Humans are unable to synthesise L-ascorbic acid (L-AA, ascorbate, vitamin C), and are thus entirely dependent upon dietary sources to meet needs. In both plant and animal metabolism, the biological functions of L-ascorbic acid are centred around the antioxidant properties of this molecule. Considerable evidence has been accruing in the last two decades of the importance of L-AA in protecting not only the plant from oxidative stress, but also mammals from various chronic diseases that have their origins in oxidative stress. Evidence suggests that the plasma levels of L-AA in large sections of the population are sub-optimal for the health protective effects of this vitamin.Until quite recently, little focus has been given to improving the L-AA content of plant foods, either in terms of the amounts present in commercial crop varieties, or in minimising losses prior to ingestion. Further, while L-AA biosynthesis in animals was elucidated in the 1960s, 1 it is only very recently that a distinct biosynthetic route for plants has been proposed. 2 The characterisation of this new pathway will undoubtedly provide the necessary focus and impetus to enable fundamental questions on plant L-AA metabolism to be resolved.This review focuses on the role of L-AA in metabolism and the latest studies regarding its biosynthesis, tissue compartmentalisation, turnover and catabolism. These inter-relationships are considered in relation to the potential to improve the L-AA content of crops. Methodology for the reliable analysis of L-AA in plant foods is brie¯y reviewed. The concentrations found in common food sources and the effects of processing, or storage prior to consumption are discussed. Finally the factors that determine the bioavailability of L-AA and how it may be improved are considered, as well as the most important future research needs.

L-Ascorbate (the reduced form of vitamin C) participates in diverse biological processes including pathogen defence mechanisms, and the modulation of plant growth and morphology, and also acts as an enzyme cofactor and redox status indicator. One of its chief biological functions is as an antioxidant. L-Ascorbate intake has been implicated in the prevention/alleviation of varied human ailments and diseases including cancer. To study the regulation of accumulation of this important nutraceutical in fruit, the expression of 24 tomato (Solanum lycopersicon) genes involved in the biosynthesis, oxidation, and recycling of L-ascorbate during the development and ripening of fruit have been characterized. Taken together with L-ascorbate abundance data, the results show distinct changes in the expression profiles for these genes, implicating them in nodal regulatory roles during the process of L-ascorbate accumulation in tomato fruit. The expression of these genes was further studied in the context of abiotic and post-harvest stress, including the effects of heat, cold, wounding, oxygen supply, and ethylene. Important aspects of the hypoxic and post-anoxic response in tomato fruit are discussed. The data suggest that L-galactose-1-phosphate phosphatase could play an important role in regulating ascorbic acid accumulation during tomato fruit development and ripening.

Humans are unable to synthesise L‐ascorbic acid (L‐AA, ascorbate, vitamin C), and are thus entirely dependent upon dietary sources to meet needs. In both plant and animal metabolism, the biological functions of L‐ascorbic acid are centred around the antioxidant properties of this molecule. Considerable evidence has been accruing in the last two decades of the importance of L‐AA in protecting not only the plant from oxidative stress, but also mammals from various chronic diseases that have their origins in oxidative stress. Evidence suggests that the plasma levels of L‐AA in large sections of the population are sub‐optimal for the health protective effects of this vitamin. Until quite recently, little focus has been given to improving the L‐AA content of plant foods, either in terms of the amounts present in commercial crop varieties, or in minimising losses prior to ingestion. Further, while L‐AA biosynthesis in animals was elucidated in the 1960s,1 it is only very recently that a distinct biosynthetic route for plants has been proposed.2 The characterisation of this new pathway will undoubtedly provide the necessary focus and impetus to enable fundamental questions on plant L‐AA metabolism to be resolved. This review focuses on the role of L‐AA in metabolism and the latest studies regarding its biosynthesis, tissue compartmentalisation, turnover and catabolism. These inter‐relationships are considered in relation to the potential to improve the L‐AA content of crops. Methodology for the reliable analysis of L‐AA in plant foods is briefly reviewed. The concentrations found in common food sources and the effects of processing, or storage prior to consumption are discussed. Finally the factors that determine the bioavailability of L‐AA and how it may be improved are considered, as well as the most important future research needs. © 2000 Society of Chemical Industry

Rosemary extracts containing the phenolic diterpenes carnosic acid and its derivative carnosol are approved food additives used in an increasingly wide range of products to enhance shelf-life, thanks to their high anti-oxidant activity. We describe here the elucidation of the complete biosynthetic pathway of carnosic acid and its reconstitution in yeast cells. Cytochrome P450 oxygenases (CYP76AH22-24) from Rosmarinus officinalis and Salvia fruticosa already characterized as ferruginol synthases are also able to produce 11-hydroxyferruginol. Modelling-based mutagenesis of three amino acids in the related ferruginol synthase (CYP76AH1) from S. miltiorrhiza is sufficient to convert it to a 11-hydroxyferruginol synthase (HFS). The three sequential C20 oxidations for the conversion of 11-hydroxyferruginol to carnosic acid are catalysed by the related CYP76AK6-8. The availability of the genes for the biosynthesis of carnosic acid opens opportunities for the metabolic engineering of phenolic diterpenes, a class of compounds with potent anti-oxidant, anti-inflammatory and anti-tumour activities.

Transgenic tobacco (Nicotiana tabacum L. cv. Xanthi) plants expressing cucumber ascorbate oxidase (EC.1.10.3.3) were used to examine the role of extracellular ascorbic acid in mediating tolerance to the ubiquitous air pollutant, ozone (O 3 ). Three homozygous transgenic lines, chosen on the basis of a preliminary screen of AO activity in the leaves of 29 lines, revealed up to a 380-fold increase in AO activity, with expression predominantly associated with leaf cell walls. Overexpression of AO resulted in no change in the total ascorbate content recovered in apoplast washing fluid, but the redox state of ascorbate was reduced from 30% in wild-type leaves to below the threshold for detection in transgenic plants. Levels of ascorbic acid and glutathione in the symplast were not affected by AO overexpression, but the redox state of ascorbate was reduced, while that of glutathione was increased. AO overexpressing plants exposed to 100 nmol mol -1 ozone for 7 h day -1 exhibited a substantial increase in foliar injury, and a greater pollutant-induced reduction in both the light-saturated rate of CO 2 assimilation and the maximum in vivo rate of ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation, compared with wild-type plants. Transgenic plants also exhibited a greater decline in CO 2 assimilation rate when exposed to a brief ozone episode (300 nmol mol -1 for 8 h). Stomatal conductance, hence O 3 uptake, was unaffected by AO over-expression. Our findings illustrate the important role played by ascorbate redox state and sub-cellular compartmentation in mediating the tolerance of plants to ozone-induced oxidative stress.Abbreviations AO: ascorbate oxidase AE A sat : CO 2 assimilation rate under light-saturated conditions AE AA: L-ascorbic acid AE AWF: apoplast washing fluid AE c a : atmospheric CO 2 concentration AE c i : intercellular CO 2 concentration AE CFA: charcoal/Purafil-filtered air AE DAT: days after transfer to the respective treatments AE DHA: dehydroascorbate AE GSH: reduced glutathione AE GSSG: oxidised glutathione AE g H2O : stomatal conductance to water vapour AE PPFD: photosynthetic photon flux density AE ROS: reactive oxygen species AE V cmax : maximum in vivo rate of Rubisco carboxylation Planta (2003) 216: 918-928