L-Ascorbate biosynthesis in peach: cloning of six L-galactose pathway-related genes and their expression during peach fruit development.

The L-ascorbate (AsA) content and the expression of six L-galactose pathway-related genes were analyzed in peach flesh during fruit development. Fluctuation of AsA during peach fruit development was divided into four phases based on the overall total AsA (T-AsA) content per fruit: AsA I, 0-36 days after full bloom (DAFB); AsA II, 37-65 DAFB; AsA III, 66-92 DAFB and AsA IV, 93-112 DAFB. Phase AsA III was a lag phase for AsA accumulation, but did not coincide with the lag phase for fruit development. The T-AsA concentration was highest at the early stage until 21 DAFB [2-3 micromol per gram of fresh weight (g(-1) FW)], and decreased to 1/4 and 1/15 of this value at 50 and 92 DAFB, respectively. T-AsA then remained at 0.15-0.20 micromol g(-1) FW until harvest at 112 DAFB. More than 90% of the T-AsA was in the reduced form until 21 DAFB. The proportion of reduced form of AsA then decreased concomitantly with the decrease in AsA concentration. To determine the main pathway of AsA biosynthesis and the AsA biosynthetic capacity of peach flesh, several precursors were incubated with immature whole fruit (59 DAFB). The AsA concentration increased markedly with L-galactono-1,4-lactone or L-galactose (Gal), but d-galacturonate and L-gulono-1,4-lactone failed to increase AsA, indicating dominance of the Gal pathway and potent AsA biosynthetic capabilities in immature peach flesh. The expression of genes involved in the last six steps of the Gal pathway was measured during fruit development. The genes studied included GDP-d-mannose pyrophosphorylase (GMPH), GDP- d-mannose-3',5'-epimerase (GME), GDP- L-galactose guanylyltransferase (GGGT), L-galactose-1-phosphate phosphatase (GPP), L-galactose-1-dehydrogenase (GDH) and L-galactono-1,4-lactone dehydrogenase (GLDH). GMPH, GME and GGGT had similar expression patterns that peaked at 43 DAFB. GPP, GDH and GLDH also had similar expression patterns that peaked twice at 21 and 91 DAFB, although the expression of GDH was quite low. High level of T-AsA concentration was roughly correlated with the level of gene expression in the early period of fruit development (AsA I), whereas no such relationships were apparent in the other periods (e.g. AsA III and IV). On the basis of these findings, we discuss the regulation of AsA biosynthesis in peach fruit.