Effect of L-Methionine Sulphoximine on the Products of Photosynthesis in Wheat (<i>Triticum aestivum</i>) Leaves

Walker, Kevin A.; Keys, A. J.; Givan, Curtis V.

doi:10.1093/jxb/35.12.1800

Cited by 43 publications

(20 citation statements)

References 26 publications

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“…To demonstrate that nitrogen enters the photorespiratory pathway through the reaction catalyzed by serine-glyoxylate transaminase, AOA (an inhibitor of transamination reactions and glycine decarboxylation [5]) has been used in conjunction with MSX, since when supplied together, ammonium production resulting from photorespiration was assumed to be abolished (13,29). Similar results were obtained in the presence of MSX together with other inhibitors of glycine decarboxylation, isonicotinyl hydrazide (12,13,27) or aminoacetonitrile (1), and these results supported the idea that most, if not all, of the ammonium detected in vivo was derived from photorespiration.However, photorespiratory ammonium production is not the only source of ammonium detected within leaf tissues in the presence of MSX. Increased levels of '5N [ammonium] in the presence of MSX and 2% 02 were observed after the addition of ['5N]KNO3 indicating active nitrate and nitrite reductase activities (6).…”

supporting

confidence: 75%

“…Results obtained by using MSX + AOA (13,29), MSX + isonicotinyl hydrazide (12,13,27), or MSX + aminoacetonitrile (1) have been presented as support for the idea that the major source of ammonium in leaves (an order of magnitude higher than primary assimilation [10]) is from photorespiration. Similar conclusions have also been reported from studies using only MSX (2, 7, 10, 17).…”

Section: Below)mentioning

confidence: 95%

“…Reduced rates of photosynthesis (measured as net CO2 fixation) have also been observed in the presence of aminoacetonitrile (3), and AOA (9), supporting the idea that reduced carbon-cycling inhibits photosynthesis. Decreased rates of net CO2 fixation are not a result of stomatal closure in the presence of MSX (17,27), nor does MSX inhibit photosynthesis in isolated chloroplasts (15). However, other mechanisms for the inhibition of photosynthesis by MSX may exist.…”

Section: Below)mentioning

confidence: 99%

“…It has been suggested that the effect of MSX on CO2 fixation results from uncoupling of electron transport by increased ammonium (1,15,17). Since directly supplied ammonium has a lesser effect on photosynthesis (7,27), an alternate mechanism explaining the effect of MSX inhibition might be the reduction of recycling of carbon skeletons from photorespiration to the Calvin cycle (2, 7, 27). Reduced rates of photosynthesis (measured as net CO2 fixation) have also been observed in the presence of aminoacetonitrile (3), and AOA (9), supporting the idea that reduced carbon-cycling inhibits photosynthesis.…”

Section: Below)mentioning

confidence: 99%

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Effect of Methionine Sulfoximine on Asparaginase Activity and Ammonium Levels in Pea Leaves

Sieciechowicz

Joy²,

Ireland³

1989

Plant Physiol.

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In developing leaves of Pisum sativum the levels of ammonium did not change during the light-dark photoperiod even though asparaginase (EC 3.5.1.1) did; asparaginase activity in detached leaves doubled during the first 2.5 hours in the light. When these leaves were supplied with I millimolar methionine sulfoximine (MSX, an inhibitor of glutamine synthetase, GS, activity) at the beginning of the photoperiod, levels of ammonium increased 8-to 10-fold, GS activity was inhibited 95%, and the light-stimulated increase in asparaginase activity was completely prevented, and declined to less than initial levels. When high concentrations of ammonium were supplied to leaves, the light-stimulated increase of asparaginase was partially prevented. However, it was also possible to prevent asparaginase increase, in the absence of ammonium accumulation, by the addition of MSX together with aminooxyacetate (AOA, which inhibits transamination and some other reactions of photorespiratory nitrogen cycling). AOA alone did not prevent light-stimulated asparaginase increase; neither MSX, AOA, or elevated ammonium levels inhibited the activity of asparaginase in vitro. These resuits suggest that the effect of MSX on asparaginase increase is not due solely to interference with photorespiratory cycling (since AOA also prevents cycling, but has no effect alone), nor to the production of high ammonium concentration or its subsequent effect on photosynthetic mechanisms. MSX must have further inhibitory effects on metabolism.It is concluded that accumulation of ammonium in the presence of MSX may underestimate rates of ammonium tumover, since liberation of ammonium from systems such as asparaginase is reduced by the effects of MSX.The degradation of asparagine is important in supplying nitrogen for amino acid and protein synthesis in developing leaves, and '5N-labeling studies have indicated that up to 75% of the nitrogen required for growth in half-expanded pea leaves was derived from the amide-nitrogen of asparagine (25). The amide-nitrogen of asparagine can be liberated directly by asparaginase (EC 3.5.1.1.) (8) port (21), and, asparaginase activity is also regulated by other factors (22). Increased asparaginase activity should result in increased production of aspartate and ammonium, which are quickly metabolized (25). The oxidation of glycine during photorespiration produces appreciable amounts ofammonium in the light, and estimates based on treatments with MSX3 (an irreversible inhibitor of GS [24]), have suggested that release of photorespiratory ammonium exceeds by 10-fold or more the rate ofproduction from nitrate reduction (10). To demonstrate that nitrogen enters the photorespiratory pathway through the reaction catalyzed by serine-glyoxylate transaminase, AOA (an inhibitor of transamination reactions and glycine decarboxylation [5]) has been used in conjunction with MSX, since when supplied together, ammonium production resulting from photorespiration was assumed to be abolished (13,29). Similar results were obtained in t...

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supporting

confidence: 75%

Section: Below)mentioning

confidence: 95%

Section: Below)mentioning

confidence: 99%

Section: Below)mentioning

confidence: 99%

See 2 more Smart Citations

Effect of Methionine Sulfoximine on Asparaginase Activity and Ammonium Levels in Pea Leaves

Sieciechowicz

Joy²,

Ireland³

1989

Plant Physiol.

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“…Previous studies in wheat (7,20) have shown that the MSOinduced inhibition of photosynthesis was delayed by the addition of externally added amino acids, although this resulted in a marked increase in ammonia levels. The lower rate of decline of photosynthesis has been attributed to the increased availability of amino donors to the photorespiratory cycle and the maintenance of carbon flux through the photorespiratory pathway (19).…”

Section: Discussionmentioning

confidence: 95%

Photorespiratory Ammonia Does Not Inhibit Photosynthesis in Glutamate Synthase Mutants of Arabidopsis

Morris

Layzell²,

Canvin³

1989

Plant Physiol.

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Exposure of ferredoxin-dependent glutamate synthase (EC 1.4.7.1) mutants of Arabidopsis thallana to photorespiratory conditions resulted in the accumulation of NH4+ and the inhibition of photosynthesis. However, upon transfer from 2% 02, 350 microliters per liter C02, to 21% 02, 350 microliters per liter C02, net photosynthesis declined at a slower rate in methionine sulfoximine treated leaf discs relative to controls. The recovery of photosynthesis was also more rapid in MSO-treated leaf discs although ammonia levels were more than threefold higher. Photosynthesis in leaf discs treated with azaserine was inhibited more than controls when transferred to 21% 02 and recovered less than controls when retumed to 2% 02 although NH4+ levels were not significantly different. The results obtained are consistent with the view that the rapid inhibition of photosynthesis in the glutamate synthase mutants in photorespiratory conditions is not due to the accumulation of NH4+ but rather to the depletion of amino donors for glyoxylate and the consequent effects of glyoxylate on the lack of return of carbon to the chloroplast.The exposure ofglutamate synthase mutants ofArabidopsis thaliana to normal air results in the accumulation of NH4' and glutamine and the inhibition of photosynthesis. The inhibition of photosynthesis in these mutants has been attributed to (a) a decline in the return of carbon skeletons to the Calvin cycle (1,16,18), (b) a triose phosphate limitation on photosynthesis (1), (c) a buildup of inhibitory metabolites such as glyoxylate (4) mutants cannot be attributed to a decline in Rubisco activity and may be directly related to the accumulation of ammonia.In Arabidopsis, glutamine and glutamate make up more than 60% of the total soluble reduced N in the leaf and are the largest potential amino donors to the photorespiratory N cycle ( 12). However, the availability of glutamine as a source of amino donors to glyoxylate is limited by the low rates of NADH-GluS and Fd-GluS activity (12). Because MSO would inhibit the reassimilation of photorespiratory ammonia (14), the addition of MSO to the leaves of GluS mutants might be expected to result in a more rapid increase in ammonia levels and a more rapid inhibition of photosynthesis if photophosphorylation was uncoupled. However, by preventing the utilization of glutamate by GS, MSO might also increase the availability of glutamate as an amino donor to glyoxylate and thus help to maintain the flux of carbon through the photorespiratory pathway. This paper examines the effect of MSO and AZA, an inhibitor of glutamate synthase on the accumulation of ammonia and the inhibition of photosynthesis in the GluS mutants of Arabidopsis thaliana.While this paper was in preparation, Grumbles (5) published a study which compared the photosynthetic decline of glutamate synthase mutants to that in wild type plants which were fed with ammonia. Grumbles concluded that ammonia was not a factor in the inhibition of photosynthesis and suggested that the inhibition of photosyn...

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Photosynthesis: Carbon Metabolism: By Day and by Night

Holtum

Kelly

Latzko

Thirty Years of Photosynthesis 1974–2004

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Effect of L-Methionine Sulphoximine on the Products of Photosynthesis in Wheat (Triticum aestivum) Leaves

Cited by 43 publications

References 26 publications

Effect of Methionine Sulfoximine on Asparaginase Activity and Ammonium Levels in Pea Leaves

Effect of Methionine Sulfoximine on Asparaginase Activity and Ammonium Levels in Pea Leaves

Photorespiratory Ammonia Does Not Inhibit Photosynthesis in Glutamate Synthase Mutants of Arabidopsis

Photosynthesis: Carbon Metabolism: By Day and by Night

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