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.

/pdf/biofortification-of-crops-with-seven-mineral-elements-often-3g2xc3trnk.pdf

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

Publisher Summary This chapter discusses the biosynthesis and metabolism in myo -inositol. The awareness about the role of myo -inositol in carbohydrate metabolism emerges not only from an appreciation of the biosynthetic origin and metabolic fate of myo -inositol in higher plants, but also from the realization that hitherto unknown or poorly understood biochemical processes may at present be reconsidered in the light of new observations. Virtually, nothing is known concerning the regulation of myo -inositol biosynthesis, myo -Inositol-1-P synthase appears to be the only enzyme involved in myo-inositol biosynthesis, plant or animal. Conversion of myo -inositol-1-P to inositol must be studied more thoroughly as the free inositol pool is a very important nutritional reserve to the newly germinating seed or the germinating pollen grain. In regard to the metabolism of myo -inositol, there is very little information available regarding the fate of myo -inositol as derived from the reserves of phytic acid in the germinating seed. The information gathered suggests that the core substance of phytic acid is every bit as useful to the new plant as the phosphate it sheds.

myo-Inositol: Biosynthesis and Metabolism

Localization of constitutive phytases in lily pollen and properties of the pH 8 form

Determination of ascorbic acid in algae by high-performance liquid chromatography on strong cation-exchange resin with electrochemical detection.

The product of myo-inositol-1-phosphate synthase, EC 5.5.1.4, from mature pollen of Lilium longiflorum Thunb., cv Ace (Easter lily) and that of myo-inositol kinase, EC 2.7.1.64, from wheat germ has been identified as 1l-myo-inositol-1-phosphate by gas chromatography of its trimethylsilyl-methyl phosphate derivative on a glass capillary column bearing a chiral phase.

Enantiomeric Form of myo-Inositol-1-Phosphate Produced by myo-Inositol-1-Phosphate Synthase and myo-Inositol Kinase in Higher Plants

d-Glucose 6-phosphate cycloaldolase is inhibited 83% by 066 mm EDTA and stimulated 17-fold by 06 mm KCl Dihydroxyacetone phosphate, an analog of the last three carbons in the proposed intermediate, d-xylo-5-hexulose 6-phosphate, acts as a partially competitive inhibitor Treatment with NaBH4 in the presence of dihydroxyacetone phosphate does not cause permanent inactivation as would be expected if a Schiff base were being formed In these properties it resembles a type II, metal-containing aldolase Photooxidation in the presence of Rose Bengal inactivates this enzyme NAD+ partially protects against this photooxidation Cells grown on medium lacking myoinositol had four times as much enzyme activity as cells grown on medium containing 100 mg of myoinositol per liter

/pdf/d-glucose-6-phosphate-cycloaldolase-inhibition-studies-and-kzequll4cz.pdf

Frank A. Loewus

Papers

myo-Inositol: Biosynthesis and Metabolism

Localization of constitutive phytases in lily pollen and properties of the pH 8 form

Determination of ascorbic acid in algae by high-performance liquid chromatography on strong cation-exchange resin with electrochemical detection.

Enantiomeric Form of myo-Inositol-1-Phosphate Produced by myo-Inositol-1-Phosphate Synthase and myo-Inositol Kinase in Higher Plants

d-Glucose 6-Phosphate Cycloaldolase: Inhibition Studies and Aldolase Function