Abstract
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.
Highlights
Ascorbate or ascorbic acid (AsA) is an important micronutrient essential for plant growth [1,2] which plays major roles in the interaction of plants with the environment, pathogens and oxidizing agents [3,4,5]
Fruit pericarp dry weight content was not affected by temperature or irradiance (Figure 1B), and we chose to express all data per fresh weight
This increased AsA content with light was dependent on temperature and decreased as temperature increased, so that at 27°C or 31°C there was no more effect of light on total or reduced AsA content
Summary
Ascorbate or ascorbic acid (AsA) is an important micronutrient essential for plant growth [1,2] which plays major roles in the interaction of plants with the environment, pathogens and oxidizing agents [3,4,5]. AsA is an essential antioxidant for humans. Besides the well-known role of vitamin C in preventing scurvy, AsA is involved in preventing various oxidative stress-related illnesses such as cancers, cardio-vascular diseases and aging [6,7,8]. Since humans have lost the ability to synthesize AsA, fruits and vegetables are the major source of dietary AsA for humans. AsA concentration in tomato fruit depends on the cultivar [9]. Owing to its large consumption, tomato fruit constitute an important source of AsA intake in the human diet. Fruit AsA content strongly varies with the pre- and post-harvest environment [10,11], and an improved understanding of the regulation of AsA accumulation in fruits is required
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