The value of mutants unable to carry out photorespiration.

Manipulation of the CO2 concentration of the atmosphere allows the selection of photorespiratory mutants from populations of seeds treated with powerful mutagens such as sodium azide. So far, barley lines deficient in activity of phosphoglycolate phosphatase, catalase, the glycine to serine conversion, glutamine synthetase, glutamate synthase, 2-oxoglutarate uptake and serine: glyoxylate aminotransferase have been isolated. In addition one line of pea lacking glutamate synthase activity and one barley line containing reduced levels of Rubisco are available. The characteristics of these mutations are described and compared with similar mutants isolated from populations of Arabidopsis. As yet, no mutant lacking glutamine synthetase activity has been isolated from Arabidopsis and possible reasons for this difference between barley and Arabidopsis are discussed. The value of these mutant plants in the elucidation of the mechanism of photorespiration and its relationships with CO2 fixation and amino acid metabolism are highlighted.

Barley mutants lacking chloroplast glutamine synthetase-biochemical and genetic analysis.

Eight mutants of barley (Hordeum vulgare cv Maris Mink) lacking the chloroplast isozyme of glutamine synthetase (EC 6.3.1.2.) were isolated by their inability to grow under photorespiratory conditions. The cytoplasmic isozyme of glutamine synthetase was present in the leaves of all the mutants, with activities comparable to the wild-type (10-12 nanokatals per gram fresh weight). The mutant plants developed normally and were fully fertile under conditions that minimize photorespiration. In 1% O(2) the rate of CO(2) fixation in leaves of one of the mutants, RPr 83/32, was the same as the wild-type, but in air this rate declined to 60% of the wild-type after 30 minutes. During this time the ammonia concentration in leaves of the mutant rose from 1 to 50 micromoles per gram fresh weight. Such ammonia accumulation in air was found in all the mutant lines. In back-crosses with the parent line, F(1) plants were viable in air. In the F(2) generation, nonviability in air and the lack of chloroplast glutamine synthetase co-segregated, in both the lines tested. These two lines and four others proved to be allelic; we designate them gln 2a-f. The characteristics of these mutants conclusively demonstrate the major role of chloroplast glutamine synthetase in photorespiration and its associated nitrogen recycling.

Characteristics of a photorespiratory mutant of barley (Hordeum vulgare L.) deficient in phosphogly collate phosphatase.

A barley mutant RPr84/90 has been isolated by selecting for plants which grow poorly in natural air, but normally in air enriched to 0.8% CO2. After 5 minutes of photosynthesis in air containing(14)CO2 this mutant incorporated 26% of the(14)C carbon into phosphoglycollate, a compound not normally labelled in wild type (cv. Maris Mink) leaves.The activity of phosphoglycollate phosphatase (EC 3.1.1.18) was 1.2 nkat mg(-1) protein at 30°C in RPr 84/90 compared to 19.2 nkat mg(-1) protein in the wild-type leaves. Phosphoglycollate phosphatase activity was not detected after protein separation by electrophoresis of leaf extracts from the mutant on polyacrylamide gels; on linear 5% acrylamide gels three bands with enzyme activity were separated from extracts of wild type plants. Gradient gel electrophoresis followed by activity staining showed two bands in Maris Mink tracks of MW 86,000 and 96,000, but no bands in 84/90. This is the first report of isozymes of phosphoglycollate phosphatase in barley which were absent in the mutant extracts. Our results confirm an earlier report of isozymes of this phosphatase in Phaseolus vulgaris [18].The photosynthetic rate of RPr 84/90 in 1% O2, 350 µl CO2 l(-1) was 9-12 mg CO2 dm(-2) h(-1) at 20°C, whereas the wild-type rate was 27-29 mg CO2 dm(-2) h(-1) at 20°C. In 21% O2, 350 µl CO2 l(-1) the rate was 2-3 mg CO2 dm(-2) h(-1) in the mutant and 20 mg CO2 dm(-2) h(-1) in the wild type.Genetic analysis has shown that the mutation segregates as a single recessive nuclear gene.

Carbon and nitrogen metabolism in barley (Hordeum vulgare L.) mutants lacking ferredoxin-dependent glutamate synthase.

Five mutant lines of barley (Hordeum vulgare L.), which are only able to grow at elevated levels of CO2, contain less than 5% of the wild-type activity of ferredoxin-dependent glutamate synthase (EC 1.4.7.1). Two of these lines (RPr 82/1 and RPr 82/9) have been studied in detail. Leaves and roots of both lines contain normal activities of NADH-dependent glutamate synthase (EC 1.4.1.14) and the other enzymes of ammonia assimilation. Under conditions that minimise photorespiration, both mutants fix CO2 at normal rates; on transfer to air, the rates drop rapidly to 15% of the wild-type. Incorporation of (14)CO2 into sugar phosphates and glycollate is increased under such conditions, whilst incorporation of radioactivity into serine, glycine, glycerate and sucrose is decreased; continuous exposure to air leads to an accumulation of (14)C in malate. The concentrations of malate, glutamine, asparagine and ammonia are all high in air, whilst aspartate, alanine, glutamate, glycine and serine are low, by comparison with the wild-type parent line (cv. Maris Mink), under the same conditions. The metabolism of [(14)C]glutamate and [(14)C]glutamine by leaves of the mutants indicates a very much reduced ability to convert glutamine to glutamate. Genetic analysis has shown that the mutation in RPr 82/9 segregates as a single recessive nuclear gene.

Carbon and nitrogen metabolism in a barley (Hordeum vulgare L.) mutant with impaired chloroplast dicarboxylate transport.

A mutant line, RPr79/2, of barley (Hordeum vulgare L. cv. Maris Mink) has been isolated that has an apparent defect in photorespiratory nitrogen metabolism. The metabolism of (14)C-labelled glutamine, glutamate and 2-oxoglutarate indicates that the mutant has a greatly reduced ability to synthesise glutamate, especially in air, although in-vitro enzyme analysis indicates the presence of wild-type activities of glutamine synthetase (EC 6.3.1.2) glutamate synthase (EC 1.4.7.1 and EC 1.4.1.14) and glutamate dehydrogenase (EC 1.4.1.2). Several characteristics of RPr79/2 are very similar to those described for glutamate-synthase-deficient barley and Arabidopsis thaliana mutants, including the pattern of labelling following fixation of (14)CO2, and the rapid rise in glutamine content and fall in glutamate in leaves on transfer to air. The CO2-fixation rate in RPr79/2 declines much more slowly on transfer from 1% O2 to air than do the rates in glutamate-synthase-deficient plants, and RPr79/2 plants do not die in air unless the temperature and irradiance are high. Analysis of (glutamine+NH3+2-oxoglutarate)-dependent O2 evolution by isolated chloroplasts shows that chloroplasts from RPr79/2 require a fivefold greater concentration of 2-oxoglutarate than does the wild-type for maximum activity. The levels of 2-oxoglutarate in illuminated leaves of RPr79/2 in air are sevenfold higher than in Maris Mink. It is suggested that RPr79/2 is defective in chloroplast dicarboxylate transport.

An investigation into the interaction between nitrogen nutrition, photosynthesis and photorespiration.

Photosynthetic CO2 assimilation, photorespiration and levels of glycollate oxidase and ribulose bisphosphate (RuBP) carboxylase were measured in barley, wheat and maize plants grown on media containing nitrate or ammonium or in plants transferred from nitrate to ammonium. The CO2 compensation point and photorespiratory CO2 release were not altered by the nitrogen growth regime nor by transfer from nitrate to ammonium. In barley and wheat plants grown on ammonium the levels of glycollate oxidase and RuBP carboxylase per unit leaf area were higher than in nitrate grown material. These differences were not evident when the results were expressed on a protein or chlorophyll basis. The ratio of glycollate oxidase activity to RuBP carboxylase activity was not altered by the nitrogen regime.

Temperature dependence of the enzymic carboxylation and oxygenation of ribulose 1,5-bisphosphate in relation to effects of temperature on photosynthesis.

Carboxylase and oxygenase activities of ribulose bisphosphate carboxylase purified from wheat were measured over the temperature range 5 to 35 degrees C either at constant O(2) and CO(2) concentrations or where the O(2) and CO(2) simulated the concentrations in water equilibrated at each temperature with the same gaseous phase. At constant CO(2) (14 micromolar) and O(2) (0.34 millimolar), the oxygenase to carboxylase ratio remained constant at 0.21 between 5 and 25 degrees C but increased to 0.26 at 35 degrees C. At O(2) and CO(2) concentrations near those expected in water equilibrated with air (21% [v/v] O(2)) containing 300 mul/l CO(2) at the various temperatures, the ratio of oxygenase to carboxylase activity increased 2.2-fold between 15 and 35 degrees C. At CO(2) and O(2) concentrations expected in water in equilibrium with subatmospheric concentrations of CO(2) in air (21% [v/v] O(2), 210 mul/l CO(2)), the oxygenase to carboxylase ratio increased from 0.25 at 10 degrees C to 0.56 at 35 degrees C. Between 20 and 30 degrees C, the apparent Q(10) value for the oxygenase reaction was 1.78 and that for the carboxylase was 1.26. Hence, the different responses of photosynthesis and photorespiration to temperature are due more to changes in the relative solubilities of CO(2) and O(2) (the solubility ratio) than to changes in kinetic parameters of the reactions catalyzed by ribulose bisphosphate carboxylase.

The enzymatic determination of bicarbonate and CO2 in reagents and buffer solutions.

A method for the determination of bicarbonate in buffer solutions between pH 7.5 and 8.75 and in stock solutions of NaHCO3 is described. The HCO-3 is reacted with phosphoenolpyruvate (PEP) in the presence of PEP carboxylase (EC 4.1.1.31) and the oxaloacetate formed reduced to malate by NADH in the reaction catalyzed by malate dehydrogenase (EC 1.1.1.37). The extent of oxidation of NADH is measured spectrophotometrically. Experiments using standard solutions show that 1 mol of NADH is oxidized per mol of HCO-3 added. The method was used to establish the precautions needed to prepare buffer solutions containing less than 1% of the bicarbonate which would be present in the same buffers in equilibrium with air.

Storage and maintaining activity of ribulose bisphosphate carboxylase/oxygenase.

Purified ribulose-1,5-bisphosphate carboxylase/oxygenase in 50% saturated (NH(4))(2)SO(4) was stable when frozen as small beads in liquid nitrogen and stored at -80 C. When stored as a slurry at 4 C most of the activity was lost within four weeks. This loss was due not only to enzyme polymerization. Activity in old preparations purified from spinach leaves, but not tobacco or tomato leaves, can be restored to the level of newly purified enzyme after storage at 4 C by treatment with 50 to 100 millimolar dithiothreitol for several hours followed by dialysis against buffer and 1 millimolar dithiothreitol before CO(2) and Mg(2+) activation and assay. Some enzyme oligomers that had been formed were not converted back to native enzyme by treatment with 100 millimolar dithiothreitol.The purified enzyme contained about 2 gram-atoms iron per mole enzyme that could not be removed by chelating agents. When the enzyme was incubated with 100 millimolar dithiothreitol and exposed to O(2), a purple dithiothreitol-iron complex was formed which could be removed by dialysis. The activities of ribulose-1,5-bisphosphate carboxylase and oxygenase were not altered by reducing the iron content to 0.7 mole per mole enzyme by treatment with dithiothreitol followed by exhaustive dialysis against iron free buffer.