Abstract:

【Objective】 The objective of this experiment was to further investigate whether apple fruit has the capability of AsA biosynthesis. 【Method】 L-galactose dehydrogenase (GalDH) cDNA which encode a enzyme involved in synthesizing AsA in plants was isolated from young fruit of Malus domestica Borkch cv. Gala,then the relationship of the mRNA expression level and enzyme activities of GalDH and L-galactono-1,4-lactone dehydrogenase(GalLDH) on AsA in different tissues of apple was studied. 【Result】 cDNA of GalDH, which was cloned from young fruit exocarp of apple, has an 975 bp open reading frame and encodes a protein of 324 amino acid residues with a molecular mass of 34.97 kD, its accession no in GenBank is GQ131419. The mRNA expression levels and enzyme activities of GalDH and GalLDH were detected in leaves as well as different tissues of apple fruits. Both of mRNA expression levels and activities of the two enzymes in leaves were higher than that in fruit, in young fruit was higher than that in mature fruit, while in peel was higher than that in flesh. Meanwhile, AsA content in apple peel was regulated by light condition. The mRNA expression levels and enzyme activities of both in sun side peel were higher than that in shaded side peel, but no significant differences between sun side flesh and shaded side flesh. A significant positive correlation was detected between AsA content and enzymes activities of GalDH and GalLDH in different tissues of apple. 【Conclusion】 These results further suggest that apple fruit has the capability of AsA biosynthesis by L-galactose pathway,and synthesis may be a major reason of AsA formation in apple fruit.

Key words: apple fruits, L-galactose dehydrogenase, L-galactono-1, 4-lactone dehydrogenase, ascorbate