP, GGP, GPP, GDP mannose 3′5’-epimerase (GME), GalLDH, myo-inositol oxygenase (MIOX), AO and ascorbate peroxidase (APX) displayed significant correlations with AsA pool. In conclusion, GalLDH was a key enzyme for hardy kiwifruit AsA biosynthesis, MDHAR was more efficient than DHAR in AsA regeneration in ‘Yilv’, AO and APX might be more efficient in AsA oxidation in ‘Lvmi-1′ and ‘Yilv’ respectively. This study explored the different AsA content and potential mechanism in two hardy kiwifruit cultivars, providing a reference for further genetic improvement and high AsA content material selection in cultivation.