Trends in Plant Science
Biosynthesis of L-ascorbic acid in plants: new pathways for an old antioxidant
Section snippets
Intrigue of an equilibrium constant
The enzyme GDP-mannose 3′,5′-epimerase was known to catalyse the conversion of GDP-D-mannose to GDP-L-galactose [10]. However, abnormal values in the apparent equilibrium constant reached for this inter-conversion catalysed by both the recombinant and the native enzyme led the authors to formulate the hypothesis that GDP-L-galactose was not the only epimerization product [15]. The new compound was identified as GDP-L-gulose, which results from the 5′-epimerization of the GDP-D-mannose.
Myo-inositol: a new player in the game?
The functionality in plants of a pathway similar to the one described in animals has never been discarded. Overexpression in lettuce and tobacco of the rat gene encoding the L-gulono-1,4-lactone oxidase increased the L-ascorbic acid level of the transgenic plants by up to sevenfold [17]. More recently, it has been shown that ectopic expression of this gene rescued leaf L-ascorbic acid content in all five Arabidopsis (vtc) mutants that were deficient in L-ascorbic acid [18]. This means that
Regulation is being disclosed
L-ascorbic acid is present in plant species in concentrations that range from an estimated 300 mM in the chloroplast stroma to less than 20 mM in other plant tissues [21]. In spite of this high and wide-ranged value, the multiple roles played by this molecule in crucial physiological processes mean that the steady-state values are tightly controlled [21]. This is achieved at various levels, such as enzyme activity, gene expression in response to developmental and environmental cues, regeneration
Specific pathways for different cell types
Fluxes in the network depicted in Figure 1 are dependent on cell specialization and metabolic regulation. It is known that L-ascorbic acid content in plants changes with light [26], hour of the day 27, 30, age [31], plant tissue [19] and cell compartment [32]. Theoretically, all these changes should be explained by the functioning of a complete metabolic network of L-ascorbic acid biosynthesis, catabolism and recycling. For example, D-galacturonate reductase activity has been reported in
Black hole of catabolism
In vivo levels of L-ascorbic acid result from the balance between synthesis and degradation. This is important because there is some evidence of a relatively high turnover rate of this compound in some plant tissues as well as suggestions that the turnover rate might contribute to controlling L-ascorbic acid content [22]. The first catabolic step is oxidation, which produces successively the monodehydroascorbate radical and dehydroascorbate. Dehydroascorbate undergoes irreversible hydrolysis to
Acknowledgements
We thank José R. Botella for critically reading the manuscript. This work was supported by grant BIO2001–1958-C04–01 from the Ministry of Education and Science (Spain).
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