The contamination of soils and water with metals has created a major environmental problem, leading to considerable losses in plant productivity and hazardous health effects. Exposure to toxic metals can intensify the production of reactive oxygen species (ROS), which are continuously produced in both unstressed and stressed plants cells. Some of the ROS species are highly toxic and must be detoxified by cellular stress responses, if the plant is to survive and grow. The aim of this review is to assess the mode of action and role of antioxidants in protecting plants from stress caused by the presence of heavy metals in the environment.

Glutamate occupies a central position in amino acid metabolism in plants. The acidic amino acid is formed by the action of glutamate synthase, utilizing glutamine and 2-oxoglutarate. However, glutamate is also the substrate for the synthesis of glutamine from ammonia, catalysed by glutamine synthetase. The alpha-amino group of glutamate may be transferred to other amino acids by the action of a wide range of multispecific aminotransferases. In addition, both the carbon skeleton and alpha-amino group of glutamate form the basis for the synthesis of gamma-aminobutyric acid, arginine, and proline. Finally, glutamate may be deaminated by glutamate dehydrogenase to form ammonia and 2-oxoglutarate. The possibility that the cellular concentrations of glutamate within the plant are homeostatically regulated by the combined action of these pathways is examined. Evidence that the well-known signalling properties of glutamate in animals may also extend to the plant kingdom is reviewed. The existence in plants of glutamate-activated ion channels and their possible relationship to the GLR gene family that is homologous to ionotropic glutamate receptors (iGluRs) in animals are discussed. Glutamate signalling is examined from an evolutionary perspective, and the roles it might play in plants, both in endogenous signalling pathways and in determining the capacity of the root to respond to sources of organic N in the soil, are considered.

Photorespiration results from the oxygenase reaction catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase. In this reaction glycollate-2-phosphate is produced and subsequently metabolized in the photorespiratory pathway to form the Calvin cycle intermediate glycerate-3-phosphate. During this metabolic process, CO2 and NH3 are produced and ATP and reducing equivalents are consumed, thus making photorespiration a wasteful process. However, precisely because of this inefficiency, photorespiration could serve as an energy sink preventing the overreduction of the photosynthetic electron transport chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic CO2 assimilation. Furthermore, photorespiration provides metabolites for other metabolic processes, e.g. glycine for the synthesis of glutathione, which is also involved in stress protection. In this review we describe the use of photorespiratory mutants to study the control and regulation of photorespiratory pathways. In addition, we discuss the possible role of photorespiration under stress conditions, such as drought, high salt concentrations and high light intensities encountered by alpine plants.

Interest in plant asparagine has rapidly taken off over the past 5 years following the report that acrylamide, a neurotoxin and potential carcinogen, is present in cooked foods, particularly carbohydrate-rich foods such as wheat and potatoes which are subjected to roasting, baking or frying at high temperatures. Subsequent studies showed that acrylamide could be formed in foods by the thermal degradation of free asparagine in the presence of sugars in the Maillard reaction. In this article, our current knowledge of asparagine in plants and in particular its occurrence in cereal seeds and potatoes is reviewed and discussed in relation to acrylamide formation. There is now clear evidence that soluble asparagine accumulates in most if not all plant organs during periods of low rates of protein synthesis and a plentiful supply of reduced nitrogen. The accumulation of asparagine occurs during normal physiological processes such as seed germination and nitrogen transport. However, in addition, stress-induced asparagine accumulation can be caused by mineral deficiencies, drought, salt, toxic metals and pathogen attack. The properties and gene regulation of the enzymes involved in asparagine synthesis and breakdown in plants are discussed in detail.

The roles of two cytosolic maize glutamine synthetase isoenzymes (GS1), products of the Gln1-3 and Gln1-4 genes, were investigated by examining the impact of knockout mutations on kernel yield. In the gln1-3 and gln1-4 single mutants and the gln1-3 gln1-4 double mutant, GS mRNA expression was impaired, resulting in reduced GS1 protein and activity. The gln1-4 phenotype displayed reduced kernel size and gln1-3 reduced kernel number, with both phenotypes displayed in gln1-3 gln1-4. However, at maturity, shoot biomass production was not modified in either the single mutants or double mutants, suggesting a specific impact on grain production in both mutants. Asn increased in the leaves of the mutants during grain filling, indicating that it probably accumulates to circumvent ammonium buildup resulting from lower GS1 activity. Phloem sap analysis revealed that unlike Gln, Asn is not efficiently transported to developing kernels, apparently causing reduced kernel production. When Gln1-3 was overexpressed constitutively in leaves, kernel number increased by 30%, providing further evidence that GS1-3 plays a major role in kernel yield. Cytoimmunochemistry and in situ hybridization revealed that GS1-3 is present in mesophyll cells, whereas GS1-4 is specifically localized in the bundle sheath cells. The two GS1 isoenzymes play nonredundant roles with respect to their tissue-specific localization.

A catalase‐deficient mutant (RPr 79/4) and the wild‐type (cv. Maris Mink) barley (Hordeum vulgare L.) counterpart, were grown for 3 weeks in high CO2 (0.7%) and then transferred to air and ozone (120 nl 1−1) in the light and shade for a period of 4 days. Leaves and roots were analysed for catalase (CAT, EC 1.11.1.6), superoxide dismutase (SOD, EC 1.15.1.1) and glutathione reductase (GR, EC 1.6.4.2) activities. CAT activity in the leaves of the RPr 79/4 catalase‐deficient mutant was around 5‐10% of that determined in Maris Mink, but in the roots, both genotypes contained approximately the same levels of activity. CAT activity in Maris Mink increased in the leaves after transferring plants from 0.7% CO2 to air or ozone, reaching a maximum of 5‐fold, after 4 days in shade and ozone. For the catalase‐deficient mutant, only small increases in CAT activity were observed in light/air and light/ozone treatments. In the roots, CAT activity decreased consistently in both genotypes, after plants were transferred from 0.7% CO2. The total soluble SOD activity in the leaves and roots of both genotypes increased after plants were transferred from 0.7% CO2. The analysis of SOD isolated from leaves following non‐denaturing PAGE, revealed the presence of up to eight SOD isoenzymes classified as Mn‐SOD or Cu/Zn‐SODs; Fe‐SOD was not detected. Significant changes in Mn‐ and Cu/Zn‐SOD isoenzymes were observed; however, they could not account for the increase in total SOD activity. In leaves, GR activity also increased in Maris Mink and RPr 79/4, following transfer from 0.7% CO2; however, no constant pattern could be established, while in roots, GR activity was reduced after 4 days of the treatments. The data suggest that elevated CO2 decreases oxidative stress in barley leaves and that soluble CAT and SOD activities increased rapidly after plants were transferred from elevated CO2, irrespective of the treatment (light, shade, air or ozone).

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Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity is modulated in vivo either by reaction with CO2 and Mg2+ to carbamylate a lysine residue in the catalytic site, or by the binding of inhibitors within the catalytic site. Binding of inhibitors blocks either activity or the carbamylation of the lysine residue that is essential for activity. At night, in many species, 2-carboxyarabinitol-1-phosphate (CA1P) is formed which binds tightly to Rubisco, inhibiting catalytic activity. Recent work has shown that tight-binding inhibitors can also decrease Rubisco activity in the light and contribute to the regulation of Rubisco activity. Here we determine the influence that such inhibitors of Rubisco exert on catalytic activity during drought stress. In tobacco plants, 'total Rubisco activity', i.e. the activity following pre-incubation with CO2 and Mg2+, was positively correlated with leaf relative water content. However, 'total Rubisco activity' in extracts from leaves with low water potential increased markedly when tightly bound inhibitors were removed, thus increasing the number of catalytic sites available. This suggests that in tobacco the decrease of Rubisco activity under drought stress is not primarily the result of changes in activation by CO2 and Mg2+ but due rather to the presence of tight-binding inhibitors. The amounts of inhibitor present in leaves of droughted tobacco based on the decrease in Rubisco activity per mg soluble protein were usually much greater than the amounts of the known inhibitors (CA1P and 'daytime inhibitor') that can be recovered in acid extracts. Alternative explanations for the difference between maximal and total activities are discussed.