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.

Corrigendum to “Biosynthesis and metabolism of ascorbic acid in plants and of analogs of ascorbic acid in fungi”: [Phytochemistry 52 (1999) 193–210]

Further Studies on myo-Inositol-1-phosphatase from the Pollen

Addition ofmyo-inositol topentaerythlutolbased germination media repressed theconversion ofD_11-14CIghicose tolabeled uronosyl and pentosyl units oftube wall pectic substance inMly pollen (Liilumlngffiorum Thunb.). Conversion ofn-11-14Clglucose tolabeled glucosyl, galactosyl, andrhamnosyl umats wasunaffected. Thereverse experiment, adtion of D-glucose topentaerythritolbased media, failed toaffect theconversion of myo-12-"Hlnositol touronosyl adpentosyl uitsaltbhogh theflow oflabel into products ofmyo-inositaol-nked glucogenesis wasblocked. Results of these experiments arediscussed intermsofafunctional myo-nositol oxidation pathway. nI1-14ClGhuose-labeled polln tubes contain alabeed, 70%ethyl alcohol-soluble, acidic Compoun whose formation Isblocked bythemyoinositol antaIst, 2-O,C-methylene-myo-1nositol (Chen etal. 1977 Plant Physiol. 59:6S8). Tlds compound hasbeenidendfied asL-malic acid.

Evidence for a Functional myo-Inositol Oxidation Pathway in

Sporoplasts (pollen protoplasts) are produced in quantity from pollen of Lilium longiflorum Thunb. by exposure to 4-methylmorpholine N-oxide (MMNO). Treatment for 1 h at 25°C with aqueous MMNO, 15% w/v, in buffered solution, pH 5.3, containing 0.5% cellulase, 0.25% pectinase and 0.1% bovine serum albumin loosens the attachment of the sporoplast to exine but fails to effect release. Displacement from exine is achieved by a series of three treatments. First, MMNO-treated pollen is suspended in a buffer containing 0.5 mM EDTA, 0.6 M mannitol and 10 mM MES, pH 5.5 which effects release of sporoplasts from exines (25–30%). Then it is suspended in a buffer containing 5 mM CaCl2, 0.6 M mannitol and 5 roM MES, pH 5.5. Finally, it is suspended in 5 mM CaCl2, 0.3 M pentaerythritol and 3 mM MES, pH 5.3. Separation of sporoplasts from exines and undigested pollen is accomplished on a discontinuous sucrose density gradient. Purified sporoplasts are readily ruptured by passing the suspension through a 15 um3 nylon mesh. Data is presented on cell-free distribution within selfgenerated Percoll gradients of phytase and a phytate-rich component.

Frank A. Loewus

Papers

Corrigendum to “Biosynthesis and metabolism of ascorbic acid in plants and of analogs of ascorbic acid in fungi”: [Phytochemistry 52 (1999) 193–210]

Further Studies on myo-Inositol-1-phosphatase from the Pollen

Evidence for a Functional myo-Inositol Oxidation Pathway in

Preparation of Sporoplasts for Studies of Pollen Physiology