Plant, Cell and Environment

Ascorbate is oxidized into the radical monodehydroascorbate (MDHA) through ascorbate oxidase or peroxidase activity or non-enzymatically by reactive oxygen species. Regeneration of ascorbate from MDHA is ensured by the enzyme monodehydroascorbate reductase (MDHAR). Previous work has shown that growth processes and yield can be altered by modifying the activity of enzymes that recycle ascorbate, therefore we have studied similar processes in cherry tomato (Solanum lycopersium L.) under- or overexpressing MDHAR. Physiological and metabolic characterization of these lines was carried out under different light conditions or by manipulating the source-sink ratio. Independently of the light regime, slower early growth of all organs was observed in MDHAR silenced lines, decreasing final fruit yield. Stomatal conductance and photosynthesis were altered as was the accumulation of hexoses and sucrose in a light-dependent manner in plantlets. Sucrose accumulation was also repressed in young fruits and final yield of MDHAR silenced lines showed a stronger decrease under carbon limitation. Ascorbate and monodehydroascorbate appear to be involved in control of growth and sugar metabolism in cherry tomato and these enzymes could be potential targets for yield improvemen

Variation in Tomato Fruit Ascorbate Levels and Consequences of Manipulation of Ascorbate Metabolism on Drought Stress Tolerance

Tomato is an important crop worldwide and one of the major sources of vitamin C (ascorbate) in the human diet. Ascorbate contributes both to tomato nutritional quality and has roles in stress tolerance and adaptation to the environment. In this study we show the variability that exists in tomato germplasm in terms of ascorbate content (10 to 90 mg/100 g fwt) which could be starting point for evaluating correlations with physiological traits potentially linked to ascorbate. We have then manipulated genes involved in ascorbate metabolism, using RNA interference, to investigate their influence on fruit ascorbate levels, fruit physiology and yield under both normal and drought stress conditions. There is some evidence that one of the genes chosen affects fruit size and yield under different conditions, but clear effects on the fruit ascorbate pool are not seen. We conclude that ascorbate metabolism is complex and can also have wider effects on fruit physiology and growth

High temperature inhibits ascorbate recycling and light stimulation of the ascorbate pool in tomato despite increased expression of biosynthesis genes

Understanding how the fruit microclimate affects ascorbate (AsA) biosynthesis, oxidation and recycling is a great challenge in improving fruit nutritional quality. For this purpose, tomatoes at breaker stage were harvested and placed in controlled environment conditions at different temperatures (12, 17, 23, 27 and 31 °C) and irradiance regimes (darkness or 150 µmol m(-2) s(-1)). Fruit pericarp tissue was used to assay ascorbate, glutathione, enzymes related to oxidative stress and the AsA/glutathione cycle and follow the expression of genes coding for 5 enzymes of the AsA biosynthesis pathway (GME, VTC2, GPP, L-GalDH, GLDH). The AsA pool size in pericarp tissue was significantly higher under light at temperatures below 27 °C. In addition, light promoted glutathione accumulation at low and high temperatures. At 12 °C, increased AsA content was correlated with the enhanced expression of all genes of the biosynthesis pathway studied, combined with higher DHAR and MDHAR activities and increased enzymatic activities related to oxidative stress (CAT and APX). In contrast, at 31 °C, MDHAR and GR activities were significantly reduced under light indicating that enzymes of the AsA/glutathione cycle may limit AsA recycling and pool size in fruit pericarp, despite enhanced expression of genes coding for AsA biosynthesis enzymes. In conclusion, this study confirms the important role of fruit microclimate in the regulation of fruit pericarp AsA content, as under oxidative conditions (12 °C, light) total fruit pericarp AsA content increased up to 71%. Moreover, it reveals that light and temperature interact to regulate both AsA biosynthesis gene expression in tomato fruits and AsA oxidation and recycling