title: Translocation and the alternative D-galacturonate pathway contribute to increasing the ascorbate level in ripening tomato fruits together with the D-mannose/L-galactose pathway.
abstract: The D-mannose/L-galactose pathway for the biosynthesis of vitamin C (L-ascorbic acid; AsA) has greatly improved the understanding of this indispensable compound in plants, where it plays multifunctional roles. However, it is yet to be proven whether the same pathway holds for all the different organs of plants, especially the fruit-bearing plants, at different stages of development. Micro-Tom was used here to elucidate the mechanisms of AsA accumulation and regulation in tomato fruits. The mRNA expression of the genes in the D-mannose/L-galactose pathway were inversely correlated with increasing AsA content of Micro-Tom fruits during ripening. Feeding L-[6-(14)C]AsA to Micro-Tom plants revealed that the bulk of the label from AsA accumulated in the source leaf was transported to the immature green fruits, and the rate of translocation decreased as ripening progressed. L-Galactose feeding, but neither D-galacturonate nor L-gulono-1,4-lactone, enhanced the content of AsA in immature green fruit. On the other hand, L-galactose and D-galacturonate, but not L-gulono-1,4-lactone, resulted in an increase in the AsA content of red ripened fruits. Crude extract prepared from insoluble fractions of green and red fruits showed D-galacturonate reductase- and aldonolactonase-specific activities, the antepenultimate and penultimate enzymes, respectively, in the D-galacturonate pathway, in both fruits. Taken together, the present findings demonstrated that tomato fruits could switch between different sources for AsA supply depending on their ripening stages. The translocation from source leaves and biosynthesis via the D-mannose/L-galactose pathway are dominant sources in immature fruits, while the alternative D-galacturonate pathway contributes to AsA accumulation in ripened Micro-Tom fruits.
authors: Department of Applied Bioscience and Biotechnology, Faculty of Life and Environmental Science, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan.
authors: Adebanjo Ayobamidele
authors: AA
authors: Badejo
authors: Keiko
authors: K
authors: Wada
authors: Yongshun
authors: Y
authors: Gao
authors: Takanori
authors: T
authors: Maruta
authors: Yoshihiro
authors: Y
authors: Sawa
authors: Shigeru
authors: S
authors: Shigeoka
authors: Takahiro
authors: T
authors: Ishikawa
mesh_terms: D001205:Ascorbic Acid / Q000378:metabolism*; D001345:Autoradiography; D001483:Base Sequence; D001692:Biological Transport; D017931:DNA Primers; D004579:Electron Transport / Q000187:drug effects; D005690:Galactose / Q000378:metabolism*; D006603:Hexuronic Acids / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000254:growth & development*; D008358:Mannose / Q000378:metabolism*; D010788:Photosynthesis; D060888:Real-Time Polymerase Chain Reaction
chemical_list: D017931:DNA Primers; D006603:Hexuronic Acids; C007819:galacturonic acid; D008358:Mannose; D001205:Ascorbic Acid; D005690:Galactose