Showing papers by "Christine H. Foyer published in 1999"

Photorespiratory glycine enhances glutathione accumulation in both the chloroplastic and cytosolic compartments

Abstract: phosphoglycerate (PGA), phosphoenolpyruvate, glyc- erate, and hydroxypyruvate did not. Glycine produced Transformed poplars overexpressing c-glutamylcyst- from glycollate was therefore required for maximal eine synthetase (c-ECS) in the chloroplast (Lggs) were GSH accumulation in both the chloroplastic and cyto- used to investigate chloroplastic biosynthesis of gluta- solic compartment. Production of glycine from PGA thione (GSH). In Lggs leaves, GSH contents were failed to meet the demand of increased GSH synthetic enhanced by up to 3.7-fold. In general, the highest capacity. GSH contents were observed in lines with highest c- glutamylcysteine (c-EC) contents. These lines had rela- Key words: Glutathione, c-glutamyl cysteine, amino acids, tively low glycine. In darkness, foliar GSH decreased photorespiration, transformed poplar. and c-EC increased. Illumination of pre-darkened Lggs in air resulted in a 5-fold decrease in the c-EC5GSH ratio. This light-induced decrease was largely abol- Introduction ished if leaves were illuminated at high CO 2 . Con- Current interest in factors influencing GSH accumulation sequently, the c-EC5GSH ratio of illuminated leaves in plant tissues has been fostered by awareness of the was much higher at high CO 2 than in air. At high CO 2 plethora of physiological roles played by this tripeptide

Acclimation of photosynthesis, H2O2 content and antioxidants in maize (Zea mays ) grown at sub-optimal temperatures

Abstract: Maize plants were grown at 14, 18 and 20 °C until the fourth leaf had emerged. Leaves from plants grown at 14 and 18 °C had less chlorophyll than those grown at 20 °C. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity was decreased at 14 °C compared with 20 °C, but the activation state was highest at 14 °C. Growth at 14 °C increased the abundance (but not the number) of Rubisco breakdown products. Phosphoenolpyruvate carboxylase (PEPC) activity was decreased at 14 °C compared with 20 °C but no chilling-dependent effects on the abundance of the PEPC protein were observed. Maximal extractable NADP-malate dehydrogenase activity increased at 14 °C compared with 20 °C whereas the glutathione pool was similar in leaves from plants grown at both temperatures. Foliar ascorbate and hydrogen peroxide were increased at 14 °C compared with 20 °C. The foliar hydrogen peroxide content was independent of irradiance at both growth temperatures. Plants grown at 14 °C had decreased rates of CO2 fixation together with decreased quantum efficiencies of photosystem (PS) II in the light, although there was no photo-inhibition. Growth at 14 °C decreased the abundance of the D1 protein of PSII and the PSI psaB gene product but the psaA gene product was largely unaffected by growth at low temperatures. The relationships between the photosystems and the co-ordinate regulation of electron transport and CO2 assimilation were maintained in plants grown at 14 °C.

Leaves in the dark see the light.

Abstract: It is not only animals that suffer from sunburn. Plants exposed to excess sunlight suffer photodamage caused by the production of oxygen free radicals, eventually resulting in a shutdown in photosynthesis. Karpinski et al . now report that plants have developed a sophisticated systemic messenger system in which signals produced in leaves exposed to high light intensities travel to distant parts of the plant (that have not been exposed to bright light) and switch on expression of protective antioxidant genes. In their Perspective, Foyer and Noctor discuss the putative identity of the systemic messenger molecule.

Changes in apoplastic antioxidants induced by powdery mildew attack in oat genotypes with race non-specific resistance

Abstract: Three oat (Avena sativa L.) lines which show differential responses to attack by the biotrophic fungal pathogen Blumeria graminis DC f. sp. avenae Marchal, which causes powdery mildew, were studied: Maldwyn shows the strongest resistance in adult plants; Selma shows greater susceptibility; while a Selma × Maldwyn hybrid, OM1387, has a similar degree of resistance to Maldwyn. Host responses to pathogen attack were complete 48 h after inoculation but largely accomplished within the first 24 h, the point when material was taken for enzyme and metabolic assays. In Maldwyn and OM1387 about 80% of attacked cells showed localized autofluorescent host-cell responses but this fell to less than 20% in Selma. A cytoplasmic marker enzyme, glucose 6-phosphate dehydrogenase, was used to determine contamination of the apoplastic extracts by cellular components. After correction for cytoplasmic contamination, up to 4% of the total foliar activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase activities were detected in the apoplast. The apoplast contained about 2% of the total foliar glutathione pool and dehydroascorbate, but not ascorbate, at values amounting to 10% of the total foliar ascorbate plus dehydroascorbate pool. Twenty-four hours after inoculation the foliar or apoplastic ascorbate pools were similar in inoculated and control leaves. Foliar catalase activity increased in both susceptible and resistant responses. Resistance correlated with increased total foliar glutathione, an increase in the ratio of reduced to oxidized glutathione and with decreased total activities of foliar ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase.

Starch synthesis in tomato remains constant throughout fruit development and is dependent on sucrose supply and sucrose synthase activity

Abstract: Studies designed to investigate the cellular pathway of phloem unloading were conducted on two tomato lines with either high or low fruit invertase activities. Experiments were based on determination of the degree to which 3 H label from [ 3 H]-(fructosyl)-sucrose was randomized between fructose and glucose following exposure of excised fruit to a pulse of labelled sucrose delivered through the pedicels. Fruit from the low invertase line harvested 10, 20 and 40 d after anthesis had similar sucrose uptake kinetics to the high invertase line. A positive correlation was found between sucrose synthase activity and sucrose uptake in both low and high invertase lines. In contrast, no correlation was observed between acid or neutral invertase activities and sucrose uptake. Within the putative apoplasmic sap collected from fruit, label in [ 3 H]-(fructosyl)-sucrose was randomized between the free hexoses and sucrose hexose moieties. Label asymmetry was retained in sucrose on arrival within the tissues. Randomization patterns were similar in both the low and high acid invertase lines. These data support the view that sucrose imported into the fruit was not exposed to extracellular hydrolysis. This suggests that movement from the phloem is likely to occur predominantly through a symplastic pathway. About 25% of the sucrose taken up by the fruit was converted into starch regardless of fruit age, suggesting that starch turnover remains constant throughout fruit development and that starch synthesis was dependent on sucrose supply.

Overexpression of sucrose phosphate synthase increases sucrose unloading in transformed tomato fruit

Abstract: Sucrose unloading and sink activity were examined in tomato plants (Lycopersicon esculentum) overexpressing sucrose phosphate synthase (SPS; EC 2.3.1.14). Like the leaves, the fruit of the transformed tomato plants had elevated (2.4-fold) SPS activity. SPS overexpression in tomato fruit did not significantly change acid invertase, and only slightly reduced ADPglc ppase activity, but enhanced sucrose synthase activity by 27%. More importantly, the amount of sucrose unloaded into the fruit was considerably increased. Using [ 3 H]- (fructosyl)-sucrose in in vitro unloading experiments with harvested 20-d-old fruit, 70% more sucrose was unloaded into the transformed fruits compared to the untransformed controls. Furthermore, the turnover of the sucrose unloaded into the fruit of transformed plants was 60% higher than that observed in the untransformed controls. Taken together, these results demonstrate that SPS overexpression increases the sink strength of transformed tomato fruit.

Relationships Between Antioxidant Metabolism and Carotenoids in the Regulation of Photosynthesis

Abstract: The large driving forces produced in photosynthesis require precise control and regulation to prevent potentially destructive side reactions involving active oxygen species that damage pigments and proteins. Oxygen can be activated either by energy transfer, yielding singlet oxygen, or by reduction, yielding superoxide or hydrogen peroxide. At Photosystem I, the lifetime of the excited chlorophyll singlet state within the antenna pigment bed is short and little threat is posed by formation of highly reactive singlet oxygen. The situation is completely different in Photosystem II. Here, the lifetime of singlet chlorophyll is sufficiently long to allow significant formation of chlorophyll triplet states able to transfer energy to ground state triplet oxygen, generating singlet oxygen. Carotenoids intervene to control these processes in at least two important ways. First, pigments such as β-carotene are capable of directly quenching both triplet chlorophyll and singlet oxygen states (the so-called triplet valve mechanism). Second, the xanthophyll cycle is involved in lowering the yield of triplet chlorophyll formation by pre-emptive quenching of excited singlet state chlorophyll, a mechanism that can have a high quantum yield value for energy dissipation. The chief difference between these two mechanisms is that the xanthophyll cycle is inducible and subject to regulation, whereas the triplet valve pathway is constitutive and unregulated. Although formation of singlet oxygen must be avoided or controlled, chloroplasts have exploited the potential of oxygen chemistry to drive and regulate metabolism while minimizing the deleterious effects of uncontrolled interactions with oxygen. Hence, while the potential of the chloroplast as a source of oxidative stress is large, in reality this organelle offers minimal risk because of pre-emptive regulation and effective defense. The production of superoxide and hydrogen peroxide by the thylakoid membranes is limited by efficient control of electron transport. This regulation limits the potential for oxidative damage and prevents high rates of electron transport to oxygen. Effective antioxidant defense ensrures rapid elimination of active forms of oxygen further preventing oxidative damage.

Photoinhibition of photosystem II in tobacco plants overexpressing glutathione reductase and poplars overexpressing superoxide dismutase

Abstract: We studied photoinhibition in two cultivars of tobacco (Nicotiana tabacum L.) expressing the bacterial gor gene in the cytosol and in four lines of poplar (Populus tremula×P. alba) expressing the FeSOD gene of Arabidopsis thaliana in the chloroplast. The respective total activities of glutathione reductase (EC 1.6.4.2) in leaves of gor tobaccos and superoxide dismutase (EC 1.15.1.1) in the FeSOD poplars were 5–8 times higher than in the respective untransformed control plants. Leaves of control and transformed plants were subjected to high-light stress at 20°C, and photoinhibition of photosystem II (PSII) was measured by oxygen evolution and chlorophyll fluorescence. The leaves were illuminated both in the presence and absence of lincomycin, which inhibits chloroplast protein synthesis. In both cases, the time course of loss of PSII activity was identical in plants overproducing superoxide dismutase (SOD) and in the untransformed controls, suggesting that the ability to convert superoxide to hydrogen peroxide is not a limiting factor in protection against photoinhibition, or in the repair of photoinhibitory damage or that the site of O2− production is not accessible to the transgene product. The rate constant of photoinhibition, measured in lincomycin-treated leaves, was smaller in glutathione reductase (GR) overproducing tobacco cv. Samsun than in the respective wild-type, but this difference was not seen in cv. Bel W3. The steady-state level of PSII activity measured when the PSII repair cycle was allowed to equilibrate with photoinhibitory damage under high light was not higher in the GR overproducing cv. Samsun, suggesting that the repair of photoinhibitory damage was not enhanced in plants overproducing GR in the cytosol.

Ascorbate metabolism in potato leaves supplied with exogenous ascorbate.

Abstract: (1999). Ascorbate Metabolism in Potato Leaves Supplied with Exogenous Ascorbate. Free Radical Research: Vol. 31, No. sup1, pp. 171-179.

Acclimation of the summer annual species, Lolium temulentum, to CO2 enrichment

Abstract: Lolium temulentum L. Ba 3081 was grown hydroponically in air (350 mmol mol(-1) CO(2)) and elevated CO(2) (700 mmol mol(-1) CO(2)) at two irradiances (150 and 500 mmol m(-2) s(-1)) for 35 days at which point the plants were harvested. Elevated CO(2) did not modify relative growth rate or biomass at either irradiance. Foliar carbon-to-nitrogen ratios were decreased at elevated CO(2) and plants had a greater number of shorter tillers, particularly at the lower growth irradiance. Both light-limited and light-saturated rates of photosynthesis were stimulated. The amount of ribulose-1, 5-bisphosphate carboxylase-oxygenase (Rubisco) protein was increased at elevated CO(2), but maximum extractable Rubisco activities were not significantly increased. A pronounced decrease in the Rubisco activation state was found with CO(2) enrichment, particularly at the higher growth irradiance. Elevated-CO(2)-induced changes in leaf carbohydrate composition were small in comparison to those caused by changes in irradiance. No CO(2)-dependent effects on fructan biosynthesis were observed. Leaf respiration rates were increased by 68% in plants grown with CO(2) enrichment and low light. We conclude that high CO(2) will only result in increased biomass if total light input favourably increases the photosynthesis-to-respiration ratio. At low irradiances, biomass is more limited by increased rates of respiration than by CO(2)-induced enhancement of photosynthesis.