Effect of mitochondrial ascorbic acid synthesis on photosynthesis.

Ascorbic acid (AA) is synthesized in plant mitochondria through the oxidation of l-galactono-1,4-lactone (l-GalL) and then distributed to different cell compartments. AA-deficient Arabidopsis thaliana mutants (vtc2) and exogenous applications of l-GalL were used to generate plants with different AA content in their leaves. This experimental approach allows determining specific AA-dependent effects on carbon metabolism. No differences in O2 uptake, malic and citric acid and NADH content suggest that AA synthesis or accumulation did not affect mitochondrial activity; however, l-GalL treatment increased CO2 assimilation and photosynthetic electron transport rate in vtc2 (but not wt) leaves demonstrating a stimulation of photosynthesis after l-GalL treatment. Increased CO2 assimilation correlated with increased leaf stomatal conductance observed in l-GalL-treated vtc2 plants.

Increased ascorbate content delays flowering in long-day grown Arabidopsis thaliana (L.) Heynh.

Flowering requires the integration of different inductive stimuli, including light, temperature and hormones. In an attempt to assess whether ascorbate (ASC) could contribute to the control of flowering time, we analyzed the effects of increased ASC content on the transition to the reproductive stage in the facultative long-day plant Arabidopsis thaliana. ASC content was increased by spraying leaves with the ASC precursor L-galactono-gamma-lactone. Our data show that increased ASC content did not affect vegetative growth, whereas a significant delay (5 days in average) in flower production occurred in ASC-overproducing plants. Higher ASC availability resulted in delayed expression of LEAFY (LFY), the gene encoding for a key transcription factor integrating different flowering-inductive pathways. On the contrary, spraying with gibberellin under the same condition caused both early LFY expression and early flowering. The possible role of ASC in the transition to the reproductive stage is discussed.

Hopes, disillusions and more hopes from vitamin C.

In the current view of most biochemists and physiologists, the role of L-ascorbic acid (AA) in cell metabolism would be more or less confined to the scavenging of reactive oxygen species. Nevertheless, many data have been collected in our and other laboratories concerning the involvement of AA in many different aspects of cell metabolism. At the present time the molecular sites of action of AA have not been completely elucidated, but recent findings on the specific requirement of AA for the activity of several 2-oxoacid-dependent dioxygenases involved in cell signalling and the activation of transcription factors open new fascinating perspectives for further research.

Short- and long-term effects of dehydroascorbate in Lupinus albus and Allium cepa roots.

Administration of 1 mM dehydroascorbate (DHA) results in a rapid and large increase in cellular ascorbate (AA) content in both Lupinus albus L. and Allium cepa L. root tips. Uptake of DHA from the medium occurs at a high rate within 10-12 h of incubation, and is slowed down thereafter. In the first few h, DHA reduction to AA is apparently correlated to GSH depletion and slightly higher DHA reductase activity. DHA incubation also seems to induce new GSH synthesis. Longer DHA incubation (24 h) affects root growth by inhibiting cell proliferation. At this stage, an apparently generalised oxidation of SH-containing proteins is observed in DHA-treated roots. Treatment with 1 mM L-galactono-gamma-lactone, the last precursor of AA biosynthesis, results in an increase in AA content similar to that obtained with DHA, but stimulates growth and affects the redox state of SH-containing proteins in the opposite way. A possible multi-step mechanism of DHA reduction/removal is suggested and the hypothesis that DHA inhibits cell cycle progression by affecting the redox state of SH-containing proteins is discussed.

The multiplicity of enzymatic DNA reduction: a new purification procedure for a DHA reducing protein from potato tubers.

The multiplicity of DHA reducing proteins has been observed by means of a native-PAGE technique in several plant species, thus demonstrating, in accordance with recent literature, that several differently evolved proteins are likely to perform DHA reduction. Moreover, a research strategy coupling the use of native-PAGE with chromatographic separation procedure, tentatively performed in Solanum tuberosum, proved to be a useful tool for the separation and partial identification of the proteins involved in DHA reduction.