To cope with environmental fluctuations and to prevent invasion by pathogens, plant metabolism must be flexible and dynamic. Active oxygen species, whose formation is accelerated under stress conditions, must be rapidly processed if oxidative damage is to be averted. The lifetime of active oxygen species within the cellular environment is determined by the antioxidative system, which provides crucial protection against oxidative damage. The antioxidative system comprises numerous enzymes and compounds of low molecular weight. While research into the former has benefited greatly from advances in molecular technology, the pathways by which the latter are synthesized have received comparatively little attention. The present review emphasizes the roles of ascorbate and glutathione in plant metabolism and stress tolerance. We provide a detailed account of current knowledge of the biosynthesis, compartmentation, and transport of these two important antioxidants, with emphasis on the unique insights and advances gained by molecular exploration.

ASCORBATE AND GLUTATHIONE: Keeping Active Oxygen Under Control

Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Antioxidants, Oxidative Damage and Oxygen Deprivation Stress: a Review

http://ucce.ucdavis.edu/files/datastore/234-17.pdf

Preharvest and postharvest factors influencing vitamin C content of horticultural crops.

Summary
The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of ‘promoter’ substances, such as ascorbate, β-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of ‘antinutrients’, such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

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Biofortification of crops with seven mineral elements often lacking in human diets--iron, zinc, copper, calcium, magnesium, selenium and iodine.

A general procedure for the preparation of labeled l-ascorbic acid from labeled d-glucose☆

1L-myo-inositol 1-phosphate synthase from pollen of Lilium longiflorum. An ordered sequential mechanism.

Methods for preparation of plant protoplasts are well established but the extension of such methods to the release of exine-free gametophytes from pollen appears to be limited (Bajaj 1974; Bajaj and Davey 1974; Bhojwani and Cocking 1972; Power 1973; Takegami and Ito 1975; Zhu et al. 1984). We have discovered that 4-methylmorpholine N-oxide monohydrate (MMNO·H2O) is an effective solvent of the intine layer when pollen grains of Lilium longiflorum Thunb. (trumpet lily) are dispersed in MMNO·H2O at its melting point, 75°C (Loewus et al. 1985). Exine-free gametophytes, which we term sporoplasts, are quickly released from their exine enclosures. With time, MMNO also disperses the empty exine ‘shells’ into immiscible droplets. Prolonged heating ruptures the sporoplasts to produce empty sporoplast envelopes or ‘ghosts’ which remain intact in the MMNO·H2O melt.

Dissolution of Pollen Intine and Release of Sporoplasts

SUMMARY: Germinated lily pollen contains a glucogenic process for the conversion of myo-inositol to starch over the myo-inositol oxidation pathway and the pentose phosphate pathway. The key intermediates between these two pathways are UDP-D-xylose and free D-xylose which allow elongating pollen tubes to utilize exogenous L-arabinose and D-xylose as carbon sources for the synthesis of glucose. Evidence is presented for the functional role of the myo-inositol oxidation pathway in pectin biosynthesis. Evidence is also presented to support the view that the cyclization of D-glucose 6-phosphate to myo-inositol 1-phosphate includes a significant isotope rate effect at carbon 5 of the substrate.

Frank A. Loewus

Papers

A general procedure for the preparation of labeled l-ascorbic acid from labeled d-glucose☆

1L-myo-inositol 1-phosphate synthase from pollen of Lilium longiflorum. An ordered sequential mechanism.

Dissolution of Pollen Intine and Release of Sporoplasts

ASPECTS OF myo -INOSITOL METABOLISM AND BIOSYNTHESIS IN HIGHER PLANTS

Recovery of exine from mature pollens and spores